EPA-600/1-
October 1975
VAPOR-PHASE ORGANIC POLLUTANTS
VOLATILE HYDROCARBONS AND OXIDATION PRODUCTS
Panel on Vapor-Phase Organic Pollutants
Committee on Medical and Biologic Effects
of Environmental Pollutants
National Research Council
Washington, D.C. 20Ul8
Contract No. 68-02-05^2
Project Officer
F. Gordon Hueter
Criteria and Special Studies Office
Health Effects Research Laboratory
Research Triangle Park, N.C., 27711
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
HEALTH EFFECTS RESEARCH LABORATORY
RESEARCH TRIANGLE PARK, N.C. , 27711
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DISCLAIMER
This report has been reviewed by the Health Effects Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection
Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
NOTICE
The project which is the subject of this report was approved
by the Governing Board of the National Research Council, acting in
behalf of the National Academy of Sciences. Such approval reflects
the Board's judgement that the project is of national importance and
appropriate with respect to both the purposes and resources of the
National Research Council.
The members of the committee selected to undertake this project
aod prepare this report were chosen for recognized scholarly
competence and with due consideration for the balance of disciplines
appropriate to the project. Responsibility for the detailed aspects
of this report rests with that committee.
Each report issuing from a study committee of the National
Research Council is reviewed by an independent group of qualified
individuals according to procedures established and monitored by the
Report Review Committee of the National Academy of Sciences. Distribution
of the report is approved, by the President of the Academy, upon
satisfactory completion of the review process.
11
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PANEL ON VAPOR-PHASE ORGANIC POLLUTANTS
BENJAMINL. VAN DUUREN, Institute of Environmental Medicine,
New York University Medical Center, New York, Co-Chairman
JOHN R. GOLDSMITH, Department of Public Health, Berkeley,
California, Co-Chairman
JULIAN Bo ANDELMAN, Graduate School of Public Health, University
of Pittsburgh, Pittsburgh, Pennsylvania
JOHN C. CRAIG, Department of Pharmaceutical Chemistry, University
of California, San Francisco, California
HANS L. FALK, National Institute of Environmental Health Sciences,
Research Triangle Park, North Carolina
JEROME KLEINERMAN, Department of Pathology, St. Luke's Hospital,
Cleveland, Ohio
ROBERT W. MURRAY, Department of Chemistry, University of Missouri,
St. Louis, Missouri
DANIEL SWERN, Department of Chemistry, Temple University,
Philadelphia, Pennsylvania
ELIZABETH E. FORCE, Division of Medical Sciences, National Research
Council, Washington, D. C., Staff Officer
T. D. BOAZ, JR., Division of Medical Sciences, National Research
Council, Washington, D. C., Staff Officer
iii
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CONSULTANTS
OSCAR J. BALCHUM, Los Angeles County - USC Medical Center, Los Angeles,
California
HECTOR BLEJER, State of California Department of Public Health, Los Angeles
MALCOLM J. CAMPBELL, College of Engineering, Washington State University,
Pullman, Washington
NEAL CASTAGNOLI, Department of Pharmaceutical Chemistry, University of
California, San Francisco, California
T. TIMOTHY CROCKER, University of California College of Medicine,
Irvine, California
JOHN W. DALY, National Institutes of Health, Bethesda, Maryland
FREDERICK J. deSERRES, National Institute of Environmental Health
Sciences, Research Triangle Park, North Carolina
JOHN FROHLIGER, Graduate School of Public Health, University of
Pittsburgh, Pittsburgh, Pennsylvania
JULIAN HEICKLEN, Department of Chemistry, Pennsylvania State University,
University Park, Pennsylvania
DONALD M. JERINA, National Institutes of Health, Bethesda, Maryland
HAROLD KALTER, Children's Hospital Research Foundation, Cincinnati,
Ohio
E. BINGHAM MATTHEIS, Department of Environmental Health, University
of Cincinnati, Cincinnati, Ohio
ROBERT E. MC MAHON, Lilly Laboratory, Indianapolis, Indiana
PAUL MAZEL, Department of Pharmacology, George Washington University
School of Medicine, Washington, D. C.
iv
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PAUL R. ORTIZ DE MONTELLANO, Department of Pharmaceutical Chemistry,
University of California, San Francisco, California
JEROME J. PERRY, Department of Microbiology, North Carolina State
University, Raleigh, North Carolina
REINHOLD A. RASMUSSEN, College of Engineering, Washington State
University, Pullman, Washington
MICHAEL TRESHOW, Department of Biology, University of Utah, Salt
Lake City, Utah
CHARLES S. TUESDAY, Environmental Sciences, General Motors Corporation,
Warren, Michigan
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COMMITTEE ON MEDICAL AND BIOLOGIC EFFECTS OF ENVIRONMENTAL POLLUTANTS
HERSCHEL E. GRIFFIN, Graduate School of Public Health, University of
Pittsburgh, Chairman
DAVID M. ANDERSON, Industrial Relations Department, Bethlehem Steel
Corporation, Bethlehem, Pennsylvania
RICHARD U. BYERRUM, College of Natural Science, Michigan State University,
East Lansing
RONALD F. COBURN, University of Pennsylvania School of Medicine,
Philadelphia
T. TIMOTHY CROCKER, University of California College of Medicine, Irvine
SHELDON K. FRIEDLANDER, California Institute of Technology, Pasadena
SAMUEL A. GUNN, University of Miami School of Medicine, Miami, Florida
ROBERT I. HENKIN, National Heart and Lung Institute, National Institutes
of Health, Bethesda, Maryland
IAN T. T. HIGGINS, School of Public Health, University of Michigan,
Ann Arbor
JOE W. HIGHTOWER, Department of Chemical Engineering, Rice University,
Houston, Texas
ORVILLE A. LEVANDER, Agricultural Research Center, Beltsville, Maryland
DWIGHT F. METZLER, Kansas State Department of Health and Environment,
Topeka
JAMES N. PITTS, JR., Statewide Air Pollution Control Center, University
of California, Riverside
vi
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I. HERBERT SCHEINBERG, Albert Einstein College of Medicine, Bronx,
New York
RALPH G. SMITH, School of Public Health, University of Michigan,
Ann Arbor
GORDON J. STOPPS, Department of Health, Toronto, Ontario, Canada
F. WILLIAM SUNDERMAN, University of Connecticut School of Medicine,
Farmington
BENJAMIN L. VAN DUUREN, New York University Medical Center, New York
BERNARD WEISS, University of Rochester Medical Center, Rochester,
New York
T. D. BOAZ, JR., Division of Medical Sciences, National Research Council,
Washington, D.C., Executive Director
vii
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PREFACE
In the Spring of 1970, the Division of Medical Sciences,. National
Academy of Sciences - National Research Council, entered into a contract
with what has since become the Environmental Protection Agency to produce
background documents for certain selected pollutants. When the panel to
prepare a study on polycyclic organic matter was formed, it was immedi-
ately apparent to its members that the task was too broad for a single
study, and so they recommended to EPA that two studies be conducted.
One of these was completed by the panel and published by the Academy in
August 1972 under the title of Particulate Polycyclic Organic Matter.
A second panel was formed to deal with other aspects of the subject, and
completed its task in the Spring of 1975, resulting in the present
document, to which has been given the title of Vapor-phase Organic
Pollutants: Volatile Hydrocarbons and Oxidation Products.
The purpose of the document is to prepare a balanced and compre-
hensive survey of certain organic pollutants in relation to health for
the information of the scientific community and the general public and
for the guidance of standard-setting and regulatory agencies. The report
describes sources, physical and chemical properties, measurement, biologic
effects, and interrelationships for a number of pollutants, and offers
recommendations for further research. Statements contained in the report
are supported by references to the scientific literature whenever possible,
or are based on a consensus of members of the Panel.
viii
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CONTENTS
1 Introduction 1
2 Sources of Atmospheric Hydrocarbon 5
3 Possible Mechanisms of Formation of Oxygenated
Organic Compounds in the Atmosphere 60
4 Atmospheric Reactions of Organic Molecules With
Nitrogen and Sulfur Oxides, Hydroxyl Radicals,
and Oxygen Atoms 190
5 Metabolism of Vapor-Phase Organic Pollutants
in Mammalian Systems 222
6 Biologic Effects of Vapor-Phase Organic
Pollutants in Humans and Other Mammalian
Systems 264
7 Epidemiologic Appraisal of Human Effects 311
8 Interactions and Effects on Total Environment 346
9 General Summary and Conclusions 400
10 Recommendations for Future Research 412
Appendix A: Collection and Sampling Techniques for
Vapor-Phase Organic Air Pollutants 421
Appendix B: Analytic Techniques for Vapor-Phase
Organic Air Pollutants 447
Appendix C: Airborne Contaminants 466
Appendix D: Toxicity Data on Occupational
Exposure to Selected Substances 468
References 478
IX
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CHAPTER 1
INTRODUCTION
This report, a companion to an earlier document, Particulate
Polycyclic Organic Matter, concerns vapor-phase substances likely
to be produced as community pollutants in sufficient amounts to
affect health and well-being.
A number of compounds, some of which are less likely to
have effects as community pollutants, have been included to
illustrate: reactions that occur in the atmosphere or to indicate
the health and environmental effects of such materials and their
metabolic transformations. The relationship between the concen-
trations likely to produce unfavorable reactions and those likely
to be present in polluted atmospheres is considered in Chapter 6.
Interpretations are related to several practical questions of
policy and environmental protection.
In general, materials with low vapor pressure or high molecular
weight have been treated only briefly, but are introduced to
illustrate basic reaction pathways.
The Panel did not*, consider the occupational health signifi-
cance of vapor-phase pollutants, particularly those of industrial
origin.
There are four major parts of this document. The first
(Chapter 2) deals with the sources of.vapor-phase organic
pollutants. Natural sources of some of the lower-molecular-weight
compounds and technologic sources (mostly from transportation)
are considered. Chapter 2 is supported by Appendixes A and B,
on collection and sampling techniques and analytic methods.
-------
The second section (Chapters 3 and 4) is a thorough treatment of the
possible mechanisms of formation of oxygenated organic hydrocarbon compounds
in the atmosphere and of atmospheric reactions of oxides of nitrogen and
sulfur.
The third section (Chapters 5, 6, 7, and 8) deals with the toxicologic,
pathophysiologic, and epidemiologic information on vapor-phase organic
pollutants, their metabolism, and their effects on the total environment.
Oxidized compounds are given special attention, because oxidation in
the atmosphere is the principal process by which nature degrades vapor-phase
hydrocarbons. Oxidation, often by photochemical mechanisms, produces
materials that may be harmful and have variable half-lives in the atmosphere.
In Chapter 7, the likely importance of the various exposures by class are
considered. Special attention is given to the importance of formaldehyde,
ozone, and other oxidants and to the significance of benzene, in view of the
possibility of an increase in the aromatic content of motor fuel. Vapor-
phase emission is not determined solely by fuel content, inasmuch as the
processes of combustion cause important transformations of hydrocarbons.
The interpretations concerning health effects depend largely on the evidence
available from exposures in the Los Angeles area, because insufficient data
are available for most other areas.
-2-
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Little is known regarding the effects of short-lived (but potentially
toxic) species (e.g., the toxic peroxyacetylnitrates). Although a balanced
overview of the several possible mechanisms (involving singlet oxygen,
hydroxyl radicals, and ozone) by which atmospheric, and especially photo-
chemical, oxidation occurs has been presented, present knowledge does not
permit accurate assessment of the relative importance of the different
pathways.
The report stresses the importance of oxidation reactions in the vapor
phase and the human health hazards produced from the (more or less transient)
products of oxidation. The review of metabolism indicates that, although
vapor-phase hydrocarbon pollutants are modified usually by enzymatic oxida-
tion within mammalian systems from nonpolar to polar compounds (which are
then excreted by the kidney), this sometimes occurs with the production of
toxic intermediates. These reactions occur mostly in the liver and to a
lesser extent in the kidney, intestine, and lung.
A major gap in knowledge is represented by the uncertainties concerning
atmospheric oxidation mechanisms. That is because the resources for study-
ing vapor-phase reactions have been substantially less than those for
studying liquid-phase reactions. A great deal more information is needed
about human exposure to low dosages over long periods and their health
implications. Most of our information is based on exposure of animals to
high dosages of a small number of compounds; there is a lack of information
about the carcinogenicity and mutagenicity of most of the compounds in
question. Vapor-phase pollutants have not been studied in as great detail
-------
as the polycyclic aromatic compounds treated by the Panel on
Particulate Polycyclic Organic Matter. The most important
practical problem lies in the possible health consequences if
a greater fraction of the fuel stock is based on aromatic hydro-
carbons. Benzene, the major representative of this class, is
known under occupational circumstances to be capable of producing
mutations and is considered by many as a carcinogen because of
its likelihood of producing leukemia, as well as interfering
with bone marrow function. The highest atmospheric concentration
of benzene is one five-thousandth of the concentration thought
likely to damage health under occupational circumstances; for
this reason, the present concentrations are not considered
harmful. However, increases in these concentrations should not
be permitted without careful evaluation of their possible health
implications.
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CHAPTER 2
SOURCES OF ATMOSPHERIC HYDROCARBON
Total hydrocarbon emission in the United States from mobile
(transportation and stationary manmade and from natural sources
381, 917a, 1238a, 1290, 1293, 1295
is summarized in Table 2-1. The
differences between estimates are due primarily to differences in
underlying assumptions and methods of calculation. Most investi-
gators agree that emission estimates are improving as more is
learned about pollution, so the more recent estimates in Table 2-1
are probably the more reliable. In particular, the most recent
1293
estimate of 25. 4 million tons/year from stationary manmade
sources is significantly higher than all previous estimates because
of improved knowledge and the increased number of stationary sources.
One estimate of worldwide hydrocarbon emission is also given in
Table 2-1. The U. S. emission is about 40% of the world total for
both mobile and stationary manmade sources. Although natural hydro-
carbon emission dwarfs manmade emission on a global basis, it generally
occurs in relatively unpopulated areas. As a result, tonnage comparisons
do not accurately reflect importance.
A detailed hydrocarbon emission inventory for the United States
1295
is presented in Table 2-2. This is the latest authoritative in-
ventory that includes both mobile and stationary manmade sources.
In discussing the various source categories, the format of Table 2-2
will be followed.
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MOBILE MANMADE SOURCES OF HYDROCARBON EMISSION
Motor vehicles are byfar the most important mobile manmade
source of gaseous hydrocarbons. According to Table 2-2, they account
for about 86%, whereas aircraft, railroads, marine vessels, and non-
highway use together account for only 14%. Nationwide total emission
from motor vehicles has been decreasing since about 1966, because
of the installation on new cars of crankcase control systems in 1963,
exhaust control systems in 1968, and evaporative control systems in
133a
1971. Total hydrocarbon emission is expected to continue to decline
133a,381
into the 1980's, even without additional controls. Some estimates
of hydrocarbon emission from automobiles and other mobile sources for
381
the years 1955-1985 are given in Table 2-3. It should be pointed out
that these National Petroleum Council estimates are somewhat lower
than the Environmental Protection Agency (EPA) estimates in Table 2-2.
Gasoline-Powered Motor Vehicles
Gasoline-powered vehicles account for about 99% of all vehicular
hydrocarbon emission, and diesels account for the remainder. Of the
emission from gasoline-powered vehicles (automobiles) in. 1967, an esti-
mated 55% came from exhaust, 25% from the crankcase (blowby), and
917a
20% from carburetor and fuel tank evaporation. Presumably, this
breakdown was for cars without any emission controls. Today, with
emission controls on many cars, the proportions are different.
Exhaust Emission. Several investigators have measured exhaust
emission from car fleets. One of the earliest studies was a Coordinating
Research Council (CRC) field survey, in which the average exhaust hydro-
carbon concentration (hexane equivalent) of 160 pre-1956 model cars from
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1358
the Los Angeles area was reported to be 1, 375 ppm. A California
Motor Vehicle Pollution Control Board (MVPCB) survey of 200 pre-1966
196
cars showed an average concentration of 828 ppm. A later survey
of 583 1966 models with first-generation controls showed an average
173
concentration of 290 ppm. On the basis of its own two surveys and
other data, the California MVPCB estimated the average hydro-
carbon emission from, automobiles to be 11.0 g/mile before controls
195
and 3.4 g/mile for 1966 models with first-generation controls.
871,1091
Differences in auto emission have been shown for different altitudes
755
and for cars of different engine size.
The development of gas chroma tog raphic procedures to measure
individual hydrocarbons has greatly increased our understanding of the
333, 568, 620, 642, 854, 1006
nature of exhaust gas. We now know that ex-
haust gas contains low-molecular-weight hydrocarbons that are not present
in the fuel, as well as fuel components. The low-molecular-weight hydro-
carbons include methane, ethane, ethylene, acetylene, propylene, the C
4
(four-carbon) olefins, and sometimes propadiene and methylacetylene.
The fuel components include hydrocarbons heavier than butane and may
number over 100. Typically, the low-molecular-weight hydrocarbons
constitute 40-60% of the total (by volume), although the exact proportions
depend on many engine-fuel variables.
The predominant hydrocarbons in gasoline exhaust have been re-
ported in three studies, each of which covered a wide range of conditions
640, 916, 1006
and included many analyses. The three agree remarkably
well (see Table 2-4), considering the diversity of the investigations. The
low-molecular-weight hydrocarbons--me thane, ethylene, acetylene, and
-------
propylene--are high on each list. Toluene and isopentane seem to be
the major fuel components. . ••-,.,
Fuel composition obviously will affect exhaust composition, but
the fuel components in exhaust do not exactly match the original fuel.
Some fuel hydrocarbons are preferentially burned, and some others
are formed in the engine. To avoid undue complexity, some investiga-
tors have used simple hydrocarbon fuels to study the relations between
428
fuel composition and exhaust composition. Fleming used blends of
isooctane, isooctene, and rn-xylene and found that the exhaust may con-
tain (aromatic) hydrocarbons heavier than those in the original fuel and
that, fuel paraffins produce more exhaust olefins than do fuel olefins.
971
Ninomiya and Biggers used blends of toluene, isooctane, and ri- ,
heptane and showed that the airrfuel ratio greatly affects the yields of
aromatic products (ethylbenzene, styrene, benzene, benzaldehyde,, and
toluene) formed from these blends. The effects on other exhaust hydro-
carbons (low-molecular-weight products and some heavier olefins) were
972 299
reported in an earlier study. Daniel found methane, ethylene,
ethane, acetylene^ propylene, and propane in the exhaust from a single-
cylinder engine burning pure propane. The concentrations depended greatly
on the airrfuel ratio, spark timing, volumetric efficiency, and compression
ratio.
The effect of gasoline composition on exhaust composition has been
337, 338,916
studied extensively by Dishart and his co-workers. They con-
cluded that ethylene is formed from saturates and olefins, propylene and
butene primarily from saturates, and diolefins primarily from olefins,
and that additional amounts of toluene, benzene, and xylenes are.formed
-------
from higher aromatics and additional 2-methyl-2-butene from higher
341
saturates. Doelling jet al_. concluded that gasoline composition had
no effect on the total hydrocarbon concentration in exhaust, but that the
percentages of aromatics, olefins, and paraffins in exhaust were correlated
1380
with fuel composition. Wigg et a_L concluded that aromatic exhaust
emission is linearly related to the aromatic content of the fuel. Similarly,
958
Neligan
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Some emission controls, however, do affect exhaust hydrocarbon
916
composition. Morris and Dishart compared an uncontrolled car,
a modified-combustion car, an air-injection car, and an advanced
thermal-reactor car. The greatest differences they found were the
high percentage (87%) of C and C hydrocarbons and the near absence
1 2
of fuel components in the exhaust of the thermal-reactor car. Adams
10
ei_ al. also found significantly higher percentages of methane and
ethylene and lower percentages of aromatics in the exhaust of a lean-
reactor car, compared with that of an unmodified car of the same model.
1363
Weaver reported that catalytic reactors selectively oxidize the
olefins and aromatics in exhaust, but that the selectivity diminishes
951
with catalyst age. Nebel and Bishop also reported that catalytic
reactors may be selective and that methane and ethylene are the most
640
difficult hydrocarbons to oxidize. Jackson found that an engine-
modification emission control system increased the photochemical
reactivity of exhaust, but that an air-injection system had no effect.
Exhaust gases contain organic compounds besides hydrocarbons,
such as aldehydes, ketones, alcohols, ethers, esters, acids, and phenols.
These partial-oxidation products are called oxygenates. The total oxygenate
concentration is about one-tenth of the total hydrocarbon concentration.
Aldehydes are generally believed to be the most important class of
oxygenates.
373,388,428
Many investigators have measured formaldehyde and
373,463,599,769,1229,1363
total aldehyde concentrations in auto ex-
249, 703, 1005, 1128
haust with sensitive chemical tests. A few investi-
599,971,972 599
gators have measured benzaldehyde and phenol, and
10
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78,374
still others have qualitatively identified various aldehydes. How-
ever, a reasonably complete quantitative analysis of exhaust aldehydes
has not been possible until recently, when sophisticated gas-chromatographic
94,448,983, 1322, 1412
techniques were developed. The results of several
of these detailed analyses are given summarized in Table 2-5. They agree
very well, considering that different engines, gasolines, and, to some ex-
tent, analytic techniques were used. Formaldehyde is by far the pre-
dominant: aldehyde, constituting about 60-70% of the total (on a volume
basis); acetaldehyde is next, at about 10%; and propionaldehyde, acrolein,
benzaldehyde, and the tolualdehydes are all found in appreciable amounts.
As might be expected, the nature of the gasoline burned influences the
aldehydes formed.
There is almost no published information on noncarbonyl oxygenates,
such as ethers, alcohols, epoxides, and peroxides. Seizinger and
Dimitriades measured 10 aldehydes, six ketones, and 16 noncarbonyl
oxygenates in exhaust from 22 different simple fuels, each containing one,
1155, 1156
two, or three hydrocarbons. They developed computational
formulas from their data that can be used to estimate the oxygenate con-
centrations in gasoline exhaust.
The use of liquefied petroleum gas (LPG) or natural gas as a motor
fuel does not in itself eliminate or even reduce exhaust hydrocarbon emis-
sion. These fuels, however, produce less complex exhaust than gasoline.
1148
Schwartz ^t a_l. reported finding mostly propane--plus small amounts
of methane, ethane, ethylene, acetylene, and propylene and traces of C
4
hydrocarbons--in the exhaust from automobiles or lift trucks burning LPG.
429
Fleming and Allsup found mostly methane and smaller amounts of
11
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other (up to C ) hydrocarbons in the exhaust from an automobile burning
4
natural.gas. They attributed the nonmethane hydrocarbons primarily to
365
the ethane in natural gas. This was confirmed by Eccleston and Fleming's
study in which natural gas and a synthetic pipeline gas derived from coal
(Synthane) were compared as motor fuels. The exhaust from Synthane--which
was about 10% hydrogen, 88% methane, and less than 0. 5% ethane--contained
noticeably less C and C hydrocarbon than the exhaust from natural gas,
2 3
whose composition was 89% methane, 9% ethane, and 2% propane and
butane. The photochemical reactivity of the Synthane exhaust was also
significantly lower. The exhausts from both the natural gas and Synthane
contained small amounts of an unidentified aldehyde (or aldehydes).
Evaporation Losses. The evaporation of gasoline from carburetors and
fuel tanks has been greatly reduced for 1971 and later automobiles, compared
959
with older models, by 88% according to one estimate, owing to the instal-
lation of evaporation control systems. Evaporative emission is still significant,
however, because many older cars are in use.
Evaporative emission consists of the lighter components of gasoline, primar-
ily C and C hydrocarbons. As many investigators have pointed out, the exact
4 5
composition depends on the gasoline used, the temperature it attains, and degree
317, 364, 368, 619, 641, 855, 930, 1242, 1329, 1371
of "weathering" or prior evaporation.
Some representative analyses of evaporative emission samples are summarized
in Table 2-6. They are not complete analyses, covering only the C , C , and
4 5
C paraffins; the C and C olefins; and the C and C aromatics. The
6 45 67
light paraffins and light olefins constituted about 70% of the carburetor emis-
sion and about 90% of the fuel-tank emission in these samples. Isopentane
and n-butane were by far the predominant hydrocarbons, together accounting
12
-------
for an average of about 50% of the total. It seems safe to generalize
that isopentane and n_-butane account for the major portion of all gasoline
evaporation losses. The amounts of other paraffins and olefins varied
from sample to sample, largely because of differences in gasoline com-
position.
641
The analyses in Table 2-6 confirm Jackson and Everett's con-
clusion that fuel-tank emission is richer in the lighter components than
carburetor emission. Presumably, this is because the gasoline iri the
carburetor is "distilled" at a higher temperature.
Blowby. Blowby or crankcase emission has been practically elimi-
nated; all 1963 and later model cars have been equipped -with positive
crankcase ventilation (PCV). The PCV cure for this problem and the
importa.nce of blowby hydrocarbon emission were first recognized by
99
Bennett and his co-workers. They measured hydrocarbon concentra-
tions of around 10, 000 ppm and flow rates of a few cubic feet per minute
for blowby and concluded that blowby accounted for about 40% of the total
hydrocarbon emission from automobiles. They also found that blowby is
predominantly carbureted mixture (85%) plus some combustion gases (15%)
and that fuel composition determines which hydrocarbons are emitted from
the crankcase. Later studies by other investigators essentially confirmed
1012, 1092, 1178
their findings on blowby composition.
Possible contamination of blowby from lubricating oil was eliminated
342
in a unique study by Domke, Lindley, and Sechrist. Using an oilless
engine with Teflon parts, they found that blowby hydrocarbon composition
was roughly 50% aromatics, 35% saturates, and 15% olefins, regardless
of the air:fuel ratio or the fuel used (gasoline or isooctane). They also
13
-------
found oxygenates at about 5 ppm in the blowby, of which only formal-
dehyde, acetaldehyde, methanol, and ethanol could be identified. Payne
1016
and Sigsworth also detected oxygenates, as •well as hydrocarbons,
in blowby gases, although they did not recognize the importance of blowby
as an emission source of their early (1952) study.
561
Hass and Scanlin measured the blowby rates of 500 cars of
various makes and ages and determined the flow-rate percentiles for
various engine sizes. This basic information has been useful to designers
1321
of PCV systems. Voelz ^t aL tested over 75, 000 vehicles in 15
metropolitan areas and found that 17% of the PCV systems needed main-
tenance and that 3. 6% of the cars were discharging some crankcase fumes
to the atmosphere. In a smaller survey of 483 cars in Cincinnati, Ohio,
29% needed PCV maintenance and 5% were discharging some crankcase
268
fumes. The detailed data from these surveys suggest that the PCV
systems in actual use are about 98% effective.
Diesel-Powered Motor Vehicles
As indicated in Table 2-2, diesel-powered motor vehicles account
for about 1% of the hydrocarbon emission from all motor vehicles and
about 1% from all mobile sources. Diesel emission originates exclusively
800,859
from the exhaust; blowby and evaporative emission is practically nil.
The gaseous hydrocarbon fraction of diesel exhaust is extremely
complicated. Many investigators have shown that it consists of light,
cracked hydrocarbons and heavy fuel-like components up to about
621, 622, 799, 800, 859, 881, 1066, 1330
C . Except for methane,
24
the light, cracked hydrocarbons are almost all olefins. Ethylene,
acetylene, and propylene are the predominant light hydrocarbons, with
14
-------
smaller amounts of C olefins' and even smaller amounts of C and C
799, 1066 4 56
olefins. Figure 2-1 shows the bimodal carbon-number distri-
bution of diesel-exhaust hydrocarbons; interestingly, the proportions of
light hydrocarbons are greater for the four-cycle engine than for the two-
621 800
cycle engine. Linnell and Scott estimated that the light hydro-
carbons constitute 10-25% of the total on a molar basis. Hum and
622
Seisinger reported that the proportions of light hydrocarbons are
highest v/hen the total hydrocarbon concentration is lowest.
Most of the analytic work on heavy hydrocarbons (C -C ) has
• . 10 24
210,350, 1205
been directed toward identifying the odorous components.
210
Caragay _e_t aL reported finding the following hydrocarbon classes
in the "oily-kerosene" fraction of diesel-exhaust condensate: indans,
tetralins, .alkylbenzenes, naphthalenes, indenes, acenaphthenes, and
benzothiophenes. Similar types of hydrocarbons were found by Dravnieks
350 1184
et^ aL in a related odor study. Skala et^ al_. identified several
high-molecular-weight aromatic carbonyl compounds in diesel exhaust
that they believe are important odorants. At present, we can say only
that the fuel-like fraction of diesel exhaust consists of at least several
hundred compounds; most of them are found in the fuel, but some--
important to the odor problem—are formed during combustion.
Low-molecular-weight aldehydes have also been found in diesel
exhaust. Formaldehyde and to a lesser extent acrolein have been re-
68, 387, 622, 799, 800, 907, 1066, 1097, 1208 1323
ported most often. Vogh
measured the following aldehydes in one diesel-exhaust sample:
formaldehyde (18. 3 ppm); acetaldehyde (3. 2 ppm); acrolein, acetone,
propionaldehyde, and isobutyraldehyde (2. 9 ppm); n.-butyraldehyde (0. 3 ppm);
15
-------
crotonaldehyde and valeraldehyde (0.4 ppm); hexaldehyde (0. 2 ppm);
and benzaldehyde (0.2 ppm). The proportions were not greatly different
from those shown in Table 2-5 for diesel auto exhaust.
387,881,1097
Several investigators have attempted to correlate
total aldehyde or formaldehyde concentration with diesel-exhaust odor,
with limited success.
Other Mobile Sources
Aircraft. Hydrocarbon emission from aircraft is estimated to be
1295
400, 000 tons/year, or about 2% of the emission from all mobile sources.
This estimate includes only gaseous hydrocarbons, not smoke or soot,
although smoke is considered to be the major air pollution problem for
aircraft.
Jet aircraft are by far the most important type from a fuel-consumption
standpoint. In general, hydrocarbon emission from turbine engines is con-
siderably lower than from comparable-sized reciprocating engines, and the
concentrations are very much lower, owing to the large air consumption of
turbine engines. Like those from reciprocating engines, the hydrocarbons
in turbine-engine exhaust consist of light, cracked products plus fuel-like
components. Figure 2-2 shows the carbon-number distribution of the hydro-
carbons in the exhaust from an aircraft turbine and in the fuel (JP-4)
229
burned to produce the exhaust. The proportions of light hydrocarbons
are much lower than for reciprocating engines powered by either gasoline
or diesel fuel. Turbine-engine exhaust is unusual in another way, as shown
in Figure 2-2: the polymerization of the fuel components to form heavy
hydrocarbons (C and heavier). Fuel polymerization does not occur in
19
16
-------
gasoline or diesel engines to nearly so great an extent, if it occurs at all.
The C and heavier hydrocarbons in turbine exhaust are probably particulate,
19
rather than gaseous.
267
Cornelius et al. measured the concentrations of individual hydro-
carbons in the exhaust from an automotive turbine engine, but reported only
that their average photochemical reactivity "compared favorably" with
733
gasoline-engine exhaust hydrocarbons. Korth and Rose measured indi-
vidual hydrocarbons in the exhaust from a turbine-powered automobile
operated on unleaded gasoline. Compared with those from a conventional
automobile (and adjusted for differences in air consumption), the turbine
exhaust concentrations were very much lower for C olefins and C
1-5 1-5
paraffins, slightly lower for C paraffins, and slightly higher for
6-8
benzene. Moreover, the exhaust from the turbine car did not contain any
aromatics above C , -whereas the exhaust from the conventional car,
8
operated on the same gasoline, did contain higher aromatics. The pro-
portion of light olefins in the turbine exhaust was much less than that in
the conventional exhaust. This agrees with Chase and Hum's carbon-
229
number distribution (Figure 2-2).
Formaldehyde and total aldehyde concentrations in turbine exhaust
229, 267,733, 815
have been measured, but no detailed aldehyde analyses
have been reported. What little information on aldehyde composition is
815
available is conflicting. In one study involving aircraft turbines
formaldehyde constituted about 70% of the total aldehyde, about the same
proportion as in gasoline- or diesel-powered engine exhaust. However,
733
in Korth and Rose's study of the turbine-powered automobile, formal-
dehyde constituted only 10% of the total aldehyde.
17
-------
Railroads. Hydrocarbon emission from railway locomotives is
1295
only about 100,000 tons/year. Because locomotives are almost all
diesel-powered, the character of their emission is essentially the same
as for diesel-powered vehicles, previously discussed.
i
Marine Vessels. Hydrocarbon emission from ships, barges, and
1295
other vessels is estimated at about 300,000 tons/year. Larger
vessels are powered by oil-fired and to a lesser extent coal-fired steam
engines. There is no information about the gaseous emission from such
vessels (although smoke is a problem on the Great Lakes), so they cannot
be discussed further here. Smaller vessels are usually diesel-powered.
The character of their emission is essentially the same as that of diesel-
powered vehicles. The special case of outboard motors is discussed in
the following section, because they are similar in design to internal-
combustion engines in some off-highway uses.
Off-Highway Use. This category includes large construction equip-
ment, farm tractors, snowmobiles, trail bikes, outboard motors, electric
generators, garden tractors, power lawnmowers, and chain saws. Hydro-
carbon emission from these sources is about 1. 9 million tons/year, or
1295
about 10% of all mobile-source emission. Off-highway emission
appears to be increasing.
Many large off-highway vehicles and machines, such as heavy con-
struction equipment and farm tractors, use gasoline or diesel engines that
are very similar to those in motor vehicles. Their emission is probably
also similar, so they will not be discussed further. However, many small
off-highway vehicles and machines use small gasoline engines, which in
18
-------
general are less efficient and emit larger quantities of hydrocarbons
343
than larger engines. Donohue^t al. estimated the hydrocarbon
emission from these small engines at 169,000 tons/year for the United
States. This is about 9% of the emission from all off-highway sources
and less than 1% of the emission from all mobile sources. Eccleston
367
and Hum tested 36 small utility engines and found the average
hydrocarbon emission to be 8 g/hp-hr for four-cycle engines and
140 g/hp-hr for two-cycle engines. This large difference was attributed
partly to the lower average size of the two-cycle engines, but mostly to
the fact that two-cycle engines are scavenged by unburned fuel-air mixture.
This scavenging action not only increases the total hydrocarbon concentra-
tion in the exhaust, but alters the composition. As shown in Figure 2-3,
two-cycle engine exhaust contains a much higher proportion of fuel com-
367
ponents.
STATIONARY MANMADE SOURCES OF HYDROCARBON EMISSION
Many industrial, commercial, and domestic activities emit gaseous
hydrocarbons and other organic compounds to the atmosphere. These
sources, both numerous and widespread, include all kinds of fuel burning,
solvent usage, and waste-disposal operations, as well as the more obvious
chemical processing and petroleum refining and marketing. The total
hydrocarbon emission from these sources is shown in Table 2-2. Accord-
ing to this 1969 estimate, stationary sources in the United States discharge
17. 6 million tons of hydrocarbon per year, compared with 19. 8 million
tons from mobile sources. A more recent (1971) survey estimates
stationary-source emission at slightly more than 25 million tons/year;
it is summarized in Table 2-7 and is considerably different from the
1969 survey and presumably more accurate.
19
-------
The composition of organic emission from various stationary
sources is discussed in the following sections.
Fuel Combustion
The fuel burned by all stationary combustion sources in the United
15
States during 1968 was the equivalent of 43. 4 x 10 BTU, of which 30%
was provided by coal, 17% by distillate and residual fuel oil, 48% by
natural gas, and 5% by miscellaneous fuels, such as coke, lignite, wood,
1294
LPG, and waste gases. The major uses of fuel are in electric
power plants, industrial processing, and space heating. In general,
the gaseous hydrocarbon and other organic emission from specific
stationary combustion sources is lower than and very different from those
of specific automotive sources.
Coal. Electric power plants account for most of the coal burned in
283, 284,481
the United States. Cuffe, Gerstle, and their co-workers re-
ported hydrocarbon emissions from several large coal-fired power plants
548
to be between 0. 1 and 0. 2 Ib/ton of fuel burned. .Hangebrauck et al
reported similar values for large power plants and considerably higher
values, 1-3 Ib/ton, for coal-fired industrial boilers. Formaldehyde
emission from the same units ranged from about 0. 001 to 0. 006 Ib/ton
of fuel, or about 1-2% of the hydrocarbon emission, and so appear to be
unimportant. However, organic acids are important and actually exceed
284
hydrocarbon emission in this type of equipment. Cuffe et aL re-
ported an average value of 12.4 Ib of organic acid (as acetic) per ton
of fuel burned for one large coal-fired plant. This was almost 70 times
1294
the hydrocarbon emission from the same plant. McGraw and Duprey
20
-------
reported that hydrocarbon emission from coal-fired equipment ranges from
0. 3 Ib/ton for large utility boilers to 3 Ib/ton for commercial and domestic
608
furnaces. Hovey et jiL reported that organic emission from the com-
bustion of hard coal was only one-eight that from soft coal.
No information is available on the composition of the hydrocarbons
or organic acids that are discharged from coal-burning equipment.
Fuel Oil. Electric power plants, oil refineries, industrial plants,
and space heating account for most of the fuel oil burned in the United
States. Like that from coal-burning equipment, gaseous hydrocarbon
231
and other organic emission from oil burners is very low. Chass et al.
reported hydrocarbon emission from oil-burning power plants, industrial
boilers, and domestic and commercial furnaces to be about 0. 2 Ib/ton;
aldehyde and ketone emission, about 0. 15 Ib/ton; and other organic emission
(presumably organic acids), 0. 4-0. 8 Ib/ton. The hydrocarbon and other
organic: emission from oil-fired equipment in petroleum refineries was
about 1 and 3 Ib/ton, considerably greater than from the other sources.
548
Hangebrauck . et al. reported hydrocarbon emission of about 0. 3 Ib/ton
and formaldehyde emission of about 0. 006 Ib/ton for large oil-fired equip-
ment; emission from smaller equipment was somewhat higher—about 1. 0
230
and 0. 03 Ib/ton, respectively. Chass and George measured aldehyde
emission from about 30 small industrial burners operated on oil, gas, or
both. The oil-fired burners discharged 0. 1-6. 7 Ib of aldehydes per ton
1348
of fuel burned and averaged 1. 0 Ib/ton. Wasser et al. studied the
effect of excess air on the emission from a small domestic oil burner.
The hydrocarbon emission was very low under optimal conditions--about
0. 06 lb/ton--but increased to over 30 Ib/ton when the air was reduced to
21
-------
1294
stoichiometric proportions. McGraw and Duprey reviewed the
literature and estimated the hydrocarbon emission from oil-fired equip-
830
ment to be about 0. 5 Ib/ton. Magill and Benoleil reported somewhat
higher emission for large oil-fired units than those quoted above in their
early (1952) study.
Natural Gas. Space heating accounts for most of the natural gas
consumed in the United States, although power plants and industrial
processes are important users. Natural gas is considered to be clean-
231
burning fuel, but it does produce some organic emission. Chass et al.
reported negligible hydrocarbon emission from gas-fired power plants,
industrial boilers, and commercial and domestic heaters. Aldehyde
emission from the same sources was less than 0. 1 Ib/ton, and other
organic emission (presumably organic acids) was between 0. 1 and 0. 2
231
Ib/ton. Chass
-------
Methane is presumably the predominant hydrocarbon in the effluent
from gas-fired equipment, but there are no published data to confirm this.
537
Hall reported that formaldehyde, acetaldehyde, and formic acid are
produced when natural gas is burned in appliances with a deficiency of
air.
Wood. Wood is used as an industrial fuel only where it is a readily
available byproduct. Hydrocarbon emission from burning it depends on
the proportions of wood and bark, the moisture content, and how -well
1294
the furnace is designed and maintained. McGraw and Duprey re-
ported typical emission from the combustion of wood in industrial boilers
as 2 Ib of hydrocarbon per ton and 0. 5 Ib of carbonyl per ton; these values
are comparable with those reported for the combustion of other fuels.
457
Fritschen et al. measured total hydrocarbon emission of
2-4 Ib/ton from the burning of pine slash samples in laboratory apparatus.
Methane was the predominant hydrocarbon, but smaller amounts of ethylene,
ethane, acetylene, and propylene and traces of C and C olefins were also
4 5
found. Several other investigators have reported a variety of oxygenated
647
organics in wood smoke. Jahnsen found 29 different organics in the
effluent from hickory sawdust that was burned in Pyrex apparatus. Acetic
acid was the principal organic acid; methyl alcohol, the principal alcohol;
diacetyl, the principal carbonyl; and guaiacol and 2, 6-methoxypyrogallol,
94
the principal phenols. Bellar and Sigsby identified methyl alcohol, ethyl
alcohol, acetone, acetaldehyde, acrolein, propyl alcohol, 2-methylpropyl
alcohol, and butyl alcohol in the effluent from a trench incinerator burning
602
wood. Hoff and Kapsalopoulou reported finding 18 different alcohols,
aldehydes, ketones, and ethers and benzene and toluene in the low-boiling
23
-------
fraction of smoke from a hickory fire used to smoke meats. Levaggi
791
and Feldstein measured amounts of formaldehyde, acetaldehyde, and
acetone in parts per million and smaller amounts of propyl alcohol and
methylethylketone in the effluent from a wood-burning fireplace.
Summary. Some typical fuel combustion emission values are shown
in Table 2-8. The purpose is to show the composition of the gaseous
organic emission from different combustion sources, not to compare
different fuels. Organic acids are the major constituent, followed by
hydrocarbons and aldehydes. This is an entirely different order from auto
exhaust, in which hydrocarbons are predominant, aldehydes are secondary,
and organic acids are negligible.
It should be pointed out that the nationwide hydrocarbon emission
summarized in Table 2-2 does not include organic acids, at least for the
fuel combustion sources. On the basis of 1968 U. S. consumption of the
three fuels and the emission values in Table 2-8, an additional 4. 5 million
tons of organic acids are emitted, of which 3. 5 million tons are from coal,
0. 5 million tons from oil, and 0. 5 million tons from gas. These must be
considered only rough estimates, but they do indicate that fuel combustion
is a more important source of organic emission than implied in Table 2-2
or Table 2-7.
Industrial Processes
A wide variety of organic pollutants are emitted by industrial
processes. Some of the more important are discussed below.
24
-------
Primary Metals. The production of coke is the main source of
gaseous hydrocarbon emission associated with the primary metals
industry. Coke-oven gases contain aromatic hydrocarbons and phenols,
as well as inorganic pollutants. The emission occurs principally when
261
coal is charged to the oven and when the coke is quenched and removed.
The recent: development of a continuous coking process may greatly reduce
76
coke-oven emission in the future. The phasing-out of old beehive
ovens, from which no attempt is even made to collect the coal tars, will
139
also alleviate the problem.
Petroleum Refining. Various refinery processing and storage opera-
tions discharge hydrocarbons to the atmosphere. Good design and house-
381
keeping minimize this emission, however. The more volatile C , C ,
4 5
and C hydrocarbons probably account for most of the refinery emission.
6
Petroleum refineries also discharge small amounts of organic sulfur com-
pounds.
Chemical Processing. Many chemical processing plants discharge a
variety of organic compounds to the atmosphere, depending on their opera-
409
tions. Fawcett inventoried the emission from a phthalic anhydride
plant and found that it included phthalic anhydride, maleic anhydride,
822
naphthoquinone, benzoic acid, and various aldehydes. Lur'e reported
that butadiene, isobutylene, styrene, benzene, and ethyl alcohol vapors
1341
are discharged from a synthetic rubber plant. Walter and Amberg re-
ported that a-pinene, methyl alcohol, and, to a lesser extent, acetone are
the major organic compounds emitted from kraft paper mills (sulfur com-
pounds excluded). No one organic compound or group of compounds pre-
dominates in chemical plant emissions.
25
-------
792
Other Industrial Sources. Levaggi and Feldstein measured
aldehyde emission from a variety of small industrial operations, such
as coffee roasting, printing, paint spraying, and foundry-core prepara-
tion, many of which involved ovens and after-burners. They found ppm
amounts of formaldehyde, acetaldehyde, and acetone in parts per million
and smaller amounts of acrolein and other C and C aldehydes. The
3 4
emission from many of these and from other small industrial operations
1294
arises from the use of chemical solvents. McGraw and Duprey have
compiled air pollutant emission rates for a variety of industrial processes.
Solid-Waste Disposal
The per capita solid-waste load in the United States is about 10 Ib/day,
1294
about half of which is disposed of by incineration. This incineration
produces a wide variety of organic air pollutants, whose amount and composi-
tion depend greatly on the nature of the wastes and how they are burned.
Domestic, Municipal, and Industrial Incinerators. These incinerators
vary widely in size, design, and effectiveness. In general, large, fully
engineered units, many of which have stack controls, discharge smaller
1279
amounts of pollutants than small, domestic units. Tuttle and Feldstein
reported much lower hydrocarbon emission from an adequately designed
multichamber incinerator than from an inadequately designed incinerator;
methane and ethylene were the predominant hydrocarbons from both units,
but many other C hydrocarbons were found with the less efficient in-
2-6 1213,1214
cinerator. Stenburg and co-workers reported hydrocarbon
emission of 1-2 Ib/ton of refuse burned and formaldehyde emission of
0. 01-0. 02 Ib/ton for a medium-sized multichamber incinerator. Carotti
26
-------
212
and Kaiser's data for a large municipal incinerator indicate hydro-
carbon emission of about 0. 2 Ib/ton, aldehyde emission of about 0. 1
Ib/ton, and organic acid emission of about 3 Ib/ton; most of the hydro-
674
carbon was methane and ethylene. Kaiser et a_l. reported somewhat
higher values for an apartment-house incinerator: organic acids, 18 Ib/ton;
esters, 10 Ib/ton; aldehydes, 4 Ib/ton; and benzene and phenol, about 0. 1
Ib/ton each. In addition, Kaiser et al. detected smaller amounts of
methane, ethylene, propylene, acetaldehyde, methyl alcohol, ethyl alcohol,
acetone, and unidentified higher-molecular-weight products totalling about
1422
0. 6 Ib/ton. Still higher emission -was reported by Yocum et aL for a
backyard, incinerator burning a high proportion of garden clippings: methyl
alcohol, 9-23 Ib/ton; ethylene, 8-61 Ib/ton; acetone, > 8 Ib/ton; methane,
23-150 Ib/ton; acetylene, 4-73 Ib/ton; olefins, >6 Ib/ton; carbon disulfide,
> 3 Ib/ton; benzene, >3 Ib/ton; organic acids, > 4 Ib/ton; phenols, > 8 Ib/ton;
830
and aldehydes, 5-64 Ib/ton. Magill and Benoliel reported organic emission
of several hundred pounds per ton for domestic incinerators burning paper or
grass clippings, compared with only 1-2 Ib/ton for municipal incinerators.
94
Bellar and Sigsby measured individual oxygenates in the effluent
from a trench incinerator burning wood. They found a much wider range
of products when no forced air was added. With forced air, methyl and
ethyl alcohol predominated. Bellar and Sigsby noted that the ratio of alcohols
to aldehydes was considerably greater in the incinerator effluent than in auto
exhaust.
482
Open Burning. Gerstle and Kemnitz reported the following emission
values from the open burning of municipal refuse, landscape refuse, and auto-
mobile components: hydrocarbons,. 30 Ib/ton; organic acids, 15 Ib/ton;
27
-------
1294
and formaldehyde, 0.01-0. 10 Ib/ton. McGraw and Duprey reported
that organic emission from open burning is generally higher than that
from incineration.
Teepee or Wigwam Burners. Teepee burners are large conical
structures in which industrial and municipal wastes are burned. They
are used only when more efficient (and costly) incinerators are not avail-
750
able. Kreichelt reported that many teepee burners do not receive
enough air for good combustion, because of poor maintenance or inadequate
blowers. The organic emission from teepee burners depends on the waste
material burned and is generally greater than that from incinerators but
less than that from open burning.
Miscellaneous Stationary Sources
The following sources account for over half the total gaseous organic
emission from all stationary sources. Solvent evaporation is the largest
single stationary source, 7. 1 million tons/year according to the most
recent EPA estimates (Table 2-7). Solid-waste combustion and agricultural
burning are the next most important stationary sources.
Forest Fires. It is nearly impossible to measure emission from a
forest fire, but at least one study has been made under field conditions
457
closely simulating forest fires. Fritschen et aL measured the gaseous
organic emission from the burning of several acres of mature Douglas fir
trees. They identified 25 different organics, ranging in molecular -weight
from that of ethylene to that of xylene, the most significant being ethylene,
ethane, propylene, propane, methyl alcohol, and ethyl alcohol. Acetone,
benzene, and toluene were also found in most of the samples. They estimated
28
-------
the total hydrocarbon emission from forest fires at 2-4 Ib/ton burned,
on the basis of these and other laboratory tests. This is considerably
410
lower than the 166-lb/ton estimate of Feldstein et al. for the open
burning of land-clearing debris.
1218
Stephens and Burleson reported an excess (compared with
normal air) of olefins over paraffins in the air near a brush fire in
Riverside, California, which is consistent with the results of laboratory
studies on the combustion of agricultural wastes.
Structural Fires. The gaseous organic emission from structural
fires, like that from forest fires, can be only roughly estimated. It is
probably similar in composition to the emission from wood combustion
and municipal-waste incineration and includes both hydrocarbons and
oxygenated compounds.
Coal-Refuse Banks. Smoldering coal-refuse banks are a conspicuous
833
source of air pollution in mining areas. Carbon monoxide and sulfur
190
dioxide are the major gases emitted, but some organics are also pro-
duced. The composition of the gaseous organics is not known.
Agricultural Burning. Agricultural wastes--such as cut grass and
weeds, straw, felled trees, and other debris--are often disposed of by
open burning. The relatively low temperatures associated -with open
burning tend to increase the emission of gaseous organics, compared
410
with incinei ration.
301
Darley ei_ al. measured the hydrocarbon produced by burning
fruit prunings, barley straw, and native bush in a special tower that
simulated field conditions. The average hydrocarbon emission from
29
-------
these agricultural wastes was about 13 Ib/ton, of which 2 Ib consisted of
ethylene, 3 Ib of other olefins below C , 1 Ib of paraffins below C ,
5 132 6
and 7 Ib of heavier hydrocarbons. Boubel^al. reported similar
values for the burning of grass and straw in the same apparatus.
482
Gerstle and Kemnitz reported a considerably higher hydrocarbon
emission, 30 Ib/ton, for the open burning of landscape refuse. Feldstein
410
et aL reported still higher values for the open burning of land-clearing
debris. They estimated (from other incinerator tests) total gaseous organic
emission at 166 Ib/ton, of which 30 Ib would consist of ethylene, 30 Ib of
other olefins, 36 Ib of saturated hydrocarbons, 11 Ib of aromatic hydro-
carbons, and 59 Ib of oxygenated organics. Many of the low-molecular-
weight alcohols and aldehydes in wood smoke are probably produced by the
open burning of agricultural wastes.
Solvent Evaporation. Some of the more important uses of solvents
include dry-cleaning, surface coatings, metal degreasing, and chemical
processing. Most solvents evaporate eventually, either inadvertently or
by plan, and end up in the atmosphere. Therefore, the overall composition
of solvent emission can be deduced accurately from usage data.
The usage or consumption of different solvents in the United States
for 1968 is shown in Table 2-9. Petroleum naphtha, a generic name for
hydrocarbon mixtures of varied composition and volatility, is by far the
most important solvent from a tonnage standpoint, accounting for about
60% of the total usage. No other solvent accounts for more than 4%.
In all, hydrocarbon-based solvents account for 70% of the total usage;
ketones and other oxygenated hydrocarbons, 14%; and chlorinated hydro-
carbons, 16%.
30
-------
It should be pointed out that the data in Table 2-9 indicate only the
quantities various chemicals consumed as solvents, rather than total
amounts produced or consumed in all uses.
Gasoline Marketing. Vapor breathing losses from storage tanks
at refineries and bulk plants and vapor displacement during filling of
tank trucks, service station tanks, and automobile tanks account for
almost all the emission associated with gasoline marketing. The vapor
lost from breathing and filling consist primarily of the more volatile C
4
C , and C hydrocarbons. The analyses of automobile evaporation
5 6
losses shown in Table 2-6 are probably typical of gasoline marketing
emission.
Various control devices and practices minimize vapor losses at
most storage facilities, but there are no vapor loss controls at retail
service stations.
NATURAL SOURCES OF ATMOSPHERIC HYDROCARBONS
Many natural processes emit hydrocarbons to the atmosphere. The
major natural sources that have been identified and for -which quantitative
estimates are available are biologic decomposition of organic matter,
seepa.ge from natural gas and oil fields, and volatile emission from plants.
However, there is information in the literature that indicates that there are
many other natural sources of hydrocarbons and oxygenates that have not
been considered heretofore. Techniques sensitive enough to measure the
minute concentrations of hydrocarbons and oxygenates present in remote
areas have only recently become available. For example, methane, the
predominant hydrocarbon in the atmosphere, was first identified as a
31
-------
890
trace constituent of the atmosphere as late as 1948. As more
sensitive analytic techniques become available and more effort is
channeled into attempts to understand the characteristics and role of
natural emission, one -would expect more and more minor sources
(particularly in the biosphere) to be identified.
This section discusses the major natural sources for which quanti-
tative estimates are available and indicates some of the minor sources
that have been identified.
Methane is produced in the anaerobic bacterial decomposition of
189, 362
organic matter in swamps, lakes, marshes, paddy fields, etc.
747 14
Koyama has estimated the global production of methane as 2. 7 x 10
g/year (300 million tons/year), on the basis of his measurements of
methane fermentation in various soils and lake sediments under controlled
experimental conditions. Koyama estimated that paddy fields contribute
624
two-thirds of the methane production and used Hutchinson's estimate
of enteric fermentation in animals to account for one-sixth. The other
one-sixth of the methane in Koyama1 s estimate came from coal fields
372
and soils in grassland and forest areas. Ehhalt pointed out that
Koyama did not include methane production from swamps, and Robinson
1084
and Robbins pointed out that methane production in humid, tropical
areas should also be considered. Robinson and Robbins have estimated
the production from swamps and humid tropical areas and added it to
Koyama1 s figure to derive an estimate of methane production of
14
14. 5x10 g/year (1. 6 billion tons/year).
Methane is the predominant hydrocarbon in natural gas, and seepage
19a
from gas, oil, and coal fields constitutes another source. However,
32
-------
372
Ehhalt considered measurements of the carbon-14 content of atmospheric
methane and concluded that up to 25% of the atmospheric methane is from
"dead" carbon and could have come from oil-field seepage or the combustion
of fossil fuels, whereas 75% is of recent biogenic origin.
Our understanding of the natural sources of methane is still highly
limited; as a result, current estimates of global production remain specu-
lative.
Plants release a variety of volatile organic substances, including
ethylene, isoprene, a-pinene, and a variety of other terpenes. Ethylene
272
production by plants was noted as early as 1910 and rediscovered in
377 183
1932. Burg has compiled an extensive list of plants that have been
shown to produce ethylene. Rates of production of ethylene in various
113
fruits have been measured by Biale ^t aL and in cotton plants, by
539
Hall et aL Although some of the fruits were thought to produce no
ethylene, later measurements by a more sensitive gas chromatographic
183, 184
method have shown that very small amounts are produced.
Ethylene is a plant hormone that is continuously produced in plants and
1,182, 281a 113
has been linked to fruit ripening. The ripening of fruits,
14 274
the fading of flowers, and injury from air pollutants have all been
2
shown to be associated with increased ethylene production. Abeles et al.
have estimated the natural production of ethylene from plants in the
United States at 20, 000 tons/year.
Other organic plant volatiles have not been studied as extensively.
632
Ivanov and Yakobson have reviewed the Russian literature and reported
that a considerable number of plant species release low-molecular-weight
hydrocarbons, aldehydes, and a wide variety of essential oil components.
33
-------
1370
Went hypothesized that the decomposition of carotenoids
(lipochromes) and phytol results in emission of volatile organics to the
atmosphere and that the fate of terpenes synthesized in plants is volatiliza-
tion into the air. Went estimated that a total of 175 million tons of volatile
organic material was emitted to the atmosphere of the -whole world each
1055 1064
year. Later, Rasmussen and Rasmussen and Went reported
ambient concentrations of plant volatiles (such as isoprene, a- and £-pinene,
limonene, and myrcene) in air at remote sites. From the average concen-
tration they measured (10 ppb), Rasmussen and Went estimated the global
production of plant volatiles at 438 million tons/year. Although the estimates
1056
of worldwide terpene emission need considerable refinement, it is
definite that there are large sources of natural, organic emission in the
1117 1052
biosphere. Furthermore, Sanadze and Dolidze and Rasmussen
have both identified isoprene as a natural volatile plant product and studied
1063,1116
the physiology of isoprene emission. Table 2-10 lists the various
1055
volatile plant products identified by Rasmussen.
From the available estimates of methane and terpene emission, it is
clear that worldwide natural hydrocarbon emission is about 2 billion tons/
year. However, there is evidence that many other organic volatiles are
naturally emitted to the atmosphere. These may be only trace amounts
emitted on a local scale, but the total worldwide production can be very
large.
Although natural gas consists predominantly of methane, it also
contains varying amounts of ethane, propane, and butane and traces of
19a
heavier hydrocarbons. It has been reported that trace quantities
of ethane, acetylene, ethylene, propane, and propylene are present as
188
products of methane fermentation.
34
-------
651
Studies of odorous compounds in natural waters have shown
that organic sulfur compounds--such as methylmercaptan, dimethylsulfide,
isobutylmercaptan, and ri-butylmercaptan--are produced. In addition,
876
geosmin has been identified as a natural product. Organic nitrogen
185, 1216, 1239
compounds are also well-known odorous natural products.
1394
Wilson at a_l. have found that ethylene and propylene are produced in
illuminated cell-free distilled water or natural seawater systems to which
dissolved organic matter produced by phytoplankton has been added.
224
Furthermore, Cavanagh e_t aL consider the small quantities of
ii-butyl alcohol, acetone, and ethyl alcohol in Pt. Barrow, Alaska, as
products of a fermentation process in the tundra cover. Aldehydes have
been identified as products of an illuminated mixture of plant extracts,
1382
oxygen, and water; and methane, ethane, ethylene, propane,
propylene, and ii-butane have been identified as products of the thermal
928
treatment of marine mud slurries.
Man is also a source of volatile organics. Body odors have been
258,984,1264
studied by the U. S. armed forces. Some atmospheric
contaminants identified during a 30-day manned experiment are low-
mole cula.r-weight aliphatic acids and aldehydes, in addition to hydro-
carbons and other compounds.
The annual hydrocarbon emission from natural sources in the United
States can be estimated from the preceding information. If natural methane
1084
emission is uniform in all land areas, Robinson and Robbin's estimate
of worldwide methane emission (1. 6 billion tons/year) can be used to calcu-
late a U. S. emission of 100 million tons/year. The assumption that natural
methane is emitted uniformly from all land areas is probably in error, in
35
-------
that emission rates are higher in tropical than in nontropical areas.
More realistically, assuming that the U. S. methane emission rate is
half the -world average, the U. S. natural methane emission would be
1064
50 million tons/year. Similarly, if Rasmussen and Went's estimate
of worldwide terpene emission (438 million tons/year) is based on uniform
distribution over the forested areas of the world, the U. S. emission would
be 22 million tons/year. Emission of ethylene from plants in the United
2
States has been estimated at 20, 000 tons/year.
Combining these estimates indicates a natural hydrocarbon emission
in the United States of about 72 million tons/year.
ATMOSPHERIC CONCENTRATIONS OF GASEOUS HYDROCARBONS AND
OXYGENATES
Many hydrocarbons are present in the atmosphere and the exact
composition varies enormously from place to place, but methane is the
predominant hydrocarbon found in all locations. In remote areas, it is
predominant by far. In populated areas, other hydrocarbons are present,
but methane has the highest concentration.
Total Hydrocarbon Concentrations
The flame ionization detector, which measures total organically
bound carbon, has been used to measure total hydrocarbon concentrations
in various cities. It has been used in the Continuous Air Monitoring
1296
Program (CAMP) network since 1962. Yearly average carbon
concentration ranges from 1. 43 ppm in Washington, D. C. , in 1962 to
3. 3 ppm in Chicago in 1962, and yearly maximal 1-hr averages range
from 8 to 17 ppm. Total hydrocarbons are also routinely measured
36
-------
197
at over 30 sites in the California air monitoring network. Total
hydrocarbon analyses have also been made for shorter periods in
33,352, 380, 1151-1153
specific studies.
Because the total hydrocarbon measurement does not discriminate
between photochemically active hydrocarbons and methane, which is rela-
tively inert, various techniques to separate the hydrocarbon mixture into
718,997 997
classes have been developed. One of these methods, which
discriminates between methane and nonmethane hydrocarbons, has been
1142, 1290
used to relate nonmethane hydrocarbons to oxidants formed in
urban atmospheres. Diurnal patterns of nonmethane hydrocarbons at
some CAMP and Los Angeles County sites are presented in Air Quality
1290
Criteria for Hydrocarbons. Ratios of nonmethane hydrocarbons to
total hydrocarbons are reported to vary from 0. 2:1 in Washington, D. C. ,
1142
to 0. 5:1 in Los Angeles.
Methane
Methane was discovered in the telluric spectrum by Migeotte in
890
1948. Later spectroscopic measurements established the average
416,499,500
methane concentration of the atmosphere to be about 1. 5 ppm.
From these spectroscopic measurements, which average over the entire
air column, it was tentatively concluded that methane is uniformly distributed
throughout the atmosphere, except for local variations near the surface.
However, vertical profiles have shown that the methane concentration is
71a,372 71a
somewhat variable in time and space. Bainbridge and Heidt
measured a decrease in the methane concentration at the tropopause, indi-
1137
eating that the stratosphere is a sink for methane. Scholz ^t a_l. have
shown that the methane concentration near the stratopause is, indeed, less
than 0. 05 ppm (by volume).
37
-------
A number of ground-level measurements of natural background
methane concentrations in remote areas have been reported. Stephens
1218
and Burleson measured 1. 39 ppm in southern California mountain
areas during the time hot, dry Santa Ana winds were blowing,Cavanagh
224
et^ al. found an average methane concentration of 1. 59 ppm at Pt.
1246,1247
Barrow, Alaska. Swinnerton e_t aL measured a mean con-
centration of 1. 25 ppm during an oceanographic cruise between
Washington, D. C. , and Puerto Rico and concentrations between 1. 25
and 1. 50 ppm over the Pacific Ocean and on the island of Hawaii. All
these measurements agree with Junge's estimate of the worldwide range
672 1169
(1. 2-1. 5 ppm). However, Shearer has reported measurements
at a rural site averaging 0. 89 ppm.
33
In inhabited areas, methane concentrations are higher.
Other Individual Hydrocarbons
Remote Locations. There have been few measurements of individual
224
hydrocarbons in remote locations. Cavanagh et aL found fractional
parts-per-billion concentrations of benzene, pentane, butane, ethane,
ethylene, acetaldehyde, and acetone at Pt. Barrow, Alaska. They also
found rL-butyl alcohol at about 100 ppb, which they attributed to biologic
1278
sources. Turk and D'Angio showed that there is a complex mixture
1064
of organic vapors in remote locations. Rasmussen and Went have
identified isoprene, a-pinene, 3-pinene, limonene, and myrcene in the
ambient air in remote locations in North Carolina, Virginia, Missouri,
and Colorado. They measured terpene concentrations from 2 to 50 ppb,
and found that ambient concentrations increased with temperature and
light intensity. Particularly high concentrations were found when conditions
38
-------
associated with the death of cells were present, such as the dying of
1052, 1060
leaves in autumn and the mowing of meadows. Rasmus sen
has also analyzed volatile organics in the ambient air in the humid
1218
tropics. Stephens and Burleson measured parts-per-billion
concentrations of ethane, ethylene, and acetylene in the mountains of
southern California during Santa Ana winds.
Populated Locations. Although the hydrocarbon composition of the
ambient air in populated areas has been studied by a number of investi-
gators, only the Los Angeles basin has been studied extensively.
956
Neligan analyzed 16 early-morning samples collected in central Los
Angeles for C hydrocarbons. He compared his results with the
2-4
composition of auto exhaust and showed that they were similar. However,
he did find a relative excess of light paraffins in the atmosphere, compared
with the composition of auto exhaust. He attributed these to natural gas
20
leakage. Altshuller and Bellar measured methane, C hydrocarbons,
2-5
and C aromatics on separate days in downtwon Los Angeles in the
6-8
fall of 1961. They followed changes in concentrations during the day
and showed that methane concentrations between 2 and 3 ppm were common,
confirmed Neligan1 s finding that light paraffins are more abundant in the
Los Angeles atmosphere than in auto exhaust, and showed that aromatic
hydrocarbons contribute appreciably to the total hydrocarbon concentration.
957
Neligan and Leonard reported the concentrations of a dozen C
6-10
aromatics at five sites in Los Angeles and concluded that aromatics
contribute significantly to the total hydrocarbon loading of urban atmospheres.
The average results from over 200 early-morning analyses of C hydro-
1-7
carbons in Los Angeles are summarized in Air Quality Criteria for Hydro-
1290 1217
carbons. Stephens and Burleson analyzed the atmopsphere in
39
-------
Riverside, California, for C hydrocarbons and also found an
1-6
abundance of light paraffins in the atmosphere, compared with auto
exhaust. Because this phenomenon appeared to be more pronounced
in the afternoon, Stephens and Burleson suggested that natural gas
losses, gasoline evaporative losses, and possibly seepage from oil
fields could contribute to the effect. In a later paper, Stephens
1219
je_t al. presented results of C analyses from various locations
1-7
in Riverside, the San Francisco bay area, and southern California oil
fields. They concluded that gasoline evaporation alone could not account
for the excess paraffins, but that natural gas leakage, seepage from oil
fields, and gasoline evaporation all contribute.
809
Lonneman _e_t al. measured methane, total hydrocarbons, and
individual C aromatics in 136 samples from Los Angeles during
6-10
the fall of 1966. The average and maximal concentrations are shown in
Table 2-11. Toluene -was the the most abundant aromatic, followed by
508
rn-xylene and benzene. Gordon j^t al. followed the diurnal patterns
of individual C hydrocarbons in downtown Los Angeles and Azusa
2-5 32
in the fall of 1967. Altshuller e_t al. also measured individual hydro-
carbons in downtown Los Angeles and Azusa in the fall of 1967, but
they measured the full range of C hydrocarbons. The average
1-10
concentrations of a number of individual hydrocarbons are shown in
Table 2-12. Diurnal patterns for various individual hydrocarbons are
shown in Figures 2-4 through 2-7. Although many hydrocarbons are
present in urban atmospheres, Altshuller e£ al. found that methane and
10 other hydrocarbons (ethane, ethylene, acetylene, n-butane, isopentane,
propane, toluene, n_-pentane, m_-xylene, and isobutane) account for about
90% of the total hydrocarbon loading.
40
-------
The hydrocarbon composition of urban areas other than Los Angeles
1384
is not so well known. Williams has identified over 30 hydrocarbons
on the University of British Columbia campus, and some measurements
992 524
from Japan are available. Grob and Grob have qualitatively
identified 108 C organics in the air of Zurich, Switzerland. Scott
6-20 1151-1153
Research Laboratories has measured individual C hydro-
1-10
carbons at various sites in New York, as well as Los Angeles, but has
810
not analyzed the results in detail. Lonneman and Kopczynski have
determined atmospheric hydrocarbon compositions in Los Angeles,
1218
Denver, and New York. Stephens and Burleson have measured the
composition of light hydrocarbons in samples from Hawaii, Denver,
New York, and Riverside and the Salinas Valley, California. They con-
cluded that, as long as samples are not deliberately taken near sources
of hydrocarbon pollution, the composition resembles that of auto exhaust,
with the addition of natural gas and of C paraffins that resemble
3-5
gasoline vapor. Samples taken in industrial areas and near a brushfire
showed distinct differences in composition.
Aldehydes and Other Oxygenates
A number of investigators have predicted that formaldehyde should
be naturally present in the atmosphere as an intermediate in the oxidation
19a, 202, 794, 850, 1368 794
of metheine. Levy calculated a steady-state day-
time concentration of 2 ppb. These predictions are generally confirmed
by the few aldehyde measurements that have been made in remote
805 418
areas. Lodge and Pate and Fischer et_ aL measured aldehyde
concentration between 1 and 10 ppb in the jungles of Panama and in
Antarctica. Although their method measured total aldehydes, it seems
reasonable to assume that formaldehyde was the major, if not the only,
41
-------
aldehyde present. Thus, there is some evidence that formaldehyde is
a natural constituent of the atmosphere at a few parts per billion.
The source of aldehyde data for populated areas in the U. S. National
Air Surveillance Networks (NASN), which measured total aldehydes on
an experimental basis in 1967. This was a pilot study. Insufficient
data were obtained to permit the computation of annual averages. The
study was discontinued after 1967. The average estimated concentrations
at 30 urban sites ranged from less than 0. 01 to 0. 06 ppm; the average
1286 1207
daily maximal concentrations ranged from 0. 01 to 0. 13 ppm. Stahl
has compiled atmospheric aldehyde data for the years before 1967. Some
indication of the composition of atmospheric aldehydes can be obtained
from total aldehyde, formaldehyde, and acrolein measurements made
1071b, 1151-1153
in the Los Angeles area. Formaldehyde was the pre-
dominant aldehyde, accounting for over 50% of the total, whereas acrolein
accounted for only about 5%. Formaldehyde has also been measured in
1420
the air of Tokyo, Japan.
There are few atmospheric measurements of oxygenates other than
aldehydes, primarily because of the lack of suitable methods of analysis.
94
However, Bellar and Sigsby have qualitatively identified methyl alcohol,
acetaldehyde, and acetone in ambient air near motor-vehicle traffic.
42
-------
TABLE 2-1
Estimates of Hydrocarbon Emissions from Manmade and Natural Sources
Hydrocarbon Emission,
Area
United States
World
Manmade
Mobile
17.6
12*
16.6
19.8
13.8
—
34
Sources
Stationary
13.9
7
15.4
17.6
—
25.4
54
million tons/year
Natural Sources Reference
72 1238a
917a
1290
1295
381
1293
2,000 1084
—Automobiles only.
43
-------
TABLE 2-2
Hydrocarbon Emission by Source Category, United States, 19695.
Source
Mobile (transportation):
Motor vehicles, gasoline
Motor vehicles, diesel
Aircraft
Railroads
Vessels
Nonhighway use
Fuel combustion—stationary:
Coal
Fuel oil
Natural gas
Wood
Industrial processes:
Primary metals
Petroleum refining
Chemical processing
Other
Solid-waste disposal:
On-site incineration
Open dumps, burned
Wigwam burners
Miscellaneous:
Forest fires
Structural fires
Coal-refuse banks
Agricultural burning
Solvent evaporation
Gasoline marketing
Hydrocarbon Emission, million tons/year
16.9
0.2
0.4
0.1
0.3
Total pollution
Natural:
Methane
Terpenes
Ethylene
1.9
0.1
0.1
0.3
0.4
0.3
2.3
0.8
2.1
0.5
1.2
0.3
2.9
0.1
0.1
1.7
3.1
1.3
sok
22£
0.04
19.8
0.9
5.5
2.0
9.2
37.4
Total pollution plus natural
72.0
109.4
a 1295
"Data from U. S. Environmental Protection Agency, except as noted.
"Derived from Robinson and Robbins, assuming U. S. methane emission per
square mile to be half world rate.
—Derived from Rasmussen and Went, assuming terpene emission to be
uniformly distributed over forested lands of the world.
44
-------
TABLE 2-3
Estimated Hydrocarbon Emissions from Automobiles and Other Mobile
Pollution Sources, United States. 1955-1985^-
Source
Automobiles?
Trucks and Buses
Aircraft
Off-Highway
Total
Estimated Hydrocarbon Emission, million tons/year
Projected Estimates
1955
9.9
1.2
0.3
0.7
1960
12.0
1.4
0.3
0.7
1965
13.0
1.7
0.2
0.7
1970
11.0
1.9
0.3
0.6
1975
5.9
1.7
0.2
0.6
1980
2.4
1.4
0.1
0.6
1985
0.9
1.4
0.1
0.5
12.1
14.4
15.6
13.8
8.4
4.5
2.9
—Derived from Environmental Conservation: The Oil and Gas Industries.
Vol. 1. Nat. Petroleum Council, June 1971.
381
45
-------
TABLE 2-4
Predominant Hydrocarbons in Auto Exhaust
Hydrocarbon
Methane^-
Ethylene^.
Acetylene—
Propylene—
n-Butane
Isopentane
Toluene
Benzene—
n.-Pentane
m- and £-Xylene
1-ButeneA
Ethane^
2-Methylpentane
n-Hexane
Isooctane
All others
Fraction of
62-Car
Survey—
16.7
14.5
14.1
6.3
5.3
3.7
3.1
2.4
2.5
1.9
1.8
1.8
1.5
1.2
1.0
22.2
Total Exhaust Hydrocarbons, vol. %
15-Fuel Engine-Variable
Studyb Study0
18 13.8
17 19.0
12 7.8
7 9.1
4 2.3
4 2.4
5 7.9
—
—
2.5
3^ 6.0l
2.3
—
—
—
30 26.9
from Papa.
1006
""Data from Morris and Dishart.
fi AO
-HData from Jackson.
d
"Combustion products.
—Includes isobutylene.
46
-------
TABLE 2-5
Exhaust Aldehyde Analyses
Fraction
Wiggs et
Formaldehyde 66.7
Acetaldehyde 9.3
Propionaldehyde^. 15.7
Acrolein r 3.2
Butyraldehydes '-
Crotonaldehyde
Valeraldehydes —
Benzaldehyde 3.2
Tolualdehydes 1.9
*
of Total Exhaust Aldehydes, vol. %
al.1380* Oberdorfer983
72.5 70.2
8.7 7.2
0.4
9.8
4.3 0.4
0.4
0.4
7.0 8.5
7.2
0.3 2.7
Wodkowski and Weaver Fracchio et al. ^°
59.9 .69.3
14.3 7.5
{7.0 0.7
2.6
3.0 1.0
1.4 0.4
—
3.3 5.4
5.9 3.1
5.2 10.0
72.9
8.5
V 6.4
1.7
0.4
—
4.3
—
5.8
Total .100 100 100 100 100 100
o
—Exhausts from two different gasolines.
—Also includes acetone of unknown proportion.
47
-------
TABLE 2-6
Partial Analysis of Carburetor and Fuel-Tank Evaporation Losses
Fraction of Total Evaporated Hydrocarbons, vol. %
Carburetor Fuel Tank
Hydrocarbon
Paraffins:
Wadeal329
Muller Q0_ Jackson &
^930 641
Caplan209 et al.^ Everett
ri- Butane
Isobutane
ri-Pentane
Isopentane
n-Hexane
2,2-Dimethylbutane
2,3-Dimethylbutane
2-Methylpentane
3-Methylpentane
Olefins:
1-Butene
trans-2-Butene
cis-2-Butene
cis- trans-
2-Pentene
1-Pentene
2-Methyl-l-butene
3-Methyl-l-butene
2-Methyl-2-butene
Aromatics:
Benzene
Toluene
11.9
0.2
2.0
45.3
0.2
0.1
3.0
2.3
1.2
0.
0.
0.
1.4
0.
0.
0.1
1.6
0.5
0.5
9.1 10.5
0.1 0.6
2.0 9.9
40.9 17.8
0.3
0.0
3.2
2.4
1.3
"15.5
^
0.
0.
0.
1.3
0.3
0.6
0.1
1.6
0.5
0.8
0
0.3
0.2
2.7
0.7
1.7
0.2
3.9
1.8
2.4
23.0
0.2
1.5
36.0
8.7
1.7
0.9
1.0
fO.8
k.
0.1
16.5
6.5
7.2
23.2
0.7
0.1
1.6
1.9
1.1
4.6
4.8
4.2
1.5
5.6
4.3
0.8
7.3
0.4
0.4
30.5
2.7
8.6
26.4
1.8
2.6
3.2
f
0.4
0.9
0.3
0.3
Subtotal
72.4
65.4
68.2
77.1
92.7
91.3
95.1
—Same gasoline at 163 and 177 F.
-Calculated values for 140 F.
—Different gasolines.
48
-------
TABLE 2-7
Most Recent (1971) Estimates of Organic Emission from
Stationary Sources in the United States^
Total Organic Emission,
Stationary Source million tons/year
Solvent evaporation 7.1
Solid-waste combustion 4.5
Agricultural burning 4.2
Forest fires 2.4-3.0
Petroleum storage and marketing 2.2
Petroleum production and refining 2.0
Chemical processing 1.4
Other industrial processing 'v 1
Fuel combustion 0.4
Coal-refuse burning 0.2
Total 25.4-26.0
"Data from U. S. Environmental Protection Agency.
49
-------
TABLE 2-8
Typical Emission of Several Classes of Compounds from
Stationary Combustion Sources
Hydrocarbons
Aldehydes
Formaldehyde
Organic acids
Emission, Ib/ton of fuel
Coal Oil Gas
0.3
unknown
0.003 '
10
1.0
0.5
0.006
5
1.0
0.5
0.008
2
50
-------
TABLE 2-9
Estimated Solvent Usage in the United States, 1968^-
Solvent
Petroleum naphtha
Perchloroethylene
Ethyl alcohol
Trichloroethylene
Toluene
Acetone
Xylenes
Fluorocarbons
Methylethylketone
1,1,1-Trichloroethane
Methylene chloride
Methyl alcohol
Ethylene dichloride
Ethylacetate
Cyclohexane
Methylisobutylketone
Hexanes
Benzene
n-Butyl alcohol
Nitrobenzene
Turpentine
Isopropylaceta.te
Ethyl ether
Monochlorobenzene
Isopropyl alcohol
Diethylene glycol
Methylacetate
Cresols
Phenol
Chloroethylene
Carbon tetrachloride
Pinene
Cyclohexyl alcohol
Cyclohexanone
Ethylbenzene
Isobutyl alcohol
Chloromethane
n-Butylacetate
Quantity Used as Solvent, tons/year
4,325,000
285,000
265,000
245,000
240,000
205,000
180,000
175,000
160,000
140,000
140,000
135,000
120,000
85,000
80,000
75,000
65,000
50,000
45,000
25,000
20,000
20,000
20,000
15,000
10,000
5,000
5,000
2,500
b
b_
b
b
b^
b
Total
7,137,500+
a 1293
"Data from U. S. Environmental Protection Agency.
-Less than 2,500.
51
-------
TABLE 2-10
Volatile Plant Products Identified by Rasmusseri3-
a-Pinene n-Heptane
8-Pinene Isoprene
Myrcene a-Ionone
D-Limonene 3-Ionone
Santene a-Irone
Camphene
aD 1055
—Rasmussen.
52
-------
TABLE 2-11
Average and Highest Concentrations of Various Aromatic
Hydrocarbons in Los Angeles§.
Concentration, ppb (by volume)
Aromatic Hydrocarbon Average Highest
Benzene 15 57
Toluene 37 129
Ethylbenzene 6 22
£-Xylene 6 25
m-Xylene 16 61
o-Xylene 8 33
Isopropylbenzene 3 12
n-Propylbenzene 2 6
3- and 4-Ethyltoluene 8 27
1,3,5-Trimethylbenzene 3 11
1,2,4-Trimethylbenzene,
isobutylbenzene, and sec-
butylbenzene 9 30
tert-Butylbenzene 2 6
Total aromatics 106 33
a 809
"Data from Lonneman et al.
53
-------
TABLE 2-12
Aliphatic Hydrocarbon and Alkylbenzene Concentrations
in Downtown Los Angeles (DLA) and AzusaJ-
Concentration, ppb (by volume)
Concentration below Which
10% of Values Occur
Hydrocarbon
DLA Azusa
Aliphatic hydrocarbons:
Methane 1,700 1,500
Ethane 4- ethylene 40 34
Acetylene + propane 30 18
Propylene 3 1
n-Butane 20 9
Isobutane 5 3
1-Butene + isobutene 2 1
2-Butene 1 1
1,3-Butadiene 1 1
n-Pentane 8 4
Isopentane 12 7
1-Pentane +
2-methyl-l-butene
Nonane + decane 2 1
Overall Average
DLA Azusa
,100
102
76
10.5
46
12
5.5
2
2
21
35
3
5
Concentration Exceeded by
10% of Values
2,200
65
43
4
21
7
3.5
1
1
10
16
DLA
3,500
180
120
21
80
20
10
5
5
35
56
Azusa
,000
100
65
8
35
12
5
2
2
16
26
Alkylbenzenes:
Toluene
Ethylbenzene
j>-Xylene
£-Xylene
m-Xylene
Propylbenzenes
Ethyltoluenes
Other Cg and CIQ
alkylbenzenes
10
1
2
2
4
2
3
6
1
1
1
2
1
1
30
5
5
6.5
12
4.5
7.5
12
14
2.5
2
3
5.5
3
3
50
9
10
11
21
8
15
23
23
4
4
6
10
6
7
13
3. o o
—Data from Altshuller et al.
54
-------
52
28
24
-e 20
§* 16
MOLE CONCENTRATI
pwcmt ef tctfll hydroe<
8S Sg o *« ~
16
12
8
4
r>
2-cycli «ngin«
—
rl II,
nli. . ill Illiiiim.
2-cycl* «ngin»
lull lood
-
I -
.1, .Jll!!,,,,..
-
.
)
-
I
4-cycU «ngin«
idlt
1.. Illlllllllll...
4-cyclt «ngin«
full load
li . IIHiiiiin..
NUMBER OF CARBON ATOMS PER MOLECULE
FIGURE 2-1. Distribution of hydrocarbons in diesel exhaust gas.
(Reprinted with permission from Hurn and Marshall.621)
55
-------
>0
to
10
0
so
to
10
I °
8 50
o
« to
g 10
o
tikg.il
12
liU
(TotOl ej FIO 686 ppmC)
I* l»»
'. a... H i H 8 § 111 a •
50
to
K>
0
50
to
10
12
It It
(T»toi
12
CmlM
k, FIO •
1C l»»
12
FIO
-------
KEY
::: Fuel on Exhoust
S 60
i
z SO
§
840
|
x
s30
z
|io
^
O
5 '0
2
t
<
* ftL
4-CTCLE ENGINE
- (Hydrocorbon—lhi* tomplt"
299 PpmCI
•
A V
/A t'
ll
/ /
* x
^ f
> >
ri
^ -
I ri
n ' I J
M?l' i^'A
p/j f 1 * ^ 1 51' * J
ri t 1 1 f 1 1 1 1 1 \A
fi
f •"»
1 [ ' t r^
r 4 ^«i f*l| Af
"
2- CYCLE ENGINE
(Hydrocarbon-
* thift iomple-*
6.935 ppmC)
""
m M
H K]
^3 fO4 m L^l
\A \A P^ f>i
,
ri
1
1
1
t
J.
1
P'Trn
M ' t/1 I Ri
M 1 1 d . (J |
CARBON NUMBER
FIGURE 2-3., Distribution of hydrocarbons in exhaust from small utility
engines. (Reprinted with permission from Eccleston and Burn.367)
57
-------
I I I I I I I I I
56 7 I 9 10 11 12 13 }l K IE
FIGURE 2-6. Diurnal variations in average hourly concentrations of n-
butane, isopentane, n-pentane, propylene, and butene in downtown Los
Angeles.
JB- «
'a
s
I-
i »
§
-i—i—i—m—i—i—
^"N, DO»N10m LOS ANGELES
? t • 10UJCNF.
I \ • XVIF..1C-S
/ **. \ •-• IUYITOI UENtS
// \ \ r :tc-ji)TYLn;.iizt?(E +
/ 0 a \ lAS-IRBETHVLBtKEI
T 1~
V •>
\.
I I I j L
1 1 1
5 6 ) I 9 10 11 12 13 14 15 16
TIKE OF DAY (PST|
FIGURE 2-7. Diurnal variations in average hourly concentrations of
toluene, xylene, ethyltoluene, sec-butylbenzene, and 1,3,5-trimethylbenzene
in downtown Los Angeles.
58
-------
i
I"
i 1.0
i i i i i
o KE THANE, DOHTOWN LOS ANGELES
•ttTHANE.AZUSA
TIKE OF DAY (PST)
2
-PJU-
FIGURE 2-4. Diurnal variations in average hourly methane concentrations
in downtown Los Angeles and Azusa. (Figures 2-4 through 2-7 reprinted
with permission from Altshuller et al.3*)
r
§ CO
8
i i i i r
J I
i i i r
DOWNTOWN LOS ANGELES
10 11 12 11
THE OF DAY (PST)
II IS
FIGURE 2-5. Diurnal variations in average hourly concentrations of
ethane, propane, and isobutane in downtown Los Angeles.
59
-------
CHAPTER 3
POSSIBLE MECHANISMS OF FORMATION OF OXYGENATED
ORGANIC COMPOUNDS IN THE ATMOSPHERE
Hydrocarbons are among the five most frequently encountered air pol-
lutants in the United States. Although their concentration in the atmosphere
is low (2-3 ppm), the total amount is large; data for 1965 are given in
Table 3-1. The emission of hydrocarbons into the atmosphere of the United
States has probably increased since 1965 as a consequence of the substantial
increase in the number of automobiles and increased industrial and electric
power production. As the table shows, approximately 63% of emitted hydro-
carbon is from automobiles, and approximately 21%, from industry.
TABLE 3-1
HYDROCARBON EMISSION INTO THE ATMOSPHERE. UNITED STATES.a 1965
Amount of Hydrocarbon Fraction of Total
Source Emitted, million tons Hydrocarbon Emitted, %
Automobiles
Industry
Electric power plants
Space heating
Refusal disposal
Total
12
4
1
1
_1
19
63
21
5
5
_5
99
a 1289
Data from U.S. Dept. of Health, Education, and Welfare. More recent figures
for mobile- and stationary-source emission will be found in Tables 2-1 and 2-2.
The reader should also consult Table 2-3 for estimated and projected figures
for mobile-source emission from 1955 to 1985. All cited figures must be
considered as estimates only.
60
-------
FORMATION FROM TRIPLET OXYGEN (ATMOSPHERIC OXYGEN)
The first part of this chapter deals mainly with the reactions of hydro-
carbons and atmospheric oxygen--molecular oxygen in its ordinary triplet ground
state--and la.ter sections with reactions of singlet oxygen and ozone. The
emphasis is on the oxidation of hydrocarbons present in the atmosphere and
the mechanisms by which selected oxidation products are produced, particu-
larly those implicated in toxic or other physiologically undesirable manifesta-
tions. In addition, the discussion includes intermediate combustion-oxidation
products of hydrocarbons derived from internal-combustion engines (excluding
particulate and polycyclic aromatic hydrocarbons). Furthermore, because
it is assumed that, at their atmospheric concentrations, hydrocarbons are
nontoxic, the role of hydrocarbons as precursors of oxygenated compounds
of relatively high reactivity--such as hydroperoxides, peroxides, epoxides,
aldehydes, and ketones--is emphasized. The report is restricted to hydro-
carbons emitted into the atmosphere because of technology; bacterial and other
production of hydrocarbons is not considered.
The volatility of hydrocarbons varies with their carbon number, and
the discussion in this chapter deals with hydrocarbons most likely to be en-
countered—those containing eight or fewer carbon atoms. Hydrocarbons
containing more than eight carbon atoms are considered to be insufficiently
volatile to contribute significantly to the air pollution problem. The emphasis
here is on hydrocarbons in urban and heavily industrialized areas, because
such areas contribute by far the largest fraction of volatile hydrocarbons.
A major problem in discussing vapor-phase organic air pollutants is the
difficulty in controlling and understanding all the variables involved in
oxidative and other transformations that hydrocarbons can undergo in the
presence of oxygen, sunlight, metals of variable valence, oxides of nitrogen
and sulfur, ozone, and peroxides.
61
-------
The reactions, kinetics, and mechanisms discussed here are based
to a large extent on well-controlled laboratory experiments on the oxidation
of hydrocarbons in the liquid phase, conducted usually at low temperatures
( < 200 C) and often in the absence of photochemical energy and the wide
variety of initiators likely to be encountered in the atmosphere. Higher-
temperature vapor-phase oxidations are also discussed, inasmuch as they
represent important reactions that occur in internal-combustion engines.
(Reactions in internal-combustion engines occur at temperatures above
200 C. ) Laboratory studies at either low or high temperatures have generally
used one initiating system (at most, two), but such limitation is not feasible
under atmospheric conditions, because components are present in uncontrolled
and varying quantities. In the atmosphere and in internal-combustion engines,
a wide variety of reactions are occurring simultaneously, proceeding from the
original hydrocarbons through the intermediate products and ultimately to the
final products, carbon dioxide and water. Unfortunately, environmental reac-
tions do not always go to completion quantitatively, and small quantities of
many oxidation products are present in the atmosphere in a more-or-less
steady state.
Atmospheric hydrocarbons vary markedly in structure, from the
relatively unreactive straight-chain saturated aliphatic hydrocarbons and
benzene to branched-chain saturated aliphatic hydrocarbons, unsaturated
hydrocarbons, alicyclic hydrocarbons, substituted aromatic hydrocarbons,
diolefins, acetylenes, andmany others. Table 3-2 is a partial list of
hydrocarbons identified in urban environments.
Because rates of oxidation of hydrocarbons vary with structure, con-
sumption is not uniform. The more reactive hydrocarbons are consumed
considerably more rapidly than the least reactive ones; thus, relative
1218
concentrations are changing rapidly.
62
-------
The rate of consumption of individual hydrocarbons is a summation of
the rates of the various reactions by which they are consumed. Therefore,
rates and mechanisms of oxidation of hydrocarbons are important to under-
stand. Considerable data are now available on rates, kinetics, mechanisms,
and activation dimensions of the oxidation of many hydrocarbons derived from
model laboratory experiments. The data from these laboratory studies are
used in discussions of oxidation of hydrocarbons under the complex sets of
conditions that exist in the atmosphere and elsewhere. Such an extrapolation
has some theoretical support; vapor-phase reaction rates have been predicted
102
from liquid-phase data by Benson, who used thermochemical information
on various classes of peroxides to estimate rates for many hydrocarbon
reactions. Benson's methods have been tested and found to be accurate
in a substantial number of reactions for which experimental data are availa-
ble, and it should be possible to extrapolate laboratory results to account
for the oxidative behavior of hydrocarbons in the vapor phase (particularly
in urban environments), including the prediction of rates of development
of secondary contaminants, as well as rates of consumption of various
hydrocarbons in the atmosphere. A computerized simulation model for the
1359
kinetics of photooxidation of propylene has already been suggested.
Methods exist for measuring total hydrocarbons and classes of hydro-
carbons, such as aldehydes, in the atmosphere, but there is no acceptable
and universally used instrumental method for measuring all the oxidation
products that are present simultaneously. It would be of considerable im-
portance to be able to determine simultaneously some well-known com-
ponents of the atmosphere that are irritants and derive from hydrocarbon
oxidation--for example, formaldehyde, acrolein, and peroxyacylnitrates.
63
-------
In addition to irritants, common alkylating agents present in the
736,1122
atmosphere include epoxides, peroxides, and lactones.
Some epoxides and peroxy compounds have been shown to be carcino-
737, 1302, 1305, 1307, 1308, 1310, 1311, 1365
genie and to have other physio-
736
logic effects. Sources of these classes of compounds are liquid
fuels, such as gasoline and diesel fuels, and partial combustion processes
in internal-combustion engines and industry. In the unburned state, liquid
fuels are reactive and readily undergo vapor-phase oxidation--especially
in the presence of light, metals, and oxides of nitrogen--to produce a broad
.range of peroxides and epoxides. The unburned gasoline and other exhaust
products emitted from automobiles contain ethylene, propylene, butenes,
and other unsaturated hydrocarbons; under atmospheric oxidation condi-
tions, these hydrocarbons can form many oxidation products.
Kinetics of Hydrocarbon Autoxidation
Kinetic studies of the liquid-phase, free-radical autoxidation of hydro-
carbons by triplet molecular oxygen are numerous and well documented in
lll,592,891a, 1335
the literature. Specific reaction rate constants and
activation dimensions have been obtained with many systems. The basic
equations of the free-radical chain reactions of oxygen with hydrocarbons
in which kinetic chains are long and hydroperoxides are major primary
products are as follows:
Initiation: X? -»• X' (where X* is any radical); (1)
Ri
RH + X- (or XCy) ± ^ R- + XH (or X02H) . (2)
64
-------
Propagation; R« + 0,
(3)
RH
R02H + R-.
(4)
Termination: 2RC>2 •
ROOR (or alcohol or
carbonyl) + 0 .
(5)
In these equations, R is rate of initiation, and k , k , and k are
i apt
specific reaction rate constants for addition, propagation, and termination,
respectively. Because k » k , termination at usual oxygen pressures
a p
is via bimolecular interaction of peroxy radicals (Eq. 5). Under these
circumstances, the rate expression for hydroperoxide formation, and for
oxygen consumption if its formation by Eq. 5 is neglected, is given in
Eq. 6:
-d[02] _ -d[RH] =
dt
dt
dt
(6)
65
-------
where R combines Eqs. 1 and 2. The reaction of Eq. 3 is usually so
i
fast and efficient that it can be neglected; it is reversible, however, at
101
high temperatures. Using thermochemical data, Benson has calculated
ceiling temperatures at which the forward and reverse rates of Eq. 3 are
equal. At 1 atm of oxygen pressure, they are above 500 C if R is alkyl,
but only 300 or 160 C if R is allyl or benzyl, respectively.
The conditions under which the autoxidation of hydrocarbons occurs in
the atmosphere are usually quite different from those under which the above
rate expressions were obtained. For example, in laboratory studies at high
temperatures or in the presence of metal catalysts, or both, negligible
hydroperoxide accumulates, autoxidation rates are high, and a number of
anomalies occur that are not immediately accommodated by the simple
kinetic scheme given. Actual autoxidation rates may be considerably higher
than would be predicted by the final rate equation and may be further in-
creased by cooxidation of relatively small amounts of more easily oxidized
substrates, typically aldehydes or ketones. Furthermore, some autoxidations
take a much longer time to reach a maximal rate than that expected to build
up a small steady-state concentration of hydroperoxide.
Many systems, however, obey Eq. 6 for consumption of oxygen and
hydrocarbon (or formation of hydroperoxide) and k 's and k 's have been
p t
determined and correlated by using non-steady-state methods. The rela-
tively low rate of Eq. 4 is particularly significant; the rate is somewhat
dependent on RO • , but is highly sensitive to R-H bond strengths and
2
the presence of electron-donating or electron-withdrawing groups.
The overall rate of autoxidation depends on the ratio of k to
1/2 p
(2k ) , about which some qualitative generalizations can be made.
t
66
-------
Experiments show that the relative ease of autoxidation of RH roughly paral-
lels the ease of breaking the C-H bond, and for hydrocarbons increases in
the series n-alkanes
-------
614
the reverse. Under these conditions, the reaction is auto catalytic;
the substituted ot-cumylhydroperoxides decompose to radicals faster than
do the unsubstituted hydroperoxides.
1335
Initiation by Thermal Decomposition of Hydroperoxides. First,
we shall consider hydrocarbon systems that contain no added catalysts; the
chief source of chain initiation, therefore, is thermal decomposition of
hydroperoxides. If the initiation involves simple unimolecular homolysis
(Eq. 7), initial scission of the hydroperoxide should be followed immediately
by the steps shown in Eqs. 8 and 9:
k
ROOM £—^RO'+HO; (7)
RQ. + RH > ROH + R-; (8)
HO- + RH > 2
H90 + R-. (9)
Because the rate constants for the processes shown in Eqs. 8 and 9 are
4 8 42,211,1336
large, probably around 10 and 10 , respectively, termina-
tion steps involving these species are highly unlikely. Beyond this point,
the autoxidation is assumed to continue by way of Eqs. 3-5, leading to the
usual steady-state expression for total radicals:
2kd[ROOH] = 2kt[ROO']2. (10)
68
-------
When the reactions are conducted under conditions in which hydroperoxide
does not accumulate--for example, at high temperatures--it also reaches a
low steady state, as shown in Eq. 11:
k [ROO-HRH] - kd[RooH] = o. (ii)
Equations 12 and 13 follow from Eqs. 10 and 11:
[ROO-] = k [RH]/kt; (12)
[ROOH] = k 2[RH]2/k k . (13)
P t d
In these equations, k is the specific reaction rate constant for hydroperoxide
d
decomposition. Hydrocarbon is consumed by two pathways, of which one is
reaction with HO' and RO (two molecules per molecule of hydroperoxide
decomposed) and the other is reaction with ROO (from Eq. 11, one molecule
per molecule of hydroperoxide decomposed). Accordingly,
-d[RH]/dt = 3kd[ROOH] = 3kp2[RH]2/kt. (14)
Because k and [ROOH] are inversely proportional (Eq. 14), both
d
disappear from the rate expression. The actual rate constant for hydro-
peroxide decomposition is unimportant, provided it is large enough to ensure
that hydroperoxide reaches a low steady-state concentration early in the reaction.
69
-------
Before this point, the autoxidation is autocatalytic; afterwards, Eq. 14 is
obeyed. If we assume E = 12-15 kcal/mole and E = 4 kcal, reasonable
P t
values for relatively unreactive hydrocarbons and secondary peroxy radicals,
then E = (24 to 30) - 4 = 20-26 kcal, about the same as predicted for
a long-chain process with E = 30-40 kcal (E , E , E , and E
i p t overa ^
indicate the activation energy for propagation, termination, initiation, and
overall process, respectively).
1335
The above analysis, which is due to Walling, produces striking
results when applied to cooxidation, the condition that exists under atmospheric
oxidation and internal -combustion conditions. Such an analysis is too complex
to be reproduced here; the original literature should be consulted. The most
interesting cases arise when one of the substances is a very easily autoxidized
material, such as an aldehyde, and the other compound is a hydrocarbon.
Equation 14 may be generalized to any initiation scheme, assuming
Eqs. 3-5 and a low steady-state concentration of ROOH; details are given
1335
in Walling1 s paper.
Metal- Catalyzed Initiation and Other Reactions. Metal-catalyzed initia-
91 111
tion has been reviewed by Bawn and, more recently, by Betts and
1335
Walling. Compounds of some variable-valence (transition) metals are
among the most important initiators for hydrocarbon autoxidation. At low
concentrations, the chief effect of transition-metal salts is to initiate
autoxidation by converting hydroperoxides to radicals, as shown in Eqs. 15
682-685, 818
and 16:
R02H — > Mn+ + RO' + OH(km) ; (15)
. + H+(kn). (16)
70
-------
In these equations, k and k are specific reaction rate constants for
m n
+ (n+1)
metal in valency states M and M , respectively. Among the m.ost.
important of these transition -metal compounds are salts of cobalt and
manganese, which can alternate between adjacent valency states and under>
go both reactions shown in Eqs. 15 and 16, thereby decomposing hydro-
peroxides catalytically by a redox cycle when equilibrium has been estab-
n+
lished between the two valency states of the metal, k [M ] [RO H] =
m 2
k [M ] [RO H ]. The initiation rate is given by Eq. 17:
n 2
R± = (km[Mn+]+kn[M(n+1)+])[R02H] = ZkJcjMHRO^J/O^ + kn), (17)
where [M] = [Mn+] + [M(n+1)+].
The autoxidation rate given by Eq. 16 depends on the square root of
the total rnetal concentration. As hydroperoxide accumulates, the autoxida-
tion accelerates, but the hydroperoxide concentration eventually attains a
maximum, at which its decomposition by Eqs. 15 and 16 exactly equals its
formation by Eq. 4. On the assumption that the alkoxyl radicals produced
in Eq. lf> react quantitatively with hydrocarbon according to Eq. 18, it may
682-685, 1068.
be shown that Eq. 19 applies:
71
-------
RO(HO.,RO • ) + RH - \ ROH(H O, RO H) + R- ; (18)
2 22
2 2
-d[0 ]/dt = k [RH] /2k . (19)
2 p t
Thus, the limiting autoxidation rate is independent of the total metal con-
centration, a result that has been observed in various autoxidation systems.
1/2
Equation 19 may not necessarily be a reliable source of k /(2k ) values,
P t
inasmuch as in its derivation no account is taken of alkoxyl radical attack on
hyd roper oxide. Moreover, the polarity of a medium rich in hydroperoxide
1/2
and metal ions would probably affect the value of k /(2k )
P t
Compounds of transition metals, particularly of cobalt and manganese, are
reported to initiate autoxidation by oxidizing particularly reactive hydro-
170, 171, 1356
carbons, such as dienoic acids and alkyl aromatics, and it
has been frequently suggested that such attack is important in producing
initiating radicals in autoxidation, as shown in Eq. 20:
M + RH — ^M + R. + H . (20)
Even in the absence of peroxidic species, cobaltous ion may react with
110,264,323
molecular oxygen, as formulated in Eqs. 21 and 22:
+ 2 +2
Co + O — ^ Co .0 (complex); (21)
2 2
+ 2 +2 +3 +2
Co -0 + Co (XH) - >Co X" + HO • + Co . (22)
2 2
Initiation is apparently effected by a reaction with oxygen to give a complex
f
that oxidizes another cobaltous ion to cobaltic. The chain reaction continues
through the activity of the hydroperoxy free radical.
72
-------
468
Kinetic evidence has been reported to show that similar reactions
567
occur with ferrous ion in aqueous media, and other work supports such
reactions for the three metallic salts--cobaltous, manganous, and cerous--
in solvent media. Similar reactions have been observed with many cobalt
837 378
compounds, and manganese phthalocyanine forms a reversible oxygen
complex, which is an efficient autoxidation catalyst similar to ferrous and
cobaltous phthalocyanine s.
In the presence of hydroperoxides, the cobaltous ion is oxidized and
free radicals are formed, as shown in Eq. 23:
+ 2 +3
Co + ROOH >Co + RO.+ OH . (23)
With cobaltic ion in the presence of hyd roper oxide, the accepted reaction
in the presence of aldehydes and monounsaturated hydrocarbons is shown in
Eq. 24:
+ 3 +2 +
Co + ROOH—> Co + H +RO • . (24)
2
Two recent studies of metal-catalyzed autoxidation involving either
152
intermediates or added materials as cooxidants are worth noting. Brill
has shown that the cobalt-catalyzed autoxidation of toluene or p_-xylene in
acetic: acid at 90 C is markedly accelerated by 2-butanone. With 1 M £-xylene,
a maximal rate of approximately 25%/hr is obtained, corresponding to
-5
-d[RH]/dt = 5. 6 x 10 mole/ (liter-sec). At this temperature,
2 ^ -7 908,909
k /2k = 2. 8 x 10 . Morimoto and Ogata have reported a
p t -5
maximal rate of 5 x 10 mole/(liter-sec) for toluene autoxidation under
the same conditions. The latter authors clearly implicate benzaldehyde as
a critical cooxidant; they show that it is present at a constant concentration
73
-------
(4. 5%, based on toluene) during most of the reactions and that the autoxida-
tion does not reach its maximal rate until this concentration is reached;
addition of benzaldehyde greatly decreases the initial induction period.
In autoxidation reactions of hydrocarbons in the atmosphere, oxygen is
present in great abundance. Thus, it may be that termination may occur
•with urisaturated, as well as with saturated, molecules, as illustrated in
Eq. 5. In their lower valency states, transition metals can also terminate
autoxidation chains by reducing alkylperoxy radicals to hydroperoxides and
110, 264, 683, 684, 1168
other products.
The behavior of transition-metal salts in the autoxidation of hydrocarbons
depends heavily on the ligands attached to them. The recent book by Reich
1068
and Stivala1 should be consulted for a more detailed account of the chemical
and kinetic behavior of transition metals in autoxidation systems.
Conclusions. This brief analysis permits prediction of limiting rates
of autoxidation that can be expected for "classic" hydroperoxide chain autoxida-
tion of hydrocarbons under different conditions of chain initiation, using values
2
of k /2k obtained under conditions of slow reaction and long kinetic chains.
P *
At these limiting rates, substantial attack on hydrocarbon occurs via alkoxy
or hydroxy radicals, and this may explain the large increases in rate that
may arise in cooxidation involving intermediate oxygenated products or added
easily autoxidized materials. Although some instances of metal-catalyzed
autoxidation proceed near the predicted maximal rates, others are notably
faster, but show kinetics inconsistent with this analysis. Under these con-
ditions, it becomes likely that hydrogen abstraction by peroxy radicals from
any substrate becomes unimportant, and either peroxy radicals participate
in red ox cycles with the metal or some entirely new chain carriers are involved.
74
-------
Although a number of such steps have been proposed, none has been con-
clusively identified, nor have their rates been established. The extensive
data on the rates of individual steps in "classic" long-chain autoxidation
may now make this possible and lead to understanding of a series of reac-
tions of unusual scientific interest and technical importance, particularly
in relation to the atmospheric oxidation of hydrocarbons.
420,891a,1313a
Products of Hydrocarbon Autoxidation
Autoxidation either in the vapor phase or in nonpolar solvents involves
free-radical chain processes. The kinetics and mechanisms of these reac-
tions are indeed complex, because numerous free-radical intermediates are
simultaneously involved and each has different lifetimes and numerous modes
of further reaction. Recombination, disproportionation, decomposition,
displacement, and addition are possible, and the natures and quantities of
the products formed during autoxidation, gaseous oxidation, and pyrolytic
decomposition depend on the relative rates of the competing processes.
The hydroperoxide theory of autoxidation has had considerable success
in describing the mechanisms of low-temperature autoxidation of hydrocarbon,
and it has long been recognized that the alkylperoxy radical RO • is a very
2
important intermediate. Equations 1-6 have been successful in describing
many instances of autoxidation of hydrocarbons conducted on a well-controlled
laboratory scale at low temperatures (<200 C). Although it is now generally
accepted that termination in autoxidation (Eq. 5) involves interaction of two
alkylperoxy radicals to form a dialkylperoxide (or other products) with libera-
tion of oxygen, under appropriate conditions the dialkylper oxide formed may
also decompose to form free radicals, thereby propagating chains, and such
75
-------
a process must be considered important in both liquid-phase and vapor-
phase oxidation (> 200 C). Alkylperoxy radicals may add to unsaturated
systems to form an alkoxy radical and epoxides, as shown in Eq. 25:
RO • + C C
/ \
RO C-C "" "~- '
.... (25)
2I
A wide variety of other oxidation products are also obtained, including saturated
and unsaturated alcohols, aldehydes, and ketones, as illustrated by the recently
233 293
published work on the autoxidation of 1-hexene and cyclopentene.
Although most oxygenated intermediates--with the possible exceptions
of ketones, lactones, and epoxides --would not be expected to build up appreci-
ably, because they are more reactive than hydrocarbons, oxygenated intermediates
are physiologically less desirable.
Recent -work has established that a further important reaction of organic
peroxy radicals is intramolecular rearrangement or isomerization, especially
at higher temperatures (over about 200 C). These rearrangement reactions
lead, by further reactions of the isomerized radical, to several interesting
types of compounds, including carbonyl compounds and alcohols -with carbon
skeletons different from those of the original organic molecules, alkenes, O-
heterocycles, and dihyd roper oxides; under favorable conditions, as in oxygen-
deficient systems, the yields of these can be quite high. Because several
of these products are effective chain-branching agents, the occurrence of
isomerization during oxidation is mechanistically important and -in many cases
may determine the overall rate, the course of reaction, and product distribu-
tion.
76
-------
Competing Reactions of Organic Peroxy Radicals
Rearrangement reactions of peroxy radicals can be important chain-
propagation steps in the oxidation of hydrocarbons only if they can compete
successfully with intermolecular H-abstraction. Radical rearrangements
involve attack by the free electron of the radical on some other bond in the
radical. These reactions occur by the intramolecular transfer of an atom
or group, usually the hydrogen atom, and an active center migration. Free-
radical isomerization is favored by high temperature and by low pressure.
A typical example is the isomerization of the 1-pentylperoxy radical by 1:6
transfer of secondary H (Eq. 26) and the intermolecular abstraction of a
secondary H atom from ri-pentane by that radical (Eq. 27):
1:6 H
CH CH CH CH CH OO« - > CH CHCH CH CH OOH; (26)
32222 transfer 3 222
CH3(CH2)3CH200. + CH3(CH2)3CH3 - - ^ CH3(CH2)3CH2OOH + CH3CHCH2CH2CH. (27)
101
It has been estimated that the activation energy for intermolecular displace-
ment by RO • of secondary hydrogen is 10. 5 kcal/mole. For 1:6 intramolecu-
2 419
lar transfer of secondary hydrogen, E = 17 kcal/mole.
Modes of Isomerization of Organic Peroxy Radicals
The simplest organic peroxy radical is CH OO*, which is formed during
3
the oxidcition of methane (Eq. 28), and it decomposes by Eq. 29. It is evident
that Eq. 29 must involve isomerization by 1:3 transfer of H from C to O
(Eq. 30); this process has a high activation energy. The reaction, sequence
is therefore usually represented by a single step (Eq. 31).
77
-------
U2 • X v,iVjUU ,
. . _., ""iv CH f) a .Oil-
. "> -CH OOH ^- CH^O +
•OH;
(28)
CH300- - CH0 + -OH (29)
CH 00- - :> -CH2OOH - ^> CH20 + -OH; (30)
CH- + 0 - ^ CH0 + -OH. (31)
2
For higher alkylperoxy radicals, also, this well-established mode of de-
composition to a carbonyl compound and a hydroperoxy radical must involve
isomerization by hydrogen transfer in order that 'OH may be formed (Eq. 32):
RR'CHOO- _ > RR'COOH - > RR'C=O + -OH. (32)
The products of photooxidation of methane, ethane, and propane illustrate
431,432
the temperature dependence of such an isomerization reaction. Below
100 C, peroxides are the only products of photooxidation; but above 100 C,
aldehydes are also produced, by the isomerization and decomposition sequence
(Eq. 32). At higher temperatures, this mode of reaction of RO • occurs ex-
2
clusively.
In more complex cases, several modes of isomerization are possible, and
each of these may be followed by several decomposition reactions, leading to a
variety of products, many of which are diagnostic of the occurrence of iso-
merization. These modes of isomerization can be grouped into three classes
of reaction: H transfer, group transfer, and ring-opening reactions.
Isomerization by H transfer is by far the most frequent mode of iso-
merization of alkylperoxy radicals, forming hydroperoxyalkyl radicals
(Eq. 33):
C H GO- - ^> -C H OOH . (33)
n 2n+ 1 n 2n
78
-------
In complex radicals, transfer of H from several different carbon atoms can
occur and can be classified as 1:3, 1:4, 1:5, or 1:6 H transfer (Eq. 34):
1:3 H transfer
CH2CH2CH2R
4-membered rin
1:4 H transfer
^
(34a)
H
5-membered rin
?
R
H
1:5 H transfer
6-membered ring
>
OOH
VCHCH2CH2R?
,OOH
CHoCHCHoR
.OOH
(34b)
(34c)
1:6 H transfer
7-membered ring
>
(34d)
Thus, for example, in the 2, 2, 4-trimethyl-l-pentylperoxy radical,
(CH ) CCH CH(CH )CH OO-, two H atoms can be transferred 1:3,
33 2 3 2
one 1:4, five 1:5, none 1:6, and nine 1:7. The relative rates of these
processes depend on the size of the ring in the necessarily cyclic transition
states involved, because, although this does not affect the enthalpy change,
account must be taken of the strain energy involved in calculating the probable
activation energy.
Isomerization of an alkylperoxy radical by transfer of an alkyl group,
producing an alkylperoxyalkyl radical (Eq. 35), is less favored energetically
than isomerization by H transfer and has been postulated much less fre-
quently in discussions of oxidation mechanisms.
79
-------
CH9
/ \
R'-CH CH-R" > R'-CHCH2CHR"OOR (35)
JO
'X
Group transfer can compete with H transfer, however, in the isomerization
of alkylperoxy radicals for which only primary H is available; the best example
of this is the neopentylperoxy radical in which H transfer (Eq. 36) and CH
1428 3
transfer (Eq. 37) compete.'
(CH3)2(j 0 H transfer > (CH3)2C-CH2OOCHV (37)
J ^1 2 CH, transfer J L *
H3C 6 J
•
-------
)—0
(39)
(40)
It has been suggested that increase in ring size is effected by Eq. 38 during
852,853 856,857
the oxidation of cyclopropane, cyclopentane, methyl-
898 575
cyclohexane, and cyclohexane, and that Eqs. 39 and 40 may play a
1430
minor part in the oxidation of cyclohexane.
Isomerization by H transfer of cycloalkylperoxy radicals is more diffi-
cult than the corresponding reactions of peroxy radicals derived from acyclic
alkanes because of restriction of rotation by the ring. In these circumstances,
ring-opening isomerization reactions appear to compete with H transfer.
Decomposition Reactions of Hydroperoxyalkyl Radicals
Hydroperoxyalkyl radicals decompose by several routes to give a variety
of stable or moderately stable products. In systems containing oxygen, oxi-
dation of these radicals will compete with their decomposition, giving hydro-
peroxyalkylperoxy radicals. The nature of the stable products of these oxida-
tion and decomposition reactions provides strong evidence of the participation
of alkylperoxy radical isomerization as a chain-propagating step in vapor-
phase oxidation reactions. The decomposition and oxidation reactions of
alkylperoxyalkyl radicals are analogous to those of hydroperoxyalkyl radicals.
81
-------
Cyclization to O-Heterocycles. Homolysis of the O-O bond (the weakest
bond) in a hydroperoxyalkyl radical is often accompanied by ring closure, to
give an O-heterocycle and a hydroxyl radical. Thus, decomposition by this
route of a-hydroperoxyalkyl radicals gives oxiranes, of 3 -radicals gives
oxetanes, of y-radicals gives tetrahydrofurans, and of 6-radicals gives
tetrahydropyrans. This important mode of decomposition is illustrated in
Eq. 41, which shows the mechanism of formation of 2, 5-dimethyltetrahydrofuran
from the 5-hydroperoxy-2-hexyl radical:
CHj—CH2 CHf_ CH2
II I
CH.* CH *CH CH > CH. CH CH CH + *OH. (41)
\ V 3
OH
In the case of simple alkanes, such as ethane, the O-heterocycles formed
are oxiranes. These are probably formed by cyclization of a hydroperoxy-
alkyl radical with elimination of >OH or by cyclization of an alkylperoxyalkyl
radical with elimination of *OR. However, the 5-hydroperoxy-2-hexyl radical
could have been formed either by alkylperoxy radical isomerization or by
720
addition of a peroxy radical to an alkene (Eq. 42), so that the formation of the
O-heterocycle is not unequivocal evidence of the occurrence of isomerization:
1:4
H transfer
'CH^CHoOOH ^L-L"aLJ-"u> H0C CH2
HO addition
82
-------
In the case of higher alkanes, however, the 0-heterocyclic products
include derivatives of oxetane, tetrahydrofuran, and tetrahydropyran, as
well as oxiranes. These products do provide definite evidence of alkylperoxy
radical isomerization; it is very difficult to write a mode of their formation
that does not involve this reaction.
1429
The slow oxidation of isobutane yields, in the second stage of the
induction period, 2,3-dimethyloxirane (isobutene oxide) and methyloxirane
(propylene oxide). In the third stage of the induction period, small amounts
of 3-methyloxetane are formed. Rearrangement by 1:4 H transfer of the
tert-butylperoxy radical or the isobutylperoxy radical followed by the elimination
of 10H gives 2,2-dimethyloxirane (Eq. 43). In the case of isobutylperoxy, two
further possibilities occur: 1:6 H transfer and loss of*OH> giving 3-methyl-
oxetane (Eq. 44); and 1:4 CH transfer and loss of*OCH , giving methyl-
3 3
oxirane (Eq. 45):
CH
/\
: >H
(GH3)2c NH ^ (c^V—™2
0—6 0 OH
(CH3)2C CH2 + "OH; (43)
,C~CHO > (CH3)29-
H \) HO-
>CH
CH CH "0 ^. CH CH CH ^, CH CH CH
3 I I / I 11+ '°H; <44>
HC *0 TTP* (i HPO
2 CH^CH CH^ ^. CH.CH
3 / \9 3 / 2 ^ 3 \
H3(/ .0>0 O \o/
6CH3
83
-------
109
The oxidation of n-butane produces 2- methyloxetane and 2-ethyloxirane
by 1:5 and 1:4 H transfer, respectively, of butylperoxy radicals and cyclization
of the hydroperoxybutyl radicals formed.
The alkylperoxy radical that should lead to a single conjugate 0-hetero-
cycle is the neopentylperoxy radical, in which 1:5 H transfer is the only
possible mode of H transfer (Eq. 36). Cyclization of the resulting 3-hydroperoxy-
2, 2-dimethyl-l-propyl radical produces 3, 3-dimethyloxetane, which has indeed
1428
been identified among the products of slow combustion of neopentane and
424
is a major product of the neopentane-oxygen cool flame.
The occurrence in combustion condensates of saturated furans and pyrans
1281a,1281b
was first reported from studies of the ii-pentane-oxygen system.
237
More recently, it has been shown that the n_-pentane-air reaction in an
annular-flow reactor gives, by 1:6 H transfer and cyclization, 2-methyltetra-
hydrofuran over a wide variety of conditions of temperature and reactant
ratio, the maximal yield being obtained at 450-500 C. In a static reactor,
by contrast, the cool flames of the ri-pentane-oxygen system at 278 C give
2-ethyl-3-methyloxirane and 2, 4-dimethyloxetane as the predominant C
288 5
0-heterocycles. It has been estimated that, in this system, 40% of the
288
pentylperoxy radicals formed isomerize and decompose to C O-heterocycles.
One of the earliest interpretations of O-heterocyclic products in terms of
alkylperoxy radical isomerization followed by cyclization is that of Bailey and
69
Norrish, who found 2, 5-dimethyltetrahydrofuran in the products of cool
flames of ri-hexane, thus providing unambiguous evidence of the participa-
tion, and isomerization of the 2-hexylperoxy radical (Eq. 46):
84
-------
H C CH H0C CH H (
21 I 2 2V Y 2 2
I ' II l
H-C—CH CH—CH ^-H,G—CH *CH—CH* > CH.CH CHCH. + OH. (46)
•5 | I 3 ' J \ J ' 3 \ / 3
0 H
0 OH
The first discovery of oxetanes in gaseous oxidation of hydrocarbons
resulted from studies of the oxidation of 2, 2, 4-trimethylpentane (isooctane)
1104
and 2, 2-dimethylbutane. In addition to 2, 2, 4, 4-tetramethyltetrahydro-
furan, isooctane gave 2-tert-butyl-3-methyloxetane, whereas 2, 3, 3-trimethyl-
oxetane was produced from 2, 2-dimethylbutane.
Oxetanes and tetrahydrofurans are much less likely to be formed by
cyclization of hydroperoxycycloalkyl radicals with elimination of «OH than
by the corresponding reaction of acyclic hydroperoxyalkyl radicals, because
of the highly strained configurations of the bicyclo products that would result.
1430
In the oxidation of cyclohexane, however, 1, 2-epoxycyclohexane is formed
by 1:4 H transfer in the cyclohexylperoxy radical and cyclization of the re-
sulting a-hyclroperoxycyclohexyl radical, and 1, 4-epoxycyclohexane
(7-ox.abicyclo[2'2-l] heptane) is formed from 1:6 H transfer in the boat con-
figuration of the cyclohexylperoxy radical followed by cyclization. Also,
the considerable yield of 5-hexenal probably arises by isomerization, cycliza-
tion, and (because of the strain in 1, 3-epoxycyclohexane) decomposition
(Eq. 47):
85
-------
1:4 H
transfer
1:5 H
transfer'
1:6 H
transfer'
—OH
cyclization
—OH
cyclization
HO—0
cyclization
(47a)
decomposi-
tion
Oil 4- OH (47b)
*OH (47c)
The gaseous oxidation of aromatic hydrocarbons has been studied less
intensively than that of alkanes. If, however, suitably positioned side chains
are present, isomerization of arylperoxy radicals can occur. Thus, the
82,807, 1124
oxidation of _o-xylene yields a •wide variety of oxygenated
products, but phthalan (jo-xylene oxide) predominates under a -wide range of
operating conditions. At low conversion, the yield of phthalan can exceed
807,1124
50%. It is very probable that the mechanism of production of
this compound involves isomerization by 1:6 H transfer of the ^-xylylperoxy
radical and cyclization (Eq. 48):
0—OH
+ OH. (48)
86
-------
It is noteworthy that no O-heterocyclic products result from the oxidation
under similar conditions of rn- and ja-xylenes, because the outer oxygen
of the peroxy group and the hydrogen of the m- or j^-methyl group cannot
get close enoxigh to each other. As a result, the kinetics of the oxidation
of ^-xylene differ markedly from those of the oxidations of m_- and p_-xylenes.
Scission of C-O. Scission of C-O (Eq. 49) is possible only when the
original isomerization occurs by 1:4 H transfer, that is, only when the
O-heterocycle derived from the hydroperoxyalkyl radical by cyclization is
an oxirane:
RR'C—CRIMR" i> RR'C—CR"'R" > RR'C = CR"'R" (49)
H 0—OH + HO'.
The proximity of the unpaired electron weakens the C-O bond in the B-position.
Alkenes result that are conjugate with the alkane from which the alkylperoxy
radical was derived. Production of alkenes by this route is held responsible
for the formation of isobutene as the major product during the induction
1429
period of the gaseous oxidation of isobutane (Eq. 50) and could account
for the production of alkenes in high yield from a variety of hydrocarbons
419
(e.g., 2-methyl-l-pentene and 2-methyl-2-pentene from 2-methylpentane,
112 1430
styrene from ethylbenzene, and cyclohexene from cyclohexane):
87
-------
Me3COO-
-H02
OOH
-H02
Me
2C = CH2. (50)
However, at temperatures over 400 C, alkenes are formed by direct abstrac-
720
tion of H by oxygen from alkyl radicals, and it has been suggested recently
that this reaction can occur at temperatures as low as 300 C, in -which case
it is in competition with addition of oxygen to alkyl radicals to form alkylperoxy
radicals, followed by their rearrangement and decomposition (Eq. 51):
°
R2C~CHRI2"
addition
isomerization
V
R2C(OOH)CR'2
displacement
C=CR' +HO
g-scission
(51)
Scission of C-C and O-O. In many 3-hydroperoxyalkyl radicals,
3-scission reactions result in the formation of an oxygenate and an alkene
('OH being eliminated) without further H shift. The production in pairs of
many of the alkenic and carbonyl scission products formed during the cool-
422,423
flame oxidation of 2-methylpentane can be accounted for quali-
tatively by this mode of decomposition (e. g. , Eqs. 52-57):
88
-------
HO-0-CH0 XCH
\
H3C
V.
CH C
OH + CH20 + CH3CH= CHC2H5; (52)
/\
KCH3)2c n o + ^2*=: CHCH.J + OH; (53)
H3C 0 OH
0— OH
I
H2C— CH— CHC2H5
CH2==CHCH3 + C^CHO + OH;
(54)
-0—OH
(CH3)2CHCH
(CH3)2CHCHO + CH2-====.CH2 +'oH; (55)
CH2
CCH3)2C— CH^
= CH2 + CH3CHO + OH; (56)
—OH
(CH3)2CH CH X
0/
OH
(CH3)2CHCH = CH2 + CH20 + 'OH. (57)
89
-------
Similar 3 -scission reactions of y~ anc^ 6-hydroperoxyalkyl radicals can
occur, but lead to an alkene and an O-heterocycle as the scission products
(Eq. 58):
H— CH— CH - >(CH)C - -pH + CH=CH + *OH. (58)
C CH9— CH9— CH9 > (CH^ ) _ C C
\ V
OH
A second group of C-C and O-O scission reactions, a scission of
a-hydroperoxyalkyl radicals, leads to oxygenated scission products in a
similar way, but H shift in an alkylidene biradical is necessary to produce
the alkene (Eq. 59):
(CH3)2C-CHCH2CH3 ^. [(CH^)9O ] + CH3CH2CHO + OH
0—OH
(59)
CH3CH=CH2.
1270
This isomerization is rapid, but the a-scission reaction involved is un-
likely to compete effectively with 3 scission of C-O in a-hydroperoxyalkyl
radicals.
Again, scission may produce two radicals, instead of two molecules
(Eq. 60); this group of reactions is energetically unfavorable and is not
very important.
90
-------
CCff3)2C-CH-CH2CH3 > (CH3)2CCHO + Ct^Ciy + "OH. (60)
HO-0
It is unlikely that the total yields of carbonyl scission compounds
formed in cool-flame combustion reactions can be explained by 6 scission
of hydroperoxyalkyl radicals. At subatmospheric pressures, these carbonyl
237,422,423
compounds are usually formed in yields considerably greater
than those of the alkenic scission products that are formed simultaneously
by such g-scission reactions. Two possible alternative routes to carbonyl
422,423
scission compounds are decomposition of alkoxy radicals and
decomposition of dihydroperoxy compounds. The discrepancies in yields
of carbonyl and alkenic scission products may also result, in part, from
attack of the TT bonds of the alkenes formed by radicals (Eq. 25).
If intramolecular group transfer occurs during scission, scission
products will result, one of which contains an arrangement of carbon
atoms not present in the hydroperoxyalkyl radical decomposed and there-
fore not present in the original alkane. Group transfer accounts for the
formation of butanone in the oxidation of 2-methylpentane (Eq. 61):
CH2CH2 H3C CK CH,
CCff3)2C CK ^ CMe CH2 ). C3
0 H 0 OH 0
6 + CH
+ 'OH. (61)
91
-------
Homolysis of 0-0 and Intramolecular Transfer of an Alkyl Group.
This reaction has been postulated to account for the production, during
gaseous oxidation, of carbonyl compounds with the same number of carbon
atoms as the alkane molecule but with different arrangements of them
1269
(e.g., pinacolone from 2,3-dimethylbutane, as in Eq. 62; 3-methyl-2-
422,423
pentanone, 2-hexanone, and 3-hexanone from 2-methylpentane; and
285
butanone from isobutane ):
H3C~(
(
J"3 rH3 ,CH3
/ 1:2
-: (J-ctlo ^ H-u-u C(CH0)_ CH, transfer /
j ' -} j 2. J
1 H 0
xo- \
VOH
CH3C-C(CH3)3 + OH.
0
(62)
130
It has been suggested that butanone arises from further reactions (Eq.
63) of acetone, which is a major product of the oxidation of isobutane:
CH3COCH3
-H-
CH3COCH2
-CH
CH3COCH2CH3
(63)
92
-------
Alternatively, it can arise by 1:4 H transfer in the tert-butylperoxy radi-
cal followed by O-O homolysis and 1:2 CH, transfer (Eq. 64):
H3C
I •
(CH3)2CCH2 »H3C-C CH2 CH-jCCH + 'OH (64)
0 OH
V
o
Similar O-O homolysis reactions, accompanied by intramolecular
transfer of an H atom (instead of an alkyl group), may compete with
hydroperoxide decomposition in forming carbonyl compounds with the
same carbon skeleton as the fuel (Eq. 65):
K-C H
P I
.CH C-O-C
CH3CH C-O-OH > (CH ) CHCC-H, + OH. (65)
II
'2*5
Homolysis of O-O and Two Other Bonds. This mode of decomposition
of hydroperoxyalkyl radicals is complex and may not occur as a single
step. Nevertheless, substantial yields of unsaturated carbonyl com-
pounds are formed in instances of combustion in which the formation of
0-heterocycles and rearranged carbonyl compounds shows that rearrange-
ment and decomposition of alkylperoxy radicals are important. Thus,
237,288
the cool-flame oxidation of ii-pentane yields acrolein and
832
crotonaldehyde, with an ethyl and a methyl radical, respectively,
being eliminated (Eq. 66):
93
-------
-CH-CH2CH ^ CH =CHCHO
n-CcH, „ J
/ I I 1
/ oo- H 6
H (66)
J Z 1 '3
60' ^ CH CH— CH-CH-CH ^ CH CH=CHCHO + H 0 + CH-
||*^ "^ £
H 0
J
H
422,423
Similarly, the cool-flame oxidation of 2-methylpentane produces
a variety of unsaturated carbonyl compounds, including acrolein,
methacrolein, l-buten-3-one, 2-pentenal, l-penten-3-one, l-penten-4-one,
trans - 2-penten-4- one, 2-methylbuten-3-one, 2-methyl-l-penten-3-one,
4-methyl-l-penten-3-one, 2-methyl-l-penten-4-one, 2-methyl-2-penten-
4-one, 2-methyl-2-pentenal, and 4-methyl-2-pentenal. Again, acrolein,
crotonaldehyde, and l-buten-3-one are products of the slow low-temperatur
1139
gaseous oxidation and cool flames of 11-heptane.
It is likely, therefore, that these unsaturated carbonyl compounds
arise from the hydroperoxyalkyl radicals known to be important in cool
flames. Their formation from such radicals necessitates the scission of
three bonds: the O-O bond, a C-H bond, and either a C-C bond or another
C-H bond (Eq. 66). If we assume that the weak O-O bond breaks first,
both of the other bonds broken are in position 3 to an atom bearing an
unpaired electron; this complex reaction is another example of a 3-scission
process.
94
-------
Oxidation of Hydroperoxyalkyl Radicals
Disubstituted products may be formed by further attack of oxygen at
the free valency of a hydroperoxyalkyl radical, that is, at the position
from which the H was transferred during the isomerization of the original
alkylperoxy radical (Eq. 67):
•C H OOH + 0 > -OOC H OOH. (67)
n 2n 2 ' n 2n
The hydroperoxyalkylperoxy radical thus formed usually abstracts
hydrogen intermolecularly (or, if a suitable hydrogen is available,
intramolecularly) to form a dihydroperoxy compound (Eq. 68); whose
decomposition leads to other dioxygenated products:
•OOC H OOH + RH > R- + HOOC H OOH. (68)
n 2n n 2n
One of the fullest studies of this reaction is the liquid-phase oxida-
50
tion of 5-methylnonane. At 90 C, attack on this hydrocarbon by
oxygen and chain carriers removes tertiary hydrogen 19 times as
frequently as secondary hydrogen, which, in turn, is removed 4 times
as frequently as primary hydrogen. The predominant resulting alkyl
radical is therefore CH (CH ) C(Me)(CH ) CH , and the alkylperoxy
323 233
95
-------
radical formed by later addition of oxygen is CH (CH ) C(Me)(OO)
3 23
(CH ) CH , in which there are three possible modes of transfer of
233
secondary hydrogen (1:4, 1:5, and 1:6 H transfer). Further oxidative
attack at the carbon atoms from which H has been transferred will lead
to three disubstituted 5-methylnonanes--4, 5-, 3, 5-, and 2, 5-disubstituted
products. The autoxidation products were reduced by lithium aluminum
hydride, converting monosubstituted products into alcohols and disubsti-
tuted products (presumably dihyd roper oxides) into the corresponding
i
diols. The resulting hydroxy compounds were analyzed by gas-liquid
chromatography, which showed that the ratios of diols produced were
2, 5-diol:3, 5-diol:4, 5-diol = 8:4:1.
Thus, 1:6 H transfer is the most frequent mode of rearrangement of
the 5-methyl-5-nonylperoxy radical, -whereas 1: 5 H transfer is more
frequent than 1:4 H transfer.
Dihydroperoxides. The formation of dihydroperoxides by Eqs. 67
and 68 has been established both in solution and in the vapor phase.
1103
Rust has studied the formation in solution of dihydroperoxides by
autoxidation of dimethyl-substituted ri-alkanes (2, 3-dimethylpentane,
2, 4-dimethylpentane, 2, 5-dimethylhexane, and 2, 6-dimethylheptane),
and it is interesting to compare his findings with those of Arndt and his
50
collaborators. By comparing dihyd roper oxide formation from these
branched hydrocarbons, each of which has two tertiary hydrogen atoms,
96
-------
Rust showed that 1: 5 H transfer was most frequent; considerable 1:6 H
transfer also took place, but rearrangement by 1:4 or 1:7 H transfer
was of little or no significance. The yields of 2, 4-dihydroperoxy-
2, 4-dimethylpentane and of 2, 5-dihydroperoxy-2, 5-dimethylhexane from
the corresponding alkanes were each about 90% of the total autoxidation
product, illustrating the high efficiency of intramolecular H transfer
when the hydrogen involved in the rearrangement reaction is tertiary
and attached to a carbon atom that is separated by one or two atoms
from the carbon atom bearing the peroxy group. The differences
between the relative susceptibilities of 1:5 and 1:6 H transfer in
50 1103
monomethyl-substituted and dimethyl-substituted hydrocarbons
are believed to result from conformational differences introduced
by the degree of chain branching. Dihydroperoxides (mainly formed by
1:6 H transfer) have also been produced during liquid-phase oxidation
1280
ofri-alkanes (e.g., ri-decane ).
Paper-chromatographic techniques have been developed for the
215
separation and identification of small amounts of organic peroxides
and applied to the products of the gaseous oxidation of ti-heptane,
216-219
2, 2, 3-trimethylbutane, ii-butane, propane, and cyclohexane.
The products from ri-heptane, the fuel most extensively studied,
included a dihydroperoxyheptane (in which the positions of the OOH
groups were not determined) and the products of its addition and
condensation reactions -with aldehydes. It was also suggested that an
unidentified peroxidic fraction of the products, completely nonvolatile
97
-------
in vacuo at room temperature, might contain trihydroperoxyheptane.
If this is true, it is likely that its mode of formation involves two
intramolecular rearrangements (Eq. 69):
H °2
CH 00-^ , \-C_H OOH
7 15 transfer7 7 14
0
•OOC H OOH ? £ > •C7H.(OOH)9 >
7 14 transfer 7 13 2 s
RH
•OOC?H (OOH)2 > C7H13(OOH)3 . (69)
No dihydroperoxide could be detected in the products of slow com-
bustion of propane or of 2, 3, 3-trimethylbutane. In the latter case,
initial attack -will remove the single tertiary hydrogen; rearrangement
of the resulting RO ' radical, (CH ) CC(OO-)(CH ) , must involve
2 33 32
primary hydrogen. Similarly, in the propane molecule, 1:5 H transfer is
possible only if both initial attack and isomerization involve primary
hydrogen and 1:4 H transfer only if one of these processes does. Butane
yields dihydroperoxybutane only under very vigorous conditions
(335-345 C; C H :O = 2:1); 1:5 H transfer involves primary hydro-
4 10 2
gen once, but 1:4 H transfer involves only secondary hydrogen. The
pattern of formation of dihydroperoxides from these fuels is thus
consistent with the relative ease of rearrangements involving 1:3,
1:4, 1:5, and 1:6 intramolecular H transfer and -with the strengths of
primary, secondary, and tertiary C-H bonds.
98
-------
Strong support for the formation of dihydroperoxides by a mechanism
involving isomerization of alkylperoxy radicals followed by oxidation of
the hydroperoxyalkyl radicals thus formed has resulted from studies
of the effect of hydrogen bromide on the gaseous n-heptane-oxygen
554
system. In the absence of this additive, the peroxidic products are
dihydroperoxides (Eq. 70); but, in its presence, monohydroperoxides
are formed, owing to the successful competition of the reaction of
alkylperoxy radicals with hydrogen bromide (which has a weak bond)
with their isomerization (Eq. 71):
0 , then
CH_ , .00- isomerization v -C-Ho^OOH _ >. CnH0
n 2n + 1 ? n. zn — ^ n 2.
CnH2n + I00' + HBr ~~} °nH2n + 1°°H + Br'' ^71^
Dicarbonyl Compounds. The radical, -OOC H OOH, or the dihydro-
n 2n
peroxide, C H (OOH) , will decompose by homolytic 0-0 scission,
n 2n 2
producing carbonyl hydroperoxides and dicarbonyl compounds. Several
369a,370,1281a,1281b
early ultraviolet spectroscopic studies of the preflame
o
combustion of hydrocarbons detected a strong absorption at 2600 A, and
85,86,1255
further work showed that this may be, in part, a result of the
production of 3-dicarbonyl compounds (e.g., CH COCH CHO from
3 2
n-C H and CH COCH COCH from n-C H ), which are later
4 10 323 5 12
99
-------
consumed in the cool flame. More recent studies suggest, however,
that unsaturated carbonyl compounds (e.g., 2-methyl-2-penten-4-one
from 2-methylpentane) are more important absorbing species. Never-
theless, g-dicarbonyl compounds are certainly formed, and yields as
high as 10% (from the oxidation of n.-pentane to pentane-2, 4-dione below
the cool-flame limit) have been reported. The involvement of
isomerization in the mechanism of their formation is supported by
the fact that none of the intermediate products of ri-butane combustion
(C alkenes, butanone, crotonaldehyde, ri-butyr aldehyde, and
4
l-buten-3-one) yields l-butanal-3-one on oxidation.
The tentative identification of (CH ) C(OH)CH COCH C(OOH)(CH )
32 22 32
in the products of autoxidation of 2, 6-dimethylheptane suggests that
1103
trihyd roper oxides decompose similarly.
Dicarbonyl compounds and related species are not, however, the
sole products of dihyd roper oxide decomposition. Homolysis of both
O-O bonds and g elimination of two hydrogen atoms or alkyl groups,
leading to a dicarbonyl compound (Eq. 72) is the major fate of 3- and
Y-dihydroperoxides. The easiest mode of decomposition of
a-dihydroperoxides, however, is homolysis to two hydroxyl radicals
and two monocarbonyl scission products (Eq. 73).
HqC CH--C-0-OH ^H0 + H* + 20H + CH0CCH0CCH0 . (72)
\/ I
C H
D
-OH
2C=0 + CH3CH2CHO + 20H. (73)
CHCHo CH o
I
OH
100
-------
Chain Cycles
The formation, isomerization, and later decomposition or further
oxidation of alkylperoxy radicals provide chain-propagation mechanisms
that are important during the oxidation of hydrocarbons, particularly
in the cool-flame and low-temperature two-stage ignition regions of
gaseous systems. The basic chain cycle involved is shown in the
sequence of Eqs. 74-77:
C H- .. + 0
n 2n+l
(74)
C H ,00' isomerizationy-C H0 OOH;
n 2n+l — > n 2n
(75)
•CnH2 OOH decomposition^products + X';
(76)
X" + C H
2n+2
(77)
X • is the radical produced by decomposition of hydroperoxyalkyl
radicals!. This radical X- can be 'OH (from cyclization and other
modes of decomposition of -C H OOH in which 0-0 is ruptured),
n 2n
101
-------
HO* (from 3 scission of C-O in a - 'C H OOH radicals), or an
2 n 2n
alkyl radical or hydrogen atom (from complex decomposition
reactions of ' C H OOH, whose stable products are unsaturated
n 2n
carbonyl compounds). The relative yields of O-heterocycles and
carbonyl scission products, of alkenes, and of unsaturated carbonyl
compounds provide an approximate measure of the relative extents
of formation of the radicals that accompany them: 'OH, HO' , and
422,423 2
R' , respectively. It has thus been shown that • OH is formed
most frequently by far. In regimes in which isomerization and later
decomposition is an important fate of alkylperoxy radicals, the
hydroxyl radical is the predominant entity that attacks the alkane,
to regenerate an alkyl radical (Eq. 78):
'OH+C H vH O + C H- . (78)
n 2n+2 ' 2 n 2n+l
The activation energy for attack of an alkane by 'OH, although
518, 600 103
difficult to determine accurately, is low (1-2 kcal/mole).
This has an important consequence. The reaction will be unselective,
being almost insensitive to C-H bond strength (cf. the attack of
721,1039
alkanes by chlorine atoms ); every alkyl radical derived from
the alkane skeleton will therefore be formed. To describe in detail
the chain-propagation steps under conditions in which isomerization
is a frequent fate of alkylperoxy radicals, it is necessary to consider
every alkylperoxy radical derived from the alkane, and not just the
tertiary radicals.
102
-------
If a substantial proportion of the hydroperoxyalkyl radicals formed
by isomerization are oxidized, the position is more complicated.
Hydroperoxyalkylperoxy radicals will also be potential abstracters of
hydrogen from an alkane, giving dihyd roper oxides (Eq. 79):
•OOC H OOH + C H ^ HOOC H OOH + C H' . (79)
n 2n n 2n+2 ^ n 2n n 2n+l
Kinetically, Eq. 79 'will resemble the abstraction of hydrogen from an
alkane by an alkylperoxy radical (Eq. 2), the preexponential factor of
Eq. 79 being somewhat lower because of the greater bulk of the radical
reactant. This reaction is therefore much more selective than attack
by 'OH, tertiary C-H bonds being attacked preferentially.
A further complication is involved if isomerization of alkylperoxy
radicals by group transfer competes with that by H transfer, although
this is rarely important. Decomposition reactions of alkylperoxyalkyl
radicals, analogous to those of hydroperoxyalkyl radicals in which 'OH
1428
is eliminated, will give 'OR as the radical product (Eq. 80),
whereas: the analogue of 6 scission of C-O in hydroperoxyalkyl radicals
1429
will produce RO • (Eq. 81):
2
0
Me3CCH200- ) Me2CCH20-OMe ) Me2C CH2 + MeO' ; (80)
• ) MeCHCH2-OOMe )MeCH =CH + Me02 . (81)
103
-------
The attack of a hydrocarbon by an alkoxy radical has an activation energy
104,1166,1167
of 3-11 kcal/mole, depending on the structure of the alkane.
It is therefore more selective than that by a hydroxyl radical, but less
selective than that by an alkylperoxy or hydroperoxyalkylperoxy radical.
The proportions of the various alkyl radicals that are formed during
alkane oxidation are therefore sensitive to the nature of the attacking
entity. The predominance of *OH (when alkylperoxy radical isomerization
is important) ensures, however, that all these will participate. It is
possible to write a qualitative chain-propagation scheme for the oxidation
of a hydrocarbon by this mechanism. Because the products of oxidation
are formed largely by propagation steps (branching being infrequent in
degenerately branched reactions, such as the oxidation of alkanes),
this scheme should account for the products found experimentally. For
2-methylpentane and ii-pentane, it has been shown that the agreement
288,422,423
between theory and experiment is good.
Competition Cyclization; The Relative Yields of 0-Heterocycles. The
mated equilibrium constants of 1:4, 1:5, 1:6, and 1:7 H transfer in alkylperoxy
radicals and the rates of cyclization of hydroperoxyalkyl radicals to
give O-heterocycles enable the relative yields of the O-heterocycles
derived from the carbon skeleton of the alkane to be calculated and com-
pared with experimental results.
In the oxidation of 2-methylpentane, for example, the chain-propagation
cycle that produces a C O-heterocycle is Eq. 82:
6
0
2
C F + 'OH ^C H' i. C H 00- ^
6 14 7 6 13 t 6 13 s
•C H OOH >C H 0 + -OH. (82)
6 12 ' 6 12
104
-------
Five species of C H ' , five of C H OO •, 21 of 'C H OOH, and
6 13 6 13 ' 6 12
11 of C H O O-heterocycles are involved. Each O-heterocycle (except
6 12
3-n_-propyloxetane) is formed by two routes (Eq. 83):
(CH3)2C(00')CH2CH2CH3
(CH3) C(OOH)CH2CH CH2 (83)
\ "2|f2
(CH ) CHCH0CH0CH9 >(CH.)0CHCH0CH0CH000 (CH-VC CH0 .
32 222 i 3 2 222 ^, 3 2 v / 2
I / 0^
I—> (CH3)2CCH2CH2CH2OOH/
At high pressures in half-stoichiometric mixtures of 2-methylpentane
•with air, the presence of substantial amounts of 3-methyltetrahydropyran
is also in agreement with the theory. At low pressures in RH:2O mixtures,
2
the absence of 3-methyltetrahydropyran is surprising, however; it appears
that, under these conditions, 6-hydroperoxyalky 1 radicals may decompose
predominantly by homolysis of O-O and C-C, accompanied by methyl
422,423
transfer to produce ii-butyraldehyde and ethylene. The amounts
of 2, 2, 4-trimethyloxetane are also higher than those predicted theoretically.
The simple theory predicts that the concentration of C oxiranes
6
(notably 2, 2-dimethyl-3-ethyloxirane, 2-methyl-2-n_-propyloxirane, and
2-methyl-3-isopropyloxirane) will be lower than those found experimentally
at both high and low pressures. This is readily explicable, however:
oxiranes may arise from the addition of RO • to alkenes followed by
2
105
-------
0 scission of C-O and by the analogous addition of HO • . But the
2
latter reaction is not very important during the oxidation of large
419,422,423
alkane molecules.
841,1104
In the oxidation of isooctane, the major O-heterocyclic
products are 2, 2, 4, 4-tetramethyltetrahydrofuran and 2-tert-butyl-
3-methyloxetane; the absence of 2-isopropyl-3, 3-dimethyloxetane
suggests that the 2, 2, 4-trimethyl-5-pentylperoxy radical (not the
2, 2, 4-trimethyl-3-pentylperoxy radical) is the precursor of
2-tert-butyl-3-methyloxetane. This is in agreement with the
theory; formation of the oxetane from the 5-alkylperoxy radical
should theoretically be 25 times as fast as from the 3-alkylperoxy
radical.
In general, it follows that, if the two routes to an O-heterocycle
involve (1) abstraction by 'OH of a primary hydrogen followed by
intramolecular transfer of a secondary or tertiary hydrogen and
(2) abstraction by -OH of a secondary or tertiary hydrogen followed
by intramolecular transfer of a primary hydrogen (Eq. 84), then the
former reaction is the faster:
(84)
CH3)3CCH2CH(CH3)CH200'
«—» (CH) CC
•OH • — ^ (,c;n3; ccm;iHui3,)Utt2Uun
then \
*•>»
00 (CHo)
1 *
;H3)3ccHC(CH3)2 7
/
OOH /
1 /
/
L-» (CH3) CCHCH(CH3)CH2
A
-C-CH CB
y
CH
3
106
-------
In other words, a high proportion of O-heterocycles results from
primary alkylperoxy radicals; this is a reflection of the unselectivity
of attack of a hydrocarbon by "OH. This suggests that the formation of
2, 2-dimethyltetrahydrofuran from 2-methylpentane occurs via the
2-methyl-5-pentylperoxy radical and that the formation of
109
2-methyloxetane from n-butane, of 2-methyltetrahydrofuran from
237,1281a,1281b
ri-pentane, and of 2-methyltetrahydropyran from
69
ii-hexane result primarily from the isomerization of primary
424
alkylperoxy radicals. The formation of oxetanes from neopentane
1429
and isobutane must, of course, involve primary radicals.
Competition between Cyclization and B Scission. The competition
between cyclization and scission reactions depends on the structure of
the decomposing hydroperoxyalkyl radical, y- and 6-Hydroperoxyalkyl
radicals cyclize rapidly, producing derivatives of tetrahydrofuran and
tetrahyd ropy ran. In the 6 scission of C-C in such radicals, small-ring
O-heterocycles (oxiranes from Y-radicals, oxetanes from 6-radicals)
are formed. In the 3-scission reaction, the strain energy that must be
overcome is greater than that in the cyclization reactions, with the
result that cyclization is the faster reaction (Eq. 85):
CH.CHCH9CH0CHCH0
Ji / ^ J
-OH
cyclization
FAST
3 scission of
C-C,SLOW
\
X0
CH
CHCH3 + OH (85a)
v/
CH2=CHCH3 + OH (85b)
107
-------
The cyclization of 3-hydroperoxyalkyl radicals is slower than
that of Y-radicals, oxetanes being formed. However, 3 scission of
C-C gives carbonyl compounds (not O-heterocycles) plus alkenes; in
this scission reaction, no strain energy is involved. In this case, the
3 scission is likely to be the faster reaction (Eq. 86):
CH.,CHCH CHCH.CH
3 2 23
0-OH
cyclization .
SLOW
^ scission of.
A
CH.HC C
3 V
rv r-un -i- PT.
CHCH2CH3+ OH
C-C, FAST
(86a)
CH3CH° + CH2=CHCH2CH3 + OH (86b)
Similarly, the cyclization of ot-hydroperoxyalkyl radicals, -which
produces oxiranes, is slow. In these radicals, 3 scission of C-O, not
C-C, occurs, producing alkenes conjugate with the original alkane and
HO radicals. Again, because no strain energy is involved in 3 scission,
2
this is the faster reaction (Eq. 87):
CH3CHCHCH2CH2CH
0-OH
cyclization
SLOW
3-scission
C-O, FAST
--
^/
CH,,CH=CHCH0CH0CH0 + HO,
J 2. £ j t
-OH (87a)
(87b)
108
-------
Competition between Decomposition and Oxidation. Further oxidation
of hydroperoxyalkyl radicals is analogous to the addition of oxygen to
98, 235
alkyl radicals. The activation energy of the reaction is zero.
Transition-state theory suggests that, at 550 K, the preexponential
-13 3
factor is about 10 cm /molecule-sec for large alkyl radicals; this
-12. 8 3
result compares with the experimental value of 10 cm /molecule-sec
720
for propyl and butyl radicals. The constant for addition is therefore
-13 3
approximately 10 cm /molecule-sec. When the partial pressure of
5.5
oxygen is 200 torr, then k [O ] = 1O /sec, which compares
addition 2
5.5 5.9
with the rough values for k of 1O /sec, 10 /sec,
decomposition
11 3.5 7.8
10 /sec, 10 /sec, and 10 /sec for the formation of oxiranes, oxetanes,
tetrahydrofurans, tetrahydropyrans, conjugate alkenes, and carbonyl
scission products, respectively.
Although these values are only very approximate, oxidation probably
competes effectively with the formation of conjugate alkenes, oxiranes,
and oxetanes, but not -with that of tetrahydrofurans, tetrahydropyrans,
and carbonyl scission products. It is, therefore, a-hydroperoxyalkyl
radicals (and, to a small extent, 3-hydroperoxyalkyl radicals) that will
add to oxygen; Y- and 6-hydroperoxyalkyl radicals will decompose
preferentially.
109
-------
The addition of oxygen to a-hydroperoxyalkyl radicals gives
a-hydroperoxyalkylperoxy radicals, which can abstract hydrogen
intermolecularly or intramolecularly to produce a-dihydroperoxides
or dihydroperoxyalkyl radicals, a-dihydroperoxides will decompose
by homolysis of both 0-0 bonds and 6 scission of the bond joining
the two peroxidized carbon atoms (Eq. 73). This reaction may be
capable of explaining the discrepancy in the yields of carbonyl
scission products and alkenic scission products from oxidation
reactions in regimes where alkylperoxy radical rearrangement is
responsible for chain propagation. Decomposition of 6-hydroxyalkyl
radicals by scission of 0-0 and C-C bonds accounts qualitatively for
the formation of these two types of scission product, but the two
members of each alkene-carbonyl "pair" should be formed in
equimolar yields if this is their sole mode of formation. In prac-
288,422,423
tice, the yields of carbonyl scission products considerably
exceed those of alkenic scission products. The decomposition of
a-dihydroperoxides to give two carbonyl scission products and no
alkenes affords a possible explanation for the relatively high yields of
422,423
the former. A further reaction contributing to the formation
of carbonyl scission products is, however, decomposition of alkoxy
288
radicals produced by 0-0 homolysis in monohydroperoxides.
The addition of oxygen to g-hydroperoxyalkyl radicals will
produce, eventually, 3-dihydroperoxides. In these compounds, there
is no C-C bond in the B-position to both 0-0 bonds; their decomposition
110
-------
will probably lead, therefore, to dicarbonyl compounds (Eq. 72).
3-dicarbonyl compounds are, indeed, formed in the low-temperature
gaseous oxidations of n-butane and n-pentane.
Importance of Other Reactions. It is prohibitively difficult to
estimate the rates of complex modes of reaction of hydroperoxyalkyl
radicals, such as 0-0 scission with group transfer, C-C and 0-0
scission with group transfer, and scission to unsaturated carbonyl
compounds. The formation, in considerable yields, of products
diagnostic: of these reactions during the gaseous oxidation of alkanes
shows, however, that they compete well with the simpler modes of
decomposition discussed above.
A further competitive reaction that must be considered is the
"reverse isomerization" of a hydroperoxyalkyl radical to an
alkylperoxy radical. The reverse isomerization of j- and
6-hydroperoxyalkyl radicals is slow, compared with their
decomposition to 0-heterocycles, but reverse isomerization of
a- and g-hydroperoxyalkyl radicals competes well with the
fastest modes of their further reaction (decomposition or oxidation).
Nevertheless, at the high temperatures characteristic of vapor-phase
oxidation (internal combustion, for example), rearrangement of
alkylperoxy radicals to give a- and 3-hydroperoxyalkyl radicals is
a viable mode of chain propagation, inasmuch as when such a
hydroperoxyalkyl radical has isomerized back to an alkylperoxy
radical, the likely fate of that radical is to isomerize again! y~
6-hydroperoxyalkyl radicals probably react directly to give stable or
111
-------
moderately stable products and a- and g-hydroperoxyalkyl radicals
eventually do so.
Related Reactions During Oxidation
Rearrangement reactions of alkylperoxy radicals containing a
variety of substituent groups—e.g., HO-, R N-, 0=C, and -0-C(=0)-
2
—are important during the oxidation reactions of substituted alkanes.
Oxidation of Alkenes. The low-temperature gaseous oxidation
of alkenes is, in general, propagated by abstraction of hydrogen
1185
attached to an allylic carbon atom, to give an allylic radical.
A competing reaction, however, involves addition of oxygen or
oxygenated radicals to the double bond, to give, eventually, carbonyl
scission products. This reaction is favored, and can predominate,
in alkenes of which the hydrogen atoms on each allylic carbon are
exclusively primary. During the early stages of the gaseous oxidation
286,287
of 2-methyl-2-butene, for example, large and equal yields of
acetone and acetaldehyde are formed. Similarly, in the gaseous
1186
oxidation of isobutene, acetone and formaldehyde are formed in
286,287
equimolar quantities. It has been proposed that these scission
products could arise from direct addition of oxygen to the double
bond, cyclization and breakdown of the moloxide formed (Eq. 88):
(CH ) C=CHCH + 0 x (CH ) C CHCH
32 32 7 3 21
0 0
(CH ) C=0 + CH CHO. (88)
112
-------
This nonradical reaction seems rather unlikely, however. A more
1186
probable route is the addition of a hydroxyl radical to the double
bond to form a hydroxyalkyl radical, followed by addition of oxygen
to this. Isomerization by 1:5 H transfer of the hydroxyalkylperoxy
radical thus formed will give a substituted alkoxy radical that can
decompose by 3 scission, forming equimolar quantities of the two
carbonyl scission products and regenerating a hydroxyl radical
(Eq. 89):
CH /\
*(CH ) C-CH2OH
(CH3) C ^ (CH3)2C=0 + CH20 + 'OH. (89)
The isomerization and decomposition of a hydroxyalkylperoxy radical
therefore appear to be responsible for propagation of the primary
chain during the gaseous oxidation of some alkenes, in a way analogous
to that in which intramolecular reactions of unsubstituted alkylperoxy
radicals propagate chains during the oxidation of alkanes.
The formation of minor products (butanone, propionaldehyde,
isopropyl alcohol, and tert-butyl alcohol) of the slow combustion of
2-methyl-2-butene, labeled in specific skeletal positions with
carbon--14, has been described in terms of the isomerization reactions
113
-------
of peroxy biradicals formed by direct addition of oxygen to the
286,287
double bond of the fuel. The formation of the two alcohols
can be explained on the basis of the mechanism involving addition of
•OH followed by addition of oxygen which accounts so well for the
major carbonyl scission products.
Oxidation of Ketones. In the gaseous oxidation of ketones,
scission reactions predominate, producing lower aldehydes and
79, 80, 80a, 81
oxides of carbon. From butanone, for example,
acetaldehyde, carbon monoxide, and carbon dioxide are important
80
products. It is likely that these are formed from ketonylperoxy
radicals by intramolecular group transfer and decomposition of the
substituted alkylperoxyalkyl radical thus formed (Eqs. 90 and 91):
CH,
0=C'
\,
CH,
o-
0
1:4 CH.,
transfer
0=C
0-OCH,
'CH'
CH,
(90)
a scission^
CO + CH3CHO + CH30;
H~C—C
0
!
]
o-
1
o
1:3 CHiCO v
transfer x
0-
•"< /
^^
0
(91)
g scission
—
CH3CHO + C02
114
-------
Similarly, acetone gives formaldehyde and oxides of carbon. The
variation of the relative yields of carbon monoxide and carbon dioxide
with temperature is interesting. Carbon dioxide decreases in
80a,81
importance as the temperature is raised; comparison of the
isomerization reactions involved suggests that the opposite should
be true (1:3 CH CO transfer will have a higher activation energy than
3
1:4 CH transfer), but comparison of the decomposition reaction leads
3
to a result in agreement -with experiment ( 3 scission is easier than
a scission). The results suggest that the mode of decomposition of
the substituted alkylperoxyalkyl radical is the rate-controlling step.
If group transfer in ketonylperoxy radicals is important, the
energetically favored H-transfer reaction should also occur (Eq. 92):
H C-CH
0
CH
H
0
1:5 H trans-v
fer '
CH3HC— C— CH2
(92)
OOH
Cyclization of the radical thus formed, with elimination of OH, -will be
more difficult than that of the corresponding unsubstituted hydroperoxyalkyl
radical, (CH CH(OOH)(CH CH' ), because the carbonyl oxygen and the
3 22
three carbon atoms that are potential members of the ring are constrained
115
-------
in a planar configuration by the carbonyl ir-bond. It is perhaps not
surprising that carbonyl-substituted O-heterocycles'have not been
formed in the products of the oxidation of ketones. If the carbonyl
group can be "ejected" from the substituted hydroperoxyalkyl radical
during ring closure, however, this constraint no longer applies, and
substituted O-heterecycles should result (Eq. 93):
0
H C-CH CH > CH HC CH0 + CO + OH . (93)
3 v -2 3 \/ 2
\0 V
OH
These O-heterocycles have, indeed, been formed among the products of
ketone oxidation reactions (e. g. , 2-ethyloxirane from 3-pentanone,
methyloxirane from butanone, and oxirane from acetone), and their
80
formation has been interpreted in this way.
A further possible mode of isomerization of ketonylperoxy radicals
is transfer of a hydrogen atom from a carbon atom on the same side of
the carbonyl group as is the peroxy group. Decomposition by 3 scission
of C-O of the resulting substituted hydroperoxyalkyl radical would
result in a conjugated unsaturated ketone (Eq. 94a):
P 0 0
_} HO 2 + CH3CCH = CH2 (943)
CH (
1 pTJ /
_,— \^fl — — __ ^
1
1
:H > CH cc — CH2
0 OH
0 H
V.
0*
1- ^ PH-
0
II
^ CH3C-OH + CH3C=0, etc. (94b)
116
-------
l-buten-3-one has, indeed, been found among the oxidation products of
80
butanone. Another mode of decomposition of a-ketohydroperoxides
is cleavage to carbonyls and carboxylic acids (Eq. 94b).
The ease with which isomerization of ketonylperoxy radicals can
occur is a function of the molecular structure of the ketone, just as
that of alkylperoxy radicals is a function of alkane structure. The
fact that, at subatmospheric pressures, 2, 2-dimethylbutan-3-one gives
no cool flame suggests that the peroxy radicals derived from it cannot
isomerize in the way characteristic of the radicals derived from acetone,
butanone, 3-pentanone, and 3-methyl-2-butanone (each of which gives
601
cool flames). If, as discussed above, the two important modes of
H transfer in ketonylperoxy radicals are 1:4 H transfer between atoms
on the same side of the carbonyl group and 1: 5 H transfer between atoms
on opposite sides, then the absence of cool flames from the oxida-
tion of 2, 2-dimethylbutan-3-one is explicable, in that neither
•OOCH C(CH ) COCH nor (CH ) CCOCH OO'can react in this
232 3 33 2
way. It is difficult to see, however, why 1:6 H transfer in these radi-
cals does not lead to cool flames, inasmuch as 1:5 transfer of primary
hydrogen is capable of so doing during the oxidation of acetone. The
probable explanation is that, because of the IT-bond, the C-C(=O)-C
o
bond angle is 120 ; that is, it is considerably higher than the
C-C(H )-C bond angle, so that 1:6 H transfer is much more diffi-
2
cult in ketonylperoxy radicals than in alkylperoxy radicals.
117
-------
Oxidation of Esters. During the oxidation of formates, 1:5 intra-
molecular H transfer in substituted alkylperoxy radicals is of major
426
importance. It is followed by 3 scission of a C-0 bond, (as well as
the usual homolysis of the O-O bond) of the substituted hydroperoxyalkyl
radical formed, to give an aldehyde, carbon dioxide, and 'OH (Eq. 95):
o- «.
0 C—0—CH—0—OH-
fiH
HO
/ C02 + CH3CHO + OH . (95)
In the oxidation of ethylformate, acetaldehyde is the sole nonperoxidic
organic product at temperatures below 250 C. The oxidation reactions
of esters of higher organic acids (acetates, propionates, etc. ) do not
exhibit an analogous temperature region, possibly because, in these
cases, H transfer must involve a ring of at least seven members. The
formation of cyclic transition states in these radicals is more difficult
than that in the corresponding unsubstituted alkylperoxy radicals because
of the constraint of planarity imposed on three ring members by the
"" -bond.
At somewhat higher temperatures (about 300 C), however, a wider
variety of substituted alkylperoxy radicals derived from esters can
isomerize. Again, the ability of esters to give cool flames has been
601
explained in terms of this reaction. At low pressures, ethylformate,
ethylacetate, ethylpropionate, n-propylformate, n_-propylacetate,
118
-------
isopropylacetate, and methyl-ii-butyrate produce cool flames or
associated phenomena, but methylformate, methylacetate,
methylacetate, methylpropionate, and tert-butylacetate do not.
It appears that 1:6 or 1:7 transfer of primary hydrogen occurs too
slowly at these temperatures to lead to cool-flame reaction, but
that 1:6 transfer of secondary hydrogen or 1:5 or 1:4 transfer of any
hydrogen is sufficiently rapid to do so. The rates of these processes
are, again, affected by steric constraints associated -with the
IT -bond.
These characteristics of the oxidation of ketones and esters show
that two H-bearing centers are necessary for the formation of cool
flames and therefore strongly support the argument that an isomeri-
zation reaction is an important chain-propagating step in this regime.
Rearrangement of Alkoxy Radicals. Direct decomposition to a
carbonyl compound and an alkyl radical is generally the predominant
mode of reaction of the alkoxy radicals formed during gaseous
oxidation; intermolecular H abstraction, forming an alcohol, is also
of considerable importance. Rearrangement reactions, by migration
of an atom or group to oxygen have only a minor role and have not
been studied as fully as the corresponding reactions of alkylperoxy
radicals.
In the vapor phase, RO • rearrangement involving 1:3 H transfer
and 2:1 CH transfer has been suggested to be responsible for the
3
formation of pinacolyl alcohol during the slow combustion of 2, 3-
dimethylbutane (Eq. 96):
119
-------
0- H OH
H-C-C - C-CH - > CH -C - C(CH,)0
3 3 . 3 2
CH3 CH3 CH3
OH OH
CH C-C(CH-),. —£2_> CH0CHC(CH-)^ (96)
J . J J •} J X
Several modes of H transfer in and skeletal scission of alkoxy
425,426
radicals play a major part in the oxidation of esters. In the
substituted alkoxy radical derived from an ester by the normal hydro-
peroxide chain mechanism and hydroperoxide decomposition (Eq. 97),
IC-OCH_R' > RC-OCHR' >RC-OCHR' ,
I! 2 II I II
0 0 OOH 0 ti-
presence of an ether linkage appears to affect the balance of com-
peting decomposition and abstraction reactions compared with those of
unsubstituted alkoxy radicals derived from alkanes. Thus, although
direct scission to a carbonyl compound, R'CHO, by detachment of the
1045
largest group, RC(=O)O-, is still important, abstraction of
hydrogen from a further substrate molecule to give RC(=O)OCH(OH)R'
120
-------
is negligible, no alcohol being formed. Two modes of isomerization
by H transfer and scission of an ether C-O bond also occur; 1:2
transfer of H from C to O (Eq. 98) gives an acid and an acyl radical,
and 1:3 transfer of H from C to C (Eq. 99) gives an aldehyde and an
acyloxy radical:
H .0
I - ,/
R-C-o-C-0- > RC=0 + R'C' ; (98)
0 R1 OH
R 0 R' .0
\ C' > RCHO + R'C; . (99)
CK H/ 0-
"o-
These reactions explain qualitatively the pattern of product formation,
its variation with temperature, and the effects of ester structure
426
(i.e. , the identities of R and R1) on the course of oxidation.
Alkoxy biradicals, formed by the addition of oxygen atoms to the
17-bonds of alkenes, are short-lived species that cyclize to give
epoxides and isomerize by H transfer or group transfer to give
128, 290-292, 650, 666,1121,1122
carbonyl compounds. The transfer
of hydrogen atoms is entirely intramolecular; that of alkyl groups
occurs by both intermolecular and intramolecular mechanisms. The
expoxides and carbonyl compounds have excess vibrational energy on
formation and decompose, unless deactivated by collision. From each
121
-------
nonaromatic alkene studied, all possible products are formed. A
particularly striking example of biradical rearrangement is the
production of carbonyl compounds from a cyclic alkene, which
involves a reduction in ring size.
Variation of the Rate of Hydrocarbon Oxidation -with Temperature
One of the most striking features of the gaseous oxidation of
hydrocarbons and their derivatives is the complicated dependence
of the rate on temperature. Rate-temperature curves for the
slow combustion of a wide range of fuels show marked regions of
"negative temperature coefficient, " and this phenomenon of
decrease in rate •with increase in temperature is also reflected
in the ignition limits of the fuels. Plots of ignition temperature
against pressure for given mixtures of a hydrocarbon with air or
oxygen show a low-temperature region, associated with the
occurrence of "cool flames," and a high-temperature region.
The former region shows "fine structure. " Several maximal and
minimal values of the pressure necessary for cool flames, or for
ignition, occur as the temperature is raised; the number of these
"lobes" is a sensitive function of the molecular structure of the
421,1339
fuel.
To explain these phenomena, it is necessary that, as tempera-
ture increases, repeated fluctuations of the net branching factor
of the chain reaction occur. The most ready explanation is that
the nature of the degenerate branching reaction (or of the reaction
producing the branching agent) changes repeatedly with temperature.
122
-------
At low temperatures, below those at which cool flames and spontaneous
ignition occur (<250 C), it is commonly held that homolysis of alkyl hydro-
peroxides (Eq. 100) is responsible for degenerate branching, as in the liquid
phase; at high temperatures, above those at which cool flames occur
(>400 C), it is probable that homolysis of hydrogen peroxide fills a similar
role (Eq. 101):
ROOH , ^RO- + -OH ; (100)
HOOH + M -» 2'OH + M . (101)
At the intermediate temperatures characteristic of cool flames,
analysis of the products of the gaseous oxidation of many hydrocarbons
has shown conclusively that alkylperoxy radical isomerization is the most
important mode of chain propagation. Because this propagation process
is unselective, these products include many species (e. g. , aldehydes
and O-heterocycles) -with labile hydrogen atoms. Further reaction of
these species with oxygen (Eq. 102) or with alkylperoxy radicals (Eq. 103)
can lead to branching:
AH + 02 >A- + H02" ; (102)
AH + RO' -»A- + ROOH; ROOH >RO' + -OH . (103)
123
-------
Alkylperoxy radical isomerization therefore produces a variety of
branching agents; it is very likely, indeed, that this process is responsi-
ble for the formation of cool flames and for the complex fine structure
of the low-temperature ignition regime of hydrocarbon oxidation.
It has been proposed that a given mode of intramolecular H transfer
in alkylperoxy radicals is responsible for a given lobe in the pressure-
421,1339
temperature locus of the ignition limit. These descriptions
must be indirect, however, because it is the branching reaction, not
the propagation reactions, that determines the "kinetic balance" of
the reacting mixture and hence the limiting conditions for nonisothermal
behavior. Moreover, there are no sharp changes in product distribu-
tions at the lobe boundaries, as there would be if a given mode of
422-424
isomerization were exclusive to a given lobe.
23
Aldehyde-Hydrocarbon Reactions
Photodecomposition of aldehydes can also result in free radicals
(Eqs. 104-106):
RCHO + hv ^RCHO* ; (104)
RCHO*-^>R' + -CHO ; (105)
RCHO* _> RH + CO . (106)
27
This mechanism was explored by Altshuller and co-workers in
664
the photooxidation of propionaldehyde and by Johnston and Heicklen
with acetaldehyde. The alkyl and formyl radicals •will either react
124
-------
with oxygen to produce peroxyalkyl and peroxyformyl radicals or,
as is much less probable, react with olefins and aromatics directly.
If the alkyl or formyl radicals react first with molecular oxygen,
the resulting peroxyformyl and peroxyalkyl radicals can react -with
an olefin or aromatic hydrocarbon (Eqs. 107-109):
RO* + He >RO He- ; (107)
RO He- >HcO- + RO- ; (108)
R02Hc- >R02H + R' . (109)
(He = Hydrocarbon)
In either case, a chain-propagating step(s) is occurring, and the photodecom-
position of aldehydes should facilitate the reaction of hydrocarbons.
Some of the reactions of methyl and methoxy, as well as ethoxy, radi-
325, 551
cals have been discussed.
26
Altshuller and co-workers showed that aliphatic aldehydes, when
o
photooxidlzed in air with radiation of 3400 A or lower, will produce
intermediate species that react with olefinic and aromatic hydrocarbons.
These reactions, even at low concentrations, proceed at significant
rates in the absence of nitrogen oxides. These reactions were investi-
o
gated with both sunlight fluorescent lamps ( E = 3100 A) and natural
max
sunlight; data are shown in Tables 3-4 and 3-5. Although conversions
of aldehyde and olefin mixtures in natural sunlight are low, the
amounts consumed are significant. Comparisons of the rates of
125
-------
reaction of olefins under these conditions with those involving oxides
of nitrogen are difficult. The rate of reaction of ethylene is approxi-
mately 10 times higher in the presence of nitrogen oxides than its
rate of reaction -with aldehydes. This comparison was made under
the most favorable conditions of hydrocarbon: nitrogen dioxide ratio
and with natural sunlight.
Ketone-Hydrocarbon Reactions •
Another group of oxygenates, the aliphatic ketones, also contri-
bute to the reaction of hydrocarbons. The primary quantum yield
o
for the photodissociation of acetone at 3130 A has been reported as
0. 9. Acetone dissociates as shown in Eq. 110, -where 0 / (0 + 0 ) =
o II I II
0. 07 at 3130 A: and ambient temperature.
CH3COCH3 + hv__>CH CO + CH3 [I] ; (110)
—>2CH3 + CO [II] .
Acetone and diethylketone have been irradiated in the presence of
2-methyl-l-butene and sunlight fluorescent lamps for periods ranging
1041
from 1 to 3 hrs. Table 3-6 shows the rates of reaction of the
olefin and the products resulting from such an irradiation.
Synergistic Effects in the Photooxidation of Organic Substances
A synergistic effect is one in which the reactivity or the amount
of product produced by a given compound is unpredictably affected
126
-------
by the presence of a second. Some indications of this effect were
970
observed in 1964 with the photooxidation of ethylene. When
ethylene was irradiated in the presence of oxygenates and some
aromatics, more formaldehyde was produced. This occurred not
because more formaldehyde was produced per molecule of
ethylene reacted, but because more ethylene reacted.
The importance of synergistic effects in actual atmospheric con-
ditions has not been determined, not because of lack of effort but
because of lack of sufficiently good data from both atmospheric
samples and laboratory measurements. Some data obtained from
the irradiation of atmospheric samples are shown in Table 3-7.
731a
The data of Kopczynski and co-workers and of Stephens and
1217
Burleson are the result of irradiating Los Angeles air samples,
591 114 2b
the data of Heuss and Glass on and Schuck and Doyle are
from the irradiation of pure components, and the data of Leach and
774
co-workers are from the irradiation of diluted auto exhaust.
114 2b
The data of Schuck and Doyle seem to disagree markedly with
those of others. The single-component data are the only data in
which synergistic effects are absent. If -we ignore the data of
Schuck and Doyle, the remaining values show that 11-butane, sec-
butylbenzene, and the olefins react more quickly when irradiated
in multicomponent mixtures. Equally apparent is that the reactivi-
ties of toluene and the xylenes are essentially unchanged. It is
not obvious why the data of Schuck and Doyle do not agree with any
of the other data. More information is needed before definitive
127
-------
conclusions can be reached on synergistic effects in polluted atmos-
pheres.
309,564
Oxidation of Aromatic Hydrocarbons
Aromatic hydrocarbons, present in the atmosphere in minor
amounts, are also subject to autoxidation by triplet oxygen in an
environment containing metallic catalysts, ozone, oxides of nitro-
gen and sulfur, peroxides, and light to form species with reactive
functional gr'oups. The only hydrocarbons whose oxidation will be
considered here are benzene, toluene, dimethylbenzene and
trimethylbenzene, and isopropylbenzene (cumene), because these
are the major ones present in the atmosphere. Their reactions
should be typical of those of other structurally similar aromatic
hydrocarbons not specifically discussed.
In the liquid phase at ambient temperatures, benzene is rela-
tively stable and it is not readily autoxidized by triplet oxygen in
the absence of catalysts. In the atmosphere, however, the situa-
tion is very complex, and one can visualize the oxidative formation
of arene epoxides and peroxides, phenol, polyhydric phenols,
quinones, ring-cleavage products such as muconic acid, and
low-molecular-weight oxidation products.
Toluene is more rapidly oxidized than benzene in the liquid
phase and undergoes side-chain oxidation to benzylhydroperoxide
as the initial product. This decomposes to benzaldehyde, which
undergoes facile oxidation to benzoic acid, a stable end product
128
-------
in solution. In the atmosphere, the initial hydroperoxidation and
later decomposition would also be expected to occur, but it is not
known whether benzoic acid would survive in the vapor phase with
the variety of catalytic species present. In principle, a wide
range of reaction products can be obtained from benzoic acid. In
addition, toluene itself may undergo the same types of ring attack
as benzene.
The al'kylbenzenes (o-m-, and ja-xylene and trimethyIbenzenes)
are even more readily autoxidized in the liquid phase than benzene
and toluene. Initial oxidation products are monohydroperoxide and
polyhydroperoxide, which are converted to an exceedingly complex
mixture containing acids, quinones, phenolic oxidation products,
and products of ring attack. Atmospheric oxidation is also likely
to be facile.
Isopropylbenzene (cumene) is rapidly oxidized by triplet oxygen
in the liquid phase to the hydroperoxide in an efficient, high-yield
process. The hydroperoxide undergoes ready cleavage to acetone
and phenol in the presence of catalytic quantities of acids and to
acetophenone in the presence of variable-valence metals. The
products of hydroperoxide decomposition (acetone, phenol, and
acetophenone) are also readily oxidized by triplet oxygen to form a
broad spectrum of products. Atmospheric oxidation may follow
similar pathways, but information on that point is not available.
591, 731a, 774,1142b, 1217
Recent studies on vapor-phase oxidation
have shown, that toluene, £- and rn-xylene, and sec-butylbenzene
129
-------
are oxidized by triplet oxygen in the presence of light at rates com-
parable with that of ethylene. The reported results are somewhat
confusing, with some authors reporting faster and some slower
oxidation. All rate differences are minor, however, and lead to
the tentative conclusion that aromatic hydrocarbons are oxidized
at a reasonable rate by triplet oxygen in the presence of light.
Oxidation products were not identified.
130
-------
Partial List of Hydrocarbons Present in the Atmosphere12
Carbon
Number
1
2
3
4
5
6
7
8
9
Saturated Aliphatic
Hydrocarbons (Paraffins)
Methane
Ethane
Propane
Butane
Isobutane
Pentane
Isopentane
Hexane
2 , 2-Dimethylbutane
2 , 3-Dimethylbutane
2-Methylpentane
3-Methylpentane
2 , 4-Dimethylpentane
2 , 3-Dimethylpentane
2-Methylhexane
2,2, 4-Tr imethylpentane
Unsaturated Aliphatic
Hydrocarbons (Olefins)
Ethylene
Propylene
1-Butene
2-Butenes, cis- and
trans-
Isobutylene
1-Pentene
2-Pentenes, cis- and
trans-
2-Methyl-l-butene
2-Methyl-2-butene
3-Methyl-l-butene
1-Hexene
2-Hexenes, cis- and
trans-
3-Hexenes, cis- and
trans-
4-Methyl-l-pentene
2-Methyl-l-pentene
4-Methyl-2-pentene
Aromatic Alicyclic
Hydrocarbons Acetylenes Diolefins Hydrocarbons
Acetylene
Methyl- Allene
acetylene
1,3-Buta-
diene
Isoprene Cyclopentene
Cyclopentane
Benzene Methylcyclo-
pentane
Cyclohexane
Toluene
Xylenes
Triraethyl-
benzenes
Derived from Air Quality Criteria for Hydrocarbons.
-------
TABLE 3-3
Rate Constants'^ for the Autoxidation of Hydrocarbons
Ni
Hydrocarbon
Propylene
1-Butene
1-Hexene
1-Octene
Neopentylethylene
Isobutylene
Tetramethylethylene
Cyclopentene
Cyclohexene
Cyclooctene
Toluene
Ethylbenzene
Cumene
1 , 1-Diphenyle thane
k /(2k )1/2x 105 k /H, k x 10~6,
p t p t
At 70 C6 At 30 (^ at 30 Cfe at 30 C
21.7
81
38
6.2 50 130
13
20
1,120 — 0.14 0.32
220 — 1.7 3.1
175 — 1.5 2.8
10
1.4 0.08 150
21 0.65 20
150 0.18 0.0075
110 0.34 0.047
In liters/per mole per second.
For abstraction only.
-------
Photooxidation of Aldehyde-Hydrocarbon Mixtures in Air by Sunlight Fluorescent Lamps
Initial Concentration, Fraction Reacted with Irradiation, %
Aldehyde and Hydrocarbon
Formaldehyde
2-Methylbutene-l
Formaldehyde
2 , 3-Dimethylbutene-2
Formaldehyde
1,3, 5-Tr imethylbenzene
Acetaldehyde
2-Methylbutene-l
Propionaldehyde
Ethylene
Prop ionaldehyde
trans-2-Butene
Propionaldehyde
2-Methylbutene-l
Pr op ionald ehyd e
2 , 3-Dimethylbutene-2
Propionaldehyde
2-Methylbutene-l
2,3-Dimethyioutene-2
Propionaldehyde
1,3, 5-Tr imethylbenzene
ppm (vol)
13
10
30
10
10
10
32
10
22
10
22
11
20
10
22
10
18
10
10
18
10
In 1 hr
66
11
57
29
50
8
33
5
53
2
53
9
49
7
51
12
49
1
50
8
In 2 hr
70
23
71
54
62
20
61
20
79
6
—
76
21
76
32
75
8
21
—
In 3 hr
79 -I
33 J
74
74
71
29
74
38
;;}
;;}
89 -1
38 J
89 "I
53 J
88 -j
15 f
36 ->
88 "I
34 J
Other Products Identified
Methylethylketone
—
—
—
Acetaldehyde, ethylhydro-
peroxide, hydrogen
peroxide
Acetaldehyde, ethylhydro-
peroxide, hydrogen
peroxide
Acetaldehyde, methylethyl-
ketone, ethylhydroperox-
ide
Acetone
Acetone, methylethylketone,
ethylhydroperoxide ,
hydrogen perioxide
Formaldehyde
-------
TABLE 3-5
Photooxidation of Aldehyde-Hydrocarbon Mixtures by Natural Sunlight
Aldehyde and Hydrocarbon
Formaldehyde
2,3-Dimethylbutene-2
Initial Concentra-
tion, ppm (vol)
15
10
Fraction Reacted with Irradiation,'
In 5 hr In 6 hr
61
36
Other Products Identified
Oxidant , hydrogen peroxide
Acetaldehyde
2-Methylbutene-l
5
10
12
4
Oxidant
Propionaldehyde
2-Methylbutene-l
19
10
25
10
Acetaldehyde, methylethyl-
ketone , oxidant , hydrogen
peroxide
-------
Ketone or
Mixture
Acetone
Acetone
2-Methyl-l-butene
Acetone
2-Methyl-l-butene
Acetone
2-Me t hy 1- 1-b u t ene
Diethylketone
TABLE 3-6
Photooxidation of Ketone-Hydrocarbon Mixtures in Air by
Fraction Reacted with Irradia-
tion, %
In In In In In
Initial Concentration. 38 60 120 150 180
ppm (vol) min min min min min
10 -- 12.4 23.3 -- 31.
5 — 10.2 22.4 — 34.
10 — 2.3 7.0 — 11.
10 — 11.4 23.8 — 32.
10 -- 4.5 8.6 15.
20 — 12.2 23.6 — 33.
10 — 7.8 17.6 — 28.
3 17.3 24.7 — 69.3 76.
Sunlight Fluorescent Lamps
Other Products Identified
In 60 min In 120 min
9 Total oxidanta Formaldehyde^
(0.14) (1.82)
Hydrogen per-
oxide*3
(0.03)
Methylhydro-
peroxide1^
Tig=33 min)
Total oxidanta
(0.267)
7
3
4
8
5
7
7 Ethylhydroper-
oxide
(T*s = 50 min)
In 180 min
Formaldehyde^*
(2.88)
Total oxidanta
(0.352)
Formaldehyde^
(0.93)
Total oxidanta
(0.29)
Formaldehyde^*
(1.8)
Total oxidanta
(0.49)
Slow oxidant
(T*5 = 32 min)
Fo rma Id ehy d eP
(3.84)
Total oxidanta
(0.84)
aFerrous thiocyanate method (absorbance per 1-liter sample)
Titanium-8-quinolinol method.
Chromotropic acid method (ppm)
Kinetic colorimetry with molybdate-catalyzed
potassium iodide.
-------
TABLE 3-7
Relative Reaction Rates of Selected Hydrocarbons (4-hr Irradiation)*2
Kopczynski e^ al.73a Stephens and
Heuss and Glasson
591
Leach et al.
774
Hydrocarbon
Ethane
Propane
n-Butane
Isobutane
n-Pentane
Isopentane
2 , 4-Dimethylpentane
Acetylene
Ethylene
Propylene
1-Butene
2-Methyl-2-butene
Toluene
m-Xylene
i-1 o-Xylene
ON sec-Butylbenzene
Relative
Rate
0.06
0.16
0.27
0.24
0.37
0.43
0.99
0.14
1.00
2.33
l.62b
9.39C
0.56
1.14
0.70
1.25^
Standard Burleson-1-^-1-'
Deviation (4-hr Irradiation)
0.09
0.09
0.13
0.26
0.07
0.13
0.27
0.07
' — .
0.42
0.52
1.99
0.34
0.45
0.28
0.45
0
0.18
0.37
0.33
—
0.58
—
0.17
1.0
2.5
2.5
—
—
—
—
—
(6-hr Irradiation)
„
—
0.11
—
—
—
—
—
1.00
1.6
1.9
2.1
0.87
1.2
1.2
0.47
(Auto Exhaust) Schuck and Doyle11
__
—
0.3
—
—
—
—
—
1.00
1.95-2.50
—
—
0.59
1.13-1.59
0.74-1.27
—
__
—
—
—
0.037
0.037
—
—
1.00
12.6
9.7
60
—
—
—
—
a
Ethylene =1.00.
Contains isobutylene.
•»
'Averaged over first hour of irradiation.
Contains 1,2,4-trimethylbenzene.
-------
FORMATION FROM SINGLET OXYGEN
The possibility that singlet oxygen can be produced in the atmosphere
and later react with selected hydrocarbons to yield oxygenated organic com-
pounds has only recently received attention. Before considering this possi-
bility in detail, it is oerhaps worth while to review briefly what singlet oxygen
is and the methods by which it is generated. This review is not meant to be
comprehensive; several excellent reviews on the subject are already availa-
435, 436 504-506, 687, 695, 696, 1029, 1360, 1405
bl-e.
Nature of Singlet Oxygen
The highest occupied molecular orbitals in molecular oxygen are a pair
of doubly degenerate antibonding orbitals containing two electrons. Applica-
tion of Hund's rule leads to a triplet state for the ground state. Other possi-
ble electronic configurations lead to the formation of three excited singlet
states. Two of these states are degenerate in energy, are 22 kcal above
1
the ground state in energy, and are referred to as the A state. The thi.r,d
1
singlet state, the Z state, lies 38 kcal above the ground state,, and is
diamagn&tic by virtue of a cancellation of orbital angular momentum in the..
two available ir orbitals.
It is possible to predict the chemical behavior of these excited states of
oxygen on the basis of orbital occupation. The first-excited state is expected
to be electrophilic in its chemical behavior--i. e. , it is expected to undergo
two-electron reactions. The second excited state has an orbital occupancy
similar to that of the ground state and is expected to undergo the one-
electron--i. e. , free-radical--reactions usually associated with ground-state
695
oxygen. Both these states have been amply identified spectroscopically.
Because singlet molecular oxygen is an excited-state species, it is im-
1
portant to consider its lifetime. The radiative lifetime for the A state
137
-------
66,967
has been found to be 45 min. The radiative lifetime for the 1 £ state is
1332
7-12 sec. These long radiative lifetimes reflect the spin-forbidden char-
acter of the transition to the ground state. Because of collisional deactivation,
the expected lifetimes of the states in condensed phases are considerably
-9 1 -3
shorter. They have been estimated to be 10 sec for I and 10 sec for
1 52 1
A . A more recent estimate of the A state, based on g-carotene
838a
quenching studies, predicts a solution lifetime of 200 ysec. Merkel
1
and Kearns have measured 0 ( A g) lifetimes of 2-200 ysec, depending
880 2 1424a 1
on the solvent. Young ^t aL recently reported 0 ( A ) lifetimes
2
in a variety of solvents that compare favorably with those of Merkel and
Kearns. These reduced lifetimes, owing to the quenching of the excited states,
1
suggest that it will be extremely difficult to observe any reactions of the £
52
state in solution.
Photosensitized Oxidation and Singlet Oxygen
Most of the recent resurgence of interest in singlet oxygen stems from
work indicating that singlet oxygen is the active oxidant in many photosensitized
oxidations. The possibility that electronically excited singlet oxygen might be
the actual oxidant in dye-sensitized photooxidation reactions was first suggested
692,693
over 30 years ago by Kautsky. The mechanism proposed by Kautsky
is shown in Eqs. 111-113:
S + hv -»• 1S ; (111)
1S ^ 3S ; (112)
33 111
S + 0 -> S + 02 ( X or A). (113)
138
-------
According to this mechanism, the sensitizer molecule, S, absorbs the light
1
and is excited to a singlet state, S. Intersystem crossing then occurs, to
3
give a sensitizer triplet, S. Finally, energy transfer occurs from the trip-
let excited sensitizer to ground-state oxygen. This energy transfer produces
oxygen in an excited singlet state and returns the sensitizer to its ground state.
The Kautsky mechanism was ignored until recently, when additional evidence
stirred renewed interest in his proposal, leading ultimately to its confirmation.
705
In 1963, Khan and Kasha investigated the red chemiluminescence that occurs
in the reaction of hydrogen peroxide and sodium hypochlorite. They identified
some of the emission bands as originating from electronically excited oxygen.
It was later shown that one of the emissions occurs from a pair of excited
51
oxygen molecules.
440,441
Beginning in 1964, Foote and co-workers have published a series
of elegant psipers that demonstrate not only that the hydrogen per oxide-sodium
hypochlorite system can effect chemical oxygenation reactions, but that the
nature and selectivity of the products and the stereochemistry of these oxygena-
tions are the same as those observed in photosensitized oxidations of the same
266
substrates. Simultaneously with the 1964 Foote paper, Corey and Taylor
showed that the excited oxygen molecules produced in a radiofrequency discharge
react with some substrates to give oxygenation products that are also identical
with those obtained in photosensitized oxidations. Since these 1964 observa-
tions, singlet oxygen has received increasing attention from scientists in a
wide range of disciplines. This increasing attention has led to the suspicion
or confirmation that singlet oxygen is involved in a variety of reactions and
phenomena. Of interest in this report is the possible involvement of singlet
oxygen in photodynamic action, carcinogenicity, and mutagenicity.
139
-------
Production of Singlet Oxygen
Largely as a result of the solution-phase work of Foote and co-workers,
it is now clear that a major source of singlet oxygen is the transfer of energy
from excited triplet states of sensitizer molecules to ground-state
435,436,440,441
oxygen. The photolysis of ozone serves as another
344,467,636,667,759,980,1424
photochemical method of producing singlet oxygen.
Oxygen in its triplet ground state absorbs very -weakly in the red end of
the visible spectrum. Because of this very low absorption probability, the
direct excitation of ground-state oxygen to the singlet state has not been con-
sidered a likely source of singlet oxygen. Recently, however, it has been
o
found that the emission at 6238 A from a helium-neon laser almost exactly
coincides with the transition required for the excitation of the ground state to
1 400 400
the A state. Using this method of excitation, Evans has been able
to produce enough singlet oxygen to oxygenate several organic substrates.
Radiofrequency discharge flow systems serve as a convenient source
of singlet oxygen. This method was first shown to give singlet oxygen by
433
Foner and Hudson in 1956. The use of the singlet oxygen thus produced
266
to oxygenate organic compounds was first shown by Corey and Taylor in 1964.
Since then, several groups have successfully used this method, as reviewed
1360 461
by Wayne and Furukawa et al.
A growing number of chemical methods for generating singlet oxygen
are available. One of the first of these is the hydrogen peroxide-sodium hypo-
440
chlorite system described by Foote and Wexler. Other methods available
862
are the reaction of bromine with hydrogen peroxide, the decomposition of
862 1350
alkaline solutions of peracids, the decomposition of photoperoxides,
613
the self-reaction of sec-butylperoxy radicals, the base-induced decompo-
1210 704
sition of peroxyacetylnitrate, the decomposition of superoxide ion, and
1020a
the aqueous decomposition of potassium perchromate. The decomposition
140
-------
, of a number of oxygen-rich products formed from the reaction of ozone with
a variety of organic compounds has also been shown to be a source of singlet
940-942,944, 945
oxygen.
Singlet Oxygen in the Upper Atmosphere. This subject has been recently
1131
reviewed and will only be summarized here. The sky emits radiation
1
known as the "airglow, " which contains some emissions from both 0 ( ^ g)
1 665 1 2
and 0 ( E ). Indeed, the first evidence of 0 ( A) molecules in the
2 2
upper atmosiphere was obtained in 1958 and -was based on the 1. 58- ym (0, 1
band) emission of this state observed from the ground. A separate emission
1 981 403,528
from 0 (A) has also been observed from aircraft, balloons, and
2
402, 560
rockets. This second emission, at 1. 27 Mm (0,0 band), is not readily
visible from the ground, because it is reabsorbed by atmospheric oxygen
1
molecules. These observations indicate that the 0 (A) concentration rises
10 32
to a maximum of 4 x 10 molecules/cm at 50 km. It then decreases again
to the limits of detection at about 100 km. There are large differences in the
intensity of the emission bands between night and day. These differences suggest
that the primary source of this singlet oxygen is photochemical. It is now be-
lieved that the source is the ultraviolet photolysis of ozone.
Singlet Oxygen in the Lower Atmosphere. Basic to any consideration of
the role of singlet oxygen in the lower atmosphere is a consideration of the
likelihood of producing singlet oxygen there. This subject has recently re-
ceived considerable attention and has led to some differences of opinion regard-
ing the role of singlet oxygen in polluted atmospheres. The polluted atmospheres
referred to here are those to which photochemical smog is a major contributor,
787
such as those in the Los Angeles basin. Leighton was the first to consider
141
-------
the possible role of singlet oxygen in oxidation reactions in these atmospheres.
i
The specific oxidation process of interest is the conversion of nitric oxide to
nitrogen dioxide. It has been known for some time that the termolecular reac-
tion between oxygen and nitric oxide cannot proceed rapidly enough to explain
the buildup of nitrogen dioxide:
2NO + O -> 2NO . (114)
2 2
Once ozone is produced, it is consumed by reaction with nitric oxide:
NO + O + NO + O
3 22 (115)
Nitrogen dioxide buildup generally precedes ozone buildup, however, so that the
reaction of nitric oxide with ozone is not usually regarded as the important
process for this oxidation. This situation has led to the speculation that singlet
787
oxygen might be involved in the nitric oxide conversion process. Leighton
first considered, then rejected, this possibility on the grounds that the calcu-
lated rate of direct excitation from ground-state oxygen to singlet oxygen was
92
too low. In 1964, Bayes recalculated this rate, taking into account his ob-
1
servation that O ( A ) is not efficiently quenched by O . On this basis, he
2 2
concluded that singlet oxygen could be involved in smog formation. Further-
1409 1
more, Bayes and Winer showed in 1966 that O ( A ), produced in a
2
radiofrequency discharge, reacts with tetramethylethylene in the gas phase,
to give the same hydroperoxide product formed in solution.
706
Beginning in 1967, Pitts and co-workers have published a series of
papers in which they revive the idea that singlet oxygen could be involved in
the oxidation of nitric oxide to nitrogen dioxide in polluted atmospheres.
Because the direct reaction of singlet oxygen (either A or £ ) with nitric
oxide is endothermic and is spin-forbidden, unless the products are produced
142
-------
1027
in an excited state, Pitts e_t al. have proposed an indirect mechanism for
1
the involvement of O in the nitric oxide oxidation. The essential steps of
2
this proposed mechanism are as follows:
10, + -C=C=C- -»- -C-C=C- ; (116)
2 I I
H 0-0-H
A 0
-C-C=C + radicals (e.g., R-£.) ; (117)
0-0-H
R-C- + 0 (ground state) -> R-C-0-0. ; (118)
R-C-0-0 + NO •> R-C-0. + NO ; (119)
N02 + hv -> NO + 0 ; (120)
0 + 02 + M->03+M (where M is a third body) ; (121)
0 + NO -»• NO + 0 . ; (122)
1
Equation 116 is simply an example of one of the known reactions of 0
436 2
with olefins--namely, the "ene" reaction. In Eq. 117, the allylic hydro-
peroxide produced in Eq. 116 is thermally decomposed, to give radicals that
include the acyl radical. The acyl radical then combines •with ground-state
oxygen (Eq. 118), to give a peroxy radical, which then reacts with the nitric
oxide (Eq. 119). Equations 120 and 121 illustrate the usually accepted mecha-
nism for the production of ozone in polluted atmospheres. Equations 118 and
119 are identical with those involved in the mechanism generally accepted for
22, 1026
the photooxidation of hydrocarbons in the presence of nitrogen oxides.
If singlet oxygen is involved in this scheme, or in any other scheme per-
mitting it to be involved in air pollution chemistry, then it seems likely that
143
-------
it must be the A state, and not the E state, that is involved. The several
quenching studies that have been reported indicate that the E state is colli-
sionally deactivated at a high rate, whereas the A state is inefficiently
52, 415,838a, 880, 947, 1424a 696
quenched. Kearns and Khan, using correla-
tion diagrams, have predicted that the E state will not react in a concerted .
reaction, such as that shown in Eq. 116. State correlation diagrams do
696
indicate that both A and E states could be involved in hydrogen abstrac-
tion reactions. Such a reaction is predicted to have an activation energy of
10 kcal for the A state and to proceed with little or no activation energy for
the E state. Again, because of the rapid deactivation of the E state, such
reactions are not expected to be observed. To date, there are no reports of
vapor-phase reactions involving the E state. Later references to vapor-phase
reactions of singlet oxygen in this report therefore pertain exclusively to the
A state.
The proposals of Pitts and co-workers suggest that, in addition to the
direct excitation process, several other possible ways for the generation of
1
O ( A ) in the atmosphere need to be considered. It should be emphasized
2
that, whereas there is still no direct evidence of the presence of singlet oxygen
in the lower atmosphere, each of the methods to be considered has been verified
as a source of singlet oxygen in laboratory experiments. In 1967, Pitts and co-
706
•workers suggested that a high yield of singlet oxygen could be obtained in a
system in which, first, the solar radiation is absorbed by a molecule capable of
photos en sitizati on and, second, there is a transfer to ground-state oxygen. One
example of possible photosensitizing molecules is the group of polycyclic aro-
matic hydrocarbons produced in some combustion processes'. It has now been
demonstrated that such energy transfer processes do occur in the vapor
263,697,757, 1198, 1211, 1349
phase.
144
-------
The sensitizers used in these studies are among those one expects to find in
fairly high concentrations in polluted atmospheres. Pitts et al. have also
1027
suggested that excited nitrogen dioxide may act as a photosensitizer.
This possibility has been realized experimentally by Frankiewicz and
108,449, 450, 450a
Berry:
NO * + 0_ -> N09 + -"-Q, . (123)
2 2 2. t.
These workers have calculated that this mechanism alone could produce a steady-
1 7 8
state O ( Ag) concentration in the atmosphere of 10 -1 0 per cubic centimeter.
2
They also point out that the concentration of oxygen atoms is seldom greater than
5 108
5 x 10 per cubic centimeter. Thus, despite its lower reactivity relative to
1
ozone and atomic oxygen, O ( A g) could make a contribution to smog formation.
175 2
Bufalini points out that the oxygen atom reaction is most important in the
initial stages of the smog producing process, and its importance decreases with
irradiation time. He concludes, on the basis of the expected concentrations and
1
the known reaction rates for oxygen atoms and O ( A g), that the contribution
1 2
of the O ( Ag) reaction is at best less than one-hundreth that of atomic oxygen.
2 1
Frankiewicz and Berry argue that the contribution of O ( A g) reactions could
2 450
be 0. 1-5%, depending on the class of compounds being considered. Jones
and Bayes have also demonstrated energy transfer from nitrogen dioxide to
666a
oxygen. More recently, it has been shown that sulfur dioxide can also
1 307
act as a photosensitizer to produce O ( £ +) from ground-state oxygen.
2 g
Using computer simulation to study the changes that occur in smog chambers
containing nitric oxide, nitrogen dioxide, trans-2-butene, and air, Calvert and
201,322 1
co-workers have concluded that O ( A ) is an unimportant reactant
2 g
in this system.
145
-------
A second possibility for the production of singlet oxygen in the lower
atmosphere referred to by Pitts is the ultraviolet photolysis of ozone. Wayne
978, 1360a
and Norrish had shown that the ultraviolet photolysis of ozone al-
most certainly leads to excited oxygen atoms in a singlet state. If this is
the case, then the other product of this reaction ought to be singlet oxygen.
467 1 1
In 1970, Gauthier and Snelling reported that both O ( A g) and O ( Z )
o 2 2
can be formed from the photolysis of ozone at 2537 A:
03(1A) + hv + 0 (^-D) + 02 (singlet). (124)
759
In 1969, Kummler et. al. suggested that this ultraviolet photolysis of ozone
could be a source of singlet oxygen in polluted atmospheres. Using the avail-
1409 1 (
able data for the collisional quenching of Q£ ( Ag) by ground-state oxygen,
these authors compute the lifetime, T , of O ( ^- A g) to be 0. 57 sec at ground
2 1
level. They then propose that a steady-state concentration of O ( A g) will
2
quickly be reached according to the following equation:
[02( Ag)] = ka[03]r , (125)
1
in which ka is the first-order rate constant for the formation of O ( A g) from
2 1
ozone photolysis. These authors contend that the concentrations of O ( A g)
2
thus calculated make singlet oxygen a more important oxidizing agent than atomic
oxygen in these polluted atmospheres.
546
However, Hamming has argued that singlet oxygen cannot be as important
as oxygen atoms in the photochemical smog process during the early morning hoj«:s,
146
-------
when neither ozone nor, presumably, singlet oxygen would be present in
measurable quantities. Additional data then became available, and Kummler
758
et al. made new calculations concerning the possible concentrations of
241
singlet oxygen available from ozone photolysis. More accurate data on
1
the quenching of O ( A g) by ground-state oxygen indicate that the effective
2
lifetime of the singlet oxygen is 0. 088 sec at ground level, a lower value than
1
that used previously. Using this lifetime for O ( A g), these authors calcu-
1 2 -8
late that the ratio of O ( Ag) to ozone will range from 8x10 :1 to
-7 2
6 x 10 :1 a.t the early morning nitrogen dioxide peak. For any later time,
1
these calculations indicate that the 0 ( Ag), produced by ozone photolysis,
2
will exceed the oxygen atom concentration. However, it had been shown by
588 1
Herron and Huie in 1970 that 0 ( A g) is less than one-thousandth as
2 758
reactive as oxygen atoms with olefins. Thus, Kummler et al. conclude
that ultraviolet photolysis of ozone is not itself sufficient to generate enough
singlet oxygen to make it an important reactant in polluted atmospheres. They
point out, however, that other mechanisms for the production of singlet oxygen
in the atmosphere could produce high enough concentrations to overweigh the
reactivity disadvantage relative to oxygen atoms. One of these other mechanisms
could be the photosensitization process suggested by Pitts and co-workers, and
another could be the chemiexcitation process discussed next.
As described earlier, it has been shown that ozone can react with some
organic substances to produce oxygen-rich intermediates that yield singlet
940-942, 944, 945
oxygen on decomposition. Most of this work has been done
940
with solutions. In one case, however, Murray and Kaplan have shown
that the singlet oxygen so produced undergoes reactions in the vapor phase.
These authors have suggested that such reactions may be important in polluted
147
-------
atmospheres. Murray _et. al. have also pointed out that these results indicate
943
that ozone may have to be regarded as latent singlet oxygen. The objec-
tion that singlet oxygen is expected to have a short lifetime in the atmosphere
may not always be decisive, in that ozone could react with biologic substrates
to produce singlet oxygen in proximity to those substrates.
756
Kummler and Bortner have identified a number of inorganic atmospheric
reactions of ozone, all of which have the potential for producing singlet oxygen.
To date, none of these inorganic reactions has been shown to give singlet oxygen.
The same authors point out that, if an organic pollutant were present at 0. 01 ppm
and reacted with ozone at a rate comparable with that of nitric oxide, then such
a reaction could produce singlet oxygen at the rate of 1. 14 ppm/hr.
Finally, Pitts and co-workers have suggested that the alkaline hydrolysis
1210
of peroxyacylnitrate to give singlet oxygen could take place slowly in the
4
lungs to produce singlet oxygen in situ.
Vapor-Phase Reactions of Singlet Oxygen
Singlet oxygen undergoes various reactions with organic compounds in con-
densed phase.s. Because most of the work to date with singlet oxygen has been
in condensed phases, a brief review might prove instructive regarding potential
vapor-phase reactions. The reaction types that have been observed are the
436 436
"ene" reaction (Eq. 126), the diene reaction (Eq. 127), the hydrogen-
840 438
abstraction reaction (Eq. 128), reaction with enamines (Eq. 129),
84,846 417
reaction to give dioxetanes (Eq. 130), reaction with amines (Eq. 131), and
4,437,439 4,6,939
reaction with sulfides and disulfides (Eqs. 132 and 133); most
of the observed reactions of singlet oxygen fall into these seven types:
148
-------
I
H
A
(126)
1
+ 0
(127)
HO
+ 0
= C^ + 0
\
NR
o = \ / = o +
0 =
R
= 0 (128)
H.
-> C = 0
RO
VC = Cx^ + 0
H/ \H
0 - 0
H/ \
(130)
VcH\= °
<
N-CHO )= 0 ; (131)
149
-------
1 0
2R-S-R + 0 > 2R-S-R ; (132)
2
1 0
2R-S-S-R + 0 > 2R-8-S-R . (133)
2
Of these general types, the reaction with amines (Eq. 131) is still a subject
83
of discussion, with some workers arguing that singlet oxygen is not involved.
1194
The recent data of Smith, however, seem to indicate clearly that singlet
oxygen can oxidize some amines. In other cases, it seems clear that the oxida-
tion does indeed involve singlet oxygen.
Singlet oxygen can be physically quenched in the vapor phase by a variety
of substances, organic and inorganic. Although this subject has received much
attention, it is distinct from reactions involving singlet oxygen and will not be
discussed here. The reader is referred to a recent excellent review for a
comprehensive description of this subject.
The number of reactions of singlet oxygen in the vapor phase that have
244, 245
been reported is very small. It reacts with ozone to give atomic oxygen:
1 3
0 ( Ag) + 0 -»• 20 + 0( P) . (134)
2 32
Reported vapor-phase reactions of singlet oxygen with organic compounds
are restricted to olefins, sulfides, disulfides, and one acetylene, 3-hexyne.
The one acetylene was used in a kinetic study in which products were not
585
isolated. Pitts and co-workers have reported that methylsulfide, ethyl-
sulfide, and methyldisulfides are oxidized to the corresponding sulfoxides.
150
-------
Many olefins have been oxidized in the vapor phase (Table 3-8). In most
cases, products have not been isolated. In the cases of the less reactive olefins,
the process measured is physical quenching, with no products being formed from
the olefin. Perhaps the first report of this type of reaction is that of Falick,
406
Mahan, and Myers, who showed in 1965 that introduction of ethylene reduces
1
the paramagnetic resonance absorption signal due to 0 ( A g). Since then,
2
additional reports have appeared. Singlet oxygen for use in vapor-phase reac-
263
tions is usually produced by the discharge method. Photosensitized oxidation
439
and triphenylphosphite ozonide have each been used in one such experiment.
In most cases, the products of these vapor-phase reactions have not been iso-
lated, as indicated in Table 3-8. Where they have been isolated, the products
are the same as those produced in solution. Where more than one product can
be produced, the vapor-phase reactions give small differences in product distri-
494
butions with a variety of olefins. Like the solution reactions, the vapor-
phase reactions do not give the usual radical oxidation processes.
To date, there have been no reports of vapor-phase reactions of singlet
oxygen with saturated hydrocarbons or terpenes.
The kinetics of the vapor-phase reactions of singlet oxygen with olefins
have recently been examined by several groups. A summary of the available
data on absolute rate constants is given in Table 3-9, and relative rate data are
given in Table 3-10. Where more than one measurement on the same substrate
is available, there is some variation in the reported values of absolute rate
constants. This variation sometimes is an order of magnitude. It should be
emphasized that some, perhaps even most, of the data were obtained under
conditions in which physical and chemical quenching cannot be separated.
There also appears to be some influence of pressure on the rate constant.
151
-------
4
It has been suggested that this may be associated with the presence of a
metastable intermediate. Both the absolute and the relative data support the
conclusion that the reaction is electrophilic in the vapor phase, as well as in
solution.
585,588
Herron and Huie have compared the rate constants for the reac-
tions of oxygen atoms and singlet oxygen with olefins and have concluded that
singlet oxygen, which is approximately one-thousandth as reactive as oxygen
atoms, will not play an important role in air pollution. This conclusion appears
to be based on a consideration of the ultraviolet photolysis as the only source of
758
singlet oxygen. Kummler et al. maintain that, when other possible atmospheric
sources of singlet oxygen are considered, this conclusion may not be correct.
Because no products could be detected from the vapor-phase reactions of
4
terminal olefins with singlet oxygen, Pitts _e_t al. have concluded that the
quenching in these cases is almost entirely physical. This means that the rate
constants for chemical reactions of these olefins would be too low to allow singlet
oxygen to compete with either oxygen atoms or ozone for removal of these olefins
from the atmosphere. In the case of internally unsaturated olefins, products
can be obtained; but the rate constants are low enough so that there is a question
as to whether singlet oxygen could compete with oxygen atoms or ozone for re-
action with these olefins in the atmosphere. What is still lacking is a good
estimate or measure of the concentration of singlet oxygen in the atmosphere.
4
Pitts e± a_l. make the point, however, that, even if singlet oxygen did not
compete favorably with ozone or oxygen atoms in the gross consumption of
atmospheric olefins, the rates are such that quantities of products of singlet
oxygen-olefin reactions of significance to air pollution could be produced. It
seems unlikely, however, that atmospheric reactions of singlet oxygen are a
major source of free radicals.
152
-------
There are no reports of vapor-phase reactions of singlet oxygen with
benzene derivatives. There are some reports of solution work with
benzene derivatives, but none with pure hydrocarbons. Saito, Matsuura,
1109, Il09a,1110, 1111
and co-workers have reported the photosensitized
oxidation of a number of methoxy derivatives of benzene. The reaction
appears to be of the diene type, proceeding through an intermediate endo-
peroxide. The reaction is illustrated here for the case of 1,2, 3, 5-
tetramethoxybenzene:
OCH
CHO
'•OCH
OH
H 0
2
OCH
CH
XOCH
OCH
CH OH
3
(135)
H
153
-------
839, 839a, 840
The same group has reported that singlet oxygen produced
via photosensitization or from a chemical source is able to accomplish
a hydrogen abstraction reaction with some phenols. A similar reaction
1021
has been reported by Pfoertner and Bose. The reaction is illustrated
here for the case of 2,.6-di-tert-butylphenol;
HO
(136)
154
-------
TABLE 3-8
Vapor-Phase Reactions of Olefins with Singlet Oxygen
Olefin
Ethylene
Propane
1-Butene
Isobutene
cis-2-Butene
Reference
5,406,585,588
5,585,588
5,585,588
585,588
4,461,585
Product
a
a
a
a
a
jtranj>-2-Butene
2-Me1;hyl-2-butene
2-Hexene
2,3-Mmethyl-2-hexene
1-Nonene
Cyclopentene
1,2-Dimethycyclo-
pentene
Cyclopentadiene
Cyclohexene
1,2-Dimethylcyclo-
hexene
1-Methylcyclo-
hexene
1-Methylcyclopentene
462 Allylic hydroperoxide
4,461 at
4,461,584,585,588 a
462,494 Allylic hydroperoxide
461,585 a.
584 _a
585 a
585 a.
584,585 a
461,585 a.
585 a
494
Allylic hydroperoxide
584,585,588
494
461,585
461,585,588
Allylic hydroperoxide
155
-------
TABLE 3-8 - continued
Olefin
4-Methylcyclo-
hexene
1,3-Cyclohexa-
diene
1,4-Cyclohexa-
diene
2,5-Dimethylfuran
Reference
585
461,585,588,940
585,588
461,584-586,588,
604
157
493
Product
Methoxyhydroperoxide
Endoperoxide
—Product not isolated.
156
-------
TABLE 3-9
Absolute Rate Constants for Vapor-Phase Reactions of Singlet Oxygen with Olefins
Olefin k, liters/mole-sec
Ethylene l.lxlO3
-------
TABLE 3-9 - continued
Olefin
1-Pentene
cis-2-Pentene
2 , 3-Dimethyl-2-pentene
1,3-Pentadiene
2-Hexene
Cyclopentadiene
1, 2-Dimethylcyclopentene
1-Methylcyclopentene
1-Methylcyclohexene
1, 2-Dimethylcyclohexene
1 , 3-Cyclohexadiene
2 , 5-Dimethylf uran
k, liters/mole-sec
1.9xl03
2 . OxlO4
7. OxlO5
6 . 3xl04
llxlO4
6.7xl03
8.8xl06
4xl05
l.SxlO4
4 . OxlO5
IxlO4
4xl05
2 . 3xl05
llxlO4
2.8xl06
l.lOxlO7
l.SxlO7
4.8xl05
1.46xl07
1.6xl07
T, K-
RT
298
b_
RT
b
298
298
b_
bi
298
298
b
298
b
298
298
300
295
300
300
Reference
5
461
588
4
538
461
461
588
588
461
461
588
461
588
461
604
584
493
585
586
—RT, = room temperature.
—Temperature not specified.
-2-10 Torr.
^1 Torr
158
-------
TABLE 3-10
Relative Rate Constants for Vapor-Phase- Reactions of Singlet Oxygen with Olefins3-
Olefin
Ethylene
Propylene
1-Butene
cis-2-Butene
2--Methyl-2-butene
2 , 3-Dimethyl-2-butene
trans-3-Methyl-2-pentene
2 , 3-Dimethyl-2-pentene
2. , 3-Dimethyl-2-hexene
1 , 3-Pentadiene
1-Methylcyclopentene
2 , 2-Dimethylcyclopentene
1 , 2-Dimethylcyclopentene
1 , 2-Dimethylcyclohexene
1 , 3-Cyclohexadiene
1 , 4-Cyclohexadiene
2 , 5-Dimethylf uran
Relative k—
<0.01
<0.01
<0.01
<0.01
<0.01
0.044
1.00
<0.01
0.69
0.7
0.66
<0.01
0.015
0.4
0.4
0.43
0.4
0.38
0.09
<0.01
17.0
Reference
585
585
585
585
585
584
584,585
585
584
585
584
585
585
585
585
584
585
584
585
585
584,585
—All experiments at 300 K.
b
—Based on 1,2-dimethyl-2-butene = 1.0.
159
-------
FORMATION FROM OZONE
Ozone in Polluted Atmospheres
Ozone in small quantities is a natural constituent of the atmosphere.
Ozone is produced in a photochemical reaction that occurs in the upper
194,521
portions of the stratosphere. Some of this ozone finds its way to
the troposphere and contributes to the background ozone concentration in
all atmospheres. A recent study by Ripperton e_t al. reported a background
1077
ozone concentration of 0. 02-0. 04 ppm. This background concentration
is generally in the range of 0-0. 1 ppm.
o
When molecular oxygen absorbs radiation of approximately 2400 A,
it dissociates into atomic oxygen; the atomic oxygen then combines with
molecular oxygen to produce ozone:
0 + hv -»- 0 + 0 ; (137)
0 +0+M+0 +M(M=a third body) . (138)
This process leads to high stratospheric concentrations of ozone, reaching
a maximum of approximately 2. 0 ppm at an altitude between 12 and 22 miles.
The ozone layer absorbs ultraviolet light, which would otherwise reach the
earth's surface and present a hazard to life.
Ozone in the troposphere has four general sources: dissociation of
oxygen as it occurs in the stratosphere, photochemical reactions involving
nitrogen oxides and hydrocarbons in polluted atmospheres, lightning, and
transfer from the stratosphere. Of these, only the second and fourth are
significant. In atmospheres in which photochemical smog is prevalent,
160
-------
the second source is the major one and can lead to ozone concentrations
as high as 1. 0 ppm. The chemistry of photochemical smog that leads
22, 23, 336, 1215, 1291
to ozone production has recently been reviewed and
will only be summarized here.
A major reaction leading to ozone formation in polluted atmospheres
has its origin in the photolysis of nitrogen dioxide, which is produced in
combustion processes:
2ND + 0 + 2N02 ; (139)
hv
N02 + NO + 0 ; (140)
0 + 02+ M ->• 0 + M
(141)
In the presence of hydrocarbons, there are clear indications of a
separate path contributing to the conversion of nitric oxide to nitrogen
dioxide. Several proposals have been offered for this alternative oxidation
process (discussed earlier and in Chapter 4). The possibility that ozone-
olefin reactions can contribute to this process is considered below.
Mechanism of Reaction of Ozone with Aliphatic Hydrocarbons
Olefins. Most of the work on the mechanism of the reaction of ozone
with olefins has been carried out with solutions. The prevailing views on
the mechanism are summarized here. Necessary modifications to the
mechanism for vapor-phase reactions are considered below.
The most extensive proposal for the mechanism of ozonolysis of
279, 280
olefins was given by Criegee, on the basis of several years of
161
-------
work by him and his co-workers. According to this mechanism, ozone
adds to the double bond to give an initial olefin-ozone adduct, J, which
is unstable and decomposes, to give a zwitterion fragment, 2, and a
carbonyl compound, 3 (Eqs. 142 and 143). Recombination of 2 and 3
leads to the normal ozonide, 4 (Eq. 144). Depending on a number of
factors, but largely the reactivity of the carbonyl compound, the zwitterions
may also react with themselves, to give diperoxides, 5, and higher
peroxides, 6 (Eq. 145). In reactive solvents, such as methyl alcohol,
the zwitterion is diverted to other products, such as methoxyhyd roper oxides,
7 (Eq. 146).
\ = C^+ 03 > N, ' ^ <^; (142)
1
vi^_^ ^
/~i X » =0 + > = 0 ; (143)
2 . 3
0 0—0 x
+ y=o > /^0/\ ; (144)
3 4
0 \ 0—0.
>=( "~""
2
162
-------
cf
V \ /°CH3
> = 0 + CH OH ^. V
3 /XOOH . (146)
7
Depending on olefin structure, the initial adduct, 1, may have
several possible structures, la-lc:
la . . lb Ic
It now seems likely that la is the most common structure for the initial ad-
Tib ~~ 1230a
duct, whereas lb is invoked when excess aldehyde is present and
71,946, 1230c
Ic is particularly likely for terminal olefins with bulky substituents.
It is also possible that Ic is formed first and then proceeds to Jja or la, de-
1230a
pending on olefin structure and specific reaction conditions. A number of
experimental results have been obtained that are not fully explained by the
Criegee mechanism. Perhaps the most significant is the failure of the
Criegee mechanism to predict stereochemical consequences in the
ozonide (or ozonides, -where more than one is formed). These results have
938
led to several additional proposals for the mechanism, all of which retain
163
-------
the basic elements of the Criegee mechanism. One of these pro-
938, 946, 1230c
posals suggests that the observed stereochemical
results can be understood if the ozonolysis reaction is viewed as
having several mechanistic paths available, one of which is the one
suggested by Criegee. Additional pathways include a direct conversion
of the initial adduct, formulated as Ic, to ozonide, and the reaction of
the initial adduct with aldehyde, to give ozonide in an aldehyde-exchange
1230a
reaction. This proposal has been amplified to include suggestions
that the initial adduct, formulated as Ib, can also undergo a Baeyer-
562
Villiger reaction, to give ozonide or to oxidize aldehyde to acid. A
second major alternative to the Criegee mechanism has been suggested
90
by Bailey and co-workers. The essence of this proposal is that olefin
stereoisomers can give different distributions of syn- and anti^-z witter ions
and that these in turn will react with the carbonyl fragment, 2, to give
different ozonide stereoisomer distributions. A further modification of
this proposal has recently been given that is claimed to be more con-
770a
sistent with the experimental observations.
Saturated Hydrocarbons. Reaction of ozone with saturated hydro-
carbons has received far less attention than reaction with olefins.
Saturated hydrocarbons are considerably less reactive than olefins. Two
1377
mechanisms have been considered for this reaction. One is a radical
mechanism in which ozone abstracts a hydrogen atom from the hydrocarbon.
Recombination of the fragments thus produced leads to a hydrotrioxide,
which decomposes further, to give an alcohol and oxygen (Eqs. 147-149):
164
-------
R-H + 0 + R.+ .OOOH ; (147)
R. + .OOOH + ROOOH ; (148)
ROOOH ->• R-OH + 0 . (149)
A variation of this mechanism has been proposed and examined -with
545
the assistance of optically active tertiary hydrocarbons. According to
this view, the first step in the reaction is regarded as an insertion reaction
of ozone into a carbon-hydrogen bond (Eq. 150). The transition state for
the insertion is postulated as having considerable radical character. The
mechanism suggests that the alcohol will be produced with retention of
configuration or with racemization, depending on the relative importance
of the two competing processes shown (Eq. 151). Experimentally, it is
found that alcohol is formed with 60-70% retention of configuration.
R - H + 0 -> [R' .OOOH] ; (150)
[R- .OOOH] + ROOOH •> ROH (retention) + 0 (151)
\
0
'R' + 0 + .OH 2^ ROH (racemized)
+ 0 + ketone +
peroxides and
fragmentation products,
165
-------
It seems likely that, if hydrotrioxides are produced in vapor-phase oxida-
tion of saturated hydrocarbons with ozone, they would decompose, to give
radicals.
100
Benson has suggested that, in vapor-phase oxidation of hydro-
carbons, the reaction could involve an initiation step that gives atomic
oxygen. The initiating atomic oxygen radicals are then postulated to
abstract-a hydrogen atom from the hydrocarbon. The reaction sequence
then has propagating steps, one of which involves ozone:
M -I- 0 —-»02 + 0 + M ; (152)
O + R-H + R'+'OH ; (153)
R- + 0 -> RO- + 02 ; (154)
RCT + R-H -+ ROH + R' . (155)
Because of the low temperatures used, it is not felt that this sequence
is important in solution reactions. But Benson suggests that the radical
abstraction process described above for solution reactions (Eqs. 150 and
151) is also not very likely, because of the endothermicity of the reaction
and the temperatures used. He concludes that an intimate ion-pair mechanism
cannot be ruled out.
0 + R - H -> [R + 0 H ]
166
-------
Vapor-Phase Reactions of Ozone
Because ozone concentrations in polluted atmospheres are consider-
ably higher than those found in the ambient tropospheric environment,
studies of the vapor-phase reactions of olefins with ozone are essential
to an understanding of the chemistry of photochemical smog. A summary
of the reported reactions is given in Table 3-11. Leighton has given an
787 •
excellent review of this subject through 1961. The data in Table 3-11
include the earlier references, as well as reports up to the present.
As indicated in Table 3-11, the products of the reaction of olefins
with ozone in the vapor phase differ somewhat from those obtained in
solution. .It has generally been assumed that the first step in the mechanism
is approximately the same as that observed in solution--!, e. , an unstable
olefin-ozone addition compound, 1, is produced. After,comparing calcu-
lated A factors for several possible transition states with the observed
322a
A factor, DeMore concluded that the transition state in the cases of
ethylene and propylene results from a concerted 1, 3 dipolar addition of
ozone to the double bond. This compound then presumably fragments to
a carbonyl compound, 3, and a zwitterion, 2. In almost all cases, the
expected aldehydes and ketones are found. In many cases, the acids
corresponding to further oxidation of these materials are also found.
Many of the reactions also gave carbon dioxide. This could arise from
further decomposition of the zwitterion, as follows:
R-CH -^ = (/ + CO + R-CH . (156)
Z H 2 J
167
-------
In at least some cases, the hydrocarbon expected from this kind of
176
decomposition has been found. The variation in product composi-
tion from corresponding solution reactions could also indicate that the
vapor-phase reactions are more likely to involve free-radical pathways.
Two recent investigations bear directly on this question. Working at a
4l6a
total pressure of 3 torr, Finlayson, Pitts, and Akimoto have obtained
evidence of OH radical production in the ozonolysis of a number of low-
molecular-weight olefins. In a related study, Atkinson, Finlayson, and
54a
Pitts have obtained evidence of a number of radical species, including
HCO and HO , in the ozonolysis of low-molecular-weight olefins at a
2
total pressure of 2 torr. A general treatment of vapor-phase ozonolysis
making use of a radical mechanism has now been offered.
The usual recombination reaction between carbonyl compound and
zwitterion to give normal ozonide, 4, seems to be completely suppressed
in the vapor phase. Although there are some reports that ozonides were
isolated in vapor-phase reactions, the conclusion is based only on infrared
spectra, and some reports have been found to be incorrect.
In a number of cases, aerosols -were reported to form. It is possible
that non-vapor-phase reactions could occur in these aerosols.to produce
ozonides and other peroxidic materials. As seen in Table 3-11, several'
groups of workers reported the formation of unstable materials. These
are presumably peroxidic materials, including ozonides.
A more recent study indicates that the dark-phase reaction of ozone
with open-chain monolefins in air produced no light-scattering aerosol.
However, similar reactions with the cyclic olefins cyclohexene and a -pinene
and the diolefin 1, 5-hexadiene produced considerable quantities of light -
107 6a
scattering aerosol.
168
-------
Decomposition of the zwitterion, 2, has also been postulated to
1150
explain some of the other lower-molecular-weight products:
H +0
^Cjz^rCT ->- H 0 + CO ; (157)
H
CH + 0
J^:C TO^ -> CH OH + CO . (158)
^ 3
Yields of the carbonyl-containing products obtained are higher when
1326
ozone is used in oxygen than in nitrogen. Thus, some of the products
probably result from further oxidation reactions after the initial attack of
the ozone on the olefin. It should be pointed out, however, that these results
are obtained when the reactants are used in the torr pressure range. Also,
in the presence of oxygen about 1. 4-2. 0 moles of olefin are consumed per
mole of ozone, whereas, when nitrogen is used as the diluent, a stoichio-
1326
metric relation is observed. Apparently, some of the products pro-
duced are themselves capable of oxidizing olefins.
176
A possible explanation for the higher aldehyde and ketone yields
in the presence of oxygen is that the zwitterion reacts with oxygen, to give
the carbonyl compound and ozone:
169
-------
C 0 0 + 0 + R-C-H + 0 . (159)
H
Niki has presented a comprehensive scheme for smog production
in which he concludes that ozone-olefin reactions are important in all
970a
aspects of the total -mechanism. According to Niki, both the time
of the induction period and the time of conversion of nitric oxide to
nitrogen dioxide are lengthened if the ozone reactions are not considered.
He proposes the following reactions as possibilities for the involvement
of the ozone-olefin reaction, using propylene as the olefin:
jf HC+HOO~ + CH-CHO
0 + CH~CH=CH CT + _
•* J L ^CH-C HOO + HCHO ; (160)
+ - 0
HCHOO + 0 = -OH + nft-OO' ; (161)
+ - 0
CH3CHOO + 02 = 'OH + CH -C-00- ; (162)
LO
0-0' + NO = NC) + H-CC
NO = NO + H-CO- ; (163)
H-C-0-0- + NO = N03 + H-C-0- ; (164)
-C-0-0- + NO = NO + CH^-CO'
CH -C-0-0- + NO = NO + CH3-CO' ; (165)
0 0
CH -C-0-0- + N02 = N03 + CH -fi-0- . (166)
170
-------
Niki, Daby, and Weinstock conclude that the later stage of the
process, -which governs the terminal concentrations of ozone and
nitrogen-containing smog products, is dominated by ozonolysis reac-
970b
tions.
646
Jaffe and Loudon have also suggested that the zwitterions
produced on ozonolysis could react -with oxygen, but they prefer the
path giving ozone and a carbonyl compound (shown earlier). They suggest
further that such reactions could be responsible for the nonstoichiometric
ratio of olefin to ozone in the presence of oxygen, which had been observed
1326
earlier by Vrbaski and Cvetanovic.
23
It has been suggested that the finding of a stoichiometric olefin-
ozone relation in the presence of nitrogen could rule out the possibility
that the zwitterion itself can attack hydrocarbons to initiate a chain oxida-
tion reaction. This may be true only when there are limited quantities
of olefins, such as were used in the kinetic studies in which the stoichio-
metric relation was found. In the gross reactions that take place in
polluted atmospheres, the possibility of reaction between zwitterions and
olefins, as well as other hydrocarbons, should probably not be ruled out.
281,763a
The zwitterion has been shown to epoxidize some olefins in solution.
A dipolar addition reaction between zwitterions and olefins has also been
1230b 543
reported. Finally, it has been suggested that the zwitterion
can initiate the oxidation of saturated hydrocarbons.
In some cases, aldehyde and ketone products are obtained that are
not normal ozonolysis products, i.e. , derived from direct cleavage of the
1363a
double bond. It has been suggested that some of these products
result from more complex rearrangements of zwitterions:
171
-------
,0 H H
CH -C-Cf I + CH -C-C
3 H x-n 3
H H
CH3-i-C-OH -> CH CHO + HCOH . (167)
Such possibilities must be regarded as highly speculative.
The observation that some of the products obtained from vapor-
phase ozonolysis are different from those obtained in solution and the
general dissatisfaction with explanations for these products using the
994b
Criegee zwitterion mechanism have prompted one group to suggest
a mechanism for the vapor-phase reaction that is based on free-radical
chemistry. According to this proposal, the Criegee zwitterion mechanism
is seen as the least important of several reaction possibilities. The other
possibilities proposed are based on a detailed kinetic and thermochemical
analysis of the vapor-phase reaction. An outline of this new proposal
follows:
H sB. H B (168)
X^ 2 \ y 2
-\ + 0 ->• Rn_, .^--PH TH TJ
\CH2CH2R3 3 1 / \ CH2CH2R3
172
-------
Ozone adds to the olefin, to give the initial ozonide or trioxolane. This
unstable intermediate is postulated as being in equilibrium with the two
possible diradicals arising from homolysis of an oxygen-oxygen bond.
The diradicals so formed have several possible fates. These are
illustrated here for one of the diradicals:
q-H abstraction path:
R1~O CH2CH2R3
n._ A..
R,-CH-t -00'
R -C -
1 6H 0
!l( C°,
H R2 t
R,-C -i—CHCH.R,
-1- 6n o—o- * J
R
!H-C-R2 + R3CH2CHO + hv ; (170)
|R 0
173
-------
Y -H abstraction path;
Criegee split:
R -C -C -CH CH R
H R2 ¥ R2 -00'
R -C-C-CH_CH9-R, + Rj-C-C-^
I \ • 2
I f '-, CHR,
0-C/ 'H'' 3
R2 0
-»• R -C -C -00- -»•
1 H ; (171)
iy
CH2-CH-R^ R.-r—V
"H
k
R.CHO +
(172)
This proposal retains the Criegee zwitterion formation as one of the
fates of the diradical. It also postulates several new fates that do explain
some of the products that are observed experimentally and that are not
readily explained via the zwitterion route. In particular, this mechanism
explains the formation of carbonyl compounds arising from cleavage of
the carbon-carbon bond adjacent to the olefinic linkage. In a recent study
174
-------
4l6b
of low-pressure vapor-phase ozone-olefin reactions, F inlay son et al.
994b
have used this free-radical scheme of O'Neal and Blumstein to explain
most of their observed products.
In the case of saturated hydrocarbons, the vapor-phase products are
1141
again approximately the same as those obtained in solution. It has been
suggested that these products are the results of free-radical reactions
1141
initiated by ozone.
A considerable number of vapor-phase olefin-ozone reactions have
been subjected to kinetic analysis. Rate data for saturated aliphatic
hydrocarbon reactions are more limited. The available data are summarized
in Tables 3-12 and 3-13. Absolute rate constants are listed in Table 3-12,
273a
and relative rate constants, in Table 3-13. Cox and Penkett have re-
ported that they also have studied vapor-phase ozone-olefin reactions. They
did not report their kinetic data, but state that second-order kinetics were
observed smd that their rate constants agreed well with those given by
787
Leighton.
32
On the basis of measured compositions of smoggy atmospheres
and the rate constants for the reaction of ozone with various species
1209a
contained in such atmospheres, Stedman and Niki have concluded
that, under typical atmospheric conditions, the carbon-containing species
primarily removing ozone are olefins.
A major problem associated with most of the vapor-phase kinetic
work is the nonstoichiometric relation between ozone and olefin. One
assumes that the stoichiometry should be 1:1 and that second-order
1363a
kinetics will be observed. As a result of the work of Wei and Cvetanovic,
it now seems clear that the failure to observe 1:1 stoichiometry is due to
175
-------
the use of oxygen as the carrier gas for ozone. When these workers
used oxygen, they observed a stoichiometry that varied from 1. 4:1 to
2.01:1 (olefin:ozone), whereas a clean 1:1 stoichiometry was achieved
in the presence of nitrogen. In a more recent study of the vapor-phase
1209b
ozonolysis of propylene and trans-2-butene, Stedman ei^ ai_. have
observed stoichiometry of unity with both air and nitrogen as diluents.
In this work, the ozone and olefin concentrations were in the parts-per-
million range, suggesting that the failure to observe unity stoichiometry
1326
in the presence of oxygen in the earlier study may be due to a con-
centration effect.
s
Because the vapor-phase ozonolysis probably proceeds through an
initial olefin-ozone adduct, it seems prudent to determine the influence
of different inert carrier gases on the rate constant, inasmuch as the
observed rate constant may depend on the deactivation ability of the
carrier gas.
As shown in Table 3-12,,when more than one measurement is avail-
able for a given olefin, the results are in fairly good agreement, although
it should be kept in mind that all these results were obtained in the
presence of oxygen. In some cases, the differences reported are more
than an order of magnitude. Examples include cis- and trans-2-butene.
176
The higher values were obtained by Bufalini and Altshuller. Such
large discrepancies appear to be restricted to 2-olefins. More recently,
23
Altshuller and Bufalini have suggested that this difference may be due
-8
to the low olefin concentrations (about 10 M) used in their work, compared
-3 1363a
with the values of about 10 M used by Wei and Cvetanovic. Studies
over a wide range of olefin concentrations are needed to understand this
apparent discrepancy further.
176
-------
These discrepancies are pointed out further in Table 3-13, where
the relative reactivities of various olefins are summarized. When ab-
solute rate constants are compared (Table 3-12), trimethylethylene
(2-methyl-2-butene) is seen to be about 300 times as reactive as ethylene,
but their relative reactivities (Table 3-13) indicate that trimethylethylene
is only about 10 times as reactive as ethylene.
Table 3-13 also shows that the relative rates in the presence of
nitrogen show the same electrophilic trend as found in solution studies--
i.e. , increasing the number of alkyl substituents at the double bond
generally increases the rate. A major departure from this trend is
found in the cases of 1, 1-dialkyl olefins, whose rates are generally lower
than those for the corresponding 1, 2-dialkyl olefins. If ozone electro-
philicity were the controlling factor, these rates should be approximately
1363a
the same. Wei and Cvetanovic have suggested that this apparent
discrepancy may be due to a greater steric hindrance to approach of
the ozone in the 1, 1-dialkyl cases. At any rate, the data do not seem
to be consistent -with an initial radical atta'ck. When ozonolysis is carried
out in the presence of oxygen, the variation from an electrophilic trend
is even greater (Table 3-13).
As shown in Table 3-12, olefins are considerably more reactive
4 5
than saturated hydrocarbons with ozone. However, olefins are 10 -10
less reactive with ozone than with atomic oxygen. Because ozone is
present in much higher concentrations than atomic oxygen, the olefin-
ozone reactions are still very important in polluted atmospheres. These
reactions -will lead to consumption of olefin, with the production of a
variety of substances (Table 3-11). They may also serve as sources of
177
-------
radical reactions. The available rate data suggest that reaction of ozone
•with saturated hydrocarbons is not likely to be significant as an atmospheric
reaction.
The reaction of ozone -with benzene derivatives in the vapor phase
176
has received very little attention. Bufalini and Altshuller have reported
that the rate constant for the reaction of ozone with 1, 3, 5-trimethylbenzene
3
is less than 0. 06 X 10 liter/mole-sec when measured in the vapor phase.
The kinetics of ozonization of a number of polyalkylbenzenes have been
949
studied in solution by Nakagawa, Andrews, and Keefer.
A review of the reaction of ozone with hydrocarbon derivatives of
70
benzene has been given by Bailey. The reaction appears to involve
both 1, 3-dipolar cyclo-addition at a carbon-carbon double bond and electro-
1378
philic ozone attack at individual carbon atoms. A study of the rate of
ozone attack on methyl-substituted benzenes reveals the following trend:
benzene < toluene < xylene < mesitylene < hexamethylbenzene. The reac-
tion thus follows a trend consistent with an electrophilic attack. The
peroxidic products of these reactions have not been characterized. The
ultimate products are the expected ones—glyoxal, methylglyoxal, biacetyl,
etc.
178
-------
TABLE 3-11
Vapor-Phase Ozonolysis of Aliphatic Hydrocarbons
Hydrocarbon
Ethylene
Propylene
1-Butene
2-Buteneb-
Isobutene
Products
Formaldehyde, formic acid, unstable inter-
mediate
Water, formaldehyde, carbon monoxide, carbon
dioxide, formic acid
Formaldehyde
Acetaldehyde, formic acid
Formic acid, unstable intermediate
a
Formaldehyde, acetaldehyde, formic acid,
acetic acid
Water, formaldehyde, acetaldehyde, carbon
monoxide, carbon dioxide, ketene, an acid
Ketene, methyl alcohol, methane, carbon
monoxide, carbon dioxide, formaldehyde,
acetaldehyde
Propylene ozonide
Acetaldehyde, propionaldehyde, acetone, formic
acid, methyl alcohol, carbon dioxide
Acetaldehyde—
a
Ethane
Propionaldehyde, propionic acid, unstable
intermediate
Acetaldehyde, propionaldehyde, n-butyral-
dehyde, formic acid, methyl alcohol,
carbon dioxide
c
Carbon dioxide, acetaldehyde, propionaldehyde
a
Acetaldehyde, acetic acid, unstable
intermediate
Acetone
Acetone, formaldehyde
Reference
154
1150
54a,176
1326
566a
193, 322a, 553, 1327
153a,15A
1150
553
322a
1326
1363a
193,1327
176
153
1326
1363a
1327
153
176
54a,553
179
-------
TABLE 3-11 - continued
Hydrocarbon Products
Acetone, formaldehyde, formic acid
Isobutyraldehyde, acetone, formic acid,
methyl alcohol, carbon dioxide
Carbon dioxide, acetaldehyde, acetone^.
a
trans-2-Butene Acetaldehyde
Methyl alcohol, carbon monoxide, acetaldehyde
Acetaldehyde, formic acid, carbon dioxide
Carbon dioxide, acetaldehyde^.
a
Reference
153
1326
1363a
1327
176
553
1326
1363a
1327
cis-2-Butene
Methyl alcohol, carbon monoxide, acetaldehyde
Acetaldehyde, formic acid, methyl alcohol,
carbon dioxide
Carbon dioxide, acetaldehyde3-
Formaldehyde, acetaldehyde, ketene, formic
acid, methyl alcohol
Acetaldehyde, 2-butanone, methylvinylketone
a
1-Pentene Butyraldehyde, formaldehyde, ozonide
Butyraldehyde , carbon dioxide, formic acid,
unstable intermediate
Water, formaldehyde, butyraldehyde, carbon
monoxide, carbon dioxide, acid
Acetic acid, propionaldehyde, butyraldehyde,
formic acid, carbon dioxide
553
1326
1363a
54a
416b
176,193,1327
553
552
1150
1326
180
-------
TABLE 3-11 - continued
Hydrocarbon
2-Penteneb-
cis-2-Pentene
trans-2-Pentene
2-Methyl-l-butene
3-Methyl-l-butene
2-Methyl-2-butene
Products
Carbon dioxide, acetaldehyde, propional-
dehyde, butyraldehyde£
a_
a
Acetaldehyde, propionaldehyde, butyral-
dehyde, methylethylketone, methyl alcohol,
carbon dioxide
Carbon dioxide, acetaldehyde, propional-
dehyde£
a_
Acetaldehyde, propionaldehyde, formic acid,
methyl alcohol, carbon dioxide
Carbon dioxide, acetaldehyde, propional-
dehyde£
a
Acetaldehyde, methylethylketone, formic
acid, methyl alcohol, carbon dioxide
Carbon dioxide, acetaldehyde, acetone,
methylethylketone0.
a
AcetaJLdehyde, isobutyraldehyde ,
acetone, formic acid, methyl alcohol,
carbon dioxide
Carbon dioxide, acetaldehyde, isobutyral-
dehyde, acetone^.
a
Acetaldehyde, acetone, formic acid,
methyl alcohol, carbon dioxide
Carbon dioxide, acetaldehyde, acetone^.
a.
Reference
1363a
193,1327
1150
1326
1363a
1327
1326
1363a
1327
1326
1363a
1327
1326
1363a
1327
1326
1363a
1327
181
-------
TABLE 3-11 - continued
Hydrocarbon
Products
Reference
1,3-Butadiene
Tetramethylethylene
1-Hexene
1-Heptene
3-Heptene^
1-Octene
1-Decene
Acetaldehyde, acrolein, formic acid,
carbon dioxide
a
Acetone0-
Formic acid, formaldehyde, acetone, methyl
alcohol, carbon dioxide, acetic acid,
esters
Acetaldehyde, acetone, formic acid, methyl
alcohol, carbon dioxide
a
Pentyl alcohol, butane
Ozonide, valeraldehyde , formaldehyde
Formaldehyde, butyraldehyde, acetylene,
carbon dioxide, water, hydroperoxides
Acetaldehyde, butyraldehyde, valeraldehyde,
acetone, formic acid, carbon dioxide
Carbon dioxide, acetaldehyde, propionaldehyde ,
butyraldehyde, valeraldehyde£
a
a
a
a
a
1326
1327
176,550,551,1363a
1191
1326
1327
176
553
1115a
1326
1363a
193, 1115b, 1327
193
1150
193
193
Cyclohexene
Styrene
Acetylene
Formic acid, adipic acid, trans-l,2-cyclo-
hexanediol
Acetaldehyde, formic acid, carbon dioxide
a
a
ji
363
1326
193,1327
176
192,322a
182
-------
TABLE 3-11 - continued
Hydrocarbon
3-Methyl-l-pentene
4-Methyl-l-pentene
2-Methyl-l-pentene
2-Hexened-
4-Methyl-2-pent ene^
2-Methyl-2-penteneb-
3-Methyl-2-pentene
OT^entene
D-Limonene
D-a-Pinene
a-Pinene
Methane
Ethane
Propane
Products
Carbon dioxide, acetaldehyde, propionaldehyde,
isovaleraldehyde, acetone£
Carbon dioxide, acetaldehyde, isobutyral-
dehyde, isovaleraldehyde, acetone°-
Carbon dioxide, acetaldehyde, propional-
dehyde, acetone, methylpropylketone£
Carbon dioxide, acetaldehyde, propional-
dehyde, butyraldehyde0-
Carbon dioxide, acetaldehyde, propional-
dehyde, isobutyraldehyde£
Carbon dioxide, acetaldehyde, propional-
dehyde, acetone0-
Carbon dioxide, acetaldehyde, acetone,
methy le thylket onefi
Diozonide
Diozonide
Ozonide
Aerosol
Formic acid, carbon dioxide, carbon
monoxide, methyl alcohol
Carbon monoxide, carbon dioxide, water,
formic acid
a
Carbon dioxide, water, formic acid, methyl
alcohol
Formic acid, carbon dioxide, methyl alcohol,
acetone
Carbon dioxide, water, acetone, formic acid,
methyl alcohol
Reference
1363a
1363a
1363a
1363a
1363a
1363a
1363a
1202a
1202a
1202a
1076a
1141
331
1140
917
1141
917
183
-------
TABLE 3-11 - continued
Hydrocarbon
It-Butane
Products
a
Formic acid, carbon dioxide, methyl alcohol
Reference
1140
1141
-§- 1140
Isobutane
a-Pentane
Formic acid, carbon dioxide, acetone, tert-
butyl alcohol
tert-Butyl alcohol, acetone, tert-butyl-
hydroxymethylperoxide
a
1141
1140
1140
—Products not isolated.
—Stereochemistry not specified.
Q
—Yields of major products greater in the presence of molecular oxygen than in its absence.
—Cis-trans mixture.
184
-------
TABLE 3-12
Absolute Rate Constants for Vapor-Phase Reactions of Ozone
with Aliphatic Hydrocarbons
Hydrocarbon
Ethylene
1-Propene
1-Butene
trans-2-Buterie
cis-2-Butene
Isobutylene
k, liters/mole-sec
1.6 x 103
0.8 x 103
1.8 x 103
1.0 x 103
0.93 x 103
b
4.9 x 103
3.8 x 103
5.1 x 103
7.5 x 103
d
6.2 x 103
3.9 x 103
260 x 103
17 x 103
165 x 103
200 x 103
2.9 x 103
13 x 103
60-92 x 103
1.4 x 103
3.7 x 103
3.6 x 103
5-14 x 103
T, K-
303
RT
RT
RT
299
Various
RT
303
RT
299
Various
—
RT
303
RT
299
303
RT
RT
RT
303
RT
RT
RT
Reference
176,193
253
1327
416b
1209b
322a
553
193
1327.
1209b
322a
176
1327
176
1327
1209b
176
553
1327
416b
176
553
1327
416b
185
-------
TABLE 3-12 - continued
Hydrocarbon
3-Methyl-l-butene
2-Methyl-l-butene
2-Methyl-2-butene
Tetraraethylethylene
Butadiene
1-Pentene
cis-2-Pentene
trans-2-Pentene
1-Hexene
1-Heptene
1-Octene
1-Decene
Acetylene
Styrene
k, liters/mole-sec
3.0 x 103
4.0 x 103
450 x 103
12 x 103
14 x 103
4.2 x 103
4.5 x 103
3.2 x 103
3.9 x 103
10 x 103
13 x 103
6.8 x 103
6.1 x 103
6.1 x 103
5.5 x 103
4.6 x 103
6.6 x 103
4.9 x 103
4.9 x 103
6.5 x 103
4.7 x 101
e
5.18 x 101
18 x 103
T, K-
RT
RT
303
RT
RT
RT
RT
303
RT
RT
RT
303
RT
303
RT
RT
299
303
303
303
303
Various
298
303
Reference
1327
1327
176
1327
1327
1327
553
193
1327
1327
1327
176
553
193
1115b
1327
1209b
193
193
193
192
322a
1209a
176
186
-------
TABLE 3-12 - continued
Hydrocarbon
Cyclohexene
Tetrafluoroethylene
Perfluoropropene
Perfluoro-2-butene—
a-Pinene
Methane
Ethane
Propane
k, liters/mole-sec
T,
Reference
n-Butane
Isobutane
35 x
14 x
81 x
13 x
1.1 x
99 x
0.85 x
0.82 x
7.4 x
5.79 x
4.1 x
4.3 x
3.75x
5.9 x
12.2 x
io3
103
IO3
IO3
io3
3
10
io-3
io-3
lO-3^
-h
1Q-3-
10-3
10-3*
io-3*1
1C'3
io-3
303
RT
RT
RT
RT
RT
312.5
298
298
298
312.5
298
298
310.5
310.5
193
1327
572a
57 2a
572a
1076a
1141
331
917
917
1141
917
917
1141
1141
— RT =. room temperature.
-log kdT1 sec'1) = (6.3 + 0.2) - (4.7 + 0.2)/2.3 RT.
1327
—This and other absolute rate constants taken from T. Vrbaski and R.J. Cvetanovic
were obtained from relative values by adopting a value of 1.8 x 10^ liters /mole-sec
for ethylene.
-log k(M-l sec'1) = (6.0+0.4) - (3.2 + 0.6)/2.3 RT.
-log k(M-1 ssec-1) = (9.5 + 0.4) - (10.8 + 0.4)/2.3 RT.
— Cis-trans mixture.
•"Oxygen added.
—No oxygen added.
187
-------
TABLE 3-13
Relative Rate Constants for Vapor-Phase Reactions
of Ozone with Aliphatic Hydrocarbons^.
Olefin
Ethylene
Propylene
1-Butene
1-Pentene
1-Hexene
3-Methyl-l-butene
3-Methyl-l-pentene
4-Methyl-l-pentene
2-Methylpropene
2-Methyl-l-butene
2-Met hy 1- 1-pent ene
trans-2-Butene
Relative k -
o2
0.51
0.32
1.41
1.30
1.08
1.10
1.07
1.10
1.28
1.25
0.84
0.95
0.90
1.15
1.00
1.00
1.11
1.30
1.30
4.56
4.30
c
Relative k —
^2 Reference
1327
0.21 1363a
1327
0.95 1363a
1327
0.85 1363a
1327
0.85 1363a
1327
1.05 1363a
1327
- 0.75 1363a
0.75 1363a
0.85 1363a
1327
1.00 1363a
1327
1.25 1363a
1.25 1363a
1327
2.20 1363a
188
-------
TABLE 3-13 - continued
Olefin
cis-2-Butene
trans-2-Pentene
cis-2-Pentene
Cyclohexene
2-Hexened-
4-Methyl-2-pentene
2-Methyl-2-butene
3-Methyl-2-pentene
2-Methyl-2-pentene
Tetramethylethylene
Butadiene
Relative k —
°2
3.69
3.40
3.50
4.20
2.84
3.50
3.74
3.60
3.50
3.24
3.50
3.40
3.40
3.80
4.80^
1.15
Relative k^ —
2 Reference
1327
2.00 1363a
1327
2.70 1363a
1327
2.60 1363a
1327
2.70 1363a
2.30 1363a
1327
3.20 1363a
3.20 1363a
3.20 1363a .
1327
5.50^ 1363a
1327
—All experiments at 298 K.
—Relative rate constant in the presence of oxygen.
«
—Relative rate constant in the presence of nitrogen,
—Cis-trans mixture.
e
—Mean value from several competition experiments.
189
-------
CHAPTER 4
ATMOSPHERIC REACTIONS OF ORGANIC MOLECULES WITH NITROGEN
AND SULFUR OXIDES, HYDROXYL RADICALS, AND OXYGEN ATOMS
The main feature of the chemistry of photochemical smog is the
conversion of nitric oxide to nitrogen dioxide in the atmosphere. A
number of possible mechanisms for the conversion have been proposed.
201, 322
The oxidants considered include 0', ' OH, 'OOH, and singlet oxygen.
Singlet oxygen is not expected to contribute significantly to the process
(see Chapter 3). Of the remaining possibilities mentioned, the interven-
tion of 'OH radicals is discussed in some detail below. The chemistry
of this conversion is not yet fully understood.
OZONE PRODUCTION
When mixtures of hydrocarbons and nitrogen oxides are subjected to
108
ultraviolet radiation in air, oxidant is produced. The course of
such an irradiation is shown in Figure 4-1 for a propylene mixture of
nitric oxide and propylene. Initially, nitrogen dioxide and aldehydes are
produced as the reactants are consumed. As the nitric oxide is exhausted,
the nitrogen dioxide concentration passes through a maximum; and as the
nitrogen dioxide is consumed, ozone and other oxidants, such as peroxy-
acetylnitrate (PAN), are produced. Such compounds as nitric acid and
nitrates must also be present to account for the nitrogen balance. It
731,766 108
has been shown that studies like that of Berry and Lehman
correlate-with what happens in the atmosphere.
The most interesting aspect of those results is that, although the
reaction is imperceptibly slow in the dark, none of the initial gases
190
-------
(nitric oxide, propylene, oxygen, and nitrogen) absorbs the ultraviolet
o
radiation (wavelength, > 2900 A). In fact, the absorbing species is
nitrogen dioxide, and trace amounts of it must be present for the reac-
tion to proceed. These are produced from the well-known, but slow,
573
reaction:
2NO + 02 •> 2N02. (1)
The photochemical reaction is autocatalytic, and the nitrogen dioxide
concentration rises at an increasing rate as long as sufficient nitric oxide
is present.
The hydrocarbon-nitric oxide photooxidation has been studied in simulated
25, 335,489,490,707, 729, 1142b
atmospheres for several hydrocarbons and aldehydes.
Measures of hydrocarbon reactivities have been based on the rates of hydro-
23
carbon consumption and nitrogen dioxide production. Altshuller and Bufalini have
reviewed hydrocarbon reactivities. Hydrocarbons and aldehydes reported
to be present in urban atmospheres (J. Garner, personal communication)
are listed in Table 4-1, which includes for some of these the relative
reactivity as calculated by one or both methods. Arbitrarily, isobutene
is considered to have a relative reactivity of 1.0.
The two scales give similar results. Reactivity can be associated
with structure. The paraffins are the least reactive. Except for iso-
octane, their relative reactivities are less than 0. 1, and often much less.
The aromatics have reactivities from 0. 15 for benzene to more than 2
for some highly substituted benzenes. Most of the olefins have reactivities
of about 0. 5 to 2. 2, although cyclopentene has a value of 6. 6. The rela-
tively high value of 2. 94 for 3-methylstyrene can be attributed to the fact
191
-------
that it is both aromatic and olefinic. The aldehyde reactivities cover
the same range as those of the aromatics, except for the surprisingly
high reactivity of _o-tolualdehyde.
It seems reasonable to assume that the reactivity of a hydrocarbon
is related to its rate coefficient for reaction with some active species in
photochemical smog that reacts readily with hydrocarbons. Such species
3
include 0( P) and HO radicals. Ozone is a product of the reaction and
does not appear until the reaction is well advanced. Thus, it is probably
not as important as oxygen or OH ', although it reacts readily with hydro-
carbons. Other conceivable intermediates, such as peroxy and alkoxy
radicals, are removed so much faster by other routes than by hydrocarbon
attack that their role can be only minor. The same is true of photoexcited
aldehydes and ketones. Nitric oxide and nitrogen dioxide react too slowly
with hydrocarbons to be important. Photoexcited nitrogen dioxide may be
quenched readily by hydrocarbons, but no report of chemical interaction
has appeared.
3
The relative reactivities of the compounds in Table 4-2 with 0( P)
and HO are also listed. Not all the available data on any compound are
3
included, but only the most likely value. For 0( P), the results from
1 28,290 583,587,618
Cvetanovic's and Herron's laboratories have been pre-
ferred, for two reasons: both have made extensive studies and are
therefore experienced; and any errors in values are likely to be internally
consistent, so the relative values from a given laboratory may be correct,
even if the absolute values are not. This is not to imply that other studies
are less accurate. In fact, the differences in values among several labora-
518-520
tories are usually less than 50%. For HO radicals, the values of Greiner
3
are preferred, for the same reasons noted for 0( P) atoms.
192
-------
3
For both 0( P) and HO, the reactivity of the hydrocarbon follows
the same trends as the photochemical reactivity, although the detailed
fit is not all that one might have expected. For methane and ethane,
the reactivities are very small, but the other paraffin reactivities with
3
0( P) and HO, although still small, are larger than the photochemical
reactivities. With the aromatics, the reverse seems to be the case,
but the measurements are sparse. With the olefins, reasonably good
matches are obtained in all cases except cyclopentene, which has an
abnormally large photochemical reactivity. With the aldehydes, the
3
data are again sparse, but the HO and 0( P) reactivities are lower by
factors of aibout 4 and 40, respectively, than the photochemical reactivi-
ties. Thus, it can be concluded that, in a general way, photochemical
3
reactivity is associated with the reactivity toward 0( P) or HO, but
that the detailed correlation must be considerably more complex.
3
Further evidence that some species in addition to 0( P) and HO are
177
attacking hydrocarbons comes from the work of Bufalini and Altshuller
28,970
and others., who noticed synergistic effects when two hydrocarbons
•were photooxidized simultaneously.
OXIDATION OF NITRIC OXIDE
A mechanism for the photochemical conversion of nitric oxide to
nitrogen dioxide in urban atmospheres has been proposed by Heicklen
571,573a
et al. in which HO radical is the important chain carrier.
With alkanes, the initiating reaction is HO radical attack to remove a
hydrogen eitom. For example, with butane, the reaction sequence
proposed was:
193
-------
C4H10 + H°' * W + H2°;
- + 02 -»• C4H902'; (3)
4H902' + NO + C4H90' + N02; (4)
C4H90- + 02 -> C3H?CHO + H02' ; (5)
H02' + NO -»• HO' + N02. (6)
The overall reaction is
C,H,n + 2NO + 20, -> C-H-jCHO + H00 + 2NO-,. (7)
'r -LU ^ J I £. Z
Note that HO- is regenerated in Eq. 6 and can reinitiate the reaction
sequence.
With olefins, the scheme is slightly modified, because HO • adds
to the double bond, rather than abstracting hydrogen. Thus, for iso-
butene, the reaction sequence might be:
C4Hg + HO' -> HOC4Hg'; (8)
HOC4Hg' + 02 -»- HOC4H802-; (9)
HOC4Hg02' + NO -* HOC4HgO' + N02; (10)
HOC4HgO' •* HOCH2' + (CH3)2CO; (11)
HOCH2' + 02 •> HOCH202'; (12)
HOCH202' + NO -*• HOCH20- + N02; (13)
HOCH20' -»• HO' + CH20. (14)
194
-------
The overall reaction -would be
C H + 2NO + 202 -> (CH3)2CO + CH20 + 2N02, (15)
1142b
which conforms to the findings of Schuck and Doyle, who observed
that formaldehyde and dimethylketone were the major products of the
photochemical oxidation of isobutene in the presence of nitric oxide.
With aromatic compounds, the cycle is probably similar to that with
olefins. The aldehydes should behave like the alkanes, but -with a slight
modification. For example, with acetaldehyde, the first few steps would
become:
CH3CHO + HO' -> CH3CO + H20; (16)
CH3CO + 02 -> CH30' + C02; (17)
and then the sequence would proceed as in Eqs. 5 and 6. Equation 17 may
not be a fundamental reaction, but may occur in several steps.
The implication of the above schemes is that carbon monoxide must
also oxidize: nitric oxide:
HO. + CO -»• H. + C02; (18)
H- + 02 + M -> H02' + M; (19)
HO ' + NO -> HO' + N02. (6)
Confirmation of the influence of carbon monoxide has been given several
488
times in recent years. The first report was by Glasson who examined
195
-------
an atmosphere with 2-ppm ethene and 1-ppm nitric oxide. The addition
of 400-ppm carbon monoxide doubled the rate of nitrogen dioxide produc-
520,914
tion. Because the rate constant for HO- attack on ethylene is
10 89
1-3x10 per mole-sec and that for HO- attack on carbon monoxide
7
is 8. 0 x 10 per mole-sec, the relative reactivity between ethylene and
carbon monoxide should be between 125 and 375. Because 200 times as
much carbon monoxide as ethylene was used in Glasson's experiment, the
expected rate of nitric oxide oxidation on the addition of carbon monoxide
would be between 1. 6 and 2. 9 times that in its absence--in excellent
agreement with the observed result. The effect of carbon monoxide
with other hydrocarbons has been confirmed.
571,573a
Simultaneously with the reports of Heicklen et al_., Weinstock
1367
and his collaborators were using a very similar mechanism to do a
complete computer study on propylene photooxidation. They estimated the
chain length (number of nitric oxide molecules converted to nitrogen dioxide
molecules per photochemical initiation step) to be about 280 and the lifetime
of the HO radical to be 56 sec. Detailed computer studies have also been
1374, 1374a 569
made by Westberg and Cohen and Hecht and Seinfeld.
It is interesting to compare the estimated chain length of 280 with
known rate coefficients for the reactions of HO- with propylene and
nitric oxide. For the HO--propylene reaction, the room-temperature rate
914, 118U 10
coefficient is 10 per mole-sec. Let us assume that three
nitric oxide molecules are converted to nitrogen dioxide molecules during
each chain cycle (one conversion per carbon atom in propylene). Then to
reach a chain length of 280, the rate coefficient for HO- removal by
8
nitric oxide should be about 10 per mole-sec. The estimated coefficient
196
-------
910,1181,1235
is at least ten times this value. Part of this dis-
crepancy can be explained by the fact that the product of the HO-
nitric oxide interaction is nitrous acid, which photolyzes to reproduce
HO- and nitric oxide; thus, the effective removal coefficient is lower.
However, a chain length of 280 seems to be too large, on the basis of
the known rate coefficient data.
The rea.ction steps outlined above were in some cases well estab-
lished, but in other cases speculative. Equations 2, 3, 16, 18, and 19
were certain. Some evidence existed for Eq. 5 with CH O and C H O
3 25
572 53,1281
radicals and for Eq. 6 at high temperatures. Equations
8-14 were all speculative and, except for Eq. 8, still are. The details
and even the validity of Eq. 17 are still unknown.
Equation 8 has now been verified for ethylene and propylene by
914
Morris et al. Studies have been undertaken by Heicklen and col-
1379
laborators to verify Eqs. 4-6. For CH O radicals, they have found
3
that, at 25 C, the relative reactivities of CH O with oxygen, nitrogen
-5 3
dioxide, and nitric oxide are 4. 7 x 10 ,0. 83, and 1. 0, respectively.
Thus, under atmospheric conditions, the reaction with oxygen is at least
1379
5 times as important as that with the oxides of nitrogen. Wiebe et al.
also found that the principal fate of HO ' was Eq. 6. The rate constant
2
662 6
for this reaction has been reported to be 1. 2 x 10 per mole-sec.
This is not a large rate constant, but Eq. 6 still must be the dominant
1181
fate of HO ' in the atmosphere. A more recent report indicates
2 8
that this rate constant is greater than 10 per mole-sec.
1204
Spicer e_t al. examined the reaction of CH O ' with nitric
3 2
oxide and nitrogen dioxide in an attempt to verify Eq. 4. In neither case
197
-------
was CH O produced. The initial products were either the adduct
3
peroxy compounds, peroxymethylnitrite and peroxymethylnitrate, or
formaldehyde and nitric and nitrous acids. These reactions can thus
explain the production of peroxy compounds and acids, but present
a dilemma for the mechanism of the conversion of nitric oxide to
nitrogen dioxide. It is not necessary that hydrocarbon peroxy radicals
react with nitric oxide to produce nitrogen dioxide; that can be done by
HO ' via Eq. 6. However, it is essential that alkoxy radicals be formed.
2
At the moment, this dilemma is unresolved, although Spicer et al. point
out that the later photodecomposition of the peroxy nitrates and nitrites
might produce alkoxy radicals. Thermal decomposition also would have
the same effect.
In regard to peroxynitrate formation, particularly peroxyacetylnitrate
549
(PAN), Hanst has offered an interesting proposal. He suggests that
PAN could be produced by
0 0
CH3C-0- + N03 -v CH3C-OON02, (20)
rather than (or in addition to)
0 0
CH3C-02' + N02 ->• CH3C-OON02. (21)
His arguments reduce to three: (1) PAN formation is delayed past the
onset of nitrogen dioxide formation and correlates with ozone production
198
-------
(see Figure 4-1); because nitrogen trioxide is produced from the ozone -
nitrogen dioxide reaction, it is the precursor to PAN formation.
(2) PAN is readily synthesized in the dark thermal reaction between
1275
acetaldehyde, oxygen, and nitrogen pentoxide, as shown by Tuesday.
(3) PAN formation correlates with the nitrogen trioxide and not the nitrogen
dioxide concentration.
Although Hanst's proposal is intriguing, it has weaknesses. The
delay in PAN production can be attributed to the delay in acetaldehyde
production, inasmuch as acetaldehyde is a necessary precursor to PAN.
Also, although it is true that there is no explanation for PAN production
in the dark thermal reaction between acetaldehyde, oxygen, and nitric
oxide via Eq,, 21, it is also true that there is no explanation of how the
dark reaction leads to Eq. 20. Nevertheless, Hanst's proposal deserves
attention, and further investigation is warranted.
The cha.in termination and initiation steps for the conversion of
nitric oxide to nitrogen dioxide may be numerous. Principally, termina-
tion may involve reaction of HO, alkoxy, and alkylperoxy radicals. How-
ever, termination by these processes would be slower than the reaction
rate constants indicate, because the products can be photolyzed to re-
generate the radicals.
Initiation occurs as a result of the nitrogen dioxide photolysis to
3
produce nitric oxide and 0( P). Most of the time, the oxygen atom
3
reacts -with oxygen to produce ozone. Occasionally, however, 0( P)
reacts with a hydrocarbon. With alkanes and aldehydes, hydrogen atom
abstraction results, and HO- is produced directly. With olefins, the
initial product is an excited epoxide, which can rearrange to an excited
199
-------
aldehyde or decompose to free radicals. If the excited aldehyde does
not decompose to radicals, but is stabilized by collision, it can photo-
decompose to produce free radicals; these free radicals then enter the
nitric oxide oxidation mechanism, and the chain is initiated.
Although an increase in hydrocarbon content increases oxidant
production, an increase in the concentration of the oxides of nitrogen
can either increase or decrease oxidant production. This effect was
532
first reported by Haagen-Smit and Fox and has since been sub-
24, 29-31,491, 531, 1090, 1221, 1274, 1275
stantiated with many hydrocarbons.
491
In one of the most recent studies, Glasson and Tuesday reported
this effect for several hydrocarbons. In particular, they found that for
propylene and rn-xylene the maximal rate of oxidation occurred for
hydrocarbon: nitric oxide ratios of about 6:1 and about 1. 5:1, respectively.
Perhaps the most dramatic demonstration of the nitric oxide inhibi-
331a,331b
tion was shown by Dimitriades. He examined the reactivity
of exhaust in an irradiated atmosphere. Because several hydrocarbons
were present, the effective hydrocarbon concentration was computed on
a per-carbon basis by summing over the products of the concentration
and relative reactivity of each hydrocarbon. The measure of reactivity
was the oxidant dosage, i. e. , the product of the oxidant concentration
and its time of existence. The results are shown in Figure 4-2. It
can be seen that the oxidant dosage increases regularly with the hydro-
carbon concentration at any concentration of the oxides of nitrogen; but
for any hydrocarbon concentration, the oxidant dosage drops as the
nitrogen oxide is increased. The effect for mixtures at a constant
hydrocarbon: nitrogen oxide ratio is shown in Figure 4-3. For ratios
200
-------
below about 4:1, the oxidant dosage actually drops as the pollutants
increase! The implication is that, if an automobile exhaust consists
of hydrocarbons and oxides of nitrogen in an effective ratio of less
than 4:1, reducing the exhaust pollutants will actually increase the
oxidant dosage!
The effect of nitric oxide is easily explained. In the conversion
of nitric oxide to nitrogen dioxide, nitric oxide is not involved in the
chain-initiating step, but it is important in removing radicals in the
termination steps. Its role in propagation is to react •with RO ' or
2
HO • . However, once enough nitric oxide is present to scavenge
2
these radicals, further increases in its concentration lead to competi-
tion between nitric oxide and hydrocarbons for the HO radicals and
between nitric oxide and oxygen for the alkoxy radicals. When either
HO or alkoxy radicals react with nitric oxide, termination of the chain
results.
An additional effect of excess oxides of nitrogen is to remove ozone
by direct reaction. The ozonation of nitric oxide gives nitrogen dioxide ,
which when ozonized gives nitrogen pentoxide and ultimately nitric acid
in the presence of water.
REACTION OF NITROGEN DIOXIDE WITH OLEFINS
Cottrell and Graham were first to report that nitrogen dioxide can
269,270
react with olefins in the vapor phase. At high temperatures,
269 270
nitrogen dioxide reacted with ethylene and propylene by a rate
law that was approximately second-order in nitrogen dioxide and first-
order in hydrocarbon pressure. The Arrhenius parameters for the rate
coefficient changed with temperature, but the activation energies were
201
-------
sufficiently large (greater than 1Z kcal/mole) that the reaction is not
of principal importance in urban atmospheres. They proposed the
reaction sequence:
N02 + olefin ^ adduct; (22)
N02 + adduct ->• products. (23)
With ethylene, the products •were almost entirely carbon dioxide, carbon
monoxide, and nitric oxide. Also produced were an oil and a small
amount of carbon-like material. The addition of nitric oxide or air had
no effect, nor did changing the surfacervolume ratio. With propylene,
the results •were similar, except that the reaction •was 3-4 times faster.
Support for the above mechanism comes from the fact that free-
radical products resulting from the addition have been observed by
669,1248
electron-spin resonance in condensed phases. It is also
known that nitrogen dioxide can induce cis-trans isomerization of un-
saturated compounds in the liquid phase. However, the first study of
the vapor-phase isomerization appears to be very recent. Sprung
e_t ^L have measured the rate coefficient for the nitrogen dioxide-
1206
catalyzed isomerization of cis-butene-2, trans-butene-2 (J. L.
Sprung, personal communication), and cis- and trans-pentene-2 (J. L.
Sprung, personal communication). They found the rate law to be first
order in both nitrogen dioxide and olefin pressure, as expected from the
above mechanism. Again, their rate constants are much too small for this
reaction to be of principal importance in urban atmospheres.
202
-------
The reaction of several olefins with nitrogen dioxide at 25-100 C
645
was studied by Jaffe whose results were very different from those
of the other investigators. He found the reaction to be first-order in
both nitrogen dioxide and olefin pressure, and the activation energies
to be between 4 and 8 kcal/mole. He proposed that the initial step was
the transfer of an oxygen atom to give excited aldehydes and epoxides,
which reacted further to give the observed products--nitric oxide,
carbon dioxide, nitrous oxide, water, methyl nitrite, methyl nitrate,
dimethyl ester, and acetaldehyde.
The reaction of nitrogen dioxide with trans-butene-2 was studied
901
in both the absence and presence of oxygen. In both cases, a liquid
product was found, but the kinetics of the reaction were different. In
the absence of oxygen, the product is presumably the addition compound
formed from the olefin with two molecules of nitrogen dioxide, i. e. the
dinitro or nitro-nitrite compound. However, with excess oxygen, the
initial adduct appears to add to oxygen before reacting with the second
nitrogen dioxide molecule, suggesting a nitroperoxynitrate product.
Although there are discrepancies in the results from the various
laboratories:, it is clear that nitrogen dioxide does react with olefins,
but not fast enough to be of major consequence in urban atmospheres.
As far as we know, no one has reported chemical reaction between
electronically excited nitrogen dioxide and hydrocarbons.
REACTIONS OF SULFUR DIOXIDE
Sulfur dioxide is an important air pollutant in some urban atmospheres,
such as those of New York, Philadelphia, and Chicago. Not only is it harm-
ful in itself,, but it can undergo oxidation to produce the more toxic sulfur
203
-------
trioxide and sulfuric acid. It may react with zwitterions and oxygenated
radicals, although almost nothing is known about such reactions. It is
203
known that methyl radicals add to sulfur dioxide but this reaction
is probably not important in the atmosphere, where alkyl radicals are
readily scavenged by oxygen.
The possible reactions of sulfur dioxide and nitric oxide with
4-
zwitterions, R 00 are of considerable interest. These zwitterions
are probably produced in the reaction of ozone with olefins, as dis-
cussed elsewhere. Presumably, the zwitterion rearranges most of
the time, but other reactions are possible. Of pertinence here is the
possibility of reaction with either nitric oxide or sulfur dioxide:
+ _
R 00 + NO -»• RO + N02, (24)
+ _
R 00 + S02 -»- RO + S03, (25)
where RO is a stable aldehyde or ketone, and not a free radical. There
is no evidence to support or refute Eq. 24; but, if it occurred, it could
regenerate nitrogen dioxide. There is evidence of Eq. 25. Cox and
273
Penkett reacted ozone with cis-2-pentene in the presence of sulfur
dioxide and moist air. In the dark, a rapid conversion of sulfur dioxide
to sulfuric acid aerosol occurred, suggesting Eq. 25 followed by hydration
of sulfur trioxide.
The influence of sulfur dioxide on photochemical smog has been the
179
subject of a recent extensive review. Therefore, the work will not
be discussed in detail here. In summary, the rate of disappearance of
204
-------
sulfur dioxide and the formation of aerosol increase when sulfur dioxide
is photolyzed in the presence of olefinic hydrocarbons and the oxides of
nitrogen.
Mixtures of sulfur dioxide, saturated and unsaturated hydrocarbons,
nitrogen dioxide, and water do not react in the dark. However, if
powdered oxides of aluminum, calcium, chromium, iron, lead, or vanadium
1298 1298
are present,, reaction occurs readily in the dark. In the same study,
it was also noticed that, in the absence of the metal oxides, the photo-
chemical oxidation of sulfur dioxide was noticeably faster in the presence
of hydrocarbon and nitrogen dioxide than in their absence.
1407
Wilson^e_t aL reported an interesting study in which several
hydrocarbons were photooxidized in the presence of sulfur dioxide and
the oxides of nitrogen under atmospheric conditions. Although sulfur
dioxide does not react -with the hydrocarbon, ozone, nitric oxide, nitrogen
dioxide, its presence reduced the nitrogen dioxide concentration in the
3
photolysis. Presumably, the reactions of sulfur dioxide with 0( P)
and nitrogen trioxide are responsible for the inhibition. The oxidant
production depended on the concentrations of water vapor, nitrogen dioxide,
and sulfur dioxide. In the case of 1-butene, 1-heptene, and isooctane,
oxidant was reduced by adding sulfur dioxide; but for toluene, the oxidant
was enhanced.
Studies of the effect of sulfur dioxide in irradiated automobile ex-
1406
hausts were made by Wilson et a^. They found that the addition of
sulfur dioxide increased aerosol formation, but produced a decrease in
eye irritation with many of the hydrocarbon-nitrogen oxide systems
studied. These results are particularly intriguing, in that they showed
205
-------
that the photochemical aerosols generated from 1-heptene and nitrogen
oxides in the absence of sulfur dioxide cause eye irritation, whereas
the sulfuric acid aerosol produced in the photooxidations of sulfur dioxide
in clean wet air did not cause eye irritation. It is clear that the role of
sulfur dioxide in photochemical smog is complex and not well understood.
Sulfur dioxide absorbs radiation that enters the lower atmosphere.
Weak absorption occurs below 3900A to a triplet state, whereas a much
stronger absorption takes place below 3400A. The latter absorption
band peaks at about 2900A, the cutoff for radiation entering the lower
atmosphere.
The primary photophysical processes in sulfur dioxide when ex-
cited into the absorption band centered at about 3000A have been studied
in detail for about a decade. These studies include the excellent work
222,517 1232
of Duncan and his co-workers, Strickler and Howell,
882-884 253,993,1049,1050
Mettee, and Calvert and his co-workers.
The details of the primary process have been elucidated through life-
time measurements of emission, fluorescence, and phosphorescence
yields during steady-state exposure and through biacetyl sensitization.
The mechanism resulting from these studies is
1S02 + S02 -> 2S02, (26a)
1S02 + S02 ->• 3S02 + S02; (26b)
1S02 -> S02 + hvf, (21 a)
->• S09, (27b)
206
-------
3
1S02 + 3S02; (27c)
3S09 -9- S09 + hv , (28a)
S02 •* S02; (28b)
S02 + S02 + 2S02, (29a)
3S02 + S02 •> S03 + SO. (29b)
The superscripts 1 and 3 refer to excited singlet and triplet states,
respectively. Rate constants have been determined for all the steps.
540-542,544, 861, 1259
Furthermore, Thrush and his co-workers
studied the emission of sulfur dioxide from the reactions of sulfur oxide
with ozone or oxygen atoms. Their results are consistent with the above
251
mechanism, except for minor discrepancies.
Electronically excited sulfur dioxide is also known to react with
oxygen to produce sulfur trioxide, although the quantum yield is small
788,1162
and the details of the process are not known. The quantum.
yield is too small to account entirely for the oxidation of sulfur dioxide
17
to sulfur trioxide in urban atmospheres. However, a more recent
study suggests that sulfur trioxide does not form to any extent from
456a
reactions of excited sulfur dioxide with oxygen, -water, or both.
294,295
Dainton and Ivin studied the photolysis of sulfur dioxide
in the presence of several paraffins and olefins. The principal products
•were sulfinic acids. The quantum yields were independent of sulfur
dioxide pressure, but increased -with the hydrocaron pressure to a
maximum of about 0. 35. A negative temperature coefficient was found
between 15 and 100 C.
207
-------
Aerosols have been seen when sulfur dioxide was irradiated in the
663
presence of hydrocarbons, and aerosol formation is enhanced in
663,730
air when sulfur dioxide is present.
1261
More recently, Timmons has reexamined the photolysis of
sulfur dioxide in the presence of alkanes. When isobutane was the added
gas, he found that, for a sulfur dioxide pressure of 20 torr, the total
quantum yield first increased with isobutane pressure and then became
constant at approximately 0. 090 for isobutane pressures above 200 torr.
The addition of excess methane had almost no effect on the quantum yield.
The negative temperature coefficient reported by Dainton and Ivin was
confirmed.
1261
Timmons also made a limited study of the photolysis of sulfur
dioxide in the presence of carbon monoxide. For four runs at 25 C and
[SO ] = 20 torr, he found that carbon dioxide was produced with a
2 -3
quantum yield increasing from 5. 2 x 10 at 21 torr of carbon monoxide
-3
to 7. 4 x 10 at 420 torr. A negative temperature coefficient -was also
found for this reaction.
140, 225
Heicklen and co-workers have studied the chemical reactions
of electronically excited sulfur dioxide. In their first studies, done
at 3130A, they examined the photolysis in the presence of carbon monoxide
and in the presence of perfluoroethylene. With carbon monoxide, carbon
dioxide was produced, although the quantum efficiency was small. How-
ever, the quantum yield of carbon dioxide H fco } , increased propor-
2
tionately with carbon monoxide pressure, but was independent of sulfur
dioxide pressure at high sulfur dioxide pressures. Such a result is
contrary to expectations based on the mechanism deduced from the
208
-------
light-emission studies, which require that ^^3O / be a function of
2
[CO]/[SO ] . Furthermore, it was found that the addition of an
2
atmosphere of nitrogen did not alter J{CO "V. Because the emitting
11 2J 883
sulfur dioxide states are readily quenched by nitrogen, this ob-
servation also cannot be explained by the light-emission measurements.
With perfluoroethylene, the photoreaction proceeds readily to pro-
duce carbonylfluoride cyclic perfluoropropylene, and polymer. At high
C F pressures, 5 fCF O} is approximately 0. 05. Again, the addition
24 2
of a large excess of nitrogen had no effect on the reaction.
At the pressures of sulfur dioxide used in these studies (i. e. , at
882,1049,1232
least 1 torr), Eqs. 27 and 28 are unimportant, and the
mechanism involving the emitting states reduces to:
1S02 + S02 + 2S02, (26a)
1S02 + S02 -> 3S02 + S02, (26b)
3S02 + S02 -* 2S02, (29a)
3S02 + S02 -»- S03 + SO. (29b)
The steady-state concentrations of the excited states are inversely
proportional to [SO ] . If these states reacted with carbon monoxide
2
or C F to produce products, then the quantum yields of product forma-
2 4
tion should depend on the sulfur dioxide pressure. This expectation
3 639
has been confirmed for SO by the results of Jackson and Calvert
3 2
in the SO -carbon monoxide system.
2
209
-------
294
The above results, as well as those of Dainton and Ivin, show
that the product quantum yields are independent of sulfur dioxide pressure
at high sulfur dioxide pressures. The addition of nitrogen, a known
883
quencher of both sulfur dioxide fluorescence and phosphorescence,
did not eliminate chemical reaction with either carbon monoxide, per-
1261
fluoroethylene, or ri-butane. The results of Timmons showed that
excess methane, another known quencher of both sulfur dioxide fluor-
883
escence and phosphorescence, did not eliminate chemical reaction.
873
Finally, some recent experiments of McQuigg and Allen have shown
that sulfur dioxide is removed when irradiated at 3130A in the presence of
-3
oxygen. The quantum yield of sulfur dioxide disappearance was 7 x 10 ,
independent of reactant pressures (oxygen, 20-400 torr; sulfur dioxide,
20-200 torr) or the addition of nitrogen or carbon dioxide.
It is clear that the electronic states predominantly involved in
chemical reaction when sulfur dioxide is irradiated at 3130A must be
different from the emitting states. Furthermore, results with, small
amounts of nitric oxide or biacetyl (two known triplet quenchers) added,
show that the reaction is inhibited, but not eliminated. Thus, two (or
more) new states must be involved, one a singlet and one a triplet.
225
Cehelnik ^t al. proposed a mechanism consistent with the known
facts. For pure sulfur dioxide, the mechanism envisioned was
S02 + hv(3130A) -> 1S02; Rate = Ia
1S02 + S02 -»• 3S02 + S02, (26b)
1S02 + S02 -»• S02* + S02; (26c)
3S02 + S02 -> 2S02, (29a)
210
-------
3S02 + S02 •*• S03 + SO; (29b)
S02* -* S02**; (30)
S02* + S02 + S02** + S02; (31)
S02** -> S02; (32)
1 3
where SO and SO are, respectively, the singlet and triplet
2 2
states that emit radiation, and SO * and SO ** are, respectively,
2 2
the singlet and triplet states that do not emit radiation. In the above
mechanism, Eq. 26a of the previous mechanism has been replaced by
Eq. 26c. Equations 27 and 28 have been omitted, because they are
unimportant at atmospheric pressure.
With carbon monoxide present, the additional reactions needed
are
1S02 + CO -»• S02* + CO, (33a)
1S02 + CO -> SO + C02; (33b)
S02* + CO -»• SO + C02; (34)
S02** + CO ->- SO + C02; (35)
3S02 + CO ->• S02 + CO, (36a)
, 3S02 + CO -»• SO + C02. (36b)
211
-------
With hydrocarbons, the mechanism is not yet known. However,
both nitrogen and oxygen are relatively insensitive quenchers of SO *
2
and SO **. Thus, the reactions of these states with hydrocarbons and
2
the oxides of nitrogen may be important in the atmosphere, especially
if they are needed only as initiation reactions for aerosol formation.
It is now necessary to relate the four photochemical states of
sulfur dioxide with the known spectroscopic states. The lowest excited
3
state of sulfur dioxide is generally agreed to be B . This state is
1
590
responsible for the weak absorption at 3900-3400A. It is also the
3
state that phosphoresces and is therefore the state designated SO
2
65,1176
The phosphorescence is readily quenched by hydrocarbons.
If the quenching is chemical, rather than physical, the quenching reactions
may play a role in the atmosphere. The second absorption band at
3400-2600A is much stronger and is probably the corresponding singlet
1
state, B .
1
One difficulty in the sulfur dioxide system is that the fluorescent
lifetime of the excited singlet state is about 70 times greater than that
517,1232
computed from the integrated absorption coefficient. This
347
anomaly has been discussed by Douglas, who has ascribed the main
effect to the mixing of the vibrational levels of the absorbing state with
1338
those of other electronic states. Walsh has suggested that an
1
optically forbidden A state should exist. Such a state has been ob-
2 - 1233
served in the electronically similar NO ion. Presumably, the
11 2 1
B and A states are strongly coupled, and it may be that B is
121 1
the absorbing state, whereas A is the emitting state.
2
212
-------
1 3
The A state should have a triplet state, A , that can be
2 2
139a
formed by intersystern crossing. Brand et aL have suggested
3
that this triplet state lies about 2. 5-3 kcal/mole above B , and this
1
would be SO **.
2
213
-------
TABLE 4-1
Reactivities of Some HydrocarbonsT" Identified
_b
In Urban Air Sampling
Reactivity
Relative Rate Constant, per mole-sec
Compound*
Paraffins
Methane
Ethane
Propane
Butane
Isobutane
n-Pentane
Isopentane
2-Methylpentane
3-Methylpentane
Cyclohexane
2 , 3-Dimethylpentane
Isooctane
Aromatics
Benzene
Toluene
o-Xylene
m-Xylene
£-Xylene
m-Ethyltoluene
1,2, 4-Tr imethylbenzene
1,3, 5-Tr imethylbenzene
Based on
Hydrocarbons
c
o.o-
__
__
__
—
0.002s-
0.002s-
0.024£
0.012s"
0.004-
—
0.30s-
1
Oo30~
--
Oo56^
c n
0.152-,0.21-
—
—
__
Based on Nitric kQ
6.2 x 10~7~
2.9 x KT5-
— 0.0072-
o.oioo-
— __
— —
0.050-
i —
lo *• j ** o H — ^
kOH
0.00016-
0.0047-
f
0.022-
0.049^,0.063^
f
0.034-
— '
—
—
00147^
0.138"
0.072-
~
Oo29^
—
• —
__
214
-------
TABLE 4-1: CONTINUED
Compound*
Aromatics
g-Methylstyrene
Olefins
Acetylene
Ethylene
Propylene
1-Butene
cis-2-Butene
trans-2-Butene
Isobutene
1,3-Butadlene
1-Pentene
cis~2-Pentene
trans-2-Pentene
2-Methyl-l-butene
2-Methyl-2-butene
3-Methyl-l-butene
2-Methyl-l,3-butadlene
Cyclopentene
1-Hexene
trans-2-Hexene
cis-3-Hexene
trans-3-Hexene
2-Methyl-l-pentene
Reactivity
Relative Rate Constant, per mole-sec
Based on
Hydrocarbons
„„
0. 045s-, 0.1-
0.57^,0.73^
0.44s-
2.43^,4.2^
4.05s-
1.0
0.80s-
0.50s-
• —
—
2.70s-
—
1.5*
—
—
—
3.5s
Based on Nitric
Oxide Oxidation
m
2.94-
o.ii
0.4*. 0.492,0.545
!-,!.<£
0.83^
2.0-
d 9^.
1.0
1.232
0.60*
•1.54*
2.2*
0.972
5.4£
0.77*
Io05*
e.e2-
00492
1.71-
^^^^
ko
—
0.0095^
0.0493',0.029i
0.191
0.253-
0.95^-
k
1.13-
1.0
k
—
k
0.90- ->
— _
3.17^
~
__
1.19^
0.26^
—
_
kOH
—
0.00292-
0.0795-
0,2-T
i
0.6CT
0.94-
i
1.1-
1.0
_
—
!.4i
__
1.84-
— —
__
__
_—
—
0,66
215
-------
TABLE 4-1: CONTINUED
Reactivity Relative Rate Constant, per mole-sec
Compound*
Olefins
cis-4-Methyl-2-pentene
trans-4-Methyl-2-pentene
Aldehydes
Formaldehyde
Acetaldehyde
Acrolein
Propionaldehyde
Butyraldehyde
Cro tonald ehyde
Benzaldehyde
£-Tolualdehyde — 6006r- — --
m-Tolualdehyde — 0.23s
£-Tolualdehyde — Ool8~
a
— The compounds reported to be present in urban air (J. Garner, personal communication),
b
— Based on a relative reactivity of 1.0 for isobutene.
- Derived from Schuck and Doyle.lll|2b
— Derived from Herron;583 based on k = 4.0 x 109 Mf1 sec"1 for l-C^Hg.
e _ . . _ _ . 519
Based on
Hydrocarbons
—
—
—
—
—
—
^«K
Based on Nitric kQ
fhrlHo Ov-MaMnn
0.922- —
10282
m t
Oo80~ 0.023-
1.34s 0.025-
Oo86~
m
2.26-
m
Io48~
1.20s
0.15s
kOH
•VIH
0.24-
0.24-
__
i
0.47- .
—
—
^MW
— Derived from Greiner,513 assuming relative rate coefficient^ 0.022 for C3H8<>
Derived from Greiner0518
Derived from Saunders and Heicklen, 1125 assuming relative k = 0.25 for
~ Derived from Herron and Huie;587 based on k = 4.0 x 109 M"1 sec"1 for l-ClfHg58-6l8o
- Derived from Morris and Niki»913
216
-------
TABLE 4-1: CONCLUDED
Derived from Altshuller and Cohen.25
k
~~ Derived from Cvetanovid.290
"~ Derived from Kopczynski,729 assuming C,H reactivity = 0.15.
m - - [-
"~ Derived from Dimitriades and Wesson, 3;5D assuming C H reactivity = 1.0.
3 6
~ Derived from Stephens and Scott.1222
~" Derived from Breen and Glass.11*5
Derived from Glasson and Tuesday.489
Derived from Atkinson and Cvetanovid.5^
~~ Derived from Morris et al.914
o
~~ Derived from Greiner.520
— Derived from Morris and Niki,912 assuming relative k = 0.025 for CH CHO.
217
-------
Figure Captions
FIGURE 4-1: Typical concentration changes during ultraviolet irradiation
of a propylene-nitric oxide mixture in air. PAN, peroxyacetylnitrate.
108
Reprinted with permission from Berry and Lehman.
FIGURE 4-2: Oxidant-dosage reactivity of exhaust as a function of nitrogen
oxide (NO ) concentration at various hydrocarbon concentrations (HC).
x 331b
Reprinted from Dimitriades.
FIGURE 4-3: Oxidant-dosage reactivity of exhaust as a function of nitrogen
oxide (NO ) concentration at various hydrocarbon: nitrogen oxide concen-
x 331b
tration ratios. Reprinted from Dimitriades.
218
-------
N>
t->
VD
NITROGEN DIOXIDE
100 150
irradiation time (min)
200
-------
160
O.I
Fig. 4-2
220
-------
160
141)
120
100
MC
NO,
«
g
CO
,10
5.5
O.Z 0.4 0.6 O.8 1.0
NO, . ppm
1.2
1.4 1.6
Fig. 4-3
221
-------
CHAPTER 5
METABOLISM OF VAPOR-PHASE ORGANIC POLLUTANTS IN
MAMMALIAN SYSTEMS
The metabolic fates of many of the substances identified as vapor—
phase organic pollutants have not been investigated, but their biologic
transformation sequences should closely parallel the metabolism of the
various functional groups to be discussed in this chapter, in -which the
emphasis is on biotransformation sequences in terms of chemical func-
tionality.
Mammals convert nonpolar xenobiotic substances by enzymatic oxi-
dation, reduction, hydrolysis, or hydration into more polar materials
that are then easily excreted either by the kidney or by the intestine via
the bile. These metabolic reactions occur principally in the liver and
to a lesser extent in the kidney, intestine, and lung. The mode of
metabolism and excretion differs widely among species. In general,
however, such compounds as hydrocarbons, aldehydes, ketones, ethers,
epoxides, esters, amides, tertiary amines, sulfides, and nitrogen com-
pounds are enzymatically converted to more polar compounds--!, e. ,
phenols, vic-glycols, acids, primary and secondary amines, sulfoxides,
and sulfones. These polar metabolites are then excreted directly or
after conjugation with glucuronic acid, sulfuric acid, glycine, or glutathione.
Epoxides, reactive alkyl and aryl halides, a, g-unsaturated carbonyl
compounds, arylhydroxylamines, and some nitrated aromatics are metab-
olized by conjugation with the thiol group of glutathione. Before excretion,
the peptide residue in the glutathione conjugate is converted to N-acetylcysteine.
Compounds with a hydroxyl or amino function--such as phenols, alcohols,
vic-glycols, amines, and hydroxylamines--are usually conjugated -with the
4-hydroxy group of glucuronic acid to form glucuronides or with sulfuric
acid to form sulfates or sulfamates. The aromatic dihydrodiols, a special
222
-------
class of vic_-diols probably produced by enzymatic hydration of intermediate
arene oxides, may be excreted directly or first conjugated with glucuronic or
sulfuric acid before excretion. Esterases and amidases convert esters
and amides to acids, which are then often conjugated with glycine to yield
hippuric acids. Carbonyl and nitrogen compounds are often reduced to
alcohols or amines. In the case of volatile organic compounds, a major
portion is often exhaled unchanged. Reviews of the metabolism and toxi-
cology of organic compounds, including many known vapor-phase organic
158, 168,473
pollutants, are available.
Although little seems to be known about the metabolism of a number
of highly reactive volatile air pollutants formed in the atmosphere--e. g. ,
peroxides, peroxy acids, and ozonides (Chapter 3)--many of these com-
pounds exhibit potent biologic effects (Chapter 6).
Initial oxidative metabolism of many foreign substances by mammals
occurs primarily in the liver and is catalyzed by membrane-bound enzymes.
The reaction incorporates one atom of the oxygen molecule into the organic
substrate, whereas the other atom is concomitantly reduced to water.
These monoxygenases thus require reducing equivalents in the form of
reduced nicotinamide adenine dinucleotide phosphate. This metabolic
system of oxidizing and reducing enzymes is frequently studied in hepatic
membrane preparations referred to as microsomes. At least two general
groups of monoxygenases have been identified in liver microsomes,. The
concentrations of the normal hepatic enzymes, referred to as cytochrome
P-450, are greatly increased by pretreatment of animals with phenobarbital
158,473
or related compounds. Spectrally distinct enzymes, referred to
as cytochrome P-448, appear in liver after pretreatment of animals with
158,473
polycyclic hydrocarbons. Both the extent of enzyme induction
223
-------
by such agents and basal concentrations of these enzymes vary greatly
259,793, 1316
in different species and in individual humans. Cytochrome
P-450 and cytochrome P-448 catalyze such reactions as aliphatic
hydroxylation, olefinic epoxidation, conversion of aromatic compounds
to arene oxides and phenols, oxidation of sulfides to sulfoxides and
sulfones, and oxidative dealkylation of amines and ethers. Because
cytochrome P-450 and cytochrome P-448 have different substrate
816
specificities, exposure of animals or humans to drugs, polycyclic
hydrocarbons, or other xenobiotic substances can markedly alter both
the quantitative and the qualitative aspects of later organic substrate
metabolism. Concomitant interaction of more than one xenobiotic sub-
stance -with the hepatic enzyme system can result in altered metabolism
of either substance. This chapter attempts to delineate representative
metabolism of vapor-phase organic air pollutants. Many of the studies
cited here have been performed with in vitro microsomal systems;
thus, it should be stressed that in vivo metabolism can be very different,
both quantitatively and qualitatively, from that observed in vitro. This
difference often depends on the route, degree, and duration of administra-
tion of a substance.
The preceding discussion has stressed some of the complex factors
pertinent to an evaluation of the pathway and extent of metabolism of
vapor-phase organic air pollutants in man and lower animals. A recent
comprehensive treatment of the metabolism of xenobiotic substances is
158 1316
available, as is a monograph on drug metabolism in man.
224
-------
HYDROCARBONS
Alkanes
Saturated hydrocarbons are metabolised both in vitro and in vivo
to alcohols,, which may be further metabolized to ketones, aldehydes,
and acids. Vapor-phase air pollutants of this type include methane,
ethane, propane, n-butane, isobutane, ri-pentane, isopentane, cyclo-
pentane, ri-hexane, 2-methylpentane, 3-methylpentane, 2, 2-dimethylbutane,
cyclohexane, methylcyclopentane, 2-methylhexane, methylcyclohexane,
2, 2, 4-trimethylpentane, and ri-octane.
The metabolism of n-butane, isobutane, n-pentane, isopentane,
cyclohexane, and methylcyclohexane has been investigated with hepatic
459
microsomes from a variety of rodents. The major metabolites from
ri-butane and n-pentane are 2-butanol and 2-pentanol, respectively.
3-Pentanol is also a significant metabolite from ri-pentane. n-Butane,
ri-pentane, and isobutane form barely detectable amounts of the primary
459
alcohol. This is in contrast with the in vitro results obtained with
306, 625, 763, 849
hexane, octane, decane, and hexadecane, for which
oxidation of the terminal methyl groups is a significant pathway. Iso-
pentane leads to all the possible alcohols, with the tertiary alcohol,
625 459
2-m ethyl -2-butanol, as the major metabolite. It-was concluded
that, in acyclic aliphatic hydrocarbons, the rate of hydroxylation at
different positions •was tertiary CH > secondary CH > primary CH
2 3
Cyclohexane is metabolized by hepatic microsomes in vitro to cyclo-
1284
hexanol, whereas cyclohexanol and trans- 1, 2-dihydroxycyclohexane
are the major urinary metabolites in vivo in ratios from 3:1 to 8:1 at
372a
different dosages. Pretreatment of rats with phenobarbital enhances
microsomal metabolism of cyclohexane, and pretreatment with a polycyclic
1284
hydrocarbon retards metabolism.
225
-------
Metabolism of methylcyclohexane in vitro by rodent microsomes
yields the following products: 3-hydroxy- 1 -methylcyclohexane > 4-
o.
hydroxy-1-methyl-cyclohexane = 1-hydroxy-1-methylcyclohexane
459
> Z-hydroxy-1-methyl-cyclohexane >> 1-hydroxymethylcyclohexane.
The in vivo metabolism of methylcyclohexane in rats is similar, except
that the 1-hydroxy-1-methyl and 1-hydroxy-methyl metabolites have not
390
been reported.
Alkenes
Olefinic compounds may be oxidized at saturated positions to yield
alkenols. However, the major metabolism of such compounds in vitro
is conversion to vic-glycols via intermediate epoxides. The epoxidation
is catalyzed by the hepatic monoxygenase system; the resulting epoxide
is converted to a vic-glycol by a trans addition of the elements of water
catalyzed by microsomal epoxide hydrase. A variety of olefins have been
reported as air pollutants, including ethylene, propylene, 1-butene, cis-
and trans-2-butene, isobutene, 1, 3-butadiene, 1-pentene, cis- and trans -
2-pentene, 2-methyl-1-butene, 2-methyl-2-butene, 3-methyl-1-butene,
2-methyl-1, 3-butadiene, cyclopentene, 1-hexene, cis- and trans-2-hexene,
cis - and trans-3-hexene, 2-methyl-1-pentene, 4-methyl-1-pentene, and
4-methyl-2-pentene. The aromatic olefin, styrene, has also been detected
as a pollutant.
1, 3-Butadiene has been reported to be metabolized to 3-butene-l, 2-
diol and erythritol with rat liver microsomes, presumably via intermediate
589
epoxides. The same products are formed from 1, 2-epoxy-3-butene.
1-Octene and trans-4-octene are converted to the corresponding epoxides
842
with liver microsomes. The intermediate epoxides are further
metabolized by the microsomal epoxide hydrase(s) to the corresponding
226
-------
842
vic-glycols. 3-Ethyl-2-pentene is also converted to the vic-glycol.
785
Styrene is converted with microsomal preparations to styrene glycol
786
via intermediate formation of styrene oxide. Volatile cycloalkenes--
such as cyclopentene, cyclohexene, and cycloheptene--are converted to
783
the corresponding trans-diols with microsomal preparations.
Epoxides have proved difficult to isolate as microsomal metabolites,
unless further metabolism to glycols by epoxide hydrase is prevented by
the addition of an inhibitor of the latter enzyme. Structure-activity
correlations of epoxides as substrates or inhibitors of hepatic epoxide
987
hydrase have been extensively studied. Monosubstituted, 1, 1-
disubstituted, and cis- 1, 2-disubstituted oxiranes are much better sub-
strates than trans-1, 2-disubstituted oxiranes, and tri- and tetra-
substituted oxiranes are nearly inactive as substrates. An alternate
metabolism of intermediate epoxides is conjugation with glutathione.
Little i« known of the relative rates of conversion of various substi-
tuted olefins to epoxides with the microsomal system. The rates of
conversion of three olefins to vic-glycols are 1-octene > trans-4-
842
octene > 3-ethyl-2-pentene. However, the oxides corresponding
to the latter two olefins are increasingly less active as substrates for
842, 987
epoxide hydrase.
In vivo metabolism of simple alkenes does not appear to have been
investigated. In vivo, styrene is converted to benzoic and mandelic
376
acids, in addition to styrene glycol. A trace amount of 4-vinylphenol
75
has been detected as a metabolite in rat.
Acetylenic and allenic compounds--such as acetylene, methylacetylene,
and propadlene--have been reported as organic air pollutants. Little is
known of the metabolism of such compounds. Phenylacetylene is metabolized
786
in vivo to phenylacetic acid.
227
-------
Arenes^
Aromatic hydrocarbons are metabolized both in vitro and in vivo
to arene oxides, -which isomerize to phenols, are enzymatically hydrated
to the so-called dihydrodiols, and are conjugated with glutathione. The
dihydrodiols are further metabolized to catechols by dehydrogenation.
A major route of metabolism of alkyl-substituted arenes consists of
aliphatic hydroxylation of a substituent, usually at the benzylic position.
The route of metabolism of an aromatic hydrocarbon has been firmly
established in the case of naphthalene, where it has been shown that
naphthalene-1, 2-oxide is the obligatory intermediate leading from naphthalene
to 1-naphthol, trans-1, 2-dihydroxy- 1, 2-dihydronaphthalene, and a gluta-
thione conjugate:
st
228
-------
Many such arene oxides are extremely unstable, thus rendering their
655,658,691
direct demonstration difficult in biologic systems. Evi-
dence of their formation is usually indirect. Thus, dihydrodiols,
catechols, and mercapturic acids formed during metabolism of aromatic
substrates are certainly derived from intermediate arene oxides. Phenolic
metabolites may, however, be formed directly from the arene by an
oxygen-insertion reaction similar to that involved in the formation of
alcohols from aliphatic hydrocarbons or may result from formation and
later isomerization of an intermediate arene oxide. Isomerization of
such oxides to phenols occurs with a concomitant migration and retention
135, 655
of isotopic hydrogen similar in magnitude to that observed during
enzymatic phenol formation from the appropriate parent arene. Thus,
*
the occurrence of the NIH shift, an almost ubiquitous phenomenon
654
associated with monoxygenase-catalyzed formation of phenols, provides
strong evidence that formation of a phenolic metabolite involves the inter-
mediacy of the corresponding arene oxide. Arene oxides, such as bromo-
benzene oxide, may react nonenzymatically with macromolecular tissue
161
constituents and thus cause tissue necrosis.
Benzene is a vapor-phase organic pollutant. Metabolism in vivo in
1008
rabbits leads primarily to phenol. Minor in vivo metabolites include
1123
trans-1, 2-dihydro-l, 2-dihydroxybenzene, catechol, hydroquinone,
1008
phenylmercapturic acid and trans-trans-muconic acid. As indicated
* ' 23
The NIH shift is the phenomenon by which the substituent ( H, H, Cl,
or Br) displaced by the entering hydroxy group during enzymatic hydroxyla-
tion of aromatic substrates migrates to an adjacent position in the aromatic
654
ring.
229
-------
earlier, the hydroxylated products may be further converted into,
and excreted as, a variety of conjugates, e. g. , glucuronides and
sulfates. In vitro, benzene is converted to phenol, but neither trans -
1, 2-dihydro-l, 2-dihydroxybenzene nor benzene oxide has been detected
657
as a product.
A variety of alkylbenzenes have been identified as organic air
pollutants, including toluene, ethylbenzene, isopropylbenzene,
n-propylbenzene, sec- and tert-butylbenzene, JD-, m-, and p_-xylene,
1, 2, 4-trimethylbenzene, and mesitylene.
Such alkylbenzenes are metabolized in vivo both to phenols and to
products resulting from side-chain oxidation. Thus, toluene is metabo-
144,1390
lized in vivo primarily to benzoic acid via intermediate forma-
tion of first benzyl alcohol and then benzaldehyde. Both o^-cresol and
£-cresol are also detected as minor in vivo metabolites of toluene.
In vivo, ethylbenzene, n-propylbenzene, and ri-butylbenzene afford side-
376a
chain oxidation products. Initial oxidation appears to occur primarily
at the benzylic position. Ethylbenzene is converted in rats to small amounts
75 75,1192
of 4-ethylphenol, but the major metabolite is methylphenylearbinol.
654
Isopropylbenzene apparently is not converted in vivo to phenolic metabolites,
nor is benzylic hydroxylation to 2-phenyl-2-propanol the major path-way for
75, 135
side-chain oxidation. Instead, 2-phenyl-1-propanol is the major
75, 1079
alcoholic metabolite. Xylenes are metabolized in vivo primarily
143
by oxidation of a methyl group to yield toluic acids. Phenols are also
75
detected as minor metabolites. The trimethylbenzenes are metabolized
75, 1390
to acids and to trace amounts of phenols.
The in vitro metabolism of alkylbenzenes has not been studied extensively.
869
Ethylbenzene yields mainly methylphenylearbinol and small amounts of
230
-------
691
2- and 4-ethylphenol. The phenolic metabolites obtained from toluene,
xyleries, and mesitylene -with rat microsomal preparations have been com-
pared with the phenols obtained on isomerization of the analogous arene
655, 691
oxides. It has been concluded that phenol formation from alkyl-
benzenes is in all instances compatible with the intermediacy of one or
more arene oxides. The proposed intermediate arene oxides are all re-
markably unstable and rearrange rapidly to phenols. Because of their
instability, it is likely that such alkylarene oxides do not survive long
enough to be enzymatically converted to trans-dihydrodiols or to pre-
mercapturic acids. Such metabolites apparently have not been detected
from alkylbenzenes.
Oxidative metabolism at the aromatic carbon atoms of three additional
volatile arenes--aniline, nitrobenzene, and chlorobenzene--should be
mentioned briefly. Aniline is metabolized in vivo to a mixture of c^- and
1008
p_-aminophenol. The proportions of these phenols differ markedly
in different species. Nitrobenzene in rabbits gives rise to m- and p_-
1008
nitrophenol and lesser amounts of ^-nitrophenol. Other metabolites
include aniline and the three aminophenols. The presence of 4-nitrocatechol
and 4-nitrophenylmercapturic acid indicates the intermediacy of 4-nitrobenzene
oxide. Chlorobenzene is metabolized in rabbits to 3-chlorophenol, 4-chlorophenol,
653
and a minor amount of 2-chlorophenol. Apparently, 4-chlorobenzene
oxide is formed as an intermediate, inasmuch as 4-chlorocatechol and its
two mono-0-methyl ethers, the corresponding dihydrodiol, and 4-chlorophenyl-
mercapturic acid are detected as metabolites. 3-Chlorobenzene oxide must
also be formed, as evidenced by the isolation of its hydration product, the
corresponding dihydrodiol (D. M. Jerina and J. W. Daly, unpublished data).
231
-------
EPOXIDES
In general, our knowledge of the metabolic fate of epoxides is de-
rived from a correlation of in vitro studies with the epoxides themselves
and in vivo studies with the precursor olefins. This is due to the chemical
instability of epoxides, particularly in acid conditions like those in the
stomach, and to the relatively recent interest in their biologic role. The
few metabolic studies in which the epoxides themselves have been ad-
ministered by injection confirm the conclusions drawn from the correlated
evidence. Owing to the paucity of data on the metabolism of volatile
epoxides, nonvolatile substances are also discussed here.
Two general pathways have been found for the metabolism and excretion
of epoxides. In one, the epoxide is hydrolyzed to a vicinal trans-diol by
the action of an epoxide hydrase enzyme. The diol is then excreted
directly or as a glucuronic acid conjugate, or it may undergo further
oxidative metabolism. Phenyl-1, 2-ethanediol obtained by the metabolism
of styrene, for example, is not only excreted as such, but is further
376
oxidized to mandelic and benzoic acids. The hydrolysis of epoxides
659
invariably yields the trans-diol and has been found to be stereospecific.
The hydrolysis of cis-stilbene oxide by rabbit liver microsomes, for
example, gives only D-threo-1, 2-diphenylethanediol, whereas only the
1351
meso-diol is obtained from trans -stilbene oxide. The rate of
epoxide hydrolysis depends heavily on the degree of substitution and the
stereochemistry about the epoxide ring. In general, the rate of hydrolysis
decreases in traversing the series mono-, di-, tri-, and tetra-substituted,
whereas cis-disubstituted epoxides are more readily hydrolyzed than the
987,1351
trans-isomers. These effects of substitution on the rate of
hydrolysis are most probably due to steric, rather than electronic, factors.
232
-------
This second major pathway for the metabolism of epoxides involves
enzyme-catalyzed conjugation with glutathione. The sulfhydryl group
of the cysteine residue in glutathione adds to the epoxide, yielding an .
a -hydroxythioether. The resulting conjugate then either is excreted
or undergoes hydrolysis of the glutathione moiety. This cleavage of
129
the glutathione group, which probably occurs in the kidney, leads
to eventual excretion of the cysteine or N-acetylcysteine conjugate.
Little is known about the structure-activity relations of this sequence.
In addition to the two major metabolic pathways summarized above,
a number of alternative biologic fates have been identified for some
epoxides. Of these, the most important is the nonenzymatic intra-
molecular rearrangement of epoxides to give the corresponding carbonyl
compounds. This transformation is observed with epoxides that are
particularly unstable. Two examples are the rearrangement of
656
1, 2-oxido-l, 2-dihydronaphthalene to give 1-naphthol and the probable
298
conversion of trichloroethylene via its epoxide to trichloroacetaldehyde.
The enzyme fumarase from swine heart, -which normally catalyzes
the addition of water to the double bond of fumarate, also catalyzes the
15
hydrolysis of trans-2, 3-epoxysuccinate to give meso-tartrate. Because
similar reactions have been observed with enzymes from nonmammalian
sources, it is possible that biosynthetic enzymes that catalyze the hydration
of double bonds will also catalyze the hydrolysis of the corresponding
epoxides.
The enzyme 2, 3-oxidosqualene-lanosterol cyclase from liver has been
found to cyclize 2, 3-oxidosqualene analogues, in some cases competing
265
with the hydrolysis of the epoxides to the diols.
233
-------
Epoxides are sufficiently reactive to undergo purely chemical
alkylation reactions with proteins and nucleic acids under physiologic
conditions. Propylene oxide, for example, alkylates the guanine and
773
adenine residues of DNA on incubation at a pH of 7. Although the
significance of these alkylation reactions in live animals is not clear,
they must be kept in mind in any discussion of the biologic fate of
epoxides.
Epoxides of Four or Fewer Carbons
The metabolic fate of the simplest epoxide, ethylene oxide, is unknown.
It is not eliminated in expired air, however, and its hydrolysis to ethylene
534,1395
glycol is considered likely. Only preliminary data are available
on ethylene, the precursor olefin, but its conversion to carbon dioxide
1313
and urinary metabolites has been reported. Trichloroacetic acid
and the glucuronide of trichloroethanol have been isolated from rat urine
298
after ingestion of trichloroethylene. It was shown that the chlorine
rearranges intramolecularly, suggesting that the corresponding trichloro-
ethylene oxide was an intermediate. This unstable species would rearrange
spontaneously to trichloroacetaldehyde, which then undergoes oxidation or
reduction to the observed products.
1, 2-Oxidobutane administered to rabbits and rats is excreted at least
649
in part as N-acetyl-S-(Z-hydroxybutyl) -L-cysteine. This is probably
derived from initial conjugation with glutathione. Butadiene and
1, 2-oxido-3-butene are converted in vitro to the diol (3-butene-l, 2-diol)
589
and tetraol (erythritol). In both cases, the concentration of the diol
rises and then falls as the erythritol concentration increases, indicating
that the hydrolysis is faster than the oxidation reaction. An early in vivo
study of the metabolism of inhaled butadiene failed to find urinary metabolites
234
-------
but it cannot be relied on, because it preceded the use of radioactive
213
labels. The diepoxide from butadiene is conjugated to glutathione
138
by rat liver preparations. Although no comprehensive study has
been carried out with the various butane epoxides, and no in vivo data
are available, the data suggest that these compounds are readily metab-
olized by the two major pathways.
2, 3-Oxidopropanol (glycidol) is conjugated with glutathione by rat
138
liver preparations, and trichloromethylethylene oxide undergoes both
986 987
• in vitro conjugation to glutathione and slow hydrolysis to the diol.
Epoxides of Five to Eight Carbons
N-Acetyl-S-(2-hydroxycyclopentyl)-L-cysteine can be isolated from
649a
rabbit urine after injection of either cyclopentene or 1, 2-oxidocyclopentane.
The conversion of cyclopentene to 1, 2-cyclopentanediol by rat liver micro-
783
somes has also been noted.
The in vitro oxidation of cyclohexene to 1, 2-oxidocyclohexane and its
782
later hydrolysis to 1, 2-cyclohexanediol has been reported. The iso-
lation of the epoxide in this case is probably due to the relatively slow
987
hydrolysis to the diol. 1, 2-Oxidocyclohexane injected into rabbits
649a
is excreted, in the urine as N-acetyl-S-(2-hydroxycyclohexyl)-L-cysteine.
The probable origin of this product from an initial glutathione conjugate is
supported by the in vitro conjugation of 1, 2-3, 4-dioxidocyclohexane with
138
glutathione.
Cycloheptene and its epoxide injected into rabbits are excreted in the
649a
urine as N-acetyl-S-(2-hydroxycycloheptyl)-L-cysteine.
Liver preparations convert 1-octene, 4-octene, and 3-acetyl-2-pentene,
842
as well as the corresponding epoxides, to diols. The isolation of only
4, 5-oxidooctane instead of the diol when 4-octene was incubated in the
235
-------
presence of 1, 2-oxidooctane as a hydrase inhibitor indicated that the
epoxide was an obligatory intermediate between olefin and diol.
The metabolism of styrene and styrene epoxide has been extensively
studied. Both styrene and styrene epoxide are converted to phenyl-1,
785
2-ethanediol by liver microsomes. The epoxide is also conjugated
138
with glutathione in vitro. The in vivo metabolism of styrene in rabbits
resulted in the formation of phenyl-1, 2-ethanediol (as its glucuronide),
376
mandelic acid, and hippuric acid. The same authors found mandelic
and hippuric acids in rabbit urine after administration of styrene epoxide,
but no diol. Because the epoxide was given orally, however, this result
is ambiguous. The formation of mandelic and hippuric acids from styrene
diol was verified by the isolation of these products as metabolites of the
diol itself. The other products that have been isolated from styrene
1203
metabolism are benzoic acid in rabbits, carbon dioxide arising
300
from the methylene group in rats, and traces of p_-hydroxyphenyl-
75
ethylene in rats. All these are consistent with metabolism via the
epoxide and the diol.
Epoxides of More Than Eight Carbons
786
Indene in vitro gives indan-1, 2-epoxide and the corresponding
784
diol. Indan-1, 2-epoxide is hydrolyzed to the diol by rabbit or rat
784,987 138
liver preparations, but it does not give a glutathione conjugate.
Not surprisingly, indene is excreted in the urine of rabbits and rats as
163
the free glucuronide conjugate indan-1, 2-diol.
1, 2-Dihydronaphthalene is one of the most extensively studied compounds.
Both the parent olefin and its epoxide have been found to give trans-diols on
129
incubation with rat liver slices. The glutathione conjugate is also formed
from these two compounds in the same enzyme system. When the glutathiol
236
-------
conjugate is incubated with rat kidney homogenate, the glutathione group is
hydrolyzed, leaving only the cysteine residue attached at C-l of 1, 2, 3,4-
tetrahydro-2-hydroxynaphthalene. This is one of the strongest pieces of
evidence that the cysteine or N-acetylcysteine conjugates are derived from
initial glutathione conjugates. Both 1, 2-dihydronaphthalene and
1, 2, 3, 4-tetrahydro-l, 2-oxidonaphthalene are excreted in the urine of
137
rabbits in pjirt as the free glucuronide conjugate diol.
The free diol, its glucuronide conjugate, and the glutathione addition product
are all found in the bile of rats that have received either 1, 2-dihydro-
136
naphthalene or the epoxide derived from it. The two general metabolic
pathways for epoxides have therefore been demonstrated in vitro and in vivo
with these substrates.
The formation of epoxides from chlorinated insecticides, such as Aldrin,
and the later metabolism to diols have long been known. An extensive review
164
of the data on these compounds is available.
The presence of uroterpenol glucuronide in human urine as a metabolite
1328
of ingested or inhaled limonene has been established.
The in vitro hydrolysis of epoxides to diols has been demonstrated
for a number of steroids and their precursors. These include squalene
265
analogues), such as 1, l-desmethyl-2, 3-oxidosqualene, 2, 3-oxido-
1352 150
cholestanes, and 16a, 17a -epoxyestratriene-3-ol. The excretion
of this last compound as the glucuronide conjugated diol by humans has
149
been reported. Oleic acid and its epoxide are converted to threo-
1353
dihydroxystearic acid by rat liver micro somes.
237
-------
ETHERS
Ethers are oxidatively metabolized, with the exception of epoxides,
whose reactivity is dominated by the inherent ring strain. Aliphatic
ethers are cleaved to an alcohol and an aldehyde, probably via initial
hydroxylation at the a-carbon to give an unstable hemiacetal thatunder-
158
goes nonenzymatic cleavage. The initially formed alcohol and alde-
hyde are directly excreted or, more commonly, undergo further pxidative,
reductive, and conjugative transformations before elimination. p_-Nitroanisole,
62
for example, is cleaved to p_-nitrophenol and formaldehyde, but the products
primarily excreted in the urine are the corresponding glucuronide and
142
sulfate.
The rate of demethylation for a series of ring-substituted anisoles
is greater for para than for meta substitution, although a simple correlation
between the rate and the electronic properties of the substituent cannot be
62
made. However, it has been clearly shown that the rate of dealkylation
decreases as the size of the alkyl group increases through the series
867,868
ethyl > ri-propyl> isopropyl> ri-butyl > ii-hexyl. Compounds with
chains larger than propyl are often metabolized by side-chain oxidation,
900,1422a
rather than dealkylation.
The cleavage of alkylarylethers has been shown to occur by oxidative
1070
rupture of the alkyl-oxygen, rather than the aryl-oxygen, linkage.
No label appeared in the p_-hydroxyacetanilide formed by enzymic demethylation
18
of the corresponding ether in the presence of 0-enriched oxygen or water.
The enzyme system(s) mediating these dealkylations is primarily in
the liver microsomes and requires oxygen and reduced pyridine
238
-------
62
nucleotide as cofactors. The differential inhibition of codeine and
£-ethoxyacetci.nilide dealkylation by such inhibitors as SKF 525-A*
suggests the presence of more than one enzyme.
A microsomal enzyme in the liver and intestine has been found
1199, 1279a
to catalyze the cleavage of fatty acid glycerylethers. This
enzyme system requires not only oxygen and reduced pyridine nucleo-
tides, but also tetrahydropteridine. Unlike the first enzyme system
discussed, it is capable of efficiently cleaving ethers -with long alkyl
chains.
Most of the evidence on ether metabolism is based on studies of
alkylarylethers. Little is actually known about the specific metabolism
of volatile dialkylethers. There is indirect evidence that these compounds
are metabolised by the path-ways discussed above. Diethylether and related
halogenated ethers, for example, appear to be metabolized to ethanol and
514, 1313
acetaldehyde, which enter the acetate pool of normal body metabolism.
Thus, administration of labeled diethylether results in labeling of steroids
514
and fatty acids.
Cyclic saturated ethers, such as tetrahydrofuran, may be formed from
the interaction of ground-state oxygen and hydrocarbons (see Chapter 3).
The evidence available, although meager, indicates that tetrahydrofurans
are metabolized in the same manner as acyclic ethers. The substituted
tetrahydrofuran obtained from thiamine tetrahydrofurfuryldisulfide, for
example, is oxidized to the corresponding lactone, presumably via a
755a
hemiacetal intermediate.
* 833a
Diethylamino-ethyl-2, 2-diphenylvalerate.
239
-------
METABOLISM OF ALCOHOLS AND PHENOLS
The general pathway for the types of compounds under consideration
is primarily oxidative, in many instances ultimately leading to the forma-
tion of carboxylic acids. The conversion of primary alcohols to aldehydes
and thence to carboxylic acids is a common metabolic sequence in mammals.
In the case of secondary alcohols, oxidative metabolism generates ketones,
which in special cases can be further metabolized oxidatively. However,
such reactions involve carbon-carbon cleavage and are less common. An
alternative pathway is reduction to the original secondary alcohol, -which
itself can be conjugated with glucuronic acid. Conjugation of primary
alcohols is less common (presumably because of greater susceptibility
to the available oxidative path-way), -whereas tertiary alcohols, not susceptible
to oxidative enzymes, are excreted primarily either unchanged or as glucuronide
conjugates. Because of the predominance of oxidative reactions involved in
the metabolism of these types of compounds, this review will start with a
consideration of the molecular species at the lower oxidative state, namely,
the alcohol. Consideration of aldehydes and ketones will follow. We are
concerned only -with vapor-phase organic air pollutants, so our attention
will focus on small molecules (C - C ), which are the more likely
1 5
species to be found in the atmosphere we breathe.
Ethanol
The metabolism of primary alcohols (R-CH OH) has been investigated
2
more extensively than that of any other class of molecules to be considered
in this report, because of man1 s consumption of ethanol. The literature
dealing -with the metabolism of ethanol is far too extensive to be reviewed
565,819,1119
here. Key review articles and books may be consulted. As
240
-------
is the case with primary alcohols in general, the metabolism of ethanol is
dominated by the pathway involving oxidation, in this case to acetaldehyde.
This conversion can be effected by three independent enzyme systems.
Catalase mixed-function oxidase and alcohol dehydrogenase in the presence
698
of hydrogen peroxide converts ethanol to acetaldehyde and water. However,
819
convincing evidence has been presented that, although the catalase pathway
may function in vitro, its role in the net metabolism of ethanol in vivo is only
minor. A second source of enzymatic oxidation of ethanol to acetaldehyde
that has been described is the mixed-function oxidase system of the liver
996
endoplasmic reticulum. However, there is some controversy over
whether contamination of the microsomal preparation, which was used to
demonstrate this conversion -with catalase, was responsible for the observed
oxidation. By far the most important enzyme system responsible.for the
+
oxidation of ethanol is the NAD -dependent oxidoreductase, alcohol dehydro-
1253
genase. This enzyme is probably a collection of enzymes found in many
1333
tissues, as well as in the liver. In addition to ethanol, many primary
866
and secondary alcohols are oxidized by this system; therefore, it is
probably of considerable importance in the metabolism of vapor-phase organic
air pollutants. In addition to the acetaldehyde pathway, several minor routes
for the meta.bolism of ethanol have been described. Sulfate and glucuronide
131,681
conjugates have been found in the urine, and there is some evidence
507
of formation of fatty acid esters. However, these pathways are quanti-
tatively insignificant.
M ethanol
After ethanol, the metabolism of methanol has been examined more
extensively than that of any other aliphatic alcohol, in part to understand
better its toxic effects on mammals. The metabolism of methanol has
241
-------
725
been reviewed. Its primary metabolic pathway, too, is oxidative,'
presumably to formaldehyde, which is then further metabolized to formic
acid and then to carbon dioxide. The role of the oxidoreductase, aldehyde
dehydrogenase, in the oxidation of methanol is not so obvious as with ethanol.
1252
Several workers (for example, Theorell ) have been unable to demon-
strate activity with purified enzyme preparations from horse liver, although
human liver is reported to catalyze the oxidation of methanol at about one-
119
tenth the rate at -which it catalyzes the corresponding oxidation of ethanol.
In contrast to the metabolism of ethanol, catalase apparently can participate
726
in the oxidation of methanol in animal tissues. The possibility that
834
species variations are important has been stressed. Because methanol
is a C-l unit, it is not surprising that it can contribute to the C-l "active
methyl" pool, although the incorporation of methanol into the choline methyl
361
group is not a simple transmethylation process.
Higher Alcohols
It is of interest that such alcohols as isoamyl and ri-butyl alcohol dis-
465
appear much more rapidly than ethanol or methanol from rat blood.
Although this phenomenon may be a consequence of partition coefficients,
the relative initial rates of oxidation of these and several other alcohols
in vitro through the action of horse liver alcohol dehydrogenase are consider-
1408
ably higher than for ethanol. In addition to alcohol dehydrogenase,
catalase has been demonstrated to be an effective catalyst for the oxidation
464
of the propyl alcohols. Presumably, conjugations of higher alcohols,
particularly secondary and tertiary alcohols, can play an important role
681a 511
in the metabolism of these substances. In a recent report, the
metabolism of the a, 3 -unsaturated compound crotyl alcohol has been
694
examined. As is the case in the metabolism of allyl alcohol, crotyl
242
-------
alcohol is excreted in rats as a glutathione derivative, 3-hydroxy-l-
methylpropylmercapturic acid. The evidence suggests that formation
of the mercapturic acid proceeds via the corresponding crotonaldehyde.
Phenols
141
Phenols present in the atmosphere can arise from combustion
256 603
processes, including automobile exhaust and tobacco smoke.
Phenolic substances present in the vapor phase will be principally lower-
molecular-weight materials, such as phenol or the isomeric cresols, which
have boiling points between 180 and 200 C at atmospheric pressure.
Another major source of phenols in the atmosphere is the photochemical
oxidation of the parent hydrocarbons (see Chapter 3), and a third mechanism
by which man is exposed to phenols is the metabolism of aromatic hydro-
carbons found in the atmosphere (see Chapter 2). The mixed-function oxidase
297
system found in both liver and lung tissues has been demonstrated to con-
vert these aromatic hydrocarbons, both in vivo and in vitro, into arene oxides,
which isomerize to phenols.
Although relatively little information is available on the metabolic fate
of the various lower-molecular-weight phenols, it appears from animal
319
studies that they exhibit toxicity of about equal magnitude. Phenol itself
is rapidly absorbed from the stomach (40% in 1 hr) at both acid and alkaline
1129
pH. However, phenol gains access to the body from all sites of ad-
ministration and can reach the circulation even if applied to the intact skin
208 14
In a recent paper, the metabolic fate of C-labeled phenol was
studied in man and in 18 animal species. The conjugation and oxidation of
90a
phenol in vivo were first described by Baumann and Preusse. The
conjugation products of phenol and cresol are now known to be their
1007, lOlOa, 1388 (p. 278)
sulfuric and glucuronic acid esters. Detailed
243
-------
discussions of the sulfate and glucuronide conjugation mechanisms are
1388 (p. 278) . , 1098
given by Williams and Roy. Phenol and the ere sols
are further susceptible to oxidative metabolism, leading to ortho- and
. . 143a, 1388 (p. 297)
para-hydroxylated products. These oxidative metabolites
1388 (p. 278)
are then also transformed into their conjugates. Another
enzymic pathway for phenol and cresols appears to be 0-rmethylation
63,74
mediated by catechol-0-methyltransferase. Although earlier reports
indicated that substantial amounts of phenol and cresols appeared in the urine
1388 (p. 294) 14
in their free state, more recent data using C-labeled material
208, lOlOa
suggest that the conjugated metabolites predominate. Sulfate and
glucuronide conjugation appear to occur with about equal facility, although
208
significant species variation has been reported.
14
In a 24-hr urine collection from a 25-mg/kg dose of [ Cjphenol, total.
excreted, radioactivity accounted for 31-95% in the species- studied. In man,
with a dose of 0. 01 mg/kg, 90% of the excreted radioactivity appeared in the
first 24 hr, 70% as the sulfate and 16% as the glucuronide conjugate, with ,
only trace amounts of the conjugated quinols.
ALDEHYDES
The oxidative metabolism of aldehydes to the corresponding carboxylic
1042
acids can be catalyzed by three groups of enzymes, aldehyde dehydrogenase,
1043 1044
aldehyde oxidase, and xanthine oxidase. In the case of acetal-
. 820, 1102
dehyde, the aldehyde dehydrogenase appears to be the main biologic catalyst.
Chemically, aldehydes are reactive species that are readily oxidized
to the corresponding carboxylic acids and may also undergo condensation
reactions with a variety of nucleophiles. Acetaldehyde and formaldehyde,
in addition to being susceptible to oxidative enzymes, may participate in
biologic condensation reactions. For example, there is good evidence
244
-------
236,708
that acetaldehyde will combine with pyruvic acid in vivo to form acetoin.
311, 1340
Recent studies have indicated that acetaldehyde may also form
tetrahydroisoquinolines through their interaction with catecholamines. A
similar report on the in vitro and in vivo condensation of acetaldehyde with
858
5-methoxytryptamine to form a 3-carboline has also been observed.
It is likely that these condensation pathways are quantitatively minor.
However, it is not at all clear whether they may play an important role in
the overall pharmacology and toxicology of the parent alcohol ingested.
In the case of the volatile a, 8 -unsaturated aldehyde crotonaldehyde,
there is good evidence of the formation of a glutathione adduct involving
135a
a Michael addition across the activated double bond. Oxidation and
511
reduction of the aldehyde moiety may also occur. Presumably, these
metabolic reactions are characteristic of a, 8-unsaturated aldehydes.
Unlike acetaldehyde and higher aldehydes, formaldehyde may be re-
13,726
garded as a normal metabolite in mammals. Thus, it is not sur-
prising that it may undergo a number of potentially important metabolic
593
conversions, including condensations with pyruvic acid and the amino
1391
group of a-amino acids. In general, formaldehyde enters into the
metabolic pool of "one-carbon compounds" and via an intermediate adduct
with tetrahydrofolic acid may be utilized in many physiologic metabolic
106, 120, 360
processes. However, these "synthetic" reactions are pre-
sumably minor, compared with oxidation to formic acid, the principal
1376
metabolic pathway of formaldehyde. In addition to liver involvement,
monkey and human retinal extracts, in the presence of glutathione, will
713
form formic acid from formaldehyde.
245
-------
KETONES
In general, the metabolism of ketones does not involve oxidative
pathways, because the cleavage of carbon-carbon bonds requires that
165
fairly high energy processes be involved. An alternate pathway
often followed involves reduction of the keto group to a secondary alcohol,
which may then be excreted unchanged or undergo conjugation and be ex-
creted as its glucuronide conjugate.
An important exception to this generalization is the metabolic fate
14 1036,1112
of acetone. It has been well established by use of C-labeled acetone
that, at least in small doses, a large percentage of the label appears as
carbon dioxide, which implies the involvement of oxidative pathways.
With larger doses, excretion of unchanged compound via kidneys, lungs,
1011
and even skin predominates. For a summary of metabolic and toxi-
165
cologic data on low-molecular-weight ketones, see Browning.
FATTY ACIDS
The volatile acids considered are the saturated and unsaturated
aliphatic fatty acids containing up to five carbon atoms and having boiling
1388
points ranging from 100 to 200 C.
The major metabolic pathway of organic acids is an oxidative degra-
dation, leading ultimately to carbon dioxide and water. Fatty acid (FA)
metabolism occurs in muscle, adipose tissue, mammary gland, brain,
and liver. In man, FA is converted to carbon dioxide in liver and in
extrahepatic tissues at about the same rate (0. 10 and 0. 12 millimole/min)
and requires oxygen consumption of 52-62 ml/min. The enzymatic capacity
available for oxidation of short-chain FA is always in excess of the measured
458
amount of oxidation of long-chain FA by intact mammalian cells, and the
oxidation of short-chain FA occurs much more rapidly in vivo and in vitro
246
-------
813
than does the oxidation of long-chain substrates. Whereas long-
chain fatty acids are partially incorporated into triglycerides, oral ad-
ministration of short-chain fatty acids has shown them to undergo rapid
oxidation without direct incorporation into triglycerides to any significant
813
extent. Short-chain fatty acids enter the capillaries and are carried
in the portal vein to the liver, which is the major site of transformation of
124,455
FA in higher animals.
Short-chain fatty acids occur in plasma, and formate and acetate have
47,434
been found in human blood.
Formic acid is a special case, owing to its greater acidity (pK , 3.7,
a
compared with 4. 7 for other saturated aliphatic acids) and its greater ease
of oxidation. The principal hazard appears to be that of direct irritant
effect of the acid on skin, eye, and mucous membranes. No evidence of
cumulative toxicity exists, presumably because of the rapid metabolism of
the acid.
Fatty acids are metabolized by oxidative processes that can be summarized
as a-, 3-, and oj-oxidation. The enzyme sequence catalyzing the oxidation
^ 226, 460,714, 1237
of FA has been extensively reviewed.
The process of a-oxidation has been observed in mammalian tissues
and occurs by a-hydroxylation, a stereospecific replacement of an a-H
by a hydroxyl function with an overall retention of configuration, as shown
915
by the preservation or loss of a tritium label in the D or L position at C-2.
2+
In brain tissue and in liver preparations, an enzyme system (requiring Fe
and ascorbic acid and inactivated by ethylenediaminetetraacetate, EDTA)
catalyzes the oxidative decarboxylation of the a-OH acid £0 the acid with
801a,824e
one less carbon atom. Detection of the 2-keto acid suggests that
this is a:n intermediate and that the process occurs by Eq. 2:
247
-------
RCH2COOH —^ RCHOHCOOH —> RCOCOOH -> RCOOH + CO . (2)
2'
The process of w-oxidation is represented in Eq. 3 and occurs
in liver microsomes, requiring reduced triphosphopyridine nucleotide
798
(TPNH) and oxygen:
RCH2OH + H20. (3)
However, the major degradative route for FA metabolism in all
forms of life is the so-called g-oxidation pathway, originally proposed
719
by Knoop to involve oxidation to the 3-keto derivative, followed by
removal of the carboxy-terminal two-carbon fragment as acetate and
repetition of the reaction sequence.
The active form of the substrate for oxidation is the acyl coenzyme A
(acyl-CoA) derivative of the FA, with the intermediates in the oxidation
824
similarly bound to coenzyme A (CoA).
The acyl-CoA synthetase activates the FA to its CoA ester by
catalyzing the reversible Eq. 4:
RCOOH + CoASH >v RCOSCoA + H20. (4)
Acyl-CoA synthetase activity dependent on adenosine triphosphate (ATP) was
1120
found in all tissues, -with the highest concentrations in liver, heart, and kidney.
248
-------
The acyl-CoA derivative then undergoes the sequence of reactions
823
referred to as the "FA oxidase spiral," involving successive
ag-dehydrogenation, hydration, oxidation, and thiolytic cleavage by
CoA to yield the acyl-CoA derivative containing two C atoms fewer than
the original compound. The four steps are shown in Eqs. 5, 6, 7, and
8:
RCH2CH2COSCoA ^ RCH==CHCOSCoA, (5)
RCte^CHCOSCoA ^.RCHOHCH2COSCoA, (6)
RCHOHCH2COSCoA > RCOCH2COSCoA, (7)
RCOCH2COSCoA + CoASH——^ RCOSCoA + CHgCOSCoA. (8)
A general review of the enzymes catalyzing these reactions has been
513
given by Green and Wakil. Acyl dehydrogenases (Eq. 5) specific for
a narrow range of chain length have been isolated; in each case, the un-
saturated product has the trans-configuration. The enoyl hydratase enzyme
1223a
(Eq. 6) appears to be independent of chain length and proceeds with
stereospecificity, giving the L-8-hydroxyacyl CoA derivative. The
g-hydroxyacyl dehydrogenase catalyzing Eq. 7 is independent of chain
length, but specific for the L-configuration of the substrate.
249
-------
Thiolytic cleavage of the 3-ketoacyl-CoA occurs by g-ketothiolase
catalysis (Eq. 8), producing acetyl-CoA, as well as the new acyl-CoA
derivative. When this acyl-CoA product is acetoacetate, S-ketothiolase
cleavage will lead to the formation of two molecules of acetyl-CoA (Eq. 9):
CH3COCH2COSCoA + CoASH ^ > 2 CH-jCOSCoA. (9)
Acetyl-CoA is the form in which fragments then enter the citric
acid (or tricarboxylic acid) cycle, where they undergo complete oxidation
749, (p. 219)
to carbon dioxide and water.
Other pathways for acetoacetate comprise reduction to 3-hydroxybutyrate
) or cleavage to acetone and carbon dioxide; the acetone is then further cleaved
(e.g. , in muscle and other extrahepatic tissues) to acetyl fragments, which
1036
enter the acetyl pool. When plasma FA concentrations are increased,
there is evidence of increased oxidation to carbon dioxide, as well as increased
1212
formation of ketones.
The original 3-oxidation theory has been extended into the " 3-oxidation-
condensation" theory, in -which it is generally agreed that, in higher animals,
cleavage of FA into two-carbon units is followed by condensation of these units
13
into four-carbon (acetoacetate) units. Recent experimental data using C-labeled
826,827,1366
acids have entirely supported this hypothesis.
An odd-numbered FA will ultimately leave a three-carbon chain, yielding
one molecule of propionyl-CoA and one of acetyl-CoA as the final products.
The propionyl-CoA is known to undergo metabolic carboxylation to methyl-
malonyl-CoA (Eq. 10), which rearranges to succinyl-CoA (Eq. 11) and enters
690
the citric acid cycle and is oxidized further:
250
-------
CQOH
CH-CH COSCoA ^CH,-CH-COSCoA (10)
J £. *^
COOH
CH -CH-COSCoA ^ COOHCH CH2COSCoA. (11)
Propionic acid therefore does not accumulate under normal conditions.
An alternative route for propionate metabolism is the dehydrogenation
of propionyl-CoA to acrylyl-CoA, followed by hydration to L-lactoyl-CoA.
345
This undergoes dehydrogenation to pyruvate. Under aerobic conditions,
pyruvate is oxidized in animal cells to acetyl-CoA and carbon dioxide by
the pyruvate dehydrogenase system; under anaerobic conditions--e. g. , in
345
vertebrate muscle--pyruvate is reduced to lactate in a reversible reaction.
875
Little is known about the mode of oxidation of unsaturated fatty acids.
Acrylic acid is oxidized by kidney and liver enzymes. Sodium acrylate fed
to dogs in d.oses up to 1. 5 g underwent complete oxidation in vivo, no acrylate
1388
being excreted in the urine. Crotonic acid is also reported to undergo
716 801
oxidation in rat kidney preparations and with rat liver slices,
giving 8-hydroxybutyrate and acetoacetate. Under the same conditions,
801
acetoacetate was also formed from butyrate. It was postulated that
oxidation of crotonic acid may proceed by a dehydrogenation accompanied
by addition of, for example, phosphate, to form an enol phosphate inter-
mediate (Eq. 12) that would hydrolyze to the keto compound (Eq. 13):
CH3CH=CH-COOH + HO-PO(OH)2 > CH3-C=CH-COOH -I- 2H, (12)
OPO(OH)2
251
-------
CH3-C=CH-COOH + H20 > CI^COCH^COOH + H3P04. (13)
OPO(OH)
LACTONES
Some photochemical transformations (see Chapter 3) have indicated
the possible atmospheric formation of lactones. These very reactive sub-
stances may be regarded as internal esters that are readily transformed
into the corresponding hydroxy acids by lactone-hydrolyzing enzymes found
157a,755a, 1409a
in mammalian liver and kidney tissue. For most lactones
(except the B-lactones containing a strained four-membered ring), this
755a
hydrolysis is reversible. The hydroxy acids will then be further
metabolized by the routes indicated above for alcohols and carboxylic acids.
However, a second metabolic path-way may involve the interaction of lactones
with biologic amines, leading either to an amino acid or to the amide of a
hydroxy acid, and this ability to combine with cellular constituents could
634
cause toxic effects if the compounds are ingested.
ORGANIC COMPOUNDS CONTAINING NITROGEN
The principal nitrogen-containing compounds to be covered in this
section arise from the photochemical interaction of airborne hydrocarbons
and nitrogen oxides (Chapter 4). They include peroxyacyl nitrates like
peroxyacetylnitrate and may include alkyl and acyl nitrites and nitrates.
The literature contains little information on the mammalian absorption,
252
-------
fate, and excretion of the specific compounds present in air as pollutants.
However, some studies concerning the metabolic disposition and fate of
compounds closely related to those present as air pollutants are discussed
below.
Peroxyacyl Nitrates
From the toxicologic standpoint the most important of the nitrogen-
containing pollutants are the peroxyacyl nitrates, of which peroxyacetylnitrate
356,1250
(PAN) is the most widely studied. The fate of PAN and its relatives
has not yet been studied'in intact mammals. Because PAN is an important
1250
phytotoxic air pollutant, some studies of its biochemical properties
have been reported. Most importantly, PAN is known to react readily with
922, 924,925
sulfhydryl groups. For example, the reaction of PAN with
reduced glutathione produces a mixture of oxidized glutathione and S-acetyl-
922,925
glutathione. The fate of PAN during sulfhydryl oxidation has not
been reported. During the oxidation of olefins, however, PAN is converted
305
to a mixture of methylnitrate and nitromethane. In contrast, the base-
catalyzed decomposition of PAN yields acetate ion, nitrate ion, and molecular
1210
oxygen. Further studies will be required to assess the role of these
reactions in the in vivo metabolism of PAN and related compounds in mammals.
Such studies are urgently needed.
Alkylnitrate Esters
Like amylnitrite (a coronary vasodilator), alkylnitrite esters present
as air pollutants could enter the body by inhalation and be rapidly absorbed
from the lungs. Amylnitrite is destroyed by gastric juice, so unchanged
compound does not enter the body orally.
253
-------
The fate of amylnitrite, once absorbed, appears to be a straightforward
1392
hydrolysis to amyl alcohol and nitrite ion. It is currently generally
thought that amylnitrite exerts its pharmacologic effect through the intact
968
molecule, rather than by virtue of its hydrolysis to nitrite. Nickerson
has recently discussed this point and other aspects of the pharmacology of
amylnitrite.
The nature of the in vivo hydrolysis of alkylnitrite esters is unknown.
For example, it is apparently not known whether the reaction is catalyzed
by an enzyme or is a nonenzymatic process. In vitro studies with tissue
homogenates are needed to clarify this point.
Finally, there is the question of the fate of the nitrite ion formed
by hydrolysis. Studies as early as 1893 indicated that nitrite is largely
781 582
oxidized to nitrate in the intact animal. Heppel and Porterfield
have studied this reaction in some detail and found that hydrogen peroxide
serves as the oxidant and that the reaction depends on catalase. Thus,
nitrite was readily oxidized to nitrate in the presence of hydrogen peroxide-
producing enzymes, such as D-aminooxidase, when catalase was added.
Nitrite ion is also known to react with hemoglobin to produce methemoglobin.
Alkylnitrate Esters
No studies have been reported on the fate of nitrate esters of mono-
hydric alcohols, i. e. , nitrate esters of the type that would occur as air
pollutants. However, nitrate esters of polyhydric alcohols (used as coronary
vasodilators) have been extensively studied. Their pharmacology has been
968 46 802
reviewed by Nickerson and Angelakos, and Litchfield has recently
summarized the various aspects of the metabolism of nitrate esters of poly-
hydric alcohols.
254
-------
Glycerol trinitrate, "nitroglycerin, " is reasonably volatile at room
temperature. It is customarily administered sublingually and is efficiently
absorbed from the mucous membranes of the mouth. When given orally to
man, it is ra.ther ineffective, either because of poor absorption from the
1072
gut or because of destruction in the gut. In the rat, however,
14 327
[ Cjnitroglycerin given orally was found to be well absorbed.
Nitroglycerin is also readily absorbed through unbroken skin and from the
lung. This point is of considerable toxicologic importance in the case of
workers in explosive plants. These data suggest strongly that any volatile
nitrate esters present as atmospheric pollutants would readily enter the
body via skin, lungs, and oral mucosa.
No thorough study of the distribution of intact alkyInitrate esters
into body tissues has yet been reported. Some data from radiocarbon
labeling studies are available. However, because only radiocarbon con-
centrations 'were measured, it is not clear whether intact nitrate ester
was being measured or whether only metabolites were present.
Once absorbed, nitroglycerin is rapidly metabolized by loss of the
327, 954
nitrate ester groups. The initial metabolite is a mixture of
glycerol-1, 2-dinitrate and glycerol-1, 3-dinitrate. There is no apparent
initial preference for the primary or secondary ester. The ultimate
metabolites are glycerol-1-nitrate and glycerol and its degradation products.
The partially cleared esters, as well as glycerol itself, are excreted in
278
urine. Recently, Crew and co-workers have shown that the mononitrate,
dinitrate, and trinitrate esters of pentaerythritol are excreted as glucuronides
1183
in the rat. Furthermore, Sisenwine and Ruelius found that 5-isosorbide
mononitrate glucuronide undergoes in vivo denitration to isosorbide glucuronide.
1067
Reed and co-workers have also discussed the glucuronides of isosorbide
mononitrate.
255
-------
The formation of glucuronides of partially nitrated polyols is of some
interest, because it has been speculated that 3-hydroxyalkyl nitrates may
be present as air pollutants (cf. Chapter 4). The metabolism of two such
compounds, ethyleneglycol mononitrate and propyleneglycol mononitrate,
238,239
has been investigated. Both disappear rapidly from the body,
but no attempt has been made to identify glucuronides as possible metabolites.
The metabolic reaction involved in the denitration of alkylnitrates is
not a simple ester hydrolysis, in that the product is nitrite ion, rather than
46, 277, 1392
the expected nitrate:
RON02 ^-ROH + N02 . (14)
The nature of the reductive cleavage of alkylnitrate esters has been the
subject of a relatively large number of investigations. In 1940, Krantz
748
and co-workers proposed that nitrate esters are reduced to nitrite
esters, which in turn undergo facile hydrolysis to carbinol and nitrite
ion:
-4-9H HoO
RON02 > RONO -=-—^ ROH + N02~. (15)
985
Oberst and Snyder later demonstrated that incubation of nitroglycerin
with rabbit liver homogenates at 37 C and a pH of 8. 4 resulted in a rapid
581
release of nitrite ion. Heppel and Hilmoe observed that nitrate esters
reacted with reduced glutathione to produce oxidized glutathione and nitrite
ion. The reaction was catalyzed by liver homogenate. The enzymatic
256
-------
activity responsible for the reaction is localized in the soluble fraction
953, 954
of liver and has been referred to as glutathi one-organic nitrate
953
reductase. Reduced nicotinamide adenine dinucleotide phosphate
(NADPH) is required, to maintain a pool of reduced glutathione, a step
mediated by g;lutathione reductase. The rate of disappearance of NADPH
953
can be used as a measure of glutathione-organic nitrate reductase.
The overall reactions are thus:
2NADPH + GSSG ^ 2NADP+ + 2GSH; (16)
2GSH + RON02 > GSSG + H20 + RONO; (17)
RONO + H20 > ROH + HN02; (18)
2NADPH + RON02 *> 2NADP+ + ROH + HN02. (19)
Pretreatment of rats with phenobarbital stimulated the metabolism
954
of nitrate esters in the in vitro system. The effect may be a con-
sequence of an increase in glutathione reductase. In any event, later
768,952
studies indicate that tolerance to nitroglycerin is not due to
increased in vivo metabolism. Considerably more -work will be required
before the netture of the enzyme glutathione-organic nitrate reductase
(enzyme classification 1. 8. 6. 1--glutathione: polyolnitrate oxidoreductase)
is completely understood. In particular, it has not been demonstrated that
the organic nitrites are indeed true reaction intermediates. This may be
revealed as part of a substrate structure-activity study, -which should be
pursued and which should involve nitrate esters of simple aliphatic monohydric
carbinols.
257
-------
Alkylnitrate esters are also known to be readily degraded to nitrate
328
ion by blood serum. The reaction in rat serum was investigated in
some detail recently and found to require neither NADPH nor glutathione.
The reaction is nevertheless considered to be enzymatic. It is possible
that, in the intact animal, degradation in the serum is of greater importance
than in the liver.
Acylnitrates
Another class of compounds found are the anhydrides of carboxylic
acids and nitric acid. Acetylnitrate is a well-known example of this class.
Acetylnitrate is a colorless hydroscopic liquid that readily hydrolyzes to
1024
acetic acid and nitric acid. Nothing is known concerning its fate in
mammals. It is reasonable to suppose, however, that it would be readily
absorbed by humans and would undergo in vivo breakdown. Whether it
would be directly hydrolyzed to acetate and nitrate or would undergo
reductase cleavage to acetate and nitrite is open to conjecture.
Other Compounds Containing Nitrogen
In addition to the classes of compounds discussed above, other
nitrogen compounds occur in the atmosphere, particularly in some industrial
locations. These include such classes of organic solvents as aliphatic and
aromatic nitro compounds and aliphatic and aromatic amines. Compounds
of these classes are also thought to be formed photochemically in the
atmosphere in minimal concentrations. Aromatic nitro compounds are
485,886
characteristically metabolized in mammals by reductive mechanisms.
Among the products formed are aryl hydroxylamines and aromatic nitroso
compounds. These metabolites are thought to cause the methemoglobinemia
258
-------
seen after ingestion of some nitroaromati.es. The metabolism of the nitro-
paraffins has not been fully studied. The major route of metabolism, how-
920, 1388
ever, appears to be oxidative cleavage to aldehyde and nitrite.
The reactions by which amines are metabolized depend on the chemical
nature of the amine, i. e. , whether the amines are primary, secondary, or
tertiary and whether they are aromatic or aliphatic. Low-molecular-weight
aliphatic primary amines, as well as benzylamines and 3-phenethylamines,
are metabolized to aldehydes and ammonia by the soluble enzyme, monoamine
1433
oxidase. More lipid-soluble amines and those which are a-branched
1010
appear to be oxidatively deaminated by the hepatic microsomal system.
An interesting recent observation has been that ketoximes are formed in
920
the course of microsomal deamination of a-branched primary amines.
The oxidative dealkylation of tertiary and secondary amines to aldehydes
509,867
and dealkylated amines has been extensively reviewed. This reac-
tion, which appears to be catalyzed by hepatic cytochrome P-450, is a key
reaction in the inactivation of ingested amines. In contrast with aliphatic
amines, aromatic amines, such as aniline, undergo N-hydroxylation in the
1282
body. This route of metabolism is very important, because the
N-hydroxy derivatives of aromatic amines have been implicated not only
893, 1283
in methemoglobin formation, but in carcinogenesis and mutagenesis.
There is no convincing evidence of the occurrence of N-nitrosamines
in^the atmosphere. However, because they are considered to be potential
carcinogens, a systematic search for them will be necessary. Nitrosamines
are thought to be metabolized in vivo to diazo compounds, which in turn form
covalently bonded structures with macromolecules. The carcinogenic process
893
could then be initiated by these altered cell constituents.
259
-------
ORGANIC COMPOUNDS CONTAINING SULFUR
Pollution of the atmosphere by various sulfur compounds has received
8, 699, 1083
a great deal of attention in recent years.
A reading of recent reviews and a close search of the literature indicate
that information concerning volatile organic sulfur compounds in the atmosphere
699
is very limited. Thus, although a recent review by Kellogg e_t al. states
that about 95% of emissions of sulfur to the atmosphere are in the form of
"sulfur dioxide," other evidence suggests the ubiquitous occurrence of
813a
dimethylsulfide in the atmosphere.
Many organic sulfur compounds affect mammalian systems and have
1014
been discussed from the toxicologic viewpoint. Although these compounds
are not expected to be formed by any of the mechanisms discussed in Chapter
4, a knowledge of their metabolic transformations should be helpful in pre-
dicting the various ways in which other vapor-phase organic sulfur compounds
may be handled by the mammalian detoxifying system.
The metabolic pathways by which organic sulfur compounds are handled
by mammalian systems are basically similar to the limited number of pathways
for the metabolism of other foreign compounds, i.e. , by enzymatic detoxifying
systems localized in the liver. These include, in part: reduction, oxidation,
conjugation, S-methylation, and formation of thiosulfate esters.
Reduction
Bisulfides have been shown to be reduced to the corresponding
mercaptans. The general reaction is:
' % t
RSSR N RSH + R SH. (20)
260
-------
1009, 1388
Thus, diethyldisulfide is reduced to ethylmercaptan, tetra-
ethylthiuram disulfide (Antabuse) is reduced to diethyldithiocarbamic
1009,1389
acid, and dibenzyldisulfide is reduced to the corresponding
1388
benzylmercaptan. A nonspecific nucleotide-dependent disulfide
reductase is present in rat liver and catalyzes the following overall
1260
reaction:
NADPH+ + RSSR ^ 2RSH + NADP + H+. (21)
This nonspecific disulfide reductase may be responsible for the metabolism
8
of dimethyldisulfide that is emitted into the atmosphere. Reduction of
dimethyldisulfide -would yield methylmercaptan, which would then be oxidized
to carbon dioxide and inorganic sulfate.
The sulfur compounds generally undergo more than one type of bio-
transformation. Thus, after the reduction of the diethyldisulfide to yield
the mercaptan, the mercaptan may undergo methylation to form the methyl -
alkyl sulfide, which may then be oxidized to the sulfoxide and finally to the
sulfone. Furthermore, a substantial portion of the sulfur may be excreted
1009,1388
as inorganic sulfate.
Dime thy Isulfoxide has been reported to be metabolically reduced to
339,1009
dimethylsulfide, and diaminodiphenylsulfoxide is reduced to
845
diaminodiphenylsulfide.
Oxidation
Lower mercaptans have been reported to be metabolized eventually
206, 1388
to inorganic sulfate and carbon dioxide.
261
-------
1385
Dimethylsulfide has been shown to be oxidized to dimethylsulfoxide,
616, 1009
which is further oxidized to dimethylsulfone. There are a number
of cases in which sulfide compounds are initially ozidized to sulfoxides and
then to sulfones. Thus, the propylme reap tan moiety of thiamine propyl-
disulfide is oxidized partly to the sulfone and excreted in part as inorganic
973
sulfate. S-Methylthiophenol is oxidized to the sulfoxide and then to
847
the sulfone. Mustard gas (bis- 3 -chloroethylsulfide) is in part oxidized
315 486
to the sulfone, diaminodiphenylsulfide is oxidized to the sulfoxide,
316
and the thio analogue of phenylbutazone is oxidized to the sulfoxide.
A number of thioethers, including 6-methylmercaptopurine and
S-methylbenzothiazole, have been shown to be oxidatively cleaved by
844
mammalian liver enzymes. The formaldehyde formed in the process
probably enters the one-carbon metabolic pool. The other products include
mercaptans and mercaptan derivatives. Oxidative dealkylation is a pathway
to be considered in the metabolism of organic sulfur compounds. The reaction
is as follows:
RSCH £ ^. HCHO + RSH. (22)
•J O
1393
It is possible for some mercaptans to be oxidized to disulfides.
However, it is more likely that the disulfides would be reduced to the
corresponding mercaptan.
Conjugation
Conjugation with glucuronic acid is one of the main in vivo routes by
which both exogenous and endogenous substances are detoxified and inactivated.
262
-------
In a manner similar to that of hydroxy compounds, thiols form S-glucuronides.
Thus, thiophenol and 2-mercaptobenzothiazole have been shown to be metabolized
254,483, 688,847, 1234, 1388
to the S-glucuronides.
Methylation
Sulfhydryl groups are methylated in vivo. Thus, thiophenols, ethyl -
mercaptan, and thiouracil are converted to their S-methyl analogues by
847,1118, 1389
S-methyltransferases, using the donor S-adenosyl-methionine.
Formation of Thiosulfate Esters
In a manner similar to that of an alcohol or phenol, thiols might be
expected to form thiosulfate esters. Phenylthiosulfate has been reported
to be a possible metabolite of thiophenol, because an increase in sulfate
1388
ester was found after the administration of thiophenol.
It is possible for a sulfite to react with any suitable disulfide according
1098
to the following general reaction:
RSS03~. (23)
Thus, sulfate ester formation after ingestion of a disulfide must be entertained
as a distinct possibility in the metabolism of vapor-phase volatile organic
179
sulfur compounds. It has recently been suggested that both sulfinic and
sulfonic acids may be among the products obtained from the reaction of sulfur
dioxide and hydrocarbons.
263
-------
CHAPTER 6
BIOLOGIC EFFECTS OF VAPOR-PHASE ORGANIC POLLUTANTS IN HUMANS
AND OTHER MAMMALIAN SYSTEMS
ABSORPTION
Vapor-phase organic pollutants (VPOP) are absorbed through four
64
routes: the respiratory and gastrointestinal tracts, the eye, and the skin.
Discussion of routes and mechanisms of absorption of a pollutant requires
reference to its physicochemical properties, such as vapor pressure,
stability, reactivity, and solubility. The metabolism, excre-
tion, and toxicology--including ability to produce acute and chronic in-
flammatory reactions, allergenicity, and carcinogenicity--are discussed
elsewhere and are mentioned only briefly here.
Respiratory Tract
The volume of air normally breathed by a normal human adult at rest
approximates 8 liters/min, or about 11,500 liters/day. Moreover, during
vigorous exercise, the volume, and accordingly the amount of pollutants in
the breathed air, can increase by a factor of 10 or more--to 80-100 liters/min—
for relatively short periods. It can be assumed that the respiratory tract
tissues are exposed to more air contaminants than the tissues of any other
route of absorption, and more so during vigorous effort.
Generally, pollutants cross mucosal membranes--such as those of the
alveoli, conjunctivae, etc.--via three mechanisms. Through the first,
pollutants are actively absorbed, with energy derived from the cells them-
selves. The second mechanism is that of vesicular transport, or cellular
ingestion of the pollutant substance. It has also been called pinocytosis,
264
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or "drinking" by the cell. Although it is certainly the main mechanism
for the systemic absorption of large particles and colloids via the gastro-
intestinal tract, it is of little importance with regard to the absorption of
VPOP via the respiratory tract, the eye, and the skin. An exception would
be VPOP absorbed onto such particles or colloids and absorbed through
pinocytosis in the gastrointestinal tract. The third and main mechanism is
by diffusion. For inhaled soluble pollutants in the vapor phase, this is the
64
main mode of absorption.
In the; respiratory tract, where a pollutant is absorbed and how much
is absorbed depend mainly on the pollutant's solubility in water. Highly
water-soluble VPOP, such as formaldehyde, are absorbed mainly in the
upper respiratory tract, where they first come into contact with the moist
surfaces of the respiratory passages. Thus, the proximal upper respiratory
tract's nasal, buccal, nasopharyngeal, and laryngotracheal areas are the
sites of absorption of highly water-soluble compounds. However, poorly
water-soluble VPOP can reach the distal lower respiratory tract and can
affect the alveolar and bronchiolar epithelium and the interstitial tissue,
permitting entrance into the bloodstream. The chlorinated camphene
pesticide toxaphene and the incomplete combustion products of smoke from
fires and tobacco are examples of compounds absorbed mainly in the distal
122
respiratory tract.
Once VPOP reach the alveoli, the rate of alveolocapillary transfer
and of systemic absorption is high. Anesthetic gases and vapors can be
measured in distant tissues, such as the brain, within second after inhala-
tion. In a limited sense, the bronchial tree functions as a system of elastic
pipes conducting air in and out of the alveoli. Thus, although the bronchial
265
-------
mucosal and other luminar cells are exposed to most air pollutants,
these tissues absorb only the VPOP that are highly to moderately •water-
soluble, with this absorption being local and nonsystemic. VPOP that
are only poorly -water-soluble can also interact with the bronchial mucus
to some degree, but may not be directly absorbed systemically. Acute
and chronic inflammatory reactions, as well as hyperreactivity, affect
the mucosal and other cells and tissues of the nose, nasopharynx, and
bronchial tree, which are the first tissues to contact VPOP. This is
especially so for -water-soluble materials, but such reactions depend on
122
the specific toxicologic properties of the pollutant. These types of
reactions may also affect the absorption of VPOP positively or negatively.
320
Deichmann et al. noted rapid absorption of nitroolefins at various
concentrations from the respiratory tract of exposed rats, chicks, and
rabbits, as evidenced by peripheral vasodilatation and marked irritation
of the respiratory mucous membrane. At high relative humidity, there
•was greater pulmonary damage than in animals exposed to the same com-
pounds at low humidity.
Agricultural workers and pest-control operators are often adversely
affected by simultaneous respiratory, conjunctival, and dermal exposures
to aerosolized pesticides during spraying of crops, fumigation, and similar
operations. Pesticides are commonly dispersed with the aid of hydrocarbon
dilutants or propellants, such as kerosene for agricultural application and
halogenated hydrocarbons (freons) for household and other applications. This
gives particular importance to experimental and field experience with pesticides,
inasmuch as pesticide uptake could act as a tracer for the mechanism of human
uptake of vapor-phase hydrocarbons. Too few quantitative data are'available
266
-------
from uncontroiled occupational exposures to indicate the degree to which
different exposure routes contributed to the systemic absorption and result-
ing toxic effects. On the basis of clinical toxicologic evidence, dermal
exposure is thought to play an important role in the uptake of organophosphorous
pesticides. This type of exposure is of special importance, because of the
extensive household use of these compounds.
VPOP may be adsorbed onto or absorbed by finely divided atmospheric
particles, whether these belong to the natural background or man-produced
aerosols — such as asbestos, road dust, soot, fly ash, and metal fume--
or arise from biologic sources—such as animal decidua, pollens and
other vegetable particles, spores, and fungi. In urban areas, many of these
particles, notably the first five, normally enter the respiratory tract and
reach the alveoli when the particle size is 0. 5-5 ym. Only a small fraction
of particles of this size are absorbed through the alveoli and retained in the
950
body.
Gastrointestinal Tract
Most inhaled VPOP will generally reach the gastrointestinal tract,
where they will not be readily absorbed, except for highly water-soluble
compounds absorbed in the buccal and pharyngoesophageal areas. As noted,
gastrointestinal exposure to and absorption of VPOP through pinocytosis can
occur from the ingestion of air-suspended particles onto which VPOP have
been adsorbed and from the swallowing of bronchial phlegm containing similar
particles with adsorbed VPOP. Moreover, some
gastrointestinal exposure to and absorption of such pollutants can conceivably
occur from ingestion of VPOP adsorbed onto edible materials, like produce.
No quantitative data are available regarding any of these.
267
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There are data on experimentally induced gastrointestinal absorption
of liquid organic air pollutants--such as aldehydes, benzene, kerosene,
toluene, and xylene--and on the effects of accidental ingestion of liquid
pesticides. But quantitative data pertaining to absorption and oral toxicity
of organic pollutants in liquid form cannot be applied directly to the study
of gastrointestinal absorption of vapor-phase organic air pollutants.
Eye
Except during sleep, the conjunctival membranes and corneas are
almost continuously exposed to VPOP, much as the respiratory tract and
skin. However, although very small amounts of air pollutants, such as
acrolein and formaldehyde, contact these eye tissues, it is usually there
that the first uncomfortable reactions are felt. Such reactions can range
from mild irritation to acute inflammation, depending on concentration of
the VPOP, their irritant properties, etc. Highly water-soluble pollutants
122, 320
are most likely to be absorbed by the conjunctiva locally and systemically.
Skin
Scant information is available regarding the amount of percutaneous
absorption of VPOP. In general, skin absorption is a function of the solu-
bility of a substance in a water-lipid system. Thus, because most VPOP
are very soluble in lipids and few are highly water-soluble, percutaneous
absorption of VPOP at large is likely to be greater via the skin lipids, hair
follicles, and sebaceous glands for lipid-soluble VPOP, and less through
1130
sweat glands for water-soluble compounds (Scheuplein and Blank and
R. J. Feldmann, personal.communication).
268
-------
Although the low permeability of the keratinized intact stratum, corneum
has been considered one of the skin's major protections, newer data reveal
that this thin, uppermost epidermal layer permits some percutaneous ab-
sorption of vapors. In the absence of pores or air-filled interstices in
the stratum corneum, molecules able to dissolve in that intact tissue can
exhibit appreciable permeability through it. For example, the transport
of alkane vapors does indeed increase with their solubility in the epidermal
tissues. The more -water-soluble VPOP, such as acrolein and formaldehyde,
are thus more likely to penetrate. In general, the trend is as follows:
(1) the diffusion (as measured by sorption) of alcohols and alkanes decreases
with increei.se in molecular weight; (2) the presence of hydroxyl groups pro-
motes the solubility of alcohol vapors, but decreases their diffusion markedly;
and (3) the diffusion of vapors like alcohols and the higher alkanes is con-
siderably lower than for nonreactive permanent gases, such as nitrogen,
1130
carbon dioxide, and oxygen.
It has been shown that nitroolefins can produce damage to the lung by
320
percutaneous absorption. Although nitroolefins have not been shown to
be importa.nt pollutants, cutaneous absorption may be an overlooked route
for systemic toxicity.
Occupational health studies involving aerosols of the organophosphorous
pesticide parathion indicate that spraymen absorb much more parathion
through the skin than through the respiratory tract. However, these studies
also indicated that the body absorbed only a small fraction of the total dermal
exposure, whereas the absorption through the respiratory tract exposure
359
appeared to be total. Other occupational experience has shown that dermal
absorption related to exposure to pesticides is highly influenced by the type of
269
-------
pesticide, the diluent or solvent used, the physical state of the material,
the size and hydration (sweatiness) of the exposed area of skin, the part
of the body exposed, and the condition (intactness) of the exposed skin's
891
stratum corneum. Dermal absorption of VPOP is most likely in-
fluenced by the same factors.
Comparison of Various Routes of Absorption
Evaluation of common routes of human exposure to pesticides leads
to the general conclusion that toxic chemicals at equivalent dosages are
absorbed more rapidly and completely from the respiratory tract than
399
from the skin. Although not stated specifically about VPOP, it has
been demonstrated that the hazard--and, by inference, the degree of
absorption--from respiratory exposures with the same "dose" may be
3 times as great as that from dermal exposure and 10 times as great as
49
that from oral exposure. Oral exposures, as discussed elsewhere,
usually involve detoxification or modification in the liver.
PATHOPHYSIOLOGIC EFFECTS
The consequences of inhalational exposure of hydrocarbons in mammalian
organisms are complex, because the inhaled substances are always mixtures.
This intermingling of compounds makes it virtually impossible to incriminate
any single material as the villainous agent in the causation of pathologic
changes. In addition, the chemical events that occur spontaneously in the
atmosphere as a result of photo oxidation cause the formation of additional
compounds, which are derived from hydrocarbon material. These materials,
which include aldehydes and such complex oxidarits as peroxyacetylnitrate,
are among the most important irritants of the atmospheric contaminants to
which man and other mammals are exposed.
270
-------
These two factors, the complex mixtures of hydrocarbons and other
chemicals i:n the atmosphere and the spontaneous production of additional
oxidizing ag;ents involving hydrocarbon substrates, preclude the identifica-
tion of any agent as the causative factor in any pathologic change in urban
exposures. An additional factor causing confusion in the interpretation of
the effects of these agents in human exposures is the background of disease
and hyperreactivity to additional insults that may exist in the populations
under study. These responses may be the result of genetic deficiency
states,such as deficiencies of serum o^-antitrypsins; prolonged and
heavy inhalation of cigarette smoke, which may aggravate and amplify
disease states; intercurrent infection or allergic hyper sensitivity; and
underlying "sensitivity" to atmospheric materials.
These difficulties have made it necessary to resort to controlled
human exposure studies or animal exposures in order to evaluate the
responses to single agents of the hydrocarbon groups known to exist in the
polluted urban atmosphere. Even with artificial exposures, minimal
changes in the tissues are difficult.to detect. This may be due to the
insensitivity of the methods used in reported studies and to the very
limited anatomic changes produced by many of the atmospheric hydrocarbons.
271
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Methods for Detecting Effects on the Respiratory System
The effects of many common pollutants on the airways are best assessed
by measurement of specific airway conductance and resistance at a known lung
volume usually the functional residual capacity (FRC) or the lung volume at the
end of an ordinary breath. For total pulmonary resistance (TPR) measure-
ments (of lungs and chest) in such animals as the guinea pig, a pulsating
pressure is applied to the whole-body plethysmograph containing the animal,
and rate of airflow is measured by a screen pneumotachygraph applied by
means of a mask over the nose and head. TPR is the ratio of the pressure
difference to the rate of airflow.
Transfer factor (diffusion capacity), related to the rate of
transfer of tracer carbon monoxide from lung spaces into the blood, can
be measured and is useful when pollutants cause pulmonary edema, in both
man and animals. Arterial blood can be sampled and pO , pCO , pH, and
bicarbonate can be measured.
All the foregoing are reasonably objective measurements. Spirometry
in humans, which involves recording the deepest breath that can be inhaled
and then exhaled by a maximal rapid (forced) exhalation, measures vital
capacity (FVC or FEV) or the amount exhaled rapidly. Indexes related to
rate of exhalation are the FEV (the volume exhaled during the first second)
and the rate (liters/min) during the middle 50% of the vital capacity (MMEF,
or maximal midexpiratory flow rate) and during the range 200-1,200 cm
of exhalation (MEFR, or maximal expiratory flow rate). Measurements
272
-------
after the inhalation of nebulized bronchodilator are useful in assessing the
degree of reversible bronchoconstriction. The first part (40-50%) of maximal
expiratory flow is subjective, in that several recordings are necessary to
assume that maximal forced exhalation has been achieved.
Neither spirometry nor body plethysmography assesses small-airway
(bronchioles 2 mm or less in diameter) resistance in humans. Critical
closing capacity, or the volume at which small airways close during
exhalation, is being assessed for implications in inhalational effects.
Effects of Specific Classes of VPOP
Aliphatic Hydrocarbons. Straight-chain hydrocarbons are virtually
inert, and no demonstrable pathologic changes have been reported. Methane
and ethane cause death only by asphyxiation or anoxia. The higher saturated
aliphatic compounds, as well as the unsaturated or olefinic hydrocarbons,
produce anesthesia in higher concentration. Heptane and octane are re-
ported to cause incoordination and vertigo after 4 min of inhalation at
5, 000 ppm, and inhalation of octane at 10, 000 ppm or heptane at 15, 000
ppm produces narcosis within 30-60 min. Ethylene produces little or no
effect in concentrations of 5, 500 ppm for periods of several hours.
Inhalation of acetylene at 350, 000 ppm for 5 min will produce unconsciousness.
1290
This applies to hydrocarbons in the CH - C H series.
4 8 18
Alicyclic Hydrocarbons. Alicyclic hydrocarbons have effects similar
to those of the straight-chain compounds. In general, higher concentrations
act as depressants or anesthetics. They are retained but sparingly in
tissue and blood, and their toxicity is low. Cumulative toxicity due to
continued or repeated exposures to low concentrations is considered to be
273
-------
unlikely. Single exposures to cyclohexane vapor at 18,000 ppm in air
for 5 min produce slight muscle tremors in mice and rabbits; 25-30 min
of such exposure produces muscular incoordination and paralysis. This
dose does not produce effects in guinea pigs. Chronic exposures to cyclo-
hexane vapor at up to 3,330 ppm in air for 6 hr/day for 60 days in rabbits,
and 1,240 ppm for similar periods in monkeys produce no noticeable effects.
Chronic exposures to methylcyclohexane vapor (2,800 ppm for 24 hr/day
1290
for 70 continuous days) also produce no noticeable effects.
Aromatic Hydrocarbons. Aromatic hydrocarbons are biochemically
active and more irritating to mucous membranes than are aliphatic and
alicyclic hydrocarbons in equivalent concentrations. Some, particularly
benzene, produce leukopenia and anemia with chronic exposure. In general,
benzene, toluene, or xylene in acute and chronic exposures at concentra-
tions above 100 ppm may result in fatigue, weakness, confusion, skin
paresthesias, and mucous membrane irritation (eye and nose), and at
concentrations above 2,000 ppm, in prostration and unconsciousness.
Styrene in concentrations above 1,000 ppm produces mucous membrane
irritation in some animals, but unconsciousness and death in others.
Styrene concentrations of 200 ppm produce mucous irritation in man.
Apparently, no human health effects have been reported for benzene
below 25 ppm or for the other unsubstituted aromatic hydrocarbons at this
concentration. Of 102 men studied in the rotogravure printing industry,
74 had evidence of chronic benzene poisoning of various degrees of
516 1125a
severity. Savilahti found blood abnormalities in 107 of
147 workers in a shoe factory. The source of benzene was cement, and
the concentrations were reported to have ranged from 318 to 470 ppm.
1410
One death occurred. Winslow reported blood changes in workers
274
-------
where concentrations of benzene vapor below 100 ppm were found. Heimann
574
and Ford found one death from benzene poisoning and three other cases of
blood changes where air analysis for benzene showed a concentration of 105 ppm.
1403
Wilson reported three fatal cases in a plant where the average con-
555
centration of benzene vapor was 100 ppm. Hardy and Elkins reported
one death from benzene poisoning and several other cases of blood changes in a
plant where repeated air analyses indicated benzene vapor concentrations of about
60 ppm. Hematologic changes may occur without general symptoms, such
as headache, fatigue, dizziness, anorexia, nervousness, and irritability.
The opposite was also observed--that is, general symptoms of toxicity
without hematologic change.
Acute exposures in man to toluene at 50-100 ppm produce no apparent
effects, but paresthesia, fatigue, and confusion have been reported after
476, 1019
exposure to 220 ppm for 8 hr. After exposure to 600 ppm for
8 hr, confusion, dyscoordination, staggering gait, nausea, headache,
dizziness, and dilated pupils have been observed. In acute exposures,
6, 700 ppm is the LC in mice. Hematologic effects are not observed
50
with toluene toxicity.
Aldehydes. Aldehydes are formed from hydrocarbons in the
atmosphere. Formaldehyde and other mucous-membrane irritants--
such as ketones, PAN, and other oxidants--are formed in and may be
consumed in the atmosphere through photochemical actions. Their
1290
toxicity is predominantly related to their irritant properties.
The principal effect of low concentrations of aldehydes is irritation of
the skin and the mucous membranes of the eyes, nose, and throat.
The aldehydes of lower molecular weight are more soluble and act chiefly
275
-------
on the eyes and upper respiratory tract. The higher-molecular-weight
and less soluble aldehydes reach more deeply and may affect the lungs.
Aldehydes are frequent contaminants in the urban atmosphere, as
well as in industrial situations, but detailed information concerning the
effects of this family of compounds in human experience and chronic ani-
mal studies is limited. No evidence of disease or injury due to cumula-
tive exposures has been described.
Although nearly all aldehydes produce irritation of the mucosal
surfaces on exposure, the unsaturated olefinic aldehydes and the halo-
genated aldehydes generally cause greater irritative effects than do
saturated ones. Aromatic and heterocyclic aldehydes cause less severe
255
irritation. Within a given aldehyde series, the irritant effect de-
1180
creases with increasing length of the carbon chains. Low-molecular-
weight aldehydes act primarily on the eyes and upper respiratory tract,
and longer-chain aldehydes, which are less soluble, tend to affect the
lower respiratory tract and pulmonary parenchyma. Many aldehydes
have anesthetic properties, which appear to vary inversely with increasing
1187
molecular weight of the aldehyde group. In general, because they
are rapidly metabolized, aldehydes are not good anesthetic agents. Acute
exposure to high concentrations of aldehyde vapors may produce injury to
the alveolar capillary membranes, pulmonary edema, focal hemorrhage,
and exudation. A variety of sequelae after acute high-dose exposures
have been reported. These effects—which include pneumonic consolidation,
alveolar wall thickening, and parenchymal destruction—are unusual and
may be the result of complicating infection. Low concentrations of
276
-------
aldehyde cause a decrease in mucociliary activity of the respiratory
epithelium after acute exposures. Little is known of the responses of
the respiratory ciliary apparatus after chronic, intermittent, or con-
tinuous exposures to aldehydes.
Of the specific aldehydes present in the general urban atmosphere,
two are relatively important. Formaldehyde and acrolein are present
in greater concentrations than other aldehyde forms. Formaldehyde may
877,878,1223
be perceived by its odor at concentrations between 0. 060 and 0. 5 ppm.
Irritant effects on the eyes (producing lacrimation) and on the nasal and
pharyngeal mucosa, causing sneezing, rhinorrhea, cough, sore throat, a
sense of substernal oppression, or a combination of these, are reported
133,878,911, 1143
at concentrations below 1 ppm, varying from 0. 1 to 1.0 ppm.
407
Other studies have incriminated concentrations of about 2-3 ppm as
necessary to produce these symptoms. Some tolerance or adaptation
to formaldehyde exposure can occur, and concentrations of 2-3 ppm may
be tolerated by some subjects for periods of 8 hr. Above 5 ppm, discomfort
becomes very pronounced after 10-30 min in many subjects. These symptoms
persist for an hour or more after removal from the irritant atmosphere.
Serious chemical bronchitis, laryngitis, and even bronchopneumonia are
835, 1331
reported at concentrations above 5 ppm.
Formaldehyde exposures also occur in industrial settings. Dermatitis
•was observed in 25 men engaged in the manufacture of phenolformaldehyde
557
and urea-formaldehyde resins. Eczematoid reactions of an acute and
chronic type and a combination of the two were observed. These work
exposures usually involved direct contact with liquid, solid, or resinous
formaldehyde materials and differ in this respect from the usual vapor
277
-------
exposures in air pollution episodes. Skin eruptions due to formaldehyde
vapors are rare, but exacerbations of skin lesions from this material
after reexposure have been noted. Fatal formaldehyde poisonings that
442
have been reported are uniformly the result of ingestion of this agent.
Sensitization of the skin or respiratory tract in man is uncommon, and
asthma-like symptoms are seldom noted.
Animal studies have demonstrated the similar effects of irritation
of mucous membranes and skin, but with some study and confirmation
at the cellular or tissue level. Ciliary activity ceased in the anesthetized
tracheotomized rat on exposure to 0. 5 ppm for 150 sec or to 3 ppm for
275, 296,702
50 sec.
Inhalation of formaldehyde by the intact animal produces an increase
in residual volume and a decrease in tidal volume. That these effects in-
volve apparent air-way receptors and scrubbing effects of the upper air-way
due to this soluble gas is shown by the fact that increases in pulmonary
resistance and tidal volume occur in tracheotomized animals along with a
decrease in respiratory rate.
Inhalation of formaldehyde vapors as an aerosol -with sodium chloride
or other materials increases the -work of respiration -when the formaldehyde
concentration exceeds 0. 3 ppm, and the aerosol increases the time necessary
for recovery after discontinuation of the exposure. Increases in aerosol
concentration increase the resistance to air flow, and potentiates and pro-
37,764
longs the effects. Inhalation of formaldehyde vapors also can pro-
duce effects in organs other than the lung. Alkaline phosphatase activity
934
in the liver increased after exposure to vapors of 3. 5 ppm for 18 hr.
Higher concentrations (19 pprn for up to 10 hr) produce edema and
278
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1113
hemorrhage in the lungs and hyperemia of the liver. Exposure
to higher concentrations produces pleural and peritoneal fluid and
1188
pneumonitis.
Acrolein, an unsaturated aldehyde, is generally much more
toxic and irritant than the saturated aldehydes. Acute exposures produce
irritations to mucous membranes and skin, similar to those produced by
formaldehyde. Chronic toxicity studies have not been reported. Human
subjects experience a sense of irritation of the eyes and nose within 5 min
596,1190
of exposure to acrolein at 0. 25 ppm. Lacrimation occurs at
262, 578
concentrations of 0. 67 ppm after 20 sec, and at 1.04 ppm within 5 sec.
The eye irritation becomes almost intolerable at the latter concentration
after 5 min of exposure, and higher concentrations are tolerable for even
1190
shorter periods. At 21 ppm, pulmonary edema has been observed,
and inhalation of 150 ppm for 10 min is lethal in man. The American
Conference of Governmental Industrial Hygienists suggests that workplace
concentrations of acrolein vapor should not exceed 0.01 ppm, to prevent
mucosal sensory irritation.
In animal studies, exposure to acrolein vapor at 6 ppm for 87 min
produces a. 50% survival time of 87 min. This is half the survival time
related to similar formaldehyde concentrations. The LC for rats in
50
0. 5 hr is 0. 05 ppm, only one-sixth the concentration of formaldehyde
vapor needed for similar lethality. Chronic exposures of rats to acrolein
vapors at 0. 57 ppm over several weeks caused loss of •weight, decreases
in •whole-blood cholinesterase activity, a decrease in urinary coproporphyrin
527
excretion, and a change in conditioned-reflex activity. Rabbits exposed
to 0. 57 ppm for 30 days showed no apparent ill effects; at 1. 9-2. 6 ppm
279
-------
597, 885
for 4 hr, enzyme alterations in eye tissues were observed, and
934
hepatic alkaline phosphatase -was increased. Animals exposed to
higher concentrations develop pulmonary edema and desquamation and
1113,1188
degeneration of the bronchial epithelium. A concentration
1013
of 10. 5 ppm for 6 hr is fatal to 50% of mice and guinea pigs; but
cats, although they develop lacrimation, evidence of respiratory tract
irritation, and narcosis, return to apparent normal function within
2-3 hr after exposure. Residual lung damage has been reported in rats
6 months after exposure to 200 ppm for 10 min once a •week for 10 weeks.
Murphy studied the physiologic effects in guinea pigs at concentra-
tions closely approximating those present in some industrial exposures.
At 0. 06 ppm, total pulmonary resistance and tidal volume increased,
and respiratory rate decreased. These changes were accentuated with
increasing concentration and were reversed when the animals were re-
moved to clear air. The increased resistance was believed to be due
to a reflex bronchospasm mediated through the autonomic nervous
935
system. Mineral oil, sodium chloride, and silica gel aerosols
potentiate the damaging effects of acrolein. Exposure to acrolein by
aerosol potentiated the effects seen at comparable vapor concentrations,
764
as was the case with formaldehyde exposures.
Acetaldehyde is almost nonirritating in man at 50 ppm, although
it may be perceived at 25 ppm by some. Human volunteers exposed to
1013, 1179,1180
50 ppm for 15 min show some evidence of eye irritation; this
is pronounced, and nose and throat irritation is observed at 200 ppm.
Other higher-molecular-weight aldehydes have received little study.
In man, exposure to crotonaldehyde at 4 ppm for 10 min produces lacrimation
280
-------
1013
and respiratory mucosal irritation. Propionaldehyde at 134 ppm
for 30 min is mildly irritating to the exposed mucosal surfaces, where-
as butyraldehyde and isobutyraldehyde at approximately 200 ppm for
1180
30 min are almost nonirritating.
The results of animal studies involving exposures to isolated hydro-
carbons are difficult to relate to human and animal studies of true atmospheric
exposures, because many of the naturally occurring hydrocarbons are de-
graded and altered by reactions encouraged by photochemical energy and
naturally occurring oxidants in the atmosphere. As a result, additional
groups of pollutant substances are generated, including oxidants and com-
plex reaction products, such as PAN.
Hydrocarbon Mixtures. The hydrocarbons in the atmosphere are
altered to other compounds during ultraviolet irradiation by sunlight.
Hydrocarbon-oxidant relationships in experimental exposures are
often ill-defined; therefore, difficulty is encountered in determining the
cause of the effects noted.
936
Murphy ^t aL exposed guinea pigs to irradiated and non-
irradiated automobile exhaust for 4 hr. The response to nonirradiated
exhaust was relatively small, whereas flow resistance and tidal volume
increased and respiratory rate decreased during exposure to irradiated
exhaust. The latter showed.photochemical formation of aldehydes, nitrogen
dioxide, and total oxidant at the expense of nitric oxide and olefins. The
greatest response was to an atmosphere of 2.4-ppm formaldehyde, 0. 2-ppm
acrolein, 0. 8-ppm ozone, and 2. 7-ppm nitrogen dioxide. The effects pro-
duced could be simulated nearly quantitatively by exposing animals to 0. 6-ppm
acrolein, whereas 0. 7-ppm ozone produced negligible effect on flow resistance;
281
-------
even at an exhaust dilution of 1000:1 -with hydrocarbon concentrations
well within the range of community air pollution, the same effects were
detected. In this exposure, total oxidant concentration was 0.4 ppm,
and that of formaldehyde was 0. 5 ppm.
That oxidants were not responsible for these effects was shown by
study of a 150:1 dilution of nonirradiated exhaust. Although no oxidant
was present, the respiratory effects •were identical with those of irradiated
exhaust in a 1, 140:1 dilution. The effective agents are therefore present
in raw exhaust gases and are multiplied by irradiation. Because
quantitatively identical pulmonary effects are produced by acrolein
and similar effects by formaldehyde, it is a good hypothesis that these
effects are caused by aldehydes.
Eye irritation in man is similarly related to total aldehyde concei
1071
tration. Renzetti and Bryan found a good direct relationship -with
aldehyde concentrations ranging from 0. 035 to 0. 35 ppm. This direct
relationship held for formaldehyde (0. 01-0. 1 ppm), but was nonlinear
for acrolein, of -which concentrations of 0. 003-0. 015 ppm resulted in
more eye irritation than concentrations greater than 0. 015 ppm.
1276
Tuesday et al. also noted a high correlation of eye irritation -with
formaldehyde and a linear relationship between formaldehyde accumulation
and initial hydrocarbon concentration, although only light eye irritation
was noted. Initial hydrocarbon concentrations ranged from 0. 3 to 2. 6 ppm
(as hexane), and those of oxides of nitrogen, from 0.1 to 1. 1 ppm.
Formaldehyde concentrations were up to 0. 3 ppm; eye irritation occurred
with formaldehyde as low as 0. 15 ppm.
282
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1090
Romanovsky et^ aL irradiated synthetic atmospheres in a
series of tests using ethylene, propylene, isobutane and gasoline mixture,
and auto exhaust. A high correlation was noted of eye irritation with
formaldehyde content, which ranged from 0.1 to A ppm.
A series of 29 hydrocarbons, -which included 12 aromatic hydrocarbons,
591
was studied in irradiation of synthetic atmospheres by Heuss and Glasson.
The aromatic hydrocarbons, particularly styrene and B-methylstyrene,
were more potent than nonaromatic hydrocarbons in producing eye irritation.
Some systems produced peroxybenzoylnitrate, 200 times more potent as a
lachrymator than formaldehyde. In the photooxidation of styrene, formal-
dehyde accumulated to a concentration of 0. 4 ppm. It is thus likely that
formaldehyde was responsible for most of the eye irritation in this experi-
mental photochemical pollution system.
29
Altshuller et al. studied irradiated synthetic atmospheres containing
alkylbenzenes. Eye irritation -was of the same magnitude as that with
propylene photooxidation. Toluene, rn-xylene, and 1, 3, 5-trimethylbenzene
produced eye irritation greater than ethylene, and £-xylene resulted in less.
They concluded that paraffinic hydrocarbons, acetylene, and benzene in
photochemical air pollution do not contribute appreciably to eye irritation.
31
Altshuller et aL showed that the addition of propylene or toluene
to n-butane or ri- butane -ethane mixtures with nitrogen oxides produced
large increases in oxidant dosages, in formaldehyde and PAN yields, and
in eye irritation. Next to ethylene, propylene is the most abundant olefin
in the urban atmosphere. Toluene is the most abundant alkyl benzene
and, with xylene, constitutes about half the total amount of alkyl benzenes
present. The results clearly show that oxidant dosage and PAN yields
283
-------
decrease by 35-95% when propylene or toluene is removed or reduced,
depending somewhat on the nitrogen oxide concentration. When hydro-
carbon concentration is 4 ppm and nitrogen oxide concentration is 1.5
ppm, the relative eye irritation responses are as follows; to toluene, 2.3
units; to m-xylene, 1.9; to c^-xylene, 0.9; to 1,3,5-trimethylbenzene,
2.2; to ethylene, 1.1; and to 2-methyl-2-butene, 1.6.
Effects of Photochemical Reaction Products
114
Bils studied the ultrastructural alterations in lung tissue of
mice after exposure for 2-3 hr to natural urban atmospheres whose
total oxidant concentration was 0.4 ppm. The older mice showed the
115
most severe changes. Bils and Romanovsky studied the ultrastructural
effects on mice of a 3-hr exposure to synthetic photochemical smog con-
taining oxidant material at 0.50-0.75 ppm. The results were similar to
those observed in mice exposed to natural smog. The pattern of observa-
tions indicated irreversibility of lesions after 15 months of exposure,
with marked sensitivity of the endothelium and edema after 21 months of
exposure.
Because nitric oxide was one of the constituents of the synthetic smog
used in the study, it is necessary to evaluate its own effects on lung
1096
tissue. Rounds and Bils studied the effects of sodium nitrite on
organ cultures of alveolar wall cells of rabbits and rats in vitro. Ultra-
structural study of the exposed cells in vitro showed alterations in the
shape, size, and structure of the nuclei and mitochondria resulting from
a single exposure to the equivalent of nitrogen dioxide at 20 ppm for 1 hr
284
-------
and chronic exposures equivalent to nitrogen dioxide at 15 ppm for 4 hr/day
for 10 days,
114 115
Bils and Bils and Romanovsky have reported apparent ultra -
structural changes in the lungs of mice exposed to synthetic photochemical
smog. It i:3 not possible to interpret these data in relation to any specific
vapor-phase organic air pollutant.
246
Coffin and Blommer exposed mice to automobile exhaust con-
taining carbon monoxide at 12-100 ppm and oxidant materials at 0. 08-0. 67 ppm
for 4 hr. Control animals were exposed to clean air. After exposure, the
animals of both control and oxidant groups were treated with a streptococcal
aerosol (group C) of 100, 000 bacteria/animal. Animals exposed to carbon
monoxide at 100 ppm and oxidants at 0. 35-0. 67 ppm had an increased
mortality rate from streptococcal pneumonia, compared with controls.
Exposures to carbon monoxide at 25 ppm and oxidants at 0. 12 ppm ap-
parently did not compromise resistance to the streptococcal aerosol.
Ozone at 0. 08 ppm has also been reported to affect susceptibility
to respiratory infection and augment mortality.
If mice were pretreated with a-naphthylthiourea (15 mg/kg) 30 min
before exposure to irradiated and nonirradiated automobile exhaust, there
was a small but consistently greater mortality in the mice exposed to the
936
irradiated exhaust. This change, although statistically significant at
the 2% level, was not interpreted by the authors as an effect of the oxidant
materials produced in the irradiated exhaust. Because ozone and nitrogen
dioxide increase susceptibility to respiratory infection, it is not necessary
246
to attribute the effect observed by Coffin and Blommer to photooxidized
hydrocarbons alone.
285
-------
PAN is one of the better defined and studied products of the photo-
chemical oxidation of naturally occurring atmospheric hydrocarbons. It
is recognized to be exceedingly irritant, especially to the eye. The ef-
fects of short-term exposures to PAN in healthy young male volunteers
have been studied; PAN at 0.3 ppm was administered for 5 min, and the
oxygen uptake was measured during a 5-min bicycle ergometer exercise
period and during a 5-min recovery period. A 2.3% increase in oxygen
uptake was observed during exercise; this is claimed to be statistically
significant. No difference in oxygen uptake between exposed and control
groups was observed in recovery. Expiratory flow velocity was reduced
after exercise. Both these changes—the increase in oxygen uptake and
the increased flow resistance—could be due to an increase in airway
resistance, which is reflected in an increase in the work of breathing
1193
and in the oxygen cost of breathing.
Studies of the toxicity of PAN in mice have shown that mice ex-
posed to PAN at 110 ppm or more for 2 hr or more die within a month
of exposure; the higher the concentration, the earlier the death.
Most deaths occur in the second and third weeks after exposure. In-
creased age and chamber temperature tended to increase mortality, but
variation in relative humidity did not. No mortality was observed at
exposure to 97 ppm during the 4-week period of observation, and death
204
immediately after exposure was not observed. Spontaneous activity
of C-57 black male mice was considerably reduced (50%) by exposure
356
to PAN at 4 ppm for 6 hr. Dungworth and associates studied the
chronic effects of PAN on strain A mice. Exposure to 15 ppm for 130
286
-------
daily 6-hr periods over 6 months produced an 18% mortality, primarily
due to bronchopneumonia of bacterial origin. Weight loss occurred con-
sistently in the exposed group. The major features of the reaction to
injury by PAN occurred in the epithelial structures of the airways. A
papillary proliferation of epithelium was observed, with foci of squamous
metaplasia in some 50% of the animals. Mucous cells were usually re-
duced in number in the bronchial epithelium, and cilia were often
absent. Unique to this lesion is the formation of acinar structures
lined by ciliated and mucous cells. In some cases, the bronchial walls
were ulcerated and scarred; in other cases, the epithelial structures
extended into the bronchial wall. Bronchiolectasis was common, and
epithelial hyperplasia was often present; bronchiolitis fibrosa obliterans
was infrequent. Hypertrophy of peribronchiolar and adjacent alveolar
epithelium was common. Centriacinar emphysema was mild. Some focal
pneumonitis was present. Atypical squamous metaplasia was not seen, and
no neoplasia or adenomata were observed.
SYNERGISM, ANTAGONISM, AND TOLERANCE
The study of biologic effects of air pollution has been complicated
by evidence of synergism and antagonism among physical, chemical, and
biologic components of pollution that have adverse effects on susceptible
organisms. This discussion will focus on biologic interactions of reaction
products from engine exhaust, excluding particles and polycyclic aromatic
hydrocarbons, irritants like ozone, and oxides of nitrogen and sulfur.
287
-------
Evaluations of interactions that result in either unexpected severity
or unexpected mildness of toxicity suffer greatly from a lack of informa-
tion on human experience after exposure to specific air pollutants. Epi-
demiologic studies of proper design, of which there are few, could confirm
results of laboratory experiments in which some of the typical components of
pollution seem to exert synergistic or antagonistic effects.
Synergism that has been observed in experimental animals has
usually been studied only with regard to specific physiologic mechanisms
leading to an exaggerated response or death. The underlying mechanism
of the interaction, nonetheless, is often not understood, even in studies
based on clear physiologic responses.
The interaction of inert particles and vapor-phase pollutants can
alter the physiologic response to the pollutants. This could be considered
a synergistic effect, even though it is based only on physical factors that
cause adsorption of a gaseous irritant on a particle, -which leads to a
different degree of penetration and a greater concentration of the chemical
at the site of its deposition. The irritant effect, rather than being in-
creased by the very presence of particulate matter, may depend on particle
size and chemical properties, which would be responsible for the capacity
of the particle to adsorb first and allow desorption later. Thus, chemical
and physical properties of adsorbate and adsorbent are important.
37,38
Amdur noted a synergistic effect on respiratory flow resistance
and decreased compliance in guinea pigs after inhalation of formaldehyde
with a sodium chloride aerosol. The sodium chloride aerosol was of very
small particle size (average diameter, 0. 04 ym) at a concentration of
3
10 mg/m . The aerosol alone had no effect on flow resistance. The
288
-------
synergistic effect was observed with formaldehyde at 0. 3-47 ppm. It was
not present at the lowest concentration of formaldehyde, 0. 07 ppm; but
the higher the concentration of formaldehyde, the greater the resistance.
After the hour-long exposure, the resistance remained high for another
hour.
In comparable experiments with formic acid, the sodium chloride
aerosol did not increase respiratory flow resistance, but it prolonged
the period of high resistance after exposure. The increase in toxicity
of aldehydes in the presence of inert aerosols or particulate materials
764
has also been explored by La Belle and co-workers, who investigated
formaldehyde and acrolein for their toxicity when administered in the
presence and absence of specific aerosols. The greatest degree of synergism
was observed for aerosols of triethylene glycol, mineral oil, celite,
glycerin, and sodium chloride in studies with formaldehyde. In studies
with acrolein, significant synergism was seen only -with mineral oil,
*
sodium chloride, and Santocel CF. It was concluded that, when aerosol
penetration exceeds vapor penetration, toxicity is increased. It is apparent
•that aerosols are readily at hand in community air pollution, so aldehyde
toxicity may be increased.
An example of antagonism is given by the studies of Salem and
1114
Cullumbine, who studied acute aldehyde inhalation toxicity in guinea
pigs, mice, and rabbits with high doses of acetaldehyde or acrolein alone
or in combination -with kerosene smoke produced by incomplete combustion
in a kerosene lamp. Black smoke was introduced into the inhalation chamber
*
Amorphous silica preparation, Monsanto Co.
289
-------
via a baffle that limited the size of the smoke particles. The mean fatal
dose -was calculated from the concentration of aldehydes found in the
chamber and the interval until death. The concentrations of aldehydes
•were many times those encountered in the atmosphere, and extrapolation
of these data to realistic concentrations of soot and aldehyde is not possible.
The observation of reduced toxicity of air pollutants -was originally
described for ozone, -which is not a vapor-phase compound itself, but
rather a product of atmospheric chemical reactions. A lethal exposure
to ozone could be tolerated by mice and rats if they -were exposed earlier
879
for a short period to a nontoxic concentration of ozone. Tolerance
starts 1 day after the nonlethal exposure and lasts for at least 100 days.
A variety of oxidants or other chemicals can substitute for ozone for
1228
either tolerance induction or later challenge. Such protective action
could be found in several species. It is not known -whether it appears
in man.
Extending these studies to ketene, considered a likely air pollutant,
showed the existence of a similar but short-lived tolerance to lethal
doses of ketene after pretreatment -with ketene. Pretreatment with ozone
also protected against a lethal dose of ketene, and ketene pretreatment
produced tolerance to ozone. The effective dose of ketene for mice was
a 10-min exposure at 6-7 ppm, -which produced tolerance to a lethal dose
if exposure came at least 2 days after treatment. Ketene-induced tolerance
lasted for at least 2 weeks. Pretreatment with ketene produced tolerance
to ozone the very next day, but it lasted for less than a week. Pretreat-
ment with ozone at 1 ppm for 4 hr produced tolerance to ketene after
48 hr, which lasted for 2 weeks without decline.
290
-------
Later studies on other air pollutants and unrelated chemicals showed
that cross-tolerance may or may not be developed. E. g . , cumene hydro-
peroxide provided tolerance against hydrogen peroxide, but not against
ozone, nitrogen dioxide, or even cumene hydroperoxide itself. Attempts
have been made to illuminate the mechanism of tolerance; for details,
404
the reader is referred to a paper by Fairchild.
Synergism between air pollution and microbial infections in humans
515, 821, 1040
has been postulated on the basis of animal experimentation.
246-248
Diluted automobile exhaust was irradiated by Coffin et aL to
produce smog atmospheres comparable with that encountered in areas of
human habitation. Mice were exposed for 4 hr to such atmospheres con-
taining carbon monoxide at 100 ppm, oxidant at 0. 35-0. 67 ppm, nitrogen
dioxide at 0. 5-1 ppm, and nitric oxide at 0. 03-1. 96 ppm. After this
exposure, immediate inhalation of an aerosol of streptococci, calculated
to give 100,000 organisms/mouse, led to a fivefold increase in mortality,
compared with the results of streptococcal aerosol exposure after filtered-
air inhalation. When the exhaust was further diluted, the synergistic
effect disappeared, with a threshold at 0. 15-ppm oxidant, 0. 3-ppm nitrogen
dioxide, and 25-ppm carbon monoxide. It was concluded that the critical
component must be the oxidant, inasmuch as the nitrogen dioxide concen-
trations were below the toxicity threshold by a factor of 10. The oxidant
concentration, however, was similar to the threshold for synergism due
to ozone, previously determined by Coffin ejt al. , i. e. , 0. 08 ppm.
In similar experiments, the enhancing effect of cold was studied
with ozone and streptococcal infection in combination with exposure for
2 hr at 6-9 C. A 30% increase in mortality after exposure by aerosol
821
to 30,000 organisms/mouse was attributed to this additional factor.
291
-------
A suggestion regarding the mechanism was proposed by Coffin and
247
Blommer in the depression of pulmonary bactericidal activity, which
was attributed to a decrease in the number of pulmonary macrophages
and in their activity after exposure to ozone at 0. 3 ppm for 3 hr. The
synergism between artificial smog (ozonized gasoline) and influenza viral
infections was studied in C-57 black mice. The artificial smog was
maintained continuously at 1-2 ppm throughout the experiment. A few
pulmonary adenomas were observed in each group of C-57 black mice;
but squamous cell carcinomas of the lung were seen only in the group
that was subjected to a combination of continuous exposure to smog and
the three influenza virus intubations--33 of 328 survivors had squamous
247
cell cancers in their lungs.
A modification of these experiments was undertaken, with exposure
to only one virus (PR-8). Mice of the C-57 black strain were exposed to
the same "artificial smog" as in the earlier experiments. Lung tumors
964
were observed, but they were all of the adenomatous type. Introduc-
tion of the influenza viral infection reduced lung-tumor growth. The
results differed from those of the earlier studies in the lack of squamous
lung cancer.
Synergism between formaldehyde and hydrogen peroxide with respect
329, 652
to their mutagenic action has been reported for neurospora. Similar
synergistic effects on neurospora have been reported for formaldehyde
329
and ultraviolet radiation. X radiation also increased the mutagenic
1200
action of formaldehyde in drosophila spermatids. However, synergism
in drosophila was not reported for treatment with a combination of formal-
dehyde and hydrogen peroxide, and the explanation offered was that catalase
292
-------
in the flies destroyed hydrogen peroxide. Synergism could, however,
be observed if catalase •was inactivated by treatment of the flies with
1201
hydrogen cyanide or other catalase poisons. In this connection,
it can be postulated that mutagenesis will be increased not only in
species deficient in catalase or peroxidase, but also in mammalian
species if enzymatic decomposition is blocked or, more important
from the point of view of air pollution, if the hydroperoxide is more
737
complex and cannot be attacked by the enzymes. Thus, synergism
in mutagenesis was observed between tert-butylhydroperoxide and ultra-
18
violet radiation in drosophila eggs.
Synergism between simple alkanes and alkenes in mice and rats
1175
has been described by Shugaev. Butane and isobutylene were
inhaled for 2 or 4 hr, and the LD was reached for both species at
much lower brain concentrations of the hydrocarbons when combinations
1175
of the two were inhaled.
Ethylene oxide has been found to be antagonistic to ethylene. It
acts as a reversible antiripening agent for fruit and prevents aging of
flowers. This effect may turn out to be useful in allowing longer life
for flowers, but it may cause damage to fruit by preventing normal
796
ripening.
The situation is complicated further by the antagonistic interaction
1195
between ethylene and carbon dioxide in cut flowers. Ethylene at
very low concentration in the air (0.05 ppm) causes a reduction in the
vase life of cut flowers, such as carnations or narcissi, which can be
countered by addition of 5% carbon dioxide to the air. Lower concen-
trations of carbon dioxide also protect flowers from rapid wilting due
to ethylene.
293
-------
Synergistic or antagonistic interactions of air pollutants in biologic
526
systems have generally been considered as rare. Gusev stressed
that toxicologists generally were concerned only with the evaluation of
toxicity of single chemicals and did not pay enough attention to combina-
tions of toxic substances. The conclusions he reached on the basis of
available data, however, were that only an additive effect •would be ex-
pected and that toxicity could be calculated in a simple fashion for any
number of combinations of air pollutants.
1197
In a study in which a large number of solvents in a 1:1 ratio
were tested regarding their combined LD in rats that received them
50
orally, the authors concluded that most toxicologic results could be
interpreted as additive, but that 5% of all the combinations produced
either higher or lower toxicity than expected from straight additive
action. The solvents involved in the synergistic effects were limited to
three of a total of 27--formaldehyde, tetrachloroethylene, and acetonitrile.
These chemicals and the ones -with-which the synergistic effect was ob-
served deserve more study and attention. The pairs that produced less
than the expected toxicity appear to be less important, because rapid chemical
reaction between them could alter the expected toxicity, as in the case
of combinations of epoxides and alcohols or of amines and aldehydes.
MUTAGENIC EFFECTS
Air pollutants have many adverse effects; among them is their
potential for eliciting genetic changes. This review deals with published
information pertinent to the mutagenic potential of selected air pollutants.
The compilation is neither fully comprehensive nor exhaustive, but rather
is an attempt to focus attention on one aspect of the potential hazards that
such pollutants may pose to man.
294
-------
Litera.ture from the Environmental Mutagen Information Center
on selected chemicals was used in preparing this section. Detailed
information has been published on the mutagenic effects of air pollutants
392,929, 1165
in general; of specific compounds and groups of compounds,
443, 444, 498, 558, 1022, 1030, 1266
including benzene, diepoxybu-
58-60, 392, 727, 751, 838, 963, 1095 1095
tane, diepoxycyclohexane, epoxides and other
615 598
derivatives of cyclic hydrocarbons, epoxy compounds, formal-
16,56,57,686 710 105,155,257,411,484,709,710,
dehyde, nitric oxide, oxygen,
754,919 155,156,234,412,413,1032,1431,1433 39,753
ozone, phenol compounds,
444 155,234 412,413,
and toluene; and of ultraviolet radiation and x radiation.
710,1432
Methodologies for testing mutagenicity of atmospheric pollutants
are presented in detail in the National Academy of Sciences report
950
Particulate Polycyclic Organic Matter.
Atmospheric Pollutants
Fractions of particulate atmospheric pollutants collected on filters
in New York City during 1967 were tested for the induction of dominant
391,392
lethal muta.tions in mice. Although most of the samples tested
were found to be nonmutagenic, one oxyneutral pentane 28% ether fraction
was found to be significantly mutagenic, and another aliphatic air pollutant
fraction was slightly mutagenic. Organic extracts of atmospheric particu-
late pollutants collected during 1966 by continuous air monitoring over
Birmingham, Boston, and Wheeling did not significantly increase the
392
frequency of dominant lethal mutations.
295
-------
Benzene
Several studies have been reported on the effects of occupational
chronic exposure (1-26 years) to benzene on chromosome abnormalities
443,444, 558, 1030, 1266
in peripheral blood lymphocytes or bone marrow.
The concentration of benzene in vapors to which the subjects •were exposed
varied from 12 to 532 ppm. Exposure to very low concentrations, 12 ppm,
for up to 26 years did not result in significant change in frequencies of
1266
chromosome or chromatid abnormalities. Much higher concentra-
tions of benzene, however, resulted in various chromosome abnormali-
443,558, 1030
ties, such as simple chromosome or chromatid breaks, deletions,
443 444, 1030
trisomy, chromosome loss, and other unstable chromosome changes.
Similar chromosome damage was detected in workers exposed to mixtures
444
of benzene and toluene in high concentrations.
Inhaled benzene induced biochemical changes and inhibited nucleic
498
acid synthesis in pregnant albino rats. Furthermore, subcutaneous
injection of benzene into rats evoked an immediate effect--chromatid
1022
breaks in bone marrow cells --with full recovery 36 hr after treatment.
Whether such chromosome damage represents a true index of mutagenicity
or lethal cellular effects cannot be determined; nonetheless, exposure of
man, particularly chronic, to benzene should not exceed the maximal
allowable limits.
Diepoxy butane
Diepoxybutane (DEB) is an active radiomimetic substance capable of
inducing various biologic changes, such as cancer, depression of the
hemopoietic system, and mutations. It has been tested for mutagenic
potential in bacteria (Escherichia coli, Salmonella), fungi (Neurospora,
296
-------
Penicillium, Saccharomyces), insects (Drosophila, housefly), higher
plants (Vicia, Allium, barley, maize, tomato, Arabidopsis), and mammals
427
(rats, mice).
Subcutaneous injection of DEB at 1-3 mg/kg into rats resulted in
chromosome abnormalities, such as stickiness, clumping, breaks, and a
963
decrease in rnitotic index of cultured blood lymphocytes. No signifi-
cant increase in dominant lethal mutation frequency was detected, however,
963
in mice given DEB at 17 mg/kg. Chromosome abnormalities were
838
also observed in Vicia root tip cells and in maize pollen. Treat-
ment of Neurospora with DEB resulted in marked increase in adenine and
60
inositol revertants. Furthermore, DEB interacted synergistically
with ultraviolet radiation, resulting in a twofold increase in the frequency
58,59
of adenine revertants.
Formaldehyde
Formaldehyde has been tested for mutagenic potential in Drosophila,
bacteria, Neurospora, and mammals. Exposure of young male Drosophila
to formaldehyde vapors for 30-60 min did not increase the frequency of
57
recessive sex-linked lethal mutation. Inclusion of formaldehyde in the
foods of Drosophila larvae, however, resulted in a moderate to high in-
crease in recessive sex-linked lethal mutation frequency, but adenylates
16, 56,686
were necessary for expression of the formaldehyde effect. No
significant increase in frequency of dominant lethal mutations was ob-
391,392
served when mice received intraperitoneal injections of formaldehyde.
As with benzene, exposure of pregnant rats to formaldehyde during the
entire gestation period raised the requirement for ascorbic acid and
498
inhibited nucleic acid biosynthesis.
297
-------
Formaldehyde is highly reactive with a wide range of chemicals.
Several of the reaction products may be mutagenic; for example, caseine
treated with formaldehyde was mutagenic to Drosophila. This is especially
significant, because skim milk fed to animals is stabilized with formal-
427
dehyde.
Ozone
The mutagenic potential of ozone has been tested on nucleic acids
234,1032 412 413 156
in vitro. higher plants, human cells in culture, rabbits,
156 1431,1432
mice, and Chinese hamsters.
A 30-min exposure of Vicia roots to ozone resulted in chromosome
damage equivalent to that induced by 100-r x rays; most aberrations were
412
dicentric bridges and double deletions. Chinese hamsters inhaling
0. 2-ppm ozone for 5 hr showed a marked increase in chromosome aberra-
1432
tions in their circulating lymphocytes. Chromatid breaks were detected
413
after exposure of human tissue in culture to 8-ppm ozone for 5-10 min.
Mice or rabbits inhaling 0. 2-ppm ozone 5-7 hr/day for 3 weeks showed a
marked increase in neonatal mortality and slight rupture in the nuclear
envelope of myocardial fibers; these animals recovered fully 4 weeks after
156
cessation of ozone exposure. Treatment of nucleic acids with ozone
1032
in vitro resulted in base changes. j;n v^ew Of the biologic damage
induced by very low ozone concentrations in other forms of life, the limits
of ozone to which man is currently exposed may be too high.
298
-------
Oxygen under High Pressure
The effects of oxygen under pressure may resemble the toxic effects
of ozone. Therefore, studies of oxygen under pressure are included. Ex-
2
posure of higher plants to oxygen under high pressure (500-2, 000 lb/in.,
or 30-60 atm) increased the frequencies of chromosome aberrations and
105, 257,709,754,919
chlorophyll mutations. Prolonged exposure of
bacteria to oxygen under high pressure markedly increased the mutation
411,484
frequency.
Phenolic Compounds
Several phenolic compounds have been tested for their potential to
39
induce biologic changes in Vicia root tip cells. Many of these sub-
stances--such as ^-nitrophenol, p_-nitrophenol, p_-chlorophenol, 2,4-
dichlorophenol, pentachlorophenol, a-naphthol, 3-naphthol, and 2, 4-
dichloronaphthol--induced a host of cytologic and genetic changes, such
as C-mitosis, chromosome stickiness, dicentric bridges, fragments,
binucleate cells, and micronuclei. Furthermore, a marked reduction
in mitotic index was observed.
Quinone s
Several quinones were tested for mutagenic potential in the Allium
753
test for mitotic chromosome aberrations. Hydroquinone and pyro-
catechol were not very effective in inducing genetic damage. However,
other quinones, such as p_-benzoquinone and £-benzoquinone, significantly
increased the frequencies of chromosome aberrations, observed as di-
centric bridges and fragments. Chromosome stickiness and C-metaphase
were also detected.
299
-------
Miscellaneous
Several other chemicals that may become air pollutants were tested
for mutagenic potential. Several epoxides and derivatives of cyclic hydro-
carbons increased the forward-mutation frequency (8-azaguanine resistance)
in Chinese hamster cells; however, polycyclic hydrocarbons needed metabolic
615
reactivation to become mutagenically active. Four diepoxides were also
tested in mice and found to increase the frequency of dominant lethal muta-
598
tions and to induce chromosome changes.
TERATOGENIC EFFECTS
There are two ways of being alerted to the teratogenic potential of
environmental agents for human beings. Environmentally induced congenital
606
malformations may be detected by surveillance of human births and abor-
tions and reporting all agreed-on abnormalities to a recording and analyzing
headquarters; and the toxicity of chemicals may be tested prenatally in
animals. The methods for such tests have been extensively described and
73,147,452,454,456,1003,1038,1086,1272,1397,
discussed in the last 10 years.
1401
The type and frequency of embryotoxic effects of chemicals administered
to experimental animals during pregnancy have been found to depend on numer-
ous conditions or combinations of conditions, including the type and dosage
of chemical, the stage of pregnancy, and the genetic sensitivity of the stock
452,680,1397
or species.
In general, the types of embryotoxic effects, as seen at or near the end
of gestation, are in three broad categories: intrauterine growth retarda-
1396
tion, physical maldevelopment (i.e., teratogenicity),and death. Malfor-
mation ultimately can result only from interference with events that
300
-------
occur in the embryo and early fetus; and in general, the type of mal-
1346, 1398, 1401
formation produced is related to -when the chemical acts.
All other things being equal, the factor determining the relative
932,933
frequency of the three general categories of embryotoxic effect is dosage.
Below some dosage, these phenomena are rare in experimental animals.
this dosage, which by convention is termed the "no-effect" dosage, must
be determined empirically for each chemical and each set of experimental
circumstances. At the other end of the scale are dosages large enough
to kill all or most offspring. Between these extremes, typically, is a
range of dosages that permits a significant fraction of the offspring to
survive to term, but may cause some to be stunted, malformed, or
both. This "in-between" range may be so narrow for some chemicals
or in some experimental situations--the balance between normality and
lethality may be so delicate--that it is difficult to demonstrate a tera-
togenic effect at all. Commonly, however, the in-between range is
broad enough to permit detection of teratogenicity and growth retardation
readily and to allow analysis of the dose-response relations for all three
types of embryotoxicity.
The species most often used (mice, rats, hamsters, and rabbits)
are chosen usually for convenience: they cost little, they are small,
they breed rapidly, and much knowledge of their biology is already at
hand. The present system of using several rodent or other small species
or some primates to evaluate teratogenicity is the only experimental
method available.
301
-------
Few vapor-phase organic air pollutants have been tested for possible
effect on mammalian reproduction and prenatal development or for post-
natal consequences of administration during pregnancy. Many studies done
with this handful of agents were not directed specifically to teratogenesis,
and most, of the substances •were not administered by inhalation.
45
In a study using benzene, female mice received injections of
benzene at 4 ml/kg on day 6, 9, or 12 of gestation. Litter size and post-
1354
natal viability were not affected. Watanabe and Yoshida gave CF1
female mice benzene subcutaneously at 3 mg/kg on one of days 11-15 of
gestation and examined the offspring just before term for external and
skeletal defects. The only gross defects found were cleft palate and jaw
abnormalities, and the highest total frequency (7. 9%) was produced by
treatment on day 13. Fetal mortality was unaffected. No difference was
found between females with and without malformed offspring in increase of
body weight or decrease in white-cell count after treatment.
1398
Wilson gave pregnant rats carbon tetrachloride in corn oil
daily, 0. 3 ml orally or 0. 8 ml subcutaneously for 2-3 successive days,
beginning on gestation days 8-12; 21% of pregnant females died; 38% of
females resorbed their entire litters; 41% carried young to term with
9. 1% of young resorbed. Malformations were not found and only one
7
litter contained growth-retarded young. Adams e_t aJL fed pregnant
rabbits carbon tetrachloride in arachis oil, 0. 6 ml/kg on gestation
day 5 or 1 ml/kg on days 4-5--L e. , before uterine implantation.
Blastocysts were removed and examined at day 6. 5 and were found to be
normal after the smaller dose, but to have some cellular degeneration .
and (in some cases) to contain very large nuclei with prominent nucleoli after
I
the larger dose. Their later embryologic fate, however, was not determined.
302
-------
11 1048
Ranstrom gave pregnant "white" rats 6% formalin subcutarieously,
0. 25 ml twice a day throughout gestation. Young were examined just: before
term; none was growth-retarded, but the fetal adrenal weight was reduced.
Prenatal mortality and developmental abnormalities were not mentioned.
" 1134
Schnurer gave pregnant Wistar rats 2% formalin subcutaneously,
0. 25 ml twice a day throughout gestation. This dose was not toxic to
the females and produced no increased fetal death and no malformation or
growth retardation; twice this dose produced some maternal death.
497
Gofmekler and Bonashevskaya continually exposed pregnant albino
3
rats to formaldehyde by inhalation (1 or 0. 012 mg/m , time of exposure
during pregnancy unstated) and examined the offspring (age of examination
unstated) for external and internal developmental abnormalities. No gross
abnormalities were found, but histologic deviations were noted in only some
tissues (liver, kidney, etc. ) of offspring of females exposed to the higher
concentration.
1025
Piekacz administered two phthalate esters to pregnant rats
(stock unstated) by stomach tube daily in amounts corresponding to 1 and
5% of the LD for 3 months before conception and for almost the entire
50
gestation period. The mean litter size and fetal weight were reduced in
some cases, but malformations were apparently not found, nor were there
any differences between the control and test groups with respect to minor
1020
skeletal variations. Peters and Cook gave pregnant rats (stock un-
stated) intraperitoneal injections of three phthalate esters at 0. 5, 1, 2,
and 4 mg/k.g on gestation days 3, 6, and 9. One of the chemicals apparently
prevented implantation, and the others greatly reduced the mean litter size
and number of offspring weaned. Two anophthalmic offspring were noted.
303
-------
743
Kotin and Thomas exposed young C57BL male and female mice
in an inhalation chamber to smog formed by reacting gasoline with ozone
or urban atmosphere. Results indicated that low concentrations of
atmospheric pollutants impaired the conception rate, marginally
reduced the litter size, and were severely harmful to preweaning off--
617
spring. Congenital malformations were not mentioned. Hueter et al.
exposed LAFI mice to irradiated and nonirradiated automobile exhaust
and also noted marked decreases in fertility and postnatal survival of
795
neonates; again, no malformations were mentioned. Lewis et al.
in an elaborate extension of the studies noted above with irradiated
and nonirradiated automobile exhausts, essentially confirmed these
results. In addition, some exposed females were killed near term,
but no increase in fetal resorption was found. In the abovementioned
three studies, lack of effects should not be referred to vapor-phase
organics, in that ozone and other photochemical products -were probably
also present. Decreased fertility and poorer survival of the litter have
617
been noted in mice exposed to irradiated exhaust. The materials in
this exposure included ozone at 0. 6-1. 0 ppm, carbon monoxide at
60-100 ppm, hydrocarbons at 20-36 ppm, and nitrogen dioxide at
2. 9-3. 9 ppm. When males were preconditioned by exposure to arti-
fically produced oxidant smog, the nonpregnancy incidence doubled
beyond the increase produced by exposing virgin females to the
exhaust. Additional experiments indicated that the preconditioning
of the males produced an increased number of neonatal deaths
559
in the mice.
1427
Younoszai ^t al. placed pregnant Holtzman rats in chambers
where they were forced to inhale smoke from various types of cigarettes
304
-------
for 4 min five times a day on gestation days 3-22. Fetuses from all
experimental groups were growth-retarded, but those from females
exposed to tobacco cigarette smoke were most severely affected. Re-
duction in maternal food consumption may have accounted for some,
but probably not all, of the retardation. Litter size was not reduced.
Congenital malformations were not mentioned.
CARCINOGENIC EFFECTS
In studies of air pollution, much emphasis has been placed on
carcinogenic aromatic hydrocarbons, aromatic heterocyclics, and
their chemical transformation products. This is the main subject of
950
an earlier document.
Two major efforts at reviewing carcinogenic exposures are under-
way. The first is by the National Cancer Institute, which publishes
and periodically updates a survey of materials evaluated for carcino-
1291a,1291b,1291c
genie risk. This survey can be consulted for any
particular compound to see who has tested it, what animals were used, the
duration of exposure, and the results. Such a survey is not intended
to interpret the carcinogenicity. The second type of review, through
the International Agency for Research on Cancer (IARC), is a compre-
hensive and interpretative review of both experimental and human popu-
lation experience and consists of a series of monographs entitled
Evaluation of Carcinogenic Risk of Chemicals to Man. Seven such volumes
824a, 824b, 824c, 824d,824f ,824g,824h
have been published so far. Few of the compounds of
interest to this Panel have not been the subject of IARC monographs.
1099 1302
There is increasing evidence from chemical and biologic
studies that vapor-phase nonaromatic compounds in polluted air contribute
305
-------
to the total carcinogens present. It is known that aliphatic and olefinic
hydrocarbons are major vapor-phase organic air pollutants from a
variety of sources. These compounds themselves have not been shown
to be active carcinogens. However, some of their chemically reactive
transformation products obtained by interaction -with singlet oxygen,
molecular oxygen, ozone, sulfur oxides, and nitrogen oxides might be
expected to be carcinogenic. In addition to or rather than being carcino-
genic, vapor-phase organic pollutants might be tumor-promoting or co-
.1108
carcinogenic agents.
A number of basic considerations related to carcinogenesis are
route of exposure, test methods and group sizes, dose-response relations,
tolerance limits, effects of chronic irritation on carcinogenesis, and
950
aging versus carcinogenesis. These are discussed elsewhere, and many
authoritative reviews on one or more of these subjects have appeared in
242,321
recent years. They will therefore be elaborated on in this sec-
tion only if directly relevant to vapor-phase organic pollutants.
Of the many known or suspected vapor-phase organic pollutants,
only a few have been tested for carcinogenicity. Moreover, the route
of exposure most relevant to human lung cancer induction due to air
pollutants--!, e. , inhalation exposure—has not been commonly used for
known vapor-phase components. It has been used, however, in the ex-
740,741
posure of mice to ozonized gasoline. A few compounds have
1307
also been tested by intragastric instillation in rats. Most of the
compounds tested were examined by skin application, subcutaneous
414,737,1245,1302,
injection, or intraperitoneal injection in mice or rats.
1305, 1306, 1308, 1309, 1337, 1365
306
-------
A built-in difficulty in the carcinogenicity assay of the compounds
of interest is the fact that many of them are highly reactive chemically.
The chemical reactivity of these agents is discussed elsewhere in this
volume. The compounds undergo, in vivo, a -wide variety of reactions,
most of which probably bear no relation to the process of cancer induc-
tion. The bioassay of these compounds has usually resulted in the induc-
tion of tumors only at the site of .administration and only with high dosages.
An examination of the chemistry of VPOP presented in earlier sec-
tions of this document reveals that only a few compounds of interest have
actually been detected in polluted air. Many have been produced in the
laboratory in solutions, and a few have been detected in vapor-phase
experiments. These are described in other sections of this report. In
evaluating the known carcinogenicity data on VPOP, it is therefore wise
to consider structurally related chemicals, also.
On the basis of limited bioassays, the following compound types have
generally been classed as noncarcinogenic: aliphatic and olefinic hydro-
carbons, aldehydes, ketones, fatty acids and esters, and simple ethers.
However, some of these compounds, notably long-chain aliphatic hydro-
carbons and some fatty acids, have been implicated as cocarcinogens or
1108
tumor promoters, on the basis of mouse skin experiments. These
will be discussed later. Some of the known compounds in these classes
have not been adequately tested for carcinogenic activity.
Some of the oxidized products of VPOP have been shown in experi-
mental animals to have carcinogenic properties. These include epoxides,
3-lactoneis, hydroperoxides, peroxides, and the peroxy acids and esters.
1302
Van Duuren has reviewed these experimental findings.
307
-------
Very few, if any, of these oxidized hydrocarbons have been determined
to be present in air; the compounds that have been tested have been tested
by skin applications or subcutaneous injection in mice or rats, and they were
determined to be carcinogenic when used at relative high dosages--for
example, 0. 1-10 mg/application once or twice a week for 1-2 years. Be-
cause these materials are highly reactive chemically, with reactions
occurring at these dosages, the data indicate that in this system they are
weak carcinogens. Community exposures to these substances would occur,
if at all, at very low dosages. Such exposures might indeed be a matter of
concern, but cannot be evaluated without further data.
It is reasonable to question whether the induction of subcutaneous
sarcomas by such compounds at the site of injection, particularly in the
rat, implies carcinogenicity for humans. But the results of experiments in
i
which several routes of administration were used, including subcutaneous
1308
injection in rats, indicates good agreement among the various test systems.
The most extensively studied group of compounds in this series are the
epoxides. On the basis of these bioassays and studies on chemical reactivity--
1305
e. g. , rate of hydrolysis and reactions with a variety of nucleophiles it
has become possible to draw some conclusions about chemical structure and
1302
reactivity, on the one hand, and carcinogenicity, on the other. Thus,
bifunctional epoxides are more likely to be carcinogenic than monoepoxides.
This may be due, in part, to their cross-linking ability. Carcinogenicity
is more often observed in bifunctional epoxides -whose structures are
flexible, than in those whose structures are rigid. An excellent example
of this is diepoxybutane, an open-chain flexible structure, which is car-
cinogenic. Its rigid cyclic analogue, 1, 2, 3, 4-diepoxycyclohexane, is
noncarcinogenic.
308
-------
No carcinogenicity data are available on ozonides; and, even for the
other compound types listed, except epoxides, the data are extremely
sparse.
A series of aliphatic nitro compounds have been tested for carcino-
genic activityy by a variety of routes. These experiments are summarized
1362 1267.
by Weatherby and Treon and Dutra.
A number of VPOP components may not be carcinogenic, but may
exhibit cocarcinogenic or tumor-promoting activity. When used in animal
studies, these two types of agents usually result in little, if any, tumori-
genic activity. However, when they are applied at various intervals before,
with, or after another agent (usually a moderate exposure to a carcinogen),
they result, in positive tests, in a high incidence of tumors. The best-known
1303
laboratory model is the mouse skin system. In tumor-promotion tests,
a single application of 7, 1 2-dimethylbenz [a] anthracene is followed by re-
peated application of the material being tested as a tumor-promoter. In
cocarcinogenesis experiments, the two agents are applied simultaneously
and repeatedly. In these tests, the compound being tested as a cocarcino-
gen is applied with a low dose of carcinogen, such as benzo[a]pyrene. In
both tests, the combination of agents results in a much higher yield of
tumors than either agent alone, and tumors appear much more rapidly.
Not all cocarcinogens have tumor-promoting activity, and vice versa.
Most of the components expected in vapor-phase organic pollution
have not been tested for tumor-promoting or cocarcinogenic activity. An
1304
exception is; phenol. Additional studies are needed to determine
whether this kind of biologic activity is to be expected from vapor-phase
1108
pollutants. The long-chain hydrocarbon dodecane has both tumor-promoting
309
-------
and cocarcinogenic activity. Several long-chain alcohols, particularly
the CL and C,, compounds, have shown accelerating activity when applied
with benzo[a]pyrene.
It is probably well to point to the experience with another well-
known environmental carcinogen—cigarette smoke. It is generally agreed
that the carcinogenic aromatic hydrocarbons of cigarette smoke do not
1312
account satisfactorily for its carcinogenicity on mouse skin. Two-
stage carcinogenesis tests with 7,12-dimethyIbenz[a]anthracene as
initiating agent followed by repeated application of cigarette tar have
1312
resulted in a marked tumor-promoting effect. Some of the components
of cigarette smoke are known to have such promoting activity—e.g.,
1304
phenol. Long-chain aliphatic hydrocarbons, alcohols, and carboxylic
acids are expected to be biologically active cofactors (cocarcinogens or
promoters) in cigarette-smoke condensates.
310
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CHAPTER 7
EPIDEMIOLOGIC APPRAISAL OF HUMAN EFFECTS
ANALYTIC STRATEGY
Vapor-pi base organic air pollutants constitute a large class
of substances, and community exposures are usually to mixtures
of several of them. Although mechanisms of action and sometimes
of uptake and metabolism of individual pollutants are often known,
these facts are not necessarily adequate to evaluate the likely
effects on a given population of a given exposure. For such an
evaluation, epidemiologic studies are needed. However, few such
studies are known; and when effects are studied, it may be diffi-
cult to separate those of a specific substance from those of other
substances present.
Despite these difficulties and because it is believed that
the usefulness of available knowledge is related to what is done
about exposures, a strategy is proposed here to make the best
possible use of all the information we have and to focus attention
on the types of information still needed.
The strategy consists of classifying the vapor-phase organic
pollutants according to major chemical class; estimating the
average and maximal population exposures in terms of concentration
and duration for exposed groups that vary in susceptibility;
identifying available data on human population effects; classifying
estimated maximal exposures into regions of indifference, incremental
effect, imperative action, and ignorance; stipulating, on the
311
-------
basis of experimental studies, which substances might be important if suita-
ble population studies were done; and-interpreting the findings with respect
to policies that affect control and research. This strategy can be applied
to three types of exposure situation: community-wide atomospheric pollution,
occupational exposure, and household or other localized exposure. Among
community-wide exposures, motor-vehicle exhaust hydrocarbons in photochemical
smog, are of greatest concern. Occupational exposures, although not the
primary focus of this Panel's work, account for a substantial portion of our
knowledge of the effects of specific substances on human populations.
Localized exposures are the hardest to recognize and hence to study. Uses
of vapor-phase organic pesticides in the home provide the paradigm for these
exposures.
The proposed classification consists of five major classes: aliphatic
substances (A), aliphatic oxygenated substances (0), aromatic substances
(R), transient products of vapor-phase hydrocarbon pollutant reactions (T),
and relatively stable products of such reactions (S). Each class has sub-
classes. It is suggested that the subclasses of chief interest be the
following:
Al, saturated aliphatic hydrocarbons
A2, monounsaturated aliphatic hydrocarbons
A3, unsaturated aliphatic hydrocarbons of higher order
A4, sulfur- and nitrogen-containing aliphatic hydrocarbons
312
-------
01, saturated aldehydes
02, unsaturated aldehydes
03, ketones
04, organic acids
Rl, benzene
R2, toluene
R3, other substituted benzene compounds
Tl, active states of oxygen produced in hydrocarbon
reactions
T2,, free radicals produced in hydrocarbon reactions
T3., other high-energy compounds produced in hydrocarbon
reactions
SI, ozone, which may be produced in hydrocarbon
reactions
S2, PAN, nitroolefins, and homologues produced in
hydrocarbon reactions
S3, nitrogen dioxide, which may be produced in hydro-
carbon reactions
S4, other products of hydrocarbon reactions
32,1290
Table 7-1 presents the data available for the average and maximal
community exposure to air pollutants of the A, 0, and R classes in the form
of atmospheric concentrations. The data are from Los Angeles. Table 7-2 gives
198,1291
some similar data for T and S classes. Statements from the American Con-
40,41
ference of Governmental Industrial Hygienists, on the effects of specific
313
-------
substances, particularly on workmen, are given in Appendixes
C and D. Exposures of workmen are often more intense, more
sharply restricted in time, and better defined in composition
and involve less sensitive populations than community exposures.
Hatch has analyzed the role of permissible limits for hazardous
airborne substances in the working environment. This classification
procedure is based partly on his analysis.
EXPERIENCE WITH CLASSIFYING POLLUTANT EFFECTS
The next step in the strategy deals with the fundamental
relationship of the epidemiologic evidence on health effects to
the policy implications of this evidence.
When the relationships of health to environmental exposures
were first subjected to systematic study, the dramatic illness
was the phenomenon that attracted at tention--for example, typhoid
fever, cholera, lead poisoning, radiation-induced skin cancer,
and deaths or illness during air pollution disasters. Quali-
tative associations were the first objects for study; later,
epidemiologic and toxicologic studies of quantitative dose-
response relationships were made, to determine how much reduction
in exposure was needed to avoid these devastating effects.
502
In the 1950's, nondisease effects of pollutant exposures
began to be noted, and qualitative and then quantitative effects
of exposures of populations were reported. In 1959, when California
first established air quality standards on a scientific basis,
three categories were proposed—"adverse," "serious," and
"emergency," reflecting different degrees of apparent health risk.
This policy was reflected in the following statements: "The
effects of air pollutants vary both in kind and in severity. The
314
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seriousness of the effect determines the urgency of control.
A graded set of standards was established which recognized this
relationship. Three levels of air pollutants were defined as
follows:
"I. 'Adverse' Level. The first effects of air pollutants
are those likely to lead to untoward symptoms or discomfort.
Though not known to be associated with the development of disease,
even in sensitive groups, such effects are capable of disturbing
the population stability of residential or work communities.
The 'adverse' level is one at which eye irritation occurs. Also
in this category are levels of pollutants that lead to costly and
undesirable effects other than those on humans. These include
damage to vegetation, reduction in visibility, or property damage
of sufficient magnitude to constitute a significant economic or
social burden.
"II. 'Serious' Level. Levels of pollutants, or possible
combination of pollutants, likely to lead to insidious or chronic
disease or to significant alteration of important physiological
function in a sensitive group, define the 'serious' level. Such
an impairment of function implies a health risk for persons con-
stituting such a sensitive group, but not necessarily for persons
in good health.
"III. 'Emergency' Level. Levels of pollutants, or combina-
tion of pollutants, and meteorological factors likely to lead to
acute sickness or death for a sensitive group of people, define
the 'emergency1 level."
In 1963, at a World Health Organization Symposium on Air
Quality Criteria and Methods of Measurement, following Ryazanov's
suggestion, the three-category system was modified by adding a
315
-------
"no-effect" category. The system defined boundaries for pollu-
tion exposures that produce specified effects, expressed as
follows:1419
"1. Criteria for guides to air quality are the tests which
permit the determination of the nature and magnitude of the effects
of air pollution on man and his environment.
"2. Guides to air quality are sets of concentrations and
exposure times that are associated with specific effects of vary-
ing degrees of air pollution on man, animals, vegetation and on
the environment in general.
"3. In the light of present knowledge, guides to air
quality may be presented as four categories of concentrations,
exposure times and corresponding effects. These four categories
are defined by limiting values which may vary for a given pollutant
according to the anticipated effect or the criteria used and in
relation to other co-existing pollutants and the relevant physical
factors, and which take into account the varying responses of
different groups of human beings. The Symposium agreed to define
the four categories in terms of the following levels:
"Level I. Concentration and exposure time at or below which,
according to present knowledge, neither direct nor indirect
effects (including alterations of reflexes or of adaptive or
protective reactions) have been observed.
"Level II. Concentrations and exposure times at and above
which there is likely to be irritation of the sensory organs,
harmful effects on vegetation, visibility reduction, or other
adverse effects on the environment.
316
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"Level III. Concentrations and exposure times at and above
which there is likely to be impairment of vital physiological
functions or changes that may lead to chronic diseases or shortening
of life.
"Level IV. Concentrations and exposure times at and above
which there is likely to be acute illness or death in susceptible
groups of the population.
"For some known pollutants, it may not be possible to state
concentrations and exposure times corresponding to all four of
these levels because (a.) the effects corresponding to one or more
of these levels are not known to occur with the substance in
question, or (^b_) exposures producing effects corresponding to
certain levels also produce more severe effects, or (c_) the
present state of knowledge does not permit any valid quantitative
assessment (e.g., of threshold levels for carcinogenic substances).
"The possibility that some pollutants may have mutagenic
effects must be borne in mind; however, at the present time, too
little is known about this subject to permit classification of
such pollutants in the above categories."
In the meantime, "criteria" and "standards" were being
distinguished; "criteria" referred to exposures that, on the
basis of cited evidence, could produce specified effects on a
defined population--for example, sufficient exposure to photo-
chemical oxidant to increase the likelihood of an attack of asthma
in a population of adult asthmatics. ^ "Standards" were numeri-
cal objectives of an air pollution control program, formulated
by a politically authorized agency of government and presumably
related in some systematic way to criteria.
317
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DIFFICULTIES IN INTERPRETATION OF POLLUTANT-EFFECT CLASSIFICATION
Standards, whether supposedly differing in the gravity of
their health effects, as in California, or based on different
criteria, as with those set by EPA, all tended to be perceived
as though their violation implied a risk of widespread disease
or of increased mortality. Disease and nondisease effects tended
to be considered without distinction, and some types of nondisease
effects--for example, annoyance or other responses to odorous
pollution'0 --were largely ignored, in part because of this lack
of discrimination. At present, if a standard has been set, it
is inferred that it must be met, regardless of cost or secondary
effects. In the view of some, this is an extreme interpretation
of the available data and should be reconsidered.
Because the present report may become input to such a
process, this portion properly considers the classification of
effects of community exposures. It is proposed that the ideal
output from studies of the biologic effects of pollutants on
human populations consist of two sets of boundary conditions and
the dose-response function(s) connecting these boundaries.
100% '
Probability
of
on the
Populat ion
0%
Indifference i / Incremental /Imperative Action
Region i / Region / Region
No Known i / Nondisease / Disease or Fatality
Effect y Effect
ro ro
Effect
o c
HI
HI rt
Dose (in terms of concentration and duration) ro p.
rt 3
FIGURE 7-1. Diagram showing a proposed classification of pollutant
effects in terms of probability, dose, and "regions" associated with 3
different classes of effects. Broken line represents possible "no ^
appreciable effect" dose.
318
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Figure 7-1 shows diagrammatically what these boundary conditions
imply. For most pollutants, a dose can be denoted that is not
known on the basis of generally accepted data to have any effect
c /: o
(called by Hatch the upper limit of normal psychophysiologic
function). With sufficient research, the several concentration-
duration dosages that are without effect on even sensitive popu-
lation groups can be cited, along with the slightly higher ex-
posures tolerated without known effect by the more robust sectors
of the population. Such data define the lower boundary conditions
shown. The boundary conditions may not be very precisely defined,
but the variations in population sensitivity tend to make the
boundary curved. Above this boundary there is a zone of non-
disease effects within which decreased pollution is associated
with decreased health risks for the population, and increased
pollution with increased health risk. The second set of boundary
conditions is that above which, for at least some sector of the
population, there is likely to be production or aggravation of
illness or fatality.
Note that such boundary conditions require four statements
for a full description of what is known or believed to be true:
concentration of a pollutant (or index of pollution), duration
of exposure, target population, and probability of a defined
effect.
The usual dose-response relationships of interest are those
which define nondisease effects for various groups, for these
also define the magnitude of health impairment that control of
pollution, under some conditions, can prevent or avoid.
319
-------
Associated with the three regions defined by the two
boundary conditions for a pollutant or combination of pollutants
i
are appropriate regions whose names imply corresponding community
responses. Below the boundary of no health effect, there is a
region of "indifference." Although other effects of pollution
may be occurring, such as on materials or on visibility, any
change in pollution within the region is a matter of indifference
from the point of view of health. Above the upper set of
boundary conditions is a region of "imperative action" to
protect health. Action is needed, regardless of cost or dis-
ruption of other activities, for health is threatened and the
damage may be irreversible. The intermediate region is one of
"incremental health benefit." The lower the pollution, the
better health is protected. But in this region, cost of re-
ducing pollution is relevant and is appropriately compared
with health benefits and other possible applications of the
available resources to other problems.
The public health objective with respect to pollution is
ultimately to maintain conditions in the region of indifference
for all people at all times. However, such an objective cannot
be achieved at once, ignoring all other social goals. Further-
more, research tends to move the left-hand (lower) boundary to
the left as new nondisease effects of low concentration are
found. The objective must be modified when this occurs.
For many pollutants we do not know enough about the two
boundaries, much less about the dose-response relationships.
Nevertheless, we can often rank pollutant exposures according to
320
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those probably within the region of indifference and those
probably in the region of incremental health benefit, using
as a basis for reference what is known about the upper boundary
and its "distance" from the given exposure conditions. For
some pollutants, we know so little that we can say only that
we are in a "region of ignorance."
The foregoing proposal is based on exposures of human
populations. Data on animals are, as it were, on another plane;
although such data influence the nature of studies undertaken
and their interpretations, the quantitative exposure conditions
for animals define only a projection or shadow area on the
plane of dose-probability-population relationships for man.
Concerning the vapor-phase organic pollutants, we may now
apply this scheme to the available epidemiologic data on popu-
lation exposure, using occupational exposure data where no
other data are available. In carrying out such a classification,
it is. important to recognize some limitations. Short-term
effects, such as irritation, are more likely to be recognized
than are long-term effects, symptomatic effects more likely
than asymptomatic ones, and specific more likely than non-
specific, ones. Furthermore, the importance of identifying
sensitive groups and studying their responses must be stressed.
These problems can be illustrated by the discussion of specific
pollutant exposures. However, there is a further set of problems
derived from the nonspecific nature of the effects of pollutants
human populations.
321
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Although photochemical oxidant is largely ozone, ozone does not
1143
account for the eye and respiratory irritation. Ozone experimentally
causes alterations in pulmonary function that persist after exposures are
terminated, but this is not usually detected epidemiologically (with
sufficient effort, it probably could be). Association of oxidant with
asthma cannot be attributed to any single agent. Cigarette-smoking
exposures and air pollutant exposures often co-exist, and exposures to
nitrogen oxides, particles, polycyclic aromatic hydrocarbons, and carbon
monoxide occur in both smoke and pollution exposures, although in
different time-concentration relationships. Although there is an urban
excess of lung cancer that appears to be independent of the effect of
cigarette smoking, the roles of specific pollutants and even the contri-
bution of urban air pollution cannot be isolated for purposes of estimating
dose-response relationships. Yet, by weighing the available evidence
and looking at it carefully from any points of view, -we may make some
reasonable statements, as the work of groups like the ACGIH and some of
the work on water and air quality and radiation protection criteria and
standards illustrate.
INTERPRETATION OF.AVAILABLE DATA
The numbers that are available are shown in Table 7-1. The first
class of paraffins is headed by methane, which in Los Angeles from
September to November 1966 averaged 3. 22 ppm. The maxima in
1965 were three to four times these concentrations. We might
say that the maximal concentration of exposure to methane
322
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that could occur in Los Angeles is 10-12 ppm, with concentrations
of other members of the class being proportionately lower, according
to the approximate ratios derived from Table 7-1. Similar state-
ments could be made from Table 7-1 about ethylene, benzene,
toluene, total aldehydes (or formaldehyde), and acrolein. It
is likely that these are the highest community-wide values for
human population exposures from mo tor-vehicle exhaust. Locally
higher concentrations could occur because of solvent usage and
petroleum refining, distributing, and marketing.
Using the data on threshold limit values (TLV's) of the
ACGIH in Table 7-2, we see that the primary pollutants in sub-
class Al are considered "simple asphyxiants" and that, except
for ji-pentane, no TLV's are given. Thus, in subclasses Al,
A2, and A3, none of the exposures is outside the region of
ind ifferenc e.
For benzene, the TLV is 25 ppm, and for toluene, 100 ppm.
Thus, the exposure to benzene is within two orders of magnitude of
the TLV, and the effect is a long-range one. Exposures to
benzene may be in the region of indifference, but they deserve
further discussion because of the possibility of long-term
effects.
Epidemiologic Evidence on Benzene
Benzene, a widely used solvent, has had to be used with restrictions
because of its tendency to produce changes in red-cell formation, apparently
through toxic effects on bone marrow (cf. Chapter 6).
323
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The results of excessive occupational exposures are
aplastic anemia and other serious hematologic problems. The
latent period for low e.xposure is very long. In some
respects, the effects resemble those of exposures to radiation.
The lowest persistent exposures that had effects were to about
100 ppm. Exposures to 5-25 ppm have no known effects. These
findings are related to disease effects and do not necessarily
reflect the application of such methods for detecting nondisease
effects, for which an example might be mean red-cell turnover
rates. Another example of a possible nondisease effect has
recently been reported by Forni et a_l.,^^ who examined
lymphocytes from 25 people who had recovered from occupational
benzene poisoning. A high frequency of stable and unstable
chromosomal abnormalities was found in the exposed populations,
compared with control groups. For example, the cells from
control subjects showed 0.5% abnormal metaphase patterns, where-
as the cells from the subjects with past benzene hemopathy
showed 1.9% abnormal patterns.
We may assume that the affected people were regularly
exposed to more than 100 ppm 8 hr/day. Therefore, both the
concentrations of the material and the durations were much
greater than that likely in community exposures.
The data in Table 7-1 for Los Angeles mean that the
average concentration is 0.032 ppm for 1965 and 0.015 ppm for
autumn 1966 and the highest measured was 0.057 ppm in terms
that are equivalent to the data for occupational health exposure
Thus, the TLV is about 450 times the highest atmospheric concen-
tration. The average in 1966 was 0.015 ppm, about half that in'
324
-------
1965, 0.032 ppm.1290 It is important in citing such numbers
to know the concentrations in areas remote from the influence
of motor-vehicle emission and to know more about the minimal
detectable concentration and differences in concentrations.
In view of the widely discussed possibility that, in
response to the need for unleaded gasoline, a higher proportion
of aromatic fuel stocks may be used to sustain octane ratings,
we may need to assess the health importance of increasing use
of motor fuel containing benzene and related compounds. This
assessment should be based on several considerations that go
beyond the available data. Evaluation of past occupational
exposures would be of critical importance for this assessment.
First, monitoring of benzene should be systematically undertaken
at highly polluted and remote sites; second, exposures of and
possible effects in persons who fill gasoline tanks should be
assessed; and third, estimates of benzene in exhausts from the
various types of vehicles and control systems in use should be
made. From the available evidence, we conclude that community
exposure to benzene is in the region of indifference as far as
health is concerned; but if any substantial increase in aromatic
motor-vehicle fuel is contemplated, these three steps will need
to be undertaken to ensure that no avoidable health hazard
occurs.
EVALUATION OF OTHER HYDROCARBONS ;
For toluene, the TLV is 200 ppm, and maximal exposure is
so much lower that this also is in the region of indifference.
325
-------
For formaldehyde, the TLV is 2 ppm; the maximal value for
total aldehydes is 1.3 ppm. The effect of formaldehyde at low
concentrations is that of an irritant. Much of the effect of
photochemical oxidant on the eyes and respiratory tract is
attributed to the irritating effects of formaldehyde. ^
Thus, exposures to formaldehyde may very well be in the region
of incremental effects. However, these circumstances need to
be defined further; to the best of our knowledge, the effects
would be transient. The matter is discussed below.
Although the more complex compounds (subclasses 02, 03, and
04) may very well be much more toxic or irritating, their con-
centrations are almost certainly less than those shown in
Table 7-1. Chlorinated or other substituted homologues may
have greater toxicity, but are unlikely to be atmospheric
pollutants in the usual community-wide sense. They may be
occupationally important, and they may contribute to localized
pollution. Therefore, from the point of view of community
exposure, we should concentrate attention on formaldehyde.
Epidemiologic Evidence on Formaldehyde
This section of necessity deals with total aldehydes
expressed as formaldehyde, as well as with what is known of
the individual agents. Two recent reviews are available and
are appropriately summarized and compared: Air Quality Criteria
i
for Hydrocarbons129 and Community Air Quality Guides.255
To summarize from the former: "The most characteristic
and important effect of aldehydes, particularly of low-molecular
weight aldehydes in both humans and animals is the primary i
326
-------
of the eyes, upper respiratory tract, and skin. Aldehyde con-
centrations have been shown to correlate with the intensity of
odor of diesel exhaust and to some extent with the intensity of
eye irritation during natural and chemically produced smogs. ...
Aldehyde levels referred to as 'low1 in toxicological reports
are usually much greater than concentrations routinely found
in ambient air.... In most cases, the general and parenteral
toxicities of these aldehydes appear to be related mainly to
these irritant effects. The unsaturated aldehydes are several
times more toxic than the corresponding aliphatic aldehydes,
and toxicity generally decreases with increasing molecular
weight within the unsaturated and aliphatic aldehyde series ....
Sensitization of the respiratory tract is produced rarely, if
at all, by inhalation of aldehydes."
Renzetti and Bryan have shown an association between
eye irritation in a panel and aldehyde concentrations in Los
Angeles;. Regression curves of the effect of atmospheric con-
centrations of total aldehydes and formaldehyde on panel eye
irritation are given in Figures 7-2 and 7-3, respectively.
From Figure 7-2 we may infer that, with total aldehyde
above 0.05 ppm, there is likely to be a progressive increase in
eye irritation with exposure. Of course, this association does
not prove that aldehyde is the cause; other photochemical products
are also likely to be present.
However, Schuck et a_l. ^ have demonstrated (Figure 7-4)
that substantial eye irritation from irradiated synthetic
atmospheres containing nitrogen dioxide and various hydrocarbons
327
-------
can be explained by concentrations of formaldehyde. They
3
suggest that formaldehyde concentrations as low as 12 vig/m
(0.01 ppm) could cause eye irritation.
Other substances probably contribute to the eye irritation of
photochemical pollution, but most scientists accept a significant
contribution of aldehydes.
255
Community Air Quality Guides states that, "as con-
stitutents of photochemical smog, acrolein and formaldehyde
are highly suspected eye irritants." Although noting that
1071
35% of the panelists experienced eye irritation at 0. 10-ppm
formaldehyde, it nevertheless concludes that "there is no evi-
dence to suggest that sensory irritation of any form will be
present if the atmospheric aldehyde concentrations are held
under:
"0. 1 ppm formaldehyde
"0. 01 ppm acrolein
"0. 2 ppm total aldehyde as formaldehyde"
1071 1143
The evidence of Renzetti and Bryan and Schuck et al.
refutes this conclusion. Admittedly, the work leaves many ques-
tions, concerning unusually sensitive groups, other substances,
and the consistency between experimental and epidemiologic data.
The TLV's for formaldehyde and acrolein are within about
an order of magnitude of the maximal observed values in Los
Angeles. We concluded that in Los Angeles, but possibly not
elsewhere, aldehyde exposures have been in the incremental
region, on the basis of their probable role in producing eye
and respiratory tract irritation. These effects are due largely
328
-------
to photochemical reaction products, and not solely to alde-
hydes emitted as such to the atmosphere.
APPRAISAL OF TRANSIENT REACTION PRODUCTS
The transient reaction products of concern are peroxy-
acetylnitrate (PAN), peroxybenzoylnitrate and its homologues,
and singlet oxygen. In the first case, the maximal concentra-
tion is 0.1 ppm and the minimal concentration that under experi-
mental conditions produces health effects is about 0.3 ppm.
So this substance may be relevant to community exposures.
However, few systematic measurements have been made, and no
epidemic Logic data are available. We are thus in the region
of ignorance for PAN and homologous substances.
Concerning singlet oxygen, we need to know the maximal
concentration and the minimal concentration that produces health
effec'ts. We will therefore have to carry out further research
to evaluate this problem. We are in the region of ignorance
for singlet oxygen.
Concerning ozone, the maximal concentration is 0.90 ppm,
and the minimal concentration that produces health effects
(based on the TLV for 8 hr) is 0.1 ppm. From epidemiologic
data, for ozone, at least in Los Angeles, we are well into the
region of incremental effects. This is discussed more fully
below.
329
-------
Epidemiologic Evidence on Ozone and Oxidants
There are at least two separate problems to be considered
here: the contributions of specific hydrocarbons to the noxious-
ness of photochemical pollution, and the epidemiologic data on
the effects of oxidant pollution. With respect to the latter, data
from Los Angeles, scant as they are, constitute the major source
of useful information. With respect to the former, there is always
a risk that the use of yields of ozone, aldehydes, or phytotoxins
will not adequately reflect the short- or long-term health effects.
For the present, we must assume that potency with respect to
eye irritation or alterations of pulmonary function in animals is
equivalent to health impact of hydrocarbon ingredients in photo-
chemically reacted atmospheres.
A representative result for eye irritation is shown in
1591
Table 7-3. Table 7-4 is similar, but uses a different
criterion for potency, the reactivity for photooxidation of
nitric oxide to nitrogen dioxide.
591
Heuss and Glasson sought relationships between eye
1423
irritation and hydrocarbon reactivity. Yeung and Philips
have identified the structures of hydrocarbon molecules that
relate photochemical reaction to eye irritation potency. The
330
-------
most potent is that which yields benzaldehyde, a precursor of
the highly potent, but transient, peroxybenzoyInitrate. Next
in importance is the structure that yields formaldehyde, and
then that which forms aliphatic aldehydes or lower yields of
formaldehyde. Fully substituted olefins, for example, the
class that yields ketones, produce weak or minor eye irritants
with photochemical irradiation. Some compounds--for example,
1,3-butadiene—form two eye irritants (in this case, formal-
dehyde and acrolein) per reactive molecule, and special cate-
gories of reactivity need to be set up for them.
It appears that the basis for eye irritation is more complex
than a single substance (see Figure 7-^4, for example; the methods
and scales differ from those of Figure 7-3).
Data on ozone and oxidant effects have been
reviewed.198'200'1136'1291'1419 The results are summarized
in Tables 7-5 and 7-6 .
We may summarize these findings as showing evidence of
disease aggravation (asthma) at oxidant concentrations of
0.25 ppm (peak) or 0.20 ppm (hourly average) and impairment
of lung function in persons with chronic respiratory disease,
eye irritation, and impairment of athletic performance at
about 0.10 ppm (hourly average). There is evidence of an
increased likelihood of motor-vehicle accidents in Los Angeles
1300
during periods with increased oxidant. The quantitative
relationships of the exposures that produce such an effect have
not been established.
331
-------
The disease aggravation data can be interpreted as putting
a peak dose of 0. 25 ppm briefly or 0. 20 ppm for an hour in the
imperative action region involving sensitive groups. However,
this is based on a single study, and equivalent concentrations
of oxidant at other times and places may well differ from those
in Pasadena in 1956. An hourly average of 0. 1 ppm is near the
lower boundary of the region of incremental effects. The ex-
perimental and occupational exposure data in Table 7-5 are con-
sistent with the community exposure data on respiratory system
effects due to ozone. The effects on eye irritation and on
track performance are not likely to be due to ozone alone.
1431, 1432
Recent studies with hamster lymphocytes indicate
that respiratory exposures to ozone at 0. 2 ppm for 5 hr produce
stable chromosomal breaks, which are interpreted as evidence
of a mutagenic effect of the agent. The authors suggest that
the margin of safety for permitted community exposures to ozone
based on mutagenic effect is less than one-hundredth the margin
of safety permitted for radiation exposures. These findings
and interpretations are of serious import and deserve prompt
and thorough evaluation. Because a similar question arises
for exposure to benzene, ozone effects should be added to the
evaluation previously proposed for benzene as a mutagen. The
1431, 1432
authors do evaluate the interaction of ozone and
irradiation for 5 hr to 230 rad and find that ozone and radiation
effects are approximately additive.
A recent evaluation of the effects of oxidant pollution
was that of the World Health Organization's Expert
Committee on Air Quality Criteria and Guides for Urban Air
332
-------
1418
Pollutants. The foregoing analysis is entirely consistent with
1419
the WHO report.
Epidemiologic Evidence on Oxides of Nitrogen
Nitrogen dioxide is produced by photochemical reactions
of vapor-phase hydrocarbons and nitric oxide. Nitrogen dioxide
on a molar basis is several times more toxic than nitric oxide.
The epidemiologic and toxicologic bases for air quality criteria
were reviewed at an Air Pollution Control Association symposium
926
in 1969. The contributors to the Symposium and the WHO
1418
Expert Committee agree that the data do not permit quanti-
tative statements about epidemiologic aspects of nitrogen dioxide.
The WHO Expert Committee says: "A study of second-grade (6-8
years of age) schoolchildren in Chattanooga, Tenn. , USA, com-
paring two low control areas and two 'high' pollution areas
(one for NO and one for suspended particulates) showed that
2
the reported FEV values were significantly higher in the
0. 75
control areas than in the 'high' NO area. The levels of
2
suspended particulates and SO did not seem to account for the
2
health effects. Another finding was that the incidence of acute
respiratory illness in the schoolchildren, their siblings, and
their parents was significantly greater in the 'high1 NO
2
area
3
"The average NO level of 190 Wg/m (0. 10 ppm) in the
2
'high' area was exceeded on 40%, 18%, and 9% of the days
respectively at the three monitoring stations; in the control
areas this level was exceeded on 17% of the days at only one
station. " Thus, differences in exposure to nitrogen dioxide
333
-------
were not shown, even though differences in respiratory disease
were demonstrated. The WHO Committee concludes: "The Committee
believes that there is insufficient information upon which to
base specific air quality guides at this time."
It is concluded that the epidemiologic data on health
effects of nitrogen dioxide exposure is in the region of ignorance,
SUMMARY AND DISCUSSION
There appear to be three substantive questions to which
this epidemiologic appraisal of vapor-phase organic air pollutants
is relevant:
1. What is the demonstrated or reasonably inferred effect
on health of likely exposures to these pollutants in the general
community?
2. What health consequences are likely to follow a modifi-
cation of the concentration or type of such pollutants, and in
particular the increase in aromatic content of gasoline stocks?
3. . What are the crucial gaps i-n our knowledge of health
effects and with what priority should we try to fill them? In
particular, are significant health threats implied by toxicologic
or experimental data that do not match with epidemiologic data?
This task has been approached by a strategy consisting of
classifying the major pollutants; estimating the average and
maximal population exposure; relating these estimates to likely
effects on human health, based mostly on occupational health
considerations; classifying the maximal estimated exposures into
regions with respect to expected effects on human health, implying
differing attitudes or action (indifference, incremental effect,
334
-------
imperative action, or ignorance); and discussing likely or
possible effects of classes not in the region of indifference.
Judgments concerning community exposures that are classed
in the region of indifference are based largely on occupational
exposure information and must therefore be qualified.
Direct human health effects of the following classes of
pollutants under conditions of likely exposure were believed to
be in the region of indifference:
Al, saturated aliphatic hydrocarbons
A2 , monounsaturated aliphatic hydrocarbons
A3, unsaturated hydrocarbons of higher order
R1, benzene (but with qualifications — see discussion
above)
R2, toluene and xylene
In the region of incremental effects are:
01, formaldehyde
SI, ozone and oxidants (possibly in the region of
imperative action with respect to sensitive groups)
In the region of ignorance are possible effects of:
II, T2, T3, transient products
S2, PAN, nitroolefins, and homologous compounds
S3, nitrogen dioxide produced in hydrocarbon reactions
335
-------
TABLE 7-1
Community Air Concentrations of Aliphatics,
w
Aliphatic Oxygenates, and Aromatics,
1965-1967
Los Ange
les
Average Concentration, ppm
Class and
Subclass
Al
J-
A2
A3
1
°2
Rn
1
R2
a
—Data from
Sub s tance
Methane
Ethane
Propane
n-Butane
Isopentane
n-Pentane
Ethy lene
Propene
l-Butene+
isobuty lene
Acetylene
Formaldehyde
Acrolein
Benzene
Toluene
Xylenes
1965
(218
3
0
0
0
0
0
0
0
0
0
0
0
Air Quality Criteria
Sept. -Nov. 1966^
Samples)- (26 days)
.22
.098
.049
.064
.043
.035
.06
.018
.007
.039
0.05-0.12-
0.004-0.009-
.032 0.015
.053 0.037
0.03
f U A U I290
for Hydrocarbons.
Autumn
3 .
0.
0.
0.
0.
0.
0.
—
--
0.
-
-
_
0.
0.
•t_
1967^
0
08
03
07
05
03
08
08
-
-
_
048
038
—Data from Altshuller et al.32
rt
—Three to four times average unless indicated otherwise.
-1951 data; total aldehydes, 0.20-1.30 ppm.
. 25-Nov. 15, 1961.
Maximal
Concentration, ppm—
0.011
0.057
0.129
0.12
-------
TABLE 7-2
Threshold Limit Values for Chemical Substances
in the Workroom Environment, 1972JL
Class
and Subclass
A3
TLV
Substance
A2
A3
01
02
Rl
R2
SI
S2
Me thane
Ethane
Propane
n.-But ane
Isopentane
ri-Pentane
Ethylene
Propene
Acetylene
Formaldehyde
Acrolein
Benzene
Toluene
Xylene
Ozone
Nitrogen dioxide
None cited; these are
simple asphyxiants
None cited
500 1,500
Simple asphyxiant
None cited
Simple asphyxiant
0. 1
100
100
0.1
0.25
375
435
0.2
—Derived from American Conference of Governmental Industrial
Hygienis ts.41
—Not to be exceeded.
337
-------
TABLE 7-3
Eye Irritation Potency of Various Hydrocarbons
in Irradiated Synthetic Atmospheres-
Hydrocarbon
n-Butane
ri-Hexane
Isooc tane
tert-Butylbenzene
Benzene
Ethylene
1-Butene
Tetramethylethylene
cis-2-Butene
Isopropylbenzene
sec-Butylbenzene
2-Methyl-2-butene
trans-2-Butene
c^-Xylene
_p_-Xy lene
1
1.
Potency—
0
0
0.9
0.9
,0
0
1.3
1.4
1.6
1.6
1.8
1.9
2.3
2.3
2.5
Hydrocarbon Potem
m-Xylene 2.9
1,3,5-Trimethylbenzene 3.1
1-Hexene 3 . 5
Propylene 3 . 9
Ethylbenzene 4.3
Toluene 5.3
ii-Propylbenzene 5.4
Isobutylbenzene 5.7
n.-Butylbenzene 6.4
1,3-Butadiene 6.9
a-MethyIstyrene— 7.4
Allylbenzene£. 8.4
8-Methylstyrene£ 8.9
Styrene£ 8.9
a 591
"Derived from Heuss and Glasson.
b
"Conditions: hydrocarbon at 2 ppm and nitric oxide at 1 ppm, except
where noted .
c
—Conditions: hydrocarbon at 1 ppm and nitric oxide at 0.5 ppm.
338
-------
TABLE 7-4
Reactivities of Hydrocarbons Based on Ability to Participate
in Photooxidation of Nitric Oxide to Nitrogen Dioxide
Relative Reactivities
Hydrocarbon
2,3-Dimethylbutene-2
2-Methyl-2-butene
trans-2-Butene
Isobutene
Propylene
Ethylene
1,3,5-Trimethylbenzene
m-Xylene
1,2,3,5-Tetramethylbenzene
1, 2, 4-Trimethylbenzene
_o- and _p_-Xylenes;
_o- and jD-Diethy Ibenzenes
Propylbenzenes
oluene
enzene
_n-Nonane
3-Methylheptane
ri-Heptane
Methylpentanes
Pent anes
2,2,4-Trimethylpentane
Butanes
Ethane
Methane
Acetylene
Altshuller
and Cohen
(1963)25 *
2
1
1
0.4
1.2
1
0.9
0.6
0.4
0.4
0.3
0. 2
0.15
0.15
0.15
0.15
0.1
Glasson and
Tuesday489
(1970)
5.3
1.7
2
0.6
0. 6
0.3
0.9
0.7
0.6
0.6
0.4
0.3
0.2
0.2
0.05
Levy et al
'
0
0
0
0,
0.1
0
0
0.03
<0.01
0. 3
0.5
0. 3
0.1
0.02
—Arbitrarily adjusted to the Glasson and Tuesday value for trans-2-butene
to permit comparison.
339
-------
TABLE 7-5
Health Effects of Experimental and Occupational Exposures
to Ozone as an Air Pollutant.3.
Effect
Odor detection
Exposure,
ppm
0.02
Duration
Comment
Reference
Odor detected in 9 of 10 subjects Henschler et al
within 5 min
579
Respiratory irrita-
tion (nose and
throat), chest
constriction
0.3 Continuous during
working hours
(8 hr)
Occupational exposure of welders Kleinfeld e t al.
(other pollutants probably also
present)
Changes in pulmonary
functions:
Diminished FEV-j^
1^ after 8 weeks'
0.5
Small decrements in 0.2-0.3,
VC, FRC, and DLCQ
in 3, 2, and 1,
respectively, out
of 7 subjects
Impaired diffusion 0.6-0.8
capacity (DLCQ)
Increased airway 0.1-1.0
resistance
Reduced VC, severe 2.0
cough, inability
to concentrate
Acute pulmonary 9.0
edema
3 hr/day
6 days/week for
12 weeks
Continuous during
working hours
2 hr
1 hr
2 hr
Unknown
Experimental exposure; change
returns to normal 6 weeks
after exposure; no changes
observed at 0.2 ppm
Occupational exposure; all 7
subjects smoked; normal
values for VC, FRC, and
DL
CO
based on predicted value
Experimental exposure of 11
subj ects
Increase in 1 of 4 at 0.1 ppm
and A of 4 at 1.0 ppm
High temperatures; one subject
Bennett
97
Young e_t al.1426
Young et^ al.1425
J.R. Goldsmith and
J. A. Nadel
unpublished data
Griswold et al.
523
Refers to peak concentration of Kleinfeld et al.
occupational exposure; most of
exposure was to lower concentration
715
a 193
—Derived from California Air Resources Board.
-------
TABLE 7-6
Epidemiologic Effects of Oxidant Pollution in Los AngelesfL
Effeet
Eye irritation
Exposure,
ppm Durat ion
0.1
Impairment of pul- Regres-
monary function sion
(airway resistance) about 0.1
Aggravation of
respiratory
disease--asthma
0.25.!
5 min
1 week in room
containing
ambient air
Peak value
Impaired performance Regression 1 hr or more
of student athletes about 0.1
C o lumen t
Reference
Result of panel response
Subjects were smokers and non-
smokers; most subjects had
emphysema
1071a
Renzetti
and Gobran
Remmers and
Balchum1069
Dry and Hexter1299
Patients exposed to ambient air; Schoettlin and
value refers to oxidant content
at which number of attacks
increased
Exposure for 1 hr immediately
before race
Wayne e_t al.1361
a 198
—Derived from California Air Resources Board.
—Equivalent to 0.20 ppm as an hourly average
-------
NUMBER OF REPORTS IS IN PARENTHESIS
0.02 0.03 0.04 0.05 0.10
ALDEHYDE CONCENTRATIONS, ppm (log 5C°le.'-.
0.20 0.30 0.40
Figure 7-2. Regression curve of effects of aldehyde concentrations
on eye irritation in panel studies. (Reprinted with permission from
Renzetti and Bryan.10'1)
342
-------
ro
O
Q.
ID
>-
ui
O
z
i-
n:
o
o.
LU
a.
UJ
U
o:
L'J
a.
20
10
- X -••'*
s /
V*'(2083)
PROVISIONAL LINE
IX
__,_
O (609)
NUMBER OF REPORTS IS IN PARENTHESIS
2
0.01
_J L
0.02 0.03 0.04 0.05 0.10
FORMALDEHYDE CONCENTRATIONS, ppm
0.2O
Figure 7-3. Regression curve of effect of formaldehyde concen-
trations on eye irritation in panel studies. (Reprinted with
permission from Renzetti and Bryan.1^7!)
343
-------
IU
O
! - .'-a
i'i O
IU
tU
0 0.2 0.4 0.6 0.8
FORMALDEHYDE CONCENTRATIONS, ppm
x
LU
Q
uj
>-
LU
0 0.2 0.4 0.6 0.8 10.0
FORMALDEHYDE CONCENTRATIONS, ppm
Figure 7-4. Average reported eye irritation intensities of 12
subjects during photooxidation with ethylene and propylene, related
to observed formaldehyde concentrations. (Redrawn from Schuck et a
344
-------
2.0
1.0
3.0
2.0
1.0
.25 .5 .75 1.0
FORMALDEHYDE (PPM)
.005 .01 .015 .02
PEROXYBENZOYL NITRATl IPPM)
Figure 7-5. Eye irritation of peroxybenzoylnitrate
and formaldehyde. (Reprinted with permission from
Heuss and Glasson. )
345
-------
CHAPTER 8
INTERACTIONS AND EFFECTS ON TOTAL ENVIRONMENT
The basic orientation of most of this report is the assessment of the
mechanism and extent of formation of vapor-phase organic pollutants in
the atmosphere and their effects on humans, but it is also appropriate to
consider their possible effects on atmospheric properties, on materials
with which the air comes into contact, on natural waters, including aquatic
life, on microorganisms, and on vegetation.
Hardly any information is available on the effects of VPOP at their
typical ambient concentrations on building materials, clothing, paint, etc.
This chapter therefore does not attempt to consider such effects. The
available information on the incidence and effects of organic compounds in
general in natural or treated waters is also sparse. Thus, there is no
attempt here to assess the possible effects on aquatic life of VPOP that may
find their way into such waters. Consideration of the effects on natural
waters is limited for the most part to the relatively limited information on
the interaction between organic compounds in air and natural waters and
their possible effects on the latter. The contributions of VPOP to such
effects and their significance is necessarily largely speculative.
Consideration is given to the possible role of VPOP on atmospheric
properties, and their known effects on microorganisms and vegetation
are treated in some detail.
EFFECTS ON ATMOSPHERIC PROPERTIES
The reported concentrations of vapor-phase organic materials in the
32, 224, 1218
atmosphere indicate that these materials are probably not
346
-------
prevalent enough to affect the physical properties of the atmosphere
1080
significantly. Robinson attributed the major effects on visibility,
fog formation, precipitation, and solar radiation to the presence of
particulate matter in the atmosphere.
32
Altshuller et al. studied the hydrocarbon concentrations in the
Los Angeles Basin in 1967 and found that, excluding methane, over 80%
of the remaining hydrocarbon concentration can be accounted for by 10
hydrocarbons: ethane, ethylene, acetylene, ri-butane, isopentane, propane,
1Z18
toluene, n.-pentane, rn-xylene, and isobutane. Stephens and Burleson
also examined the distribution of hydrocarbons in the atmosphere and re-
ported that the composition of air samples resembled that of auto exhaust
with an addition of natural gas and of C paraffins that resemble gasoline
3-5
vapor. They determined that the background concentration of methane is
1. 39 ppm. This concentration agrees with the results reported by Fink
416 224
jst al. Cavanagh et: al. reported the methane concentration at
1218
Point Barrow, Alaska, to be 1. 59 ppm. Stephens and Burleson con-
cluded that the use of natural gas increases the methane concentration
significantly in urban areas.
Visibility
The direct effect of vapor-phase organic pollutants in decreasing
visibility ha.s not been demonstrated. Loss of visibility has been attributed
only to the effects of absorption and scattering of light by the particulate
228 397
matter in the air. Charlson and Ettinger and Royer have both
demonstrated a correlation between the aerosol mass concentration and
visibility.
347
-------
Solar Radiation
Many measurements have indicated that solar radiation is consider -
1047
ably less intense in polluted atmospheres than in clean air. Randerson
compared the spectral distribution of solar radiation in the 450- to 700-nm
band near Houston, Texas, in a clean air mass and a heavily polluted air
mass. He observed a 23% loss in solar radiation, which he attributed to
attenuation by pollution in the atmosphere. The loss of solar radiation
depends on wavelength, with the shorter wavelengths showing the greater
loss. Randerson attributed this to the greater scattering of the shorter-
wavelength radiation by the particulate matter in the atmosphere. Nader
948
and White investigated the ultraviolet radiation in urban atmospheres
in the 300- to 380-nm region and observed a loss in energy in polluted
atmospheres. They also attributed the loss to particulate matter in the
atmosphere.
Atmospheric Color
1080
Robinson discussed the effect of nitrogen dioxide, which imparts
a brown color to the sky. The change in color is due to the absorption in
particular regions of the spectrum. The vapor-phase constituent of the
atmosphere must absorb radiation in the visible region of the spectrum
(400-800 nm) if the compound is to impart a color to the sky. Most VPOP
do not absorb radiation in this region of the spectrum and therefore will not
impart a color to the sky.
Cloud Formation and Precipitation
Urban areas have been shown to have a more complex climate than
767
nonurban ones. Landsberg has pointed out that the climate of an urban
348
-------
area, when compared with the climate of a nonurban area, in the same
region, can be characterized by 5- 10% more cloudiness, 100% more
winter fog, 30% more summer fog, and 5-10% more precipitation.
1080
Robinson stated that fog persistence is related to the number of
nuclei that produce fogs with smaller drops. Little is known of droplet:
chemistry and the role that VPOP can play in fog formation and persistence.
There are no data that would link VPOP to changes in precipitation
patterns. The major effect of air pollution on such patterns is thought
1080
to be the addition of condensation nuclei to the system.
REMOVAL. FATE, AND PERSISTENCE
672
A decade ago, Junge remarked about atmospheric cleansing
mechanisms that "our knowledge of these processes is rather vague."
In the intervening years, progress has been made in understanding the
detailed life cycles of a small number of atmospheric contaminants; and
for the remaining multitude of pollutants, agreement has been reached on
the general processes by which they disappear from the atmosphere.
However, quantitative data, as are available on the sources of organic
pollutants, are largely unavailable for the removal mechanisms, and
Junge's remark remains a fair summary of the situation.
The subject has more recently been reviewed by Haagen-Smit and
533 1081
Wayne, by Robinson and Robbins and in Inadvertent Climate
626
Modification. A major aspect of the problem, precipitation scavenging,
1034
was the subject of a U. S. Atomic Energy Commission conference report.
The gas--phase reactions that transform pollutants after they enter the
atmosphere and are mixed by diffusion and turbulence are discussed else-
where. It is assumed here that any of these gas-phase reactions that
349
-------
proceed faster than the atmospheric cleansing processes have occurred.
In other words, we will concern ourselves with the ultimate mechanisms
that remove the products of organic pollutants, including unchanged com-
pounds, from the atmosphere. It is worth noting that only a very few
814
compounds, such as the Freons, appear to be without a removal mecha-
nism of the type that provides for almost all the other recognized VPOP a
residence time in the atmosphere of less than a few years. Even such
stable compounds as the freons are not immune to reactions promoted by
ambient ionization from cosmic rays, airborne radioactivity, and lightning
discharges, although their residence time against these processes may be
exceedingly long.
The low concentration of VPOP products in the atmosphere, except
close to their sources, makes it safe to assume that removal processes
will be first-order. If c is the ambient concentration at time t, we may
write, for the removal of a VPOP,
!£=:£» (1)
dt T
where T is the residence time of the VPOP before removal under the
prevailing conditions. If the VPOP is entering the well-mixed volume
V under consideration at a rate —tthen, when a steady state has been
d t
reached,
1_ dm = c. (2)
V dt T
350
-------
with c in this case being the steady-state concentration.
The assumption that VPOP are widely dispersed in a time that is
short, compared with T, will often not be true, especially when the
steady state being considered is a global one; more detailed examination
1147
of this case has been made by Schutz and co-workers. Neverthe-
less, the above result holds approximately for VPOP with residence times
of weeks or longer and can be used to derive the residence time in cases
where the ambient concentration can be measured and the production rate
estimated.
For only a handful of low-concentration atmospheric components have
residence times been estimated, and these are mostly the simpler compounds,
such as methane, carbon monoxide, and nitrous oxide. Table 8-1 shows the
current best estimates of the concentrations, production rates, and residence
times of compounds for which reasonably reliable data are available.
What mechanisms are available for removal of VPOP from the atmosphere?
It will be useful to classify these mechanisms according to the form of the
VPOP when undergoing removal. A trivial case is the degradation of a VPOP
to yield water, carbon dioxide, or other major constituents of the atmosphere;
such a pathway will be followed by a large fraction of the total VPOP load
(the oxidation processes in the atmosphere will bear analogy to slow'combus-
tion), but are of no interest here. This case apart, the products of the VPOP
will ultimately be removed as either gases or solids/liquids. Althoxigh the
mechanisms are not necessarily different, we •will consider them separately.
Removal as a Gas
672
The principles of gaseous removal were set forth by Junge. References
1034
to more recent work may be found in Precipitation Scavenging. The follow-
ing ultimate fates will be discussed briefly:
351
-------
Diffusion to the upper atmosphere
Diffusion to the ground, including ultimate utilization by plants,
animals, and microorganisms
Dissolution in cloud or rainwater, including reaction with the
•water or its solutes
Diffusion to the Upper Atmosphere. The definition of "upper atmosphere"
in this case depends somewhat on the VPOP being considered. For most pur-
poses, a VPOP penetrating the tropopause may be considered as lost to the
lower atmosphere and lost to the biosphere. However, some compounds,
such as methane, are resistant to dissociation by the ultraviolet of the lower
stratosphere and to oxidation by ozone; hence, the methane pool should be
considered as encompassing both the troposphere and the lower stratosphere.
As an extreme example, penetration of a gas into the stratosphere may lead
to its incorporation into the stratospheric aerosol, with later return to the
troposphere; however, it is unlikely that the product of this slow process
(taking years) will chemically resemble the original gas.
Crossing of the tropopause is more often the result of large-scale
turbulence, as in storms, than of diffusion on a molecular scale.
Diffusion to the Ground. This process, too, is dominated by turbulent
4 2
diffusion, with eddy diffusion coefficients of about 3x10 cm /sec being
typical. Much research on the converse question, diffusion away from the
ground, has been performed in connection with radon emissions from the
250
soil and the evaporation of ground moisture. The concept of a deposition
velocity, discussed later for aerosols, has been applied also to the case
672
of gases that diffuse to some sink at the ground surface; in this case,
the deposition velocity may be best regarded as the ratio of the eddy diffusion
352
-------
coefficient to an appropriate scale length. Values of about 1 cm/sec
250
have been found for deposition velocities of iodine vapor and sulfur
660
dioxide; these values are about 10 times higher than the deposition
velocities of aerosols.
A closer examination of diffusion to the ground raises the question
of what establishes the concentration gradient above the soil. Clearly,
there must be; an efficient sink for the diffusing gas in the soil surface
or in the biota resident within and on the surface of the soil. There is
2,628,1061
strong evidence that biologic scavenging by soil ecosystems
is, at normal ambient concentrations, an efficient sink for a wide variety
of atmospheric trace gases, including light hydrocarbons, carbon monoxide,
and sulfur dioxide.
Dissolution in Cloud or Rainwater. This is an inefficient process for
atmospheric cleansing in the case where the gas merely dissolves in the
•water droplet, -without undergoing hydration or reacting with other solutes.
672 -7
Junge has pointed out that typically only 10 of the gas in a given volume
of cloud will reside in the water, inasmuch as clouds contain only about 1 g
of water per cubic meter.
The efficiency is greater, but still low, for gases that undergo hydration
and dissociation in solution--for example, ammonia and sulfur dioxide.
Only in the case where two solutes can participate in a reaction that either
is irreversible or has a very large equilibrium constant is the rate of re-
moval appreciable. The best known example of this process is the joint
removal of ammonia and carbon dioxide, but obviously many such reactions
are potentially able to occur.
353
-------
1241
Recent experimental work supports the theory of removal of
gases, such as sulfur dioxide, that react with water; the case of "in-
soluble" gases is less clear.
Despite the low efficiency of removal of many of the trace gases,
rainout and washout will be important, because of the repeated exposure
to precipitating mechanisms, for the highly oxygenated and polar end
products of VPOP degradation and will produce small but possibly important
concentrations of some compounds in ground -water.
Removal by Aerosol Formation
A major part of atmospheric organic pollutants is olefins and other
reactive hydrocarbons. VPOP reaction products of these reactive species
are believed to be removed by free-radical polymerization processes that
result in macromolecular particles that grow in size to become the visible
haze of, for example, photochemical automobile smog. The process pro-
ducing natural haze from the olefins and terpenic emissions of vegetation
is probably similar. Light, during at least some phases of the reaction,
seems essential for aerosol production.
The formation of aerosol from manmade pollutants has been reviewed
23 533
by Altshuller and Bufalini and Haagen-Smit and Wayne, and many
other reaction schemes involving free-radical intermediates capable
of initiating radical polymerization have been suggested.
Studies on the reactions involved in the formation of aerosols are intrinsically
more difficult than those on vapor-phase reactions, and few have been attempted
under conditions resembling those in the real atmosphere; much of the available
information is gained indirectly by studying reaction rates (as measured by the
disappearance of the reactant, and perhaps the total mass of particulate matter
accumulated) as functions of reactant pressures, light intensities, and temperatures.
354
-------
The systems used to model aerosol production from automobile
exhaust have generally shown low yields of aerosol, as measured by
the ratio of carbon in the aerosol to the total carbon of the original
reactants. Much higher yields have been obtained by Rasmus sen and
1057
co-workers in experiments with cyclic olefins, such as ot-pinene.
It is not clear that any of these experiments adequately model the
processes in the real atmosphere, where, for example, small (less
than 0. 01 y m) particles are always present to act as nuclei for the
growth of the polymeric particles produced by the reactions under study.
Such nucleation may greatly facilitate aerosol production because of the
energy barrier caused by surface effects when particle production de novo
is attempted; the existence of nucleating particles will certainly alter the
size distribution of the synthetic aerosol.
496
The experiments of Goetz and Klejnot deserve attention. They
demonstrated that even large aerosol particles are often only metastable
and are prone to disappear , especially in the presence
23
of ultraviolet light. Altshuller and Bufalini point out that this metasta-
bility may have compromised the accuracy of some measurements of the
yield of aerosols in photochemical chambers from synthetic gas reactions.
Aerosol formation from VPOP in chamber experiments is promoted
by sulfur dioxide, but the resulting aerosol often contains little organic
material. Nitrogen dioxide promotes aerosol formation, especially by the
higher olefins, but nitric oxide is a powerful inhibitor. The most plausible
explanation of this inhibitory effect of nitric oxide is that nitric oxide competes
for atomic oxygen produced in the photodissociation of nitrogen dioxide more
effectively than molecular oxygen, thus limiting the later formation of ozone.
Ozone efficiently converts cyclic olefins to aerosol, even in the absence of
355
-------
1057 151
light. Other experiments indicate that aerosol formation can occur
in the absence of any of these adjuvants, provided only that ionization
produced byairborne radioactivity and cosmic rays is present.
The newly formed aerosol in the real atmosphere is certainly not a
simple polymer; at the very least, it will be a copolymer formed from the
diverse monomers present in the atmosphere, and it may incorporate
compounds that, when used as the sole monomer species in chamber experi-
ments, are insufficiently reactive to form aerosol.
Removal by Preexisting Aerosols
There is general agreement that the principal path for removal of the
products of organic pollutants is aerosol formation from the products
themselves. However, not all such products can be conceived as undergoing
polymerization, or even copolymerization, to yield macromolecules for
aerosol particle formation. We consider here a number of ways in which
these products may be removed by a preexisting aerosol. Very little exper-
imental evidence is available to judge the importance of these processes, and
the following list is not intended to be exhaustive:
Reaction with aerosols
Dissolution in aerosols
Adsorption on the surface of aerosols
Liquefaction in the interstices of aerosols
Reaction with a compound already dissolved in or attached to an aerosol
Some of these processes are well understood in contexts other than those in-
volving aerosols, and it is -worth considering them briefly before leaving the
question of aerosol removal processes.
356
-------
672
Reaction with Aerosols. This mechanism has been suggested for
the removal of ammonia, by reaction with acidic aerosols, particularly
sulfuric acid droplets. In general, young aerosols formed from VPOP
829
will be highly reactive, containing organic peroxides and hydroperoxides,
as well as low concentrations of radicals. The natural aerosol from vegetation-
1064
produced terpenes, when newly formed, may play a major role in removing
VPOP; the efficiency of this process deserves investigation.
Dissolution in Aerosols. This process and the following one may
provide paths: for the removal of unreactive products present in very low
concentrations. They are of no significance for the removal of most com-
pounds, because of their relative inefficiency, but they are effective in
changing the composition of aerosols in ways that may be biologically
important. The removal by aerosols will be competing (except for very
hydrophobic gases) with removal by rain and cloud droplets, which will
4
typically be 10 times as abundant by mass over each unit area of the
earth's surfcice. Nevertheless, clouds tend to be remote from sources of
VPOP; thus, even with hydrophilic gases, solution in the ubiquitous aerosol
may be significant. As with dissolution in cloud droplets, the most favorable
case arises when the gas-aerosol partition coefficient is unusually large
(for example, in the case of a polymeric aerosol particle that dissolves
its own monomer). In less favorable situations, the fraction of VPOP in
-14
the aerosol particles -will be too small (approximately 10 ) to be of
practical significance.
Adsorption on the Surface of Aerosols. If one treats aerosol particles
as spheres and calculates the surface area per unit volume of air, one finds
2 3 219a
areas of around 0. 1 cm /m . In fact, as Cartwright and colleagues
357
-------
and later workers have shown by electron micrography, many particles
in common aerosols are composite structures apparently formed by coagula-
tion of sizeable numbers of much smaller particles. The total area may be
10 times larger than the simplistic calculation would suggest, although it
could hardly be 100 times larger. In terms of surface area per unit mass,
such an aerosol material is comparable with active carbon. There is no
information on the adsorption properties of common aerosols, but no reason
to believe them to be poor adsorbers. Sorption by aerosols may be a signifi-
cant removal process for some VPOP. Under conditions of unusually high
aerosol density, it may be an important process.
Liquefaction in the Interstices of Aerosols. This represents an
obvious extension of the preceding discussion, relevant to VPOP with low
saturation vapor pressures at normal temperature. The most likely material
to condense in the interstices of a complex particle, where surface forces
encourage condensation that would not occur on a simple, spherical particle,
is water.
Reaction with a Compound Already Dissolved in or Attached to an
Aerosol. Just as these mechanisms are important in promoting the removal
of gases by rain and cloud droplets, and indeed are essential in causing such
removal at a significant rate in many cases, so we may speculate that they
play a comparable part in aiding removal by aerosol particles.
Removal of Aerosol Particles from the Atmosphere
In the preceding sections, we have discussed aerosol formation from
the products of organic pollutants and raised some questions about the re-
moval of VPOP by preexisting aerosols. In this section, we mention briefly
358
-------
the final proceiss, the removal of the aerosol itself from the atmosphere.
This subject has been studied in considerable depth, largely because of
the concern in the 1950's over the radioactive aerosol resulting from nuclear
weapons tests. This radioactive aerosol behaves, as far as has been ob-
served, very similarly to the ambient, nonradioactive aerosol, presumably
because of coagulation between the two. The decay of the radioisotopes in
this aerosol permits a quantification of aerosol density and an identification
of a particular aerosol as it ages and mixes with foreign aerosols, which
are otherwise difficult to attain.
The mechanism of aerosol removal has been reviewed recently by
1082
Robinson and Robbins. Removal by hydrometeors was the subiect of
1034
a 1970 U.S. Atomic Energy Commission conference.
Approximately 10% of aerosol radioactivity is deposited in dry form
on the ground and its covering; this is called "fallout. " The remainder
of the radioactivity appears in precipitation and is known as "washout"
if the aerosol is captured below the clouds and "rainout" if the capture
occurs inside the cloud. Three main physical mechanisms seem responsible
for these phenomena: impaction, the result of the aerosol particle's
possessing greater inertia than the air molecules around it; diffusion, the
Brownian movement of the smaller particles in particular; and condensation
nucleation, where the particle acts as a nucleus for the formation of a drop-
let out of supersaturated water vapor. In addition, gravitational sedimentation
will be important for very large particles (larger than a few micrometers),
and other processes, such as diffusiophoresis and electric effects, may be
found to be important.
Dry fallout is due to diffusion to the ground; to impaction on leaves,
grass, structures, etc., of the windborne aerosol; and to gravitational
359
-------
sedimentation. The varied nature of the ground cover and the varying
wind patterns make distinction between the component mechanisms academic.
It is usual to characterize the rate of fallout by a deposition velocity that
is the ratio of the (ideally, mass) deposition rate per unit area to the (mass)
density per unit volume of the aerosol; deposition velocities for typical
aerosols tend to cluster around and below 0. 1 cm/sec.
The details of wet removal, rainout, and to a lesser extent washout
are gradually becoming clear. Washout is well understood for large
particles (above 1 urn), but these particles are mostly from ground-level
sources (stack emissions, duststorms) and only rarely represent the organic
gases in their final guise. For smaller particles, impaction is less effective,
and the collection process is best visualized as diffusion of the particle across
the boundary layer around the falling raindrop. Aspects of this problem have
310
been discussed by Davies. Experimental measurements have been made
of the efficiency of washout--the ratio of the mass of aerosol collected to the
mass present in the volume swept out by the scavenging drops—and lead to
547
values much less than unity -when the aerosol particle size is 0. 1-1. 0 y m.
Rainout is generally more significant in particle removal than washout,
but is less well understood. Additional processes, such as condensation on
the particle as a nucleus and possibly electric effects, become important,
and the relatively long lifetimes of cloud droplets more than compensate
for their low velocities, compared with those of raindrops. Cycles of
coagulation of cloud droplets and reevaporation of the resulting raindrop
have the effect of increasing the size of aerosol particles by aggregating
them.
The overall lifetimes of aerosol particles with respect to wet removal
are well determined from studies on low-altitude nuclear weapons aerosols;
360
-------
TABLE 8-1
Estimated Global Concentrations, Production Rates, and Residence
Times of Some Trace Gases in the Atmosphere
Compound
Ambient Production
1 9
Concentration, p'pm (vol) Rate, 10 g/yr Residence Time, yr Reference
Methane
Carbon monoxide
Nitrous oxide
Freon
Methyl iodide
Carbon tetrachloride
1.4
0.2-0.05
0.26
50xlO~6
10~6
7xlO~5
500-1,000
700
—
0.44
40
1.7
4-7
0.7-1.5
12-13
16
0.003
1
371
671,966,1154
851
814
814
814
361
-------
they range from 40 days in the upper troposphere to about 10 in the lowest
752
kilometer. Weather variability and local peculiarities of climate will
permit much longer or shorter lifetimes at particular times or places and
perhaps more significantly will affect the size to which the aerosol particles
have grown by the time they are removed. However, coagulation and growth
of particles are univeral processes from which no newly formed
aerosol is exempt; we do not expect unusual aerosols to have a lifetime with
respect: to removal significantly longer than those of other aerosols formed
in the same location at the same time.
EFFECTS ON NATURAL WATERS
Interactions between Air and Water
In assessing the quantities of VPOP from hydrocarbons that may be
contributed by the air to terrestrial waters, one should consider the various
mechanisms for their removal from air: precipitation (rainfall, snow, and
including washout), diffusion across the interface of surface water and air,
and fallout. It should also be noted that movement takes place in the reverse
direction and can be a major source of organics in air, by such mechanisms
as evaporation and aerosol formation from sea spray. These are in addition
to more direct emissions from manmade pollutants, as well as materials
transported to the air indirectly from soil particles. These mechanisms
must be considered in relation to the specific chemical and physical forms
of the organics in air. Thus, one must distinguish among mechanisms
involving organics in the vapor form; those absorbed onto the surface of
airborne biota, such as spores, as well as minerals and other particulates;
and those dissolved or suspended in rainwater.
In general, information as to the possible contributions of VPOP from
hydrocarbons by air to surface waters is not available. Indeed, aside from
362
-------
such types of organics as polynuclear aromatic hydrocarbons and
pesticides, knowledge about the incidence of organics in precipitation
is scanty. Although some specific organics have been measured in
precipitation, the data available are often simply the total organic
concentration measured as C.O.D. (chemical oxygen demand) or weight
of organic residue per volume of precipitation.
However,, many of these data are, nevertheless, useful in assessing
possible contributions by air to surface waters and will be considered here.
1364
Thus, for example, Weibel and colleagues compared organics in rainfall
and surface runoff with those contributed by sanitary sewage in an urban and
a rural area in the United States. As noted in Table 8-2, the concentrations
of both C.O.D. and organic chloride are smaller in rainfall than in urban
or rural runoff; however, it was concluded that, in comparison with domestic
sewage, these contribute much greater quantities of organics to terrestrial
waters. Although terrestrial sources of organics, both manmade and natural,
are large, one: cannot dismiss precipitation as a contribution.
A useful comparison in this regard is the transport of DDT in the
biosphere, which was reviewed and analyzed as a case study by Woodwell
1417
et al. They conclude that the movement of DDT in the atmosphere
is the most important route, that the dominant mechanism for its removal
from the atmosphere is probably rainfall, and that most of it so removed
goes into the ocean. An input-output analysis for a steady-state distribution
of dissolved organic matter in the ocean indicates that rain (using a value
of 0. 1 mg of carbon per liter) contributes about 1% of the organic carbon
1387
to the oceans, the rivers about 0. 1%, and primary productivity the balance.
Thus, although the most important source of organics in sea water is primary
productivity, one must consider rainfall as a source, particularly when such
manmade orga.nics as DDT are of concern.
363
-------
672
Junge is a useful source of data on the incidence of organic
compounds in precipitation. Data cited by him (Table 8-3) indicate
some concentrations and ranges of organic materials measured in rain
and snow. The values for albuminoid (proteinaceous) nitrogen reported
in Table 8-3 were obtained at specific locations in the indicated countries.
It was indicated that these nitrogenous compounds were insoluble materials,
either from windborne soil or from unidentified sources. The organic
nitrogen and carbon reported for rain and snow were fairly uniform in
concentration throughout Sweden. It was speculated that about 70% of the
organic nitrogen was protein and that a sea-spray mechanism may have
been the source of the organic material. Amino acids have also been
434a
reported in rainwater.
Vapor-phase pollutants can be adsorbed directly by water by diffusion
across the water-air interface. A large number of alkanes, alkenes,
acetylenes, and aromatic hydrocarbons have been identified and quantified
32, 1290
in ambient air. Although their solubilities in pure -water are
variable and often low, accumulations of lipids at a water surface -would
1417
be likely to increase the rate of absorption from air; in general,
organic contaminants in water are capable of increasing the "solubility"
of other organic species through such mechanisms as solubilization and
43
hydrotrophy.
Methane is an example of a hydrocarbon that has been measured in
air and whose interactions -with surface -water have been studied. Most
of the concentrations measured in one study of urban atmospheres -were
between 1. 5 and 2. 5 ppm, and it was concluded that the normal geophysical
21
concentration is 1. 0-1. 2 ppm. To determine the extent of equilibration
364
-------
between air and water, samples were taken from a boat in both phases
near the air-water interface at several sample points along the Potomac
River, in the Chesapeake Bay, and 250 miles out into the Atlantic
1247
Ocean. The concentration of methane in water decreased from a
high of 500 ml/liter at Washington, D. C. , and leveled off at 250 ml/liter
downstream in the Potomac and out into the Atlantic. The high concen-
tration of methane in air was approximately 4 ppm at Washington, and
the concentration generally decreased out into the Atlantic, the lowest
being 0. 075 ppm. On the basis of solubility calculations, it was found
in all cases that the concentrations in water were higher than -would be
predicted from those in air, but the difference decreased markedly from
the Potomac River out into the Atlantic. The higher water values were
attributed to pollution in the river and bay, and it was concluded that
methane in these regions was probably transported from the water to the
atmosphere. However, it is not clear whether the ocean is acting as a
source of or a sink for methane. It may be concluded that vapor-phase
hydrocarbons and their reaction products are transferred directly across
the water-air interface. However, estimates of the magnitude of such
transfer are not available.
Finally, particulate matter in air is of interest as a source of
organics in water through the mechanisms of settling and washout. Both
natural and synthetic organics are likely to be found in such particles.
It has been noted that most of the water-insoluble materials in these
particles in polluted areas are organic substances and ashes, whereas
in unpolluted areas the ashes are replaced by -mineral dust. However
a significant amount of the particulate matter is composed of fungal spores
770
that can carry organic matter by absorption onto their surfaces.
365
-------
Uredospores of stem, and leaf rust fungi constitute an example of such
a material that is present in the summer and was found to contain alkanes
with chain lengths of C (average, 150), free fatty acids with chain
18-35
lengths of C a C epoxy acid, and other organics, including several
14-24, 18
carotenoid pigments, high-molecular--weight ke tones, and a variety of aromatic
770
compounds.
The organic material in airborne particles is often more highly con-
centrated in urban areas than in nonurban, as noted in Table 8-4, and it
has been estimated that there is a natural background concentration of about
3 672
3-6 jjg/m . The total insoluble material is about 70-80%; it contains
both minerals and combustible organic compounds, and the concentration
of organic material seems to decrease with the size of the urban community,
as noted in Table 8-4. Some data available from Los Angeles indicate some
of the percentages of types of organic compounds that have been found in
airborne particles, and these are also shown in Table 8-4. The relatively
high percentage of water-soluble species, -which include organic acids and
alcohols, indicates that these are likely to be readily leached when the
particles are deposited into surface waters or washed out from the air by
precipitation. Thus, one can conclude that aerosol particles are a source
of organics that can add to the load of terrestrial waters. However, as is
the case for the other sources in air, an estimate as to the magnitude of
their contribution is problematic.
Incidence of Organic Substances in Surface and Ground Water
Much less is known about the nature and amount of organic matter in
saline and fresh water than about the inorganic constituents, although there
is a reasonable amount of information available concerning nonspecific
366
-------
organic content, such as that shown in Table 8-5. These results were
obtained from analyses that involved either adsorption by and later
extraction from activated carbon or oxidation of organic constituents
and later measurement of the carbon dioxide produced. In surface
regions of the oceans (0-300 m), the dissolved organic content is
typically about 0. 5-1. 2 mg of carbon per liter, with an average of about
1. 0 mg/liter, -whereas, in deeper waters, the average is about 0. 5 mg/liter.
The particulate organics represent about another 10% in the surface regions,
1342, 1387
and 2-10% in the deeper ones. In many fresh waters, approximately
780
10 times these dissolved organic concentrations are found, and the
particulate concentrations, including colloidal organic matter, are similarly
small, perhaps around 10%. However, there is considerable variability in
the organic carbon content of both surface and ground fresh -waters, as shown
in Table 8-5. In some cases, this variability can be associated with sources
of pollution. Thus, -waters known to be unusually clean often contain carbon-
chloroform extract at less than 0. 025 mg/liter, whereas waters known to be
polluted with industrial -wastes have been found to contain 10 or 100 times
887
as much. Nevertheless, much of the organic matter in surface waters
780
is due to the excretory and degradative products of aquatic biota.
Polynuclear aromatic hydrocarbons and pesticides have been found in
43,803 803
marine and fresh -waters. In a survey of the scientific literature
for the period 1960-1970, aside from these two groups of compounds, perhaps
20 different organic compounds -were reported in fresh waters; most of these
are shown in Table 8-6. Several of them -were associated with specific known
sources of pollution. Specific amino acids have also been found in lake water.
367
-------
623
with concentrations of 2-23 jpg/liter, and polychlorinated biphenyls
1315
at concentrations of 0. 02- jig/liter in the Milwaukee River.
A relatively large number of organic compounds of biologic interest
1076
have been determined in seawater, including specific sugars, carboxylic
acids, amino acids, fatty acids, and urea, usually at concentrations of less
than 0. 1 mg/liter. Nevertheless, it has been estimated that only about 10%
of the total dissolved organic carbon in surface and subsurface marine waters
has been identified, using the Northeast Pacific Ocean as an example; further-
more, it was concluded that little is known about the molecular nature of such
materials in the sea and that "such identification may be as intractable as
1387
the identity of 'humus' in soils."
In trying to assess the possible effects of VPOP from hydrocarbons
in air on marine and fresh waters, we are, therefore, confronted with the
fact that there is a dearth of information about the nature and amounts of
the organic compounds therein. In turn, the problem is compounded by a
similar lack of knowledge of the specific organic contributions that air
makes to these waters. Thus, any judgment as to their possible effects
will be at best speculative and will have to rely on analogy with the limited
number of laboratory or field investigations that have attempted to define
or study the effects of model organic compounds or "organic matter" on
the behavior of natural water systems.
Effects of Organic Compounds in Water
A variety of effects of organics on water are important either because
they influence the general quality and character of the water, sediments,
and biota or because they degrade or have an economic impact on a particular
water use. In general, it is most difficult to evaluate such possible effects
368
-------
and attribute them to the contributions of VPOP from hydrocarbons in
air. This section therefore focuses on the known and hypothesized effects
of a variety of organic compounds that may influence water quality.
Several water quality criteria and standards are concerned with organic
compounds and related to both municipal and industrial use. These involve
health considerations, as well as aesthetics and the quality and economics
of industrial processes. Thus, U.S. Public Health Service Drinking Water
1288
Standards recommend that drinking water contain no more than 0. 2 mg
of carbon chloroform extract (CCE) per liter. It states that, although a
higher concentration does not necessarily indicate the presence of toxic
organic compounds, it does represent an exceptional and unwarranted
dose of ill-defined chemicals to the consumer. A recent survey of
finished waters in community water supply systems serving about 10% of
the U.S. population indicated that about 1% of the people in this group were
exposed to water exceeding this standard, the highest concentration found
848
being 0. 56 mg/liter.
Similarly, a variety of water quality criteria for surface waters
used for public water supplies—as well as waters used for food processing,
agriculture, and industrial use--specify limits for pesticides, herbicides,
1355
and unspecified organic compounds. These indicate, for example, that '
waters at the point of use should not exceed carbon tetrachloride extract at
0. 2 mg/liter for industries involving canning food, drying and freezing
fruits and vegetables, bottling soft drinks, and finishing leather, whereas
the criterion is 1 mg/liter for the hydraulic cement industry. These
criteria indicate a need to limit the concentrations of unspecified organic
compounds in raw waters, inasmuch as high concentrations will have
economic consequences as a result of requiring a higher degree of treatment
or as a result of a decrease in the quality of products.
369
-------
There are also specific effects on the taste and odor of water related
to drinking water quality. The Public Health Service Drinking Water
Standard of 1 )ig/liter for phenol is set "because of the undesirable
taste often resulting from chlorination of waters containing extremely
1288
low concentrations of phenol." However, a large number of other
organic compounds are known to impart taste and odor to drinking water.
Several of these, isolated from a river polluted with industrial wastes,
1094
are listed in Table 8-7 with their mean threshold odor concentrations.
The threshold odor concentrations vary widely. It has been noted that
chemical and refinery wastes have the greatest potential for odor, but
such organisms as actinomycetes and algae are also a common cause of
taste and odor, and there is a direct relationship between the amounts of
1093
organic material present in water and the odor intensity. Although
conventional water treatment practices have some utility in the removal
of taste- and odor-causing organic compounds, the application of powdered
activated carbon is often required as well. Nevertheless, for highly
polluted -waters, even this may not be effective. For such a water heavily
polluted with various organic industrial wastes, it was found that "in spite
of double aeration and relatively high powdered-carbon and chlorine doses
at the water plant, the odor and carbon chloroform extract (CCE) values
346
frequently exceed recommended levels." As shown in Table 8-8,
conventional water treatment is not always effective, whereas the addition
of filtration -with granular activated carbon after conventional treatment
reduced the specific organic chemicals to concentrations below the detection
limit. However, it should be noted that such a treatment technique is rarely
used, and one cannot rely on more conventional municipal treatment methods
to remove organic compounds from a raw water supply.
370
-------
There is also the possibility of contamination by organic compounds
in the distribution system after the -water leaves the treatment plant.
This can result from runoff or contributions from the air to a holding
reservoir. In one instance, it was shown that in an impoundment the
chemical oxygen demand, a measure of organic load, was typically
10 mg/liter in the cooler months and rose to as high as 18 mg/liter
260
in the warmer ones. It was concluded that in some reservoirs the
organic loEid is established by the character of the runoff.
The persistence of organic compounds in natural waters is highly
variable and is determined by such mechanisms as biologic, chemical,
and photochemical degradation, evaporation, and removal by interaction
with biota and suspended and bottom sediments. The rate of biologic
oxidation in river waters is affected by several factors, including
temperature, the nature of the biologic population, metabolic lag, con-
centration and nature of the organic compound, and the presence of
398
nutrients, such as nitrogen and phosphorus. As an indication of this
variability, it was estimated that the persistence of phenol at 4 C is about
5 days in a river with a recent history of phenolic pollution, compared
with approximately 20 days in a relatively pure river. Some variations
in the rate of biochemical oxidation of synthetic organic chemicals are
shown in Table 8-9, which lists the percentages of the theoretical maximal
biochemical oxygen demand (BOD) exerted in 10 days in a 20 C incubation
398
with nutrients and settled sewage.
Although the readily biodegradable compounds affect the health of
a stream principally by reducing its dissolved oxygen content, the more
resistant ones may cause damage owing to undesirable color, toxicity
to aquatic life, tainting of fish, and impairment of aesthetic or recreational
371
-------
398
value. In addition to the previously noted taste and odor relation-
ships and the possible effect on the quality of water for industrial use,
the presence of organic compounds in water can also result in coagulation
difficulties and a higher chlorine demand in municipal water plants.
Organic compounds in both freshwater and seawater are known to
interact with trace elements, especially through the mechanisms of com-
780,1177
plex and chelate formation. A large percentage of the copper
in seawater has been shown to be associated with organic compounds in
nonlabile combinations, as has up to 99% of the copper present in soil
solutions. Other trace elements, such as cobalt and boron, have been
similarly associated with organic materials, and it has been concluded
that the control and initiation of phytoplankton productivity in seawater
may be the most vital effect of metal-organic interactions in the environ-
1177
ment.
Through complexing mechanisms and their control on the solubility,
the sorption of trace elements on suspended and bottom sediments, and
their uptake by biota, organic compounds are likely to have a major
influence on the transport of trace elements in natural -waters. Certainly,
naturally occurring organic species like fulvic acid, which can complex
these metals and sorb onto such clay minerals as montmorillonite, are
1133
important in this regard, but organic pollutants may also play a
role. They can also concentrate at the air-water interface, where they
have been shown to enrich the concentrations of such trace elements as
353
lead, iron, copper, and nickel. Once concentrated in the surface
layer, these organics and their associated trace metals can enter the
food chain and be concentrated in higher trophic members.
372
-------
Organic compounds in water and soil-water systems can affect
the oxidation state of trace elements; this has a strong influence on
their behavior, such as their solubility and ability to adsorb and to
780 ,
exchange ions. Thus, for example, the reducing environment
6f 3+
created by organic pollution may cause the reduction of Cr to Cr ,
thereby affecting its transport in water, in that sediments and the biomass
495
have different capacities for sorbing these two species. Complexing
of trace elements by organics can also influence both the rate and the
780
equilibria of their redox reactions.
The sorption of organics onto minerals has implications for and
possible effects on both water and soil-water systems. The capacity
of soil for fixing organic substances depends on its physicochemical
728
properties, with clays in particular promoting their sorption.
There are indications that such organic materials as humic substances,
proteins, organophosphorus compounds, nucleic substances, and carbo-
hydrates sorb by clay minerals, and this favors their preservation and
gradual incorporation into the biologic cycle.
It has also been noted that the phytotoxicity of herbicides is reduced
by as much as 99% by the presence of organics in soil, and it was suggested
that this reduction is due to the sorption of herbicides by the soil organics,
1372
thereby making them unavailable to plants. Another important sorption
effect is the ability of organics in water to coat such minerals as calcite,
1237a
thereby inhibiting their equilibration with natural waters. Experi-
ments in this regard have indicated that the dissolved organic carbon in
seawater samples from different sources is reduced by 10-14% after
1237a
exposure to calcite particles. This can also be a mechanism for
the concentration and transport of organic compounds of limited solubility
43
in water.
373
-------
TABLE 8-2
Comparison of Average Concentrations of Organic Materials in
Rainfall and Runoff in Urban and Rural Areas of the United States^-
Average Concentrations
Urban Area Rural Area
Rainfall Runoff Rainfall Runoff
Chemical oxygen demand,
mg/liter 16 111 9.0 79
Organic chloride, yg/liter 0.28 1.70 0.22 0.43
-Derived from Weibel et al.1364
374
-------
TABLE 8-3
Typical Concentrations of Organic Material in Rain and Snow3-
Species
Formaldehyde
Albuminoid
326
nitrogen
392a
Organic
carbon965
Organic
nitrogen
965
Location
India
United States
Canada
India
England
Sweden
Sweden
Organic
material582a Nova Scotia
Nature of
Precipitation Concentration, mg/liter
Rain
Rain
Snow
Snow
Rain
Snow
0.5
0.4
1.1
2.6
2.0
1.7-3.4
0.8-1.9
up to 0.35
13
2.7
—Data cited by Junge. 72
375
-------
TABLE 8-4
Typical Concentrations of Organic Matter
in Aerosol Particles of the United States3.
Location
Seven cities
Five nonurban areas
Los Angeles basin
New York and Chicago
Cities under 2 million
Los Angeles
Material
Acetone-soluble
Acetone-soluble
Organic material
Organic material
Organic material
Organic acids and
other water-soluble
Saturated hydrocarbons
Two-to-five ring aromatics
Polynuclear aromatics
Concentration, yg/m
16-32
6-13
60
40
25
Approximate Fraction, %
40
20
5
5
a 672
""Derived from Junge.
376
-------
TABLE 8-5
Typical Gross Organic Content of Surface and Ground Waters
Constituent
Carbon chloroform
extract8**7
Total organic carbon
1164
Total organics by
carbon adsorption
522
Organics by
carbon adsorption-*^*"
Dissolved organic
1386C
carbon
Dissolved organic
carbon
1387'
Water
Type
Typical large U. S.
rivers
Ohio River system at
Pittsburgh
Missouri springs
Missouri deep wells
Illinois well waters
Amazon River
Northeast Pacific Ocean
0-300 m
300-3,000 m
Concentration, mg/liter
0.02-0.4
2-22
0.2-0.3
0.01-0.06
1.5-7.3
1.6-6.3
1.0
0.5
377
-------
TABLE 8-6
Some Specific Organic Compounds (Other Than Pesticides and Polynuclear
Aromatics) Found in Fresh Waters and Reported in the Period 1960-197Qg:
Compound
Location
Concentration
Methane
Aniline
Benzidine and
Naphthylamine
C~_6 carboxylic
acids
Terephthalic acid
Alkyl benzene
sulfonate
Phenol
Well water, Illinois
River, U.S.S.R.
River, Japan
Ohio River
Industrial reservoir
Various surface and
well waters
Various surface
waters
Nitrochlorobenzene Mississippi River
Methylmercuric
chloride
Agano River,
Japan
0.8-87 ml/liter
0-trace
0.29-0.39 mg/liter
0.2-25 yg/liter,
0.1 mg/liter
Up to 100 mg/liter
Up to 0.4 mg/liter
1-37 yg/liter
14.4 mg/liter
,
HDerived from Arthur D. Little, Inc.
378
-------
TABLE 8-7
Mean Threshold Odor Concentrations of Some Organic
Compounds Isolated from a Polluted RiverJL
Mean Odor Threshold
Compound Concentration, yg/liter
Naphthalene 6.8
Tetralin 18
2--Methyl-5-ethylpyridine 19
Styrene 37
Acetophenone 65
Ethylbenzene 140
Bis-(2-chloroisopropyl)ether 200
2-Ethylhexanol 270
Bis-(2-chloroethyl)ether 360
Misobutylcarbinol 1,300
Phenylmethylcarbinol 1,450
-Derived from Rosen et al.1094
379
-------
TABLE 8-8
Effect of Conventional Municipal Water Treatment Alone and in Conjunction
with Activated Carbon Filtration on the Removal of Organic
Contaminants from Polluted River Water3.
Concentration, yg/liter
Compound
Ethylbenzene
Styrene
Bis-(2-chloroethyl)ether
2-Ethylhexanol
Bis-(2-chloroisopropyl)ether
a-Methylbenzylalcohol
Acetophenone
Isophorone
Tetralin
Conventional
Treatment
Raw
11
18
55
110
24
19
90
Treated—
11
nd
10
8
nd
nd
nd
Conventional Plus
Carbon Filtration
Raw
154
48
26
29
25
57
Treated—
nd
nd
nd
nd
nd
nd
-Derived from Dostal et
b ,
~nd = not detected.
380
-------
TABLE 8-9
Biochemical Oxygen Demand of Several Organic Chemicals
Initially at Concentrations of 2.5 mg/liter in
Mineralized Dilution Water with Settle Sewage
Seed at 20 C for 10 Daysf*
Biochemical Oxygen Demand,
Chemical % of Theoretical
Monoethanolamine 58.4
Triethanolamine 0.8
Monoisopropanolamine 34.0
Butylamine 48.8
Methanol 62.7
Methylisobutylketone 49.3
Diethylketone 12.3
Acetone 71.8
Pentanedione-2,4 40.0
""Derived from Ettinger398
381
-------
Finally, the solubility of other organic compounds in water can be
increased by the presence of natural organic compounds or pollutants.
Thus, sodium humate solutions have been shown to increase greatly the
1372
solubility of DDT in water, and polynuclear aromatic hydrocarbons
are known to be similarly affected by the presence of other, more soluble
43
organic compounds.
In conclusion, there are a large variety of important effects of organic
compounds, both natural and synthetic, on natural water and soil-water re-
lationships that can influence both aqueous systems and their use by man.
The specific effects of VPOP from hydrocarbons are not readily assessable
in this regard. However, to the extent that they add to the organic load
and composition of the soil and water systems, they are of potential concern.
EFFECTS ON MICROORGANISMS
The biosphere contains three major living systems--plant, animal, and
microbial. About 50% of the living biomass on earth is microbial, with plants
representing 30-35% and animals the remainder of the cellular protoplasm.
The microbe and its functions are necessary for the continuation of plant
and animal life on earth. That microbial activities are indispenable is
•well illustrated by considering that there is a 30-year supply of carbon dioxide
in the air and that microbial degradation of organic matter produces over 80%
of the carbon dioxide released into the atmosphere. When one considers the
effects of pollution on man's well-being, it is essential to consider the effect
of pollutants on the total biosphere. The introduction of low concentrations
of toxic nonbiodegradable pollutants into the biosphere over an extended period
1158
can bring about dramatic changes in the ecology. This section is concerned
with the effects of vapor-phase hydrocarbon pollutants on the microbial populations
of the earth.
382
-------
Many studies have been conducted on the effect of air pollutants
on microbicil metabolism and growth. Estes found that exposure of
Escherichia coli to an oxidant air pollutant mixture inhibited its
395, 1004
growth. The amount of inhibition was a function of the initial
concentration of reactants. The active component in the pollution mixture
was isolated and identified as peroxyacetylnitrate (PAN). Glutamic dehydro-
genase -was isolated from E_. coli, and exposure of the enzyme to the photo-
chemical reaction products resulted in inactivation of the enzyme. Another
396
study suggested that photochemical reaction products of aldehydes would also
yield products that inhibited the activity of glutamic dehydrogehase from
923
E_. coli. Mudd found that both PAN and ozone oxidized the reduced form
of nicotinamide adenine dinucleotide. The reaction product of PAN treatment
was biologically active; that from ozone treatment -was not. The author
suggests that inactivation of enzymes by PAN is probably due to the oxi-
dation of the sulfhydryl groups in the enzymes.
The effect of vapor-phase hydrocarbons on the germination of bacterial
1087
spores was ascertained by Rode and Foster. Spore-forming bacilli are
abundant in all fertile soils. Many of the vapor-phase hydrocarbons tested
•were effective inhibitors of spore germination. Among the inhibitors were
propane, ii-butane, propylene, 1-butene, trans-2-butene, 1, 3-butene,
3-methyl-1-butene, isobutane, and chloropropanes. Ethylene was slightly
effective in inhibiting spore germination, but methane and ethane were not.
Spore suspensions of Bacillus megaterium, B_. subtilis, 13. cereus, and
B_. licheniformis remained dormant under a partial atmosphere of ri-butane,
but germinated -when the gas was removed. When pasteurized soil was ex-
posed to vapor-phase hydrocarbons, the spore formers present in the soil
did not germinate until the gas was removed. Studies with Rhizobium
383
-------
1414
meliloti demonstrated that the viability of this symbiotic nitrogen -
fixing organism was affected by exposure to nitrogen-dioxide, sulfur dioxide,
and aldehydes. The lethal effect of aldehydes was enhanced by high humidity.
643
Jacumin ^t al. , using Serratia marcescens as a test organism, measured
the effect of low concentrations of various pollutants on survival. He found
that the photochemical product of 1-hexene and nitrogen dioxide, at concen-
trations found in polluted air, gave significant killing of bacterial cells.
The toxicity of the products of the photochemical reaction in the 1-hexene-
nitrogen dioxide system was greater than the simple additive effect of the
individual components.
Luminescence in bacteria can be adversely affected by exposure to
1159, 1160
air pollutants. Serat ej: al. developed an assay for air pollutants
based on the fact that Photobacterium phosphoreum. a luminescent bacterium,
was sensitive to photochemical oxidation products. Cells of IP. phosphoreum
treated with a gas stream containing products formed by the photochemical
oxidation of cis-2-butene and nitric oxide lost both luminescence and via-
bility. The rate of luminescence decrease depended on the initial concen-
tration ratio of the reactants. Ozone, nitrogen dioxide, formaldehyde,
acetaldehyde, and PAN were also examined in this system. PAN and ozone
were effective in decreasing luminescence of P_. phosphoreum. This organism
might be useful in rapidly determining the extent of air pollution.
Several studies have been conducted on the effect of hydrocarbon
556
pollutants on soil fertility. Harper found that natural-gas seepage
killed the plant cover over the area of leakage. This was shown by
601a 466a
Hoeks and Garner to be due to the utilization of all available
oxygen from the area during aerobic hydrocarbon degradation, resulting
in a high degree of anaerobiosis. There was an increase in anaerobic
384
-------
organisms in the soil and in total nitrogen, probably owing to nitrogen
fixation by various Clostridia species. Similar results were obtained by
77, 375
other workers. The effect of halogenated hydrocarbons on soil
724
microflora has been examined. Koike found that 1, 3-dichloropropene,
1, 2-dichloropropane, and ethylene dibromide all retard nitrification in
1369
soil for an extended period. Wensley found that methylbromide
suppressed the growth of bacteria and had less effect on fungi. Members
of the penicillia and aspergilli are more abundant in fumigated soil.
Nitrifiers and cellulose decomposers were most adversely affected by
methylbromide treatment.
All organic vapor-phase air pollutants eventually are carried onto
the surface of the earth by various forms of precipitation. They dissolve
2
in water or attach to plant or soil particles. Abeles ^t aL studied the
fate of air pollutants using ethylene as a model. They estimated that
6
15 x 10 tons of ethylene were released as pollutants in 1966 and examined
soil for the ability to remove ethylene from the atmosphere. No ethylene
uptake occurred in sterilized soil or in the absence of oxygen. There was
a 24-hr lag after exposure to ethylene before uptake of the gas by soil
occurred. This suggests that microorganisms are responsible for the
ethylene assimilation. Abeles et al. made a conservative estimate that
6
7x10 tons of ethylene can be removed from the atmosphere each year
by microbial degradation.
817
Some consideration has been given by Lubowe to the effect of air
pollution on the normal microbial flora of human skin. Deposition of soot,
dust, and air pollutants on skin could affect bacterial growth and later
physiologic activities of the normal skin microflora.
385
-------
Microbes utilize and produce a number of vapor-phase air pollutants.
Anaerobic microorganisms in the intestine of ruminants and in swamps
produce great quantities of methane (marsh gas). A single cow produces
60-70 liters of methane daily. Methane accumulation from microbial
metabolism can, in rare instances, be hazardous, but in general the
methane concentration (1. 5-2. 5 ppm) in polluted urban air is not toxic,
21
nor does it react photochemically. These methanogenic organisms do
1415
create one environmental pollution problem. When elemental mercury
is present in anaerobic areas of ponds, rivers, estuaries, or oceans, the
3+
methyl group attached to Co of the vitamin B in methanogenic microbes,
12 2+
which normally is evolved as methane, is passed to the Hg molecule. Thus,
mercury is transformed to methylmercury and dimethylmercury, its most toxic
forms.
Microbes present in cow dung have been shown to produce ethane,
313
ethylene, propane, and propylene. Penicillium digitatum produces
313
acetylene, ethylene, propylene, ethane, and propane in small quantities.
969
Nickerson found that Blastomyces dermatiditis can produce ethylene at
140 mg/liter during growth in liquid culture.
Microorganisms present in soil and water and on skin and plants
throughout the surface of the earth play a very important role in prevention
of any significant buildup of vapor-phase organic hydrocarbons. These
scavengers utilize a great number of these compounds as a source of
1061
carbon and energy. In many cases, they convert potentially hazardous
775
compounds to less toxic products by cooxidation.
The ability of microorganisms to utilize hydrocarbon substrates has
446
long been known. Hydrocarbon-utilizing microbes are present in all
surface soil and water, suggesting that hydrocarbons are constantly
386
-------
introduced into the environment. Studies on the oxidation of gaseous
123, 661, 775, 776, 860, 1317, 1318
hydrocarbons are abundant in the literature
and will not be detailed here. These hydrocarbon-utilizing organisms act
to keep the amount of saturated and unsaturated vapor-phase hydrocarbons
668
in the environment at a low concentration. Jones has developed a
respirometric technique for demonstrating hydrocarbon utilization in the
natural environment.
One of the dangers associated with increased concentrations of vapor-
phase hydrocarbons in the environment is the introduction of these pollutants
into the food chain through microorganisms. The lipids in microorganisms
357, 358, 1318
can be changed drastically by growth on hydrocarbon sub-
strates, and these might also find their way into the food chains and into the
bodies of higher animals.
EFFECTS ON VEGETATION
Vapor-phase organic pollutants are universal; many are natural com-
ponents of plants and are normally vaporized into the atmosphere or re-
leased iri combustion. The organics include a varilety of partially oxidized
products of combustion, particularly C compounds of carbon and
1-10
hydrogen, plus the nitrogenated peroxyacetylnitrates.
Of these chemicals, only PAN and ethylene are known to be injurious to
vegetation, and then only when their local concentrations exceed background
concentrations by many times. Methane, benzene, acetylene, etc. , are
almost unreactive.
9
However, Adams and Ellis report that chemical and physical changes
may occur in soils saturated with natural gas. Plant growth may be retarded
or completely inhibited in gas-saturated soils. The instances are so infrequent
that studies have been limited. Mass-spectrometric analysis of affected soils
387
-------
showed the presence of 80-90% methane, with the remainder of the gas
mostly ethane. It was suggested that the disturbance in iron-manganese
relationship caused by the gases may have been one of the major factors in
the detrimental influence of gas-saturated soils on vegetative growth.
Aldehydes have occasionally been blamed for plant injury, but their
impact is more likely indirect, involving only their contribution to photo-
146, 594
chemical reactions yielding phytotoxic oxidants. Hydrocarbons
are included among the varied components of "photochemical" smog, and
herein lies their significance. Briefly, some hydrocarbons react with
nitrogen oxides in the atmosphere to produce the varied chemicals dis-
304
cussed earlier, of which the most important phytotoxicant is PAN.
PAN was not defined as a specific pollutant until after "smog" injury
889
was first described in the 1940's. Evidence that injury was associated
with automobile exhaust emissions was slow to accumulate, and it was
889
1950 before Middleton et al_. demonstrated this relation. Haagen-Smit
530
et_ aL discovered in 1952 that hydrocarbon and nitrogen oxides emitted
in exhausts react in the presence of sunlight to yield toxic gases that cause
304
plant damage, eye irritation, and reduced visibility. Parley et al.
later reported the presence of an ozone-olefin complex. Stephens and
Scott began in 1956 to isolate the toxic component that later turned out to be
1220
PAN.
Few measurements have been made of ambient PAN concentrations or
their correlations with symptoms, but controlled fumigations indicate the
concentrations at which visible symptoms might be expected. Chemical
analyses for PAN in California and Utah show concentrations of 0. 01-0. 02 ppm
303, 1262
on average days and up to 0. 05 ppm on "smoggy" days. Stephens
1222
and Scott reported that lesions developed on sensitive petunia and tobacco
388
-------
varieties after 5 hr of fumigation with 0. 1 ppm. Observations in Salt Lake
City after "natural" fumigations indicated that injury could be produced by
as little as 0. 02 ppm for 2-4 hr. Sensitive plants were injured when PAN
1250a
concentrations averaged 0. 025-0. 03 ppm for a few hours. Taylor et al.
644
and Jaffe found that sublethal effects included a reduction in dry fresh
weight at concentrations of 0. 005-0. 015 ppm for 8 hr and reduced flowering
and stem elongation at 0. 1-0. 2 ppm.
Symptoms attributed to "smog1' now seem to be caused primarily by PAN
and its hontiologues--that is, the reaction products of photolyzed nitrogen oxides
with unsaturated hydrocarbons. The symptoms produced include most of those
1250
once designated to be caused by "smog" or photochemical oxidants and are
888, 1370a
worldwide.
The precise nature of the symptoms depends as much on environmental
factors, tissue, maturity, concentrations, and duration of fumigation as on
the toxicant. Many symptoms may arise. Consequently, it is desirable to
644
designate one general category as "PAN Injury."
302
Darley ^et al_. distinguished the PAN symptoms from those caused
by products of ozone-olefin reactions. The symptoms were essentially
similar, but the ozone-olefin reaction products were most toxic to young,
fully expanded leaves, causing uniform or indiscriminately distributed glazing
over the lower leaf surface. The banding that appears on plants in the field
or -when PAN is present was not observed.
Symptoms of PAN injury cover a considerable range of expression,
depending on the concentration, duration of exposure, and nature and maturity
1250,1268
of the leaf tissue exposed. High concentrations, 0. 5-1. 0 ppm for
30 min, may cause complete collapse of the expanding leaves, but such con-
centrations have not been found to occur naturally. Lower concentrations,
0. 3-0. 5 ppm for 30 min, m^y cause the complete collapse of the tissue in a
389
-------
diffuse band across the leaf. Still lower concentrations, 0. 1-0. 3 ppm for
4-8 hr, higher than found in the field, may cause bronzing or glazing with
little or no collapse visible on the upper leaf surface. Below 0. 1 ppm,
visible symptoms consist mostly of chlorosis.
Fumigations of sensitive species at the University of Utah (F. K.
Anderson, unpublished data) with PAN at concentrations of 0. 02-0. 1 ppm for
2-6 hr provided a clear picture of symptomatology in a concentration range
characterizing urban environments. Symptoms in Ranger alfalfa, one of the
most sensitive species, first consisted of a very light yellow to -white
stippling, appearing mainly on the upper surface, but evident also on the
lower. Stippling characteristically developed between secondary veins, but
distribution along the leaf varied with maturity. Bleaching was most prominent
at the tip of terminal leaves and base of older leaves. When concentrations
approached 0. 1 ppm, local areas of tissue collapsed completely, producing
bleached, white necrotic lesions.
Lima bean plants fumigated with PAN occasionally developed a chlorotic
green to white stipple scattered over the entire upper leaf surface. More
typically, a silvery to lead-colored glaze developed in bands over the lower
surface. Bronzing was often accompanied by a tan stippling.
«
Symptoms in corn consisted of chlorotic or necrotic streaks principally
on the upper leaf surface. These streaks were largely interveinal, 0. 5-2 in.
long, and extended across the leaf at various distances from the tip. Light
gray to chlorotic stipple symptoms, developing in similar bands, appeared
on other leaves.
Endive developed a light green, overall chlorosis with slight marginal
necrosis; lower surface, interveinal, silvery to bronzed glazing was most
frequent and prominent.
390
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The symptoms of smog injury in herbaceous plants in the field, as
described in the Los Angeles area, generally consist of some type of
glazing or bronzing distributed irregularly over the underside of the
303a,886a,888a
leaf. In 1952, it became apparent that the area of the
125,492
leaf damaged was related to its maturity. When plants •were
exposed to a single fumigation, only a few leaves at a particular stage
of maturity were marked. The youngest leaf would be injured at the
tip, the next youngest about one-third of the way down from the tip,
and the oldest near the base. This banding -was especially prominent on
leaves of petunia, rye grass, and annual blue grass. Sensitive broad-
leaved plants, such as spinach, were most often marked about one-third
of the way down from the leaf tip. Banding was also observed in the field
on beets, chard, mimulus, chickweed, pigweed, dock, and others.
Banding is extremely helpful in identifying PAN injury. However,
if exposxire occurs on successive days, the newly differentiating cells
may be injured each day, so a general glazing appears over the entire
leaf. This has been the more characteristic expression in the polluted
areas of California, where toxic PAN concentrations occur daily.
Photochemical pollutants, including PAN, enter the leaf through
the mature, functional stomata. Once within the substomatal chambers,
the pollutant attacks the mesophyll cells bordering the intercellular spaces.
The morphologic, histologic effects of ozonated olefins, and later PAN,
125,492
were studied on representative sensitive species. The earliest
visible indication of injury was the oily, shiny, water-soaked appearance
of sensitive tissues on the lower leaf surface. By the time this appeared,
tissue alterations were already apparent. Tiny, raised blisters appeared;
they were formed by the swelling of guard cells and other epidermal cells
391
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nearest the stomata. These became engorged with water and increased
in width, causing the stomata to enlarge further. By the time the epi-
dermal cells collapsed, the entire leaf became turgid. Permeability
may be damaged, so that excessive water enters the affected cells, which
stretch as they engorge water, giving the underside of the leaf its shiny,
water-soaked appearance. If the fumigation does not persist, or is not
too severe, the turgid cells may recover after a few hours, leaving no
trace of visible injury.
Cells of the spongy mesophyll nearest the intercellular spaces are
1258
affected after the epidermis. Thornson et a_L , using electron
microscopy, found that small, electron-dense granules appeared in the
chloroplast stroma soon after fumigation. "Crystalline" arrays of
granules appeared later, and the shape of the chloroplast was altered.
The granules seemed to fuse into rods and then into an organized system
of plates that persisted and even seemed to continue to develop. Finally,
the integrity of the chloroplast was lost, and the membranes disrupted.
As the chloroplasts broke down and became dispersed into the cytoplasm,
the entire protoplast aggregated into a large mass, which condensed in
the interior of the cell, causing the cell to collapse. The plasmodesmatal
connections with neighboring cells appeared to persist. Although the lower
surface was usually injured first, owing to the greater amount of inter-
cellular space, the reverse may also be true.
Internal, as well as external, symptoms vary with the structure of
the plant. Broad-leaved herbaceous plants — such as table beet, sugar
beet, lettuce, and spinach--are most sensitive and characterized by a
"silvering," and sometimes bronzing, of the lower leaf surface. Mono-
cotyledonous plants--including oats, corn, barley, and grasses--appear
392
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dark green, as though water were trapped beneath the epidermis. As
cell damage progresses, the dark-green, water-soaked areas develop
into yellow streaks, which follow the zones where stomates are densest.
The yellow soon turns to brown, and longitudinal necrotic streaks appear
1115, 1268
between the larger veins.
The histologic changes and visible symptoms caused by air pollutants
are basically caused by impaired metabolic processes. Enzyme activity,
respiration, photosynthesis, ion absorption, and carbohydrate and protein
synthesis all may be impaired by PAN concentrations far lower than necessary
1250
to produce any leaf chlorosis or necrosis.
To understand how PAN might affect metabolism, it is desirable
first to know how the chemical is incorporated into the plant constituents.
1220
Stephens ^t al. synthesized PAN labeled with carbon-14, so that its
path-way in the plant might be followed. When the cells were fractionated,
much of the carbon-14 in treated plants appeared in the chloroplasts, and
355
it is in these organelles that one might first look for deleterious effects.
Early damage to the chloroplasts was shown first by histologic, micro-
scopic studies, but later physiologic studies confirmed the damaging effect
745
of PAN on the chloroplasts. The mechanism of action is not clearly
defined; it may be caused partly by injury to the enzymes necessary for
355
photophosphorylation. Bugger et al. showed that PAN inhibited photo-
synthetic carbon dioxide fixation in pinto bean plants.
PAN may also be harmful in oxidizing critical sulfhydryl groups of
921
some enzymes. Mudd showed that enzymes that contained free
sulfhydryl groups for catalytic action were especially sensitive to PAN.
354
Dugger et_ aL have shown that a correlation exists between the
sulfhydryl content of bean plants and their susceptibility to PAN. Damage
393
-------
may be prevented by spraying sulfhydryl reagents that protect the
sulfhydryl groups on intact plants or, in in vitro studies, adding the
reagents to enzyme reaction mixtures, chloroplasts, or mitochondria!
suspensions.
Oxidation of the sulfhydryl group -would impair the activity of
many enzymes. Inhibition of an enzyme, such as phosphoglucomutase,
that is vital to the synthesis of glucose may be directly responsible not
only for inhibition of glucose synthesis, but also for inhibition of the
synthesis of closely related carbohydrates--including galactose, xylose,
arabinose, and polymers that constitute such wall fractions as cellu-
994a
lose. The cell wall expansion phase of plant cell growth depends
on growth hormone-induced changes in cell wall plasticity. PAN also
has been found to inhibit one of the enzymes, glucan hydrolase, that is
necessary for the metabolic breakdown of the cell wall.
PAN inhibition of enzymes essential to both cell -wall synthesis
and degradation may well account for reduced cell enlargement and
995
growth associated with PAN damage.
A knowledge of the mechanism by which an air pollutant acts on
specific metabolic processes provides the background for understanding
the effects of air pollutants on growth and reproduction in the field.
This threat is the ultimate concern of anyone attempting to grow plants
and may be a critical factor in the well-being of natural plant populations.
The multitude of variables affecting growth and production make it
extremely difficult to measure losses that might be attributed solely to
air pollutants, and relatively few studies provide much of a clue as to
the extent of such losses.
394
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Studies of growth suppression have consisted largely of exposing
plants to ambient, polluted atmospheres and filtering air in which
1257
comparable control plants were grown. Total oxidant concentra-
tions in the field were known, but the specific composition of the polluted
air was not.
Ambient-atmosphere studies fail to delimit the effects of PAN alone
on growth and production, but they do provide an excellent picture of the
actual impact of ambient polluted atmosphere. The limited studies of the
effects of Los Angeles "smog" on plant growth were summarized by Todd
1263
£t al. Kentia palms were especially sensitive. When grown in ambient
air, plants were small, and leaves were stunted and chlorotic. Palms in
carbon-filtered atmospheres were noticeably larger, averaged one leaf more
than on plants in ambient air, and had longer leaves, which were dark green.
Avocado trees grown in ambient and filtered atmospheres responded simi-
larly, with stem diameter significantly greater in the filtered air after 6
months.
Comprehensive studies of the effect of ambient atmospheres on citrus
production in the Los Angeles area were undertaken by Thompson and his
1256, 1257
associates. Trees exposed to the naturally polluted air had
up to 30% more leaf drop, and the average yield was often only half what
it would have been in clean air. But much of this effect may have been
1250
caused by ozone.
Of the numerous hydrocarbons in urban atmospheres, only ethylene
is recognized to affect plants significantly. Ethylene is produced naturally
by plants and regulates various growth and maturation processes, including
538, 539, 570
abscission and senescence. Because of its low concentrations
in the atmosphere, measurements of ethylene concentrations are infrequent.
395
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20
Data provided by Altshuller and Bella and the California State De-
partment of Public Health suggest that background concentrations are
in the parts-per-billion range. Concentrations in the few urban •
atmospheres monitored for ethylene are 0. 004-0. 3 ppm. Higher con-
centrations have been recorded around industries producing ethylene
570
products.
Ethylene has been recognized as an air pollutant since at least
1202
1871 (Kny, cited in Sorauer ), when it was found to escape from
underground utility lines carrying illuminating gas. This gas was
produced by the combustion of coal in retorts and used in early street
lighting. The wooden or cast-iron pipes often leaked, releasing the
gas, which contained about 3% ethylene, into the surrounding soil,
where it damaged roots of nearby trees. Because coal gas is no longer
used, this source does not provide a threat to the surrounding environ-
ment; however, ethylene as a more general air pollutant has been
1088
recognized since 1943, -when it was discovered in automobile exhausts.
The internal-combustion engine is the major source of ethylene in metro-
politan atmospheres, but additional ethylene is contributed by combustion
of natural gas, coal, or wood or as a blowoff gas from the cracking of
natural gas in petrochemical plants. These and other hydrocarbon sources
have been discussed earlier.
Ethylene is sufficiently toxic to plants to be responsible for crop
648, 886a
losses near metropolitan areas. The ambient concentrations
may be a hundred times greater than the threshold for injury to the more
sensitive species. Ethylene content of samples collected in Los Angeles
over a 10-month period averaged 0. 07 ppm. Over a 4-month period,
95,330
ethylene averaged 0. 2 ppm, with peaks of 1.42 ppm.
396
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The deleterious effects of prolonged exposures to low ethylene con-
centrations, well under 1 ppm, were recognized after tremendous economic
302, 308
losses to the commercial orchid growers of Los Angeles. The
first indication of the disease was that the showy, petaloid sepals of the
flower became chlorotic and dried from the tips down just as they were
emerging from the bud; the earliest expression was the slight yellowing
of the bud. Often, the bud dropped before opening; but, if it remained,
the sepal tips were typically translucent to necrotic. Even when necrosis
was not perceptible, the flower remained fresh for only a few days, rather
308
than the 10-14 days required in the orchid trade. Davidson found that
the dry sepal disease, as it became known, could be produced by as little
as a 24-hr exposure to ethylene at 0. 002 ppm or a 6-hr exposure at 0. 05 ppm.
Even 0. 002 ppm affected the commercial value of the bloom, decreasing the
price by 25%.
The costly loss of orchids caused many growers to relocate beyond
the urban limits of Los Angeles and San Francisco, but those who remained
suffered losses estimated to exceed $100, 000 a year in the San Francisco
648
area alone. Additional greenhouse crops and many field-grown
floricultural species sustained equal or even greater losses. The demise
of the cut-flower industry in Los Angeles and San Francisco was due at
least in part to ethylene. Carnations, snapdragons, roses, camellias,
and chrysanthemums were among the most critically damaged.
The "sleepiness" disease of carnation caused an estimated $700, 000
loss to growers in 1963 (H. Jones, personal communication). This
282
ethylene-caused disease has been known since 1908, but has been more
destructive since the intensification of automobile pollution. Petals turn
yellow and wither, buds remain partly or wholly closed, and flowers open
slowly, if at all.
397
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Studies concerning the air pollution potential of ethylene received
a substantial stimulus after severe losses in cotton and other economic
570
crops near a Texas Gulf Coast polyethylene manufacturing plant.
Ethylene concentrations in the field ranged from 0. 04 to 3 ppm, depending
on distance and direction from the source. The cotton crop within a mile
of the facility was completely destroyed. Less severe field symptoms
included leaf abscission, scattered seedling death, vinelike growth habit,
and fruit abscission. Vegetative and reproductive organs were malformed,
539
with symptoms much like those produced by 2, 4-D. Young cotton plants
were severely defoliated in fields close to the ethylene source. More
distant plants exhibited "leaf puckering," reddening, and chlorosis; apical
dominance was lost, and axillary buds were forced. Flowering was stimu-
lated, but most fruits abscised.
570
Heck and Pires fumigated 93 different species -with ethylene
at concentrations of 2. 5-10 ppm. Symptoms on sensitive species consisted
of yellowing and occasionally necrosis of the lower leaves, chlorosis of
the flower buds, and inhibition of terminal growth, with an increase in
the number of nodes and young leaves. These plants recovered rapidly
when fumigation was stopped, but leaves formed during treatment never
expanded normally. A still milder type of expression was characteristic
of grasses and other more tolerant species. Plants developed no apparent
injury, but leaf elongation was permanently and measurably inhibited.
Abscission in the absence of chlorosis occurred on some plants. The
oldest small branchlets of juniper and arborvitae dropped readily, and
light shaking after fumigation caused more than 50% of the older branchlets
to fall. More than half the arborvitae cones dropped on touch. Floral
injury appeared on all 22 species that were in bloom during fumigation.
398
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Death or abscission was noted in eight species. Such an effect could
result in the complete loss of economically valuable crops.
The profound effects of ethylene on the normal development of plants
are due to its hormonal nature in being absorbed, transported, and
assimilated readily by the vegetative organs. Abscission, for instance,
is normally caused by the balance between auxin and ethylene in the
538
petiole. An artificial increase in ethylene through pollution would
upset this balance and cause early defoliation.
Various mineral oils and hydrocarbons have been used to kill
insect pests, and their phytotoxic properties have been applied in their
633
use as herbicides. Comparisons of the toxicity of hydrocarbons
appliedas sprays show the more volatile aromatic series to be most
toxic, the olefins and cycloparaffins intermediate, and the paraffins
least toxic. These hydrocarbons were toxic only at concentrations of
over 2%; most were toxic only at concentrations of over 30%. Despite
the significance of the herbicidal properties of these chemicals when
applied to the soil, toxic concentrations are unlikely to be present in
even the most polluted atmospheres.
399
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CHAPTER 9
GENERAL SUMMARY AND CONCLUSIONS
SOURCES OF HYDROCARBON EMISSIONS
Estimates of total hydrocarbon emission in the United
States are 12-20 million tons/year from mobile sources, 7-25
million tons/year from stationary sources, and 72 million
tons/year from natural sources. The U. S. emission is about
40% of the world total for both mobile and stationary sources.
Most natural emission of hydrocarbons consists of methane and
terpenes. Only a minor fraction of man-made pollutants are in
this category.
Motor vehicles are by far the most important mobile
source of gaseous emission, accounting for about 86%, whereas
aircraft, railroads, marine vessels, and nonvehicular combined
account for the rest.
Gasoline-powered vehicles account for about 99% of all
vehicular emission, and diesel-powered vehicles account for
the rest. The hydrocarbons in exhaust gases are chiefly low-
molecular-weight compounds. The total oxygenate concentration
is about one-tenth of the total hydrocarbon concentration.
Aldehydes are generally believed to be the most important class
of oxygenates.
Diesel-powered motor vehicles account for about 1^% of
the emission from all motor vehicles and about 1% of that from
all mobile sources. The gaseous hydrocarbon fraction of diesel
exhaust is extremely complicated; it consists of light, cracked
400
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hydrocarbons and heavy fuel-like components up to about C .
Low-molecular-weight aldehydes have also been found in diesel
exhaust.
Hydrocarbon emission from aircraft is estimated at 0.4 x 10°
tons/year, or about 2% of the emission from all mobile sources.
Formaldehyde and total aldehyde concentrations in turbine ex-
haust have been measured, but no detailed aldehyde analyses have
been re ported.
Emission from railway locomotives is only about 0.1 x 10"
tons/year; that from ships, barges, and other marine vessels
is estimated at about 0.3 x 10 tons/year. Such emission from
off-highway mobile sources is about 1.9 x 10 tons/year, or
about 10% of all mobile-source emission, and appears to be in-
creasing.
Total organic emission from stationary sources has been
estimated (1971) at slightly over 25 x 10 tons/year. Organic
acids are the major constituents, followed by hydrocarbons and
aldehydes. This is an entirely different order from that in
auto exhaust, where hydrocarbons predominate, aldehydes are
second, and organic acids are negligible.
Solvent evaporation is the largest single stationary
source of VPOP, 7.1 x 10 tons/year. Products from solid-waste
combustion and agricultural burning are the next most important
stationary sources.
Vapor breathing losses from storage tanks at refineries
and bulk plants and vapor displacement in the filling of tank
trucks, service-station tanks, and automobiles account for
almost all the emission associated with gasoline marketing.
401
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Various control devices and practices minimize vapor losses at most
storage facilities. There are few vapor-loss controls, however, at
retail service stations.
Of the many hydrocarbons present in the atmosphere, methane is
predominant in all locations, 1.2-1.5 ppm worldwide, and higher in
inhabited areas. Yearly averages of total hydrocarbons in urban
areas of the United States range from 1.43 to 3.3 ppm (measured as
carbon) and yearly maximal 1-hr averages range from 8 to 17 ppm.
In Los Angeles, methane and 10 other hydrocarbons (ethane, ethylene,
acetylene, n-butane, isopentane, propane, toluene, n-pentane, m-xylene,
and isobutane) account for about 90% of the total hydrocarbon load.
Formaldehyde appears to be a natural constituent of the atmosphere,
at a concentration of a few parts per billion. However, there are
few atmospheric measurements of oxygenates other than aldehydes.
POSSIBLE MECHANISMS OF FORMATION OF OXYGENATED ORGANIC COMPOUNDS IN THE
ATMOSPHERE
The atmosphere is a fertile medium for a wide variety of oxidation
processes and products, because it contains a virtually unlimited quantity
of oxidizing agents, oxidizable organic substances, and oxidation initia-
tors and catalysts. Aromatic hydrocarbons, present in the atmosphere
in minor amounts, are subject to autoxidation by triplet oxygen in an
environment containing metallic catalysts, ozone, oxides of nitrogen
and sulfur, peroxides, and light to form species with reactive func-
tional groups.
Singlet oxygen has been suggested as a possible contributor
to the chemistry of photochemical smog formation. Processes
other than direct excitation of ground-state oxygen are possible
important contributors to the production of singlet oxygen in
402
-------
the a.tmosphere . Available rate data indicate that both ozone
and atomic oxygen are more reactive toward olefins than is
singlet oxygen. It is difficult to assess the importance of
any contributions from singlet oxygen without more information
on its concentration in the atmosphere. On the basis of the
available data, it does not seem likely that (K ( Ag) is con-
tributing more than 5% to the basic reactions that convert
nitric oxide to nitrogen dioxide in polluted atmospheres.
Whatever reactions occur would be expected to produce peroxides,
hydroperoxides, and the carbonyl compounds derived from them.
Atmospheres affected by photochemical smog contain relatively
high concentrations of ozone. Ozone reacts with olefins and
saturated aliphatic hydrocarbons in the vapor phase. In the
case of saturated hydrocarbons, the products are similar to
those obtained in solution. A free radical mechanism has been
offered to explain these products.
With olefins, the vapor-phase products are similar to those
obtained in solution, except that there is no good evidence of
the formation of ozonides in the vapor phase. The products
observed are aldehydes, ketones, alcohols, and acids derived
from the olefins, along with carbon monoxide, carbon dioxide,
water, methane, and small amounts of other saturated hydro-
carbons in some cases. The reactions also produce peroxidic
materials that have not been further identified.
403
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Olefins are considerably more reactive than saturated
hydrocarbons. Rate data for olefins obtained in the presence
of oxygen show a nonstoichiometric olefinrozone ratio. When
nitrogen is used as carrier gas, the expected 1:1 stoichiometry
is observed. There is some variation in the absolute rate
data, which is probably related to the widely different concen-
trations used or the presence of oxygen. The relative rates
of olefin reaction generally follow a trend that is consistent
with initial electrophilic attack of ozone and inconsistent with
initial radical attack.
REACTIONS OF OXIDES OF NITROGEN AND SULFUR, HYDROXYL RADICALS,
AND OXYGEN ATOMS WITH ORGANIC MOLECULES IN THE ATMOSPHERE
When mixtures of hydrocarbons and nitrogen oxides are
subjected to ultraviolet radiation, nitric oxide is transformed
to nitrogen dioxide, the nitrogen dioxide photodissociates, and
oxidant (primarily ozone) is produced. The exact mechanism
for this process is uncertain, but it may involve a chain process
in which HO radical is the chain carrier. Contrary to earlier
beliefs, carbon monoxide can enter the cycle and promote the
conversion of nitric oxide to nitrogen dioxide. Nitric oxide
is involved in both chain propagation and termination. In-
creasing the nitric oxide concentration at first enhances the
production of nitrogen dioxide, but then retards the production,
as the terminating reactions become more important than the
propagating reactions. Increasing the hydrocarbon content en-
hances the conversion, because hydrocarbons are important pri-
marily in the propagation steps.
404
-------
If sulfur dioxide is also present, oxidant production may
be either enhanced or retarded, sulfuric acid is produced, and
organic aerosol formation is increased. The detailed reactions
of sulfur dioxide are not understood, but it can react with alkyl
radicals and perhaps with alkoxy and alkylperoxy radicals and
with zwitterions. Furthermore, sulfur dioxide absorbs radiation
o
above 2900A to produce at least four excited electronic states,
each of which can interact with both inorganic and organic
molecules.
METABOLISM OF VPOP IN MAMMALIAN SYSTEMS
The mammalian metabolism hydrocarbons and their oxygenated
derivatives (epoxides, ethers, alcohols, phenols, aldehydes,
and ketones) generally results in their conversion into more
polar compounds, which are excreted either as such or as their
conjugates with glucuronic or sulfuric acid. In the case of
sulfur compounds, conjugation occurs with glutathione.
Little seems to be known about the metabolism of a number
of highly reactive substances that may be formed in the atmosphere,
such as peroxides, hydroperoxides, peroxy acids, volatile
nitrates and nitrites, lactones, and ozonides, many of which
exhibit potent biologic effects.
BIOLOGIC EFFECTS OF VPOP IN HUMAN AND OTHER MAMMALIAN SYSTEMS
Absorption of VPOP
Vapor-phase organic air pollutants from hydrocarbons are
absorbed through four routes: the respiratory tract, the gastro-
intestinal tract, the eye, and the skin. Owing to the com-
paratively enormous volume of VPOP-containing air exchanged in
405
-------
human lungs at rest and during exercise, the respiratory tract
is directly and indirectly the most important route of exposure
and absorption. Almost all available data on this subject are
occupational and reveal that the respiratory tract generally
absorbs toxic chemicals at equivalent doses more rapidly and
completely than the skin and that the toxic hazard at equivalent
doses from respiratory exposure is about 3 times greater than
that from oral exposure and 10 times as great as that from
dermal exposure.
Pathophysiologic Effects
Short-term exposure to irradiated auto exhaust results in
increased flow resistance, minute volume, and tidal volume,
all of which return to normal immediately after exposure.
Agents that can cause pathophysiologic effects are also present
in raw auto exhaust gases and are multiplied by the photochemical
effects of irradiation. Similar effects are produced by acrolein
and formaldehyde; the effects of auto exhaust on respiratory
function are possibly caused by aldehydes.
Alkylbenzenes and olefins are more effective precursors
of biologically active photochemical agents than paraffins,
benzene, and acetylenes. Effective eye irritants that are
products of photochemical reactions are formaldehyde, acrolein,
peroxyacetyInitrate, and peroxybenzoyInitrate. Eye irritation
can be appreciable when oxidants are negligible.
The effects of the many hydrocarbons present in the urban
polluted atmosphere on the human and animal respiratory tract
406
-------
have not been studied sufficiently at low concentrations or
over long periods, although there are data on the effects
of concentrations much higher than are likely in community
air pollution. Studies of these effects in man are compli-
cated by the inability to separate the changes due to pollutant
materials alone from those due to other factors, such as
tobacco-smoke inhalation, spontaneous infection, genetic
factors, industrial vapor exposures, and aging. Pathologic
study of domestic or laboratory animals exposed to urban
atmospheres has yielded relatively little information.
This may be due in part to the relative insensitivity of the
methods used. Limited and incomplete studies of the effects
of real urban atmospheres on laboratory animals have been
performed with ultrastructural techniques.
Synergism, Antagonism, and Tolerance
Synergistic and antagonistic action has been demonstrated
in a few situations involving hydrocarbons and oxidizing
pollutants.
Mutagenie Effects
Some vapor-phase organic air pollutants have been shown
to have mutagenic effects on various laboratory test organisms
and may therefore pose a potential hazard to man.
Terat.ogenic Effects
The prenatal safety of exposure to vapor-phase organic
air pollutants has not been adequately assessed.
407
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Carcinogenic Effects
Most of the vapor-phase organic compounds expected to be
present in polluted atmospheres have not been tested for car-
cinogenic or cocarcinogenic activity. Several classes of com-
pounds are known to have carcinogenic activity on injection
and dermal application at relatively high dosages. These classes
of compounds are all oxygenated hydrocarbons.
EPIDEMIOLOGIC APPRAISAL OF HUMAN EFFECTS
Compounds with established unfavorable effects are formal-
dehyde, ozone, (and oxidants), and a group of lachrymators that
is not very well defined, but probably includes PAN and peroxy-
benzoylnitrate.
INTERACTIONS AND EFFECTS OF VPOP ON TOTAL ENVIRONMENT
Effects on Atmospheric Properties
There are no known effects of VPOP on the physical
properties of the atmosphere. They might influence nucleation
and the formation of droplets in the atmosphere, thereby affect-
ing fog and cloud formation. However, the significance of such
an effect is largely speculative. There is virtually no informa-
tion on their effects at their typical ambient concentrations
on building materials, clothing, and paint, although such effects
may be considerable.
VPOP from hydrocarbons in air may be transported to
terrestrial waters by precipitation, fallout, and diffusion
across the surface water-air interface. Information on the
extent of these contributions to the amounts and distributions
408
-------
of these compounds in such waters is not available. There are
some data on the gross organic-compound content of precipitation,
air, and air particles, as well as some additional characteriza-
tion by types and some specific organic compounds.
Removal, Fate, and Persistence
Two contrasting situations present themselves for vapor-
phase and aerosol removal.
Although the residence times of a few compounds that are
removed as gases have been measured, experimental confirmation
of the adequacy of the theories of removal is still sparse.
As a result,.we remain unsure of the generality of the proposed
removal mechanisms, and our ability to predict the removal
rates of gases for which no data are available is very limited.
For aerosol removal, we have no similar problem. Although
here, foo, experimental confirmation of mechanisms is sparse,
there is a large body of empirical evidence, arising mostly
from studies on radioactively labeled particles, which permits
us to estimate with some confidence the lifetime of any aerosol
before removal from the atmosphere. A weakness in this
empirical approach is that we know little about the influence
of particle size on the rate of removal and little about the
rate at which particles grow to the sizes most subject to re-
moval.
But; the most urgent problem in aerosol removal of VPOP
products is the conversion of the gas into particles. Only
the sketchiest of estimates are available of the proportion of
VPOP products that ultimately appear in aerosol form, and no
information is available on the question of whether unreactive
409
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compounds that release energy on oxidation (for example, paraffins)
can be ultimately incorporated into particles.
There is a pressing need for at least preliminary answers
to these questions.
Effects on Natural Waters
A variety of effects of organic compounds on terrestrial
waters are of possible concern because of their influence on
the general quality of the water, sediments, and biota or be-
cause they may degrade or have an impact on a particular water
use. There are quality criteria for the organic content of
water for several industrial and agricultural uses. Taste and
odor effects in drinking water have been associated with specific
organic compounds, and one cannot rely on conventional municipal
water treatment to remove all organic compounds from a raw water
supply.
In general, terrestrial waters themselves are not well
characterized as to their organic-compound composition, although
some specific types of compounds, such as pesticides and poly-
nuclear aromatic hydrocarbons, have been reasonably well studied.
Because of this dearth of information, conclusions as to the
possible effects of VPOP from hydrocarbons on the receiving
waters will at best have to be speculative or extrapolated
from studies of model organic compounds or "organic matter" in
simulated natural water systems.
410
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Through complexation processes, organic compounds are
likely to have a profound effect on the transport of trace
elements in natural waters. They can have similar effects on
other organic compounds through their ability to associate with
or solubilize them. The sorption of organic compounds onto
minerals can affect their stability and gradual incorporation
into the biologic cycle. This can also be a mechanism for
their concentration and transport in water, even when they have
limited solubility.
VPOP from hydrocarbons are only one source of the rather
complex and often undefined collection of organic compounds in
terrestrial waters that may contribute to some of these effects.
It is likely that their contributions are not decisive. However,
there is not sufficient information to make a judgment.
Effects on Microorganisms
Microorganisms are sensitive to ozone, per ox^racety 1 nitrate,
unsaturated hydrocarbons, and many pollutants that are constantly
introduced into the environment. Gaseous hydrocaron seepage
into soil results in an anaerobic condition that destroys the
normal microbial flora and causes infertility of the soil.
The activities of nitrifying bacteria in soil are disrupted
by gaseous halogenated hydrocarbons. The microbes in soil and
on plant surfaces are responsible for mineralization of vast
amounts of terpenes, carbon monoxide, ethylene, and other
naturally occurring compounds. Interfering with the mineraliza-
tion of these naturally occurring compounds by introducing an
overburden of organic pollutant into the environment can be
dangerous.
411
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Effects on Vegetation
The effects of most VPOP on vegetation are due largely to
their participation in atmospheric photochemical reactions with
the production of ozone and PAN. Concentrations of PAN in urban
atmospheres are high enough to reduce growth and production of
sensitive crop and ornamental plant species. Enzyme activity,
respiration, photosynthesis, ion absorption, and carbohydrate
and protein synthesis all may be impaired. Even for brief
periods, concentrations of 0.05-0.1 ppm cause general chlorosis
and the characteristic bronzing or glazing of the affected leaves
Broad-leaved herbaceous plants--including chard, lettuce, table
beets, and spinach--are among the most sensitive.
Ethylene is probably the only hydrocarbon that has a directly
adverse effect on higher plants at ambient concentrations.
Orchids are most sensitive, but damage has also been reported
to carnations, roses, camellias, and chrysanthemums. It is
probable that other species have also sustained significant
damage from this growth-regulating chemical, but the phytotoxic
nature of ethylene has not been studied thoroughly.
4 lib
-------
CHAPTER 10
RECOMMENDATIONS FOR FUTURE RESEARCH
MECHANISMS OF FORMATION OF OXYGENATED ORGANIC COMPOUNDS IN THE
ATMOSPHERE
Triplet Oxygen
To reduce the quantity of organic pollutants from the
oxidation of hydrocarbons in the atmosphere, research should
be undertaken to determine practical means of eliminating or
markedly reducing the quantity of hydrocarbons available for
atmospheric oxidation or of oxidizing the hydrocarbons com-
pletely to carbon dioxide and water before emission from com-
bustion processes.
There has been a plethora of research studies on the
liquid-phase oxidation of hydrocarbons, but studies of oxida-
tion in the vapor phase in the presence of metallic catalysts,
ozone, oxides of nitrogen and sulfur, and light are sparse.
Extensive research on the vapor-phase oxidation of hydrocarbons,
including kinetics and mechanisms of oxidation and quantitative
product identification and analysis, is highly desirable, with
emphasis on more efficient processes for conversion to carbon
dioxide and water.
Singlet Oxygen
More basic work should be conducted to determine the con-
centration of singlet oxygen in the atmosphere. More work is
also needed on vapor-phase reactions of singlet oxygen and the
412
-------
rates of these reactions with hydrocarbons and their oxidation
products. Polluted atmospheres are known to contain olefinic
hydrocarbons. Because some reactions of singlet oxygen with
olefins are expected to occur, the toxicity of the products
of these reactions--such as hydroperoxides, methoxyhydroperoxides,
endoperoxides, and perhaps some of the decomposition products
of these peroxidic materials needs to be determined.
Ozone
Polluted atmospheres contain a variety of hydrocarbons
with which ozone would be expected to react. Many of the
products; of these reactions are known to be toxic. More work
must be done on olefin-ozone reactions in the vapor phase with
respect to both product analysis and kinetic studies, to
assess the importance of such reactions in the atmosphere.
The question of whether ozonides are formed needs a more
decisive answer. Rate studies are needed over wide concen-
trations of olefin to attempt to determine the reason for the
present discrepancies in the reaction rate data. Additional
inert carrier gases need to be tried to determine the effect
on the observed rate constant. More work is needed on the
reaction of ozone with oxygenated products of the reaction
of ozone and olefins. The role of radical reactions in the
kinetics and production of products should be clarified.
Information obtained from research should be useful in controlling
the number and quantity of ozone-hydrocarbon reaction products.
413
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REACTIONS OF OXIDES OF NITROGEN AND SULFUR, HYDROXY RADICALS,
AND OXYGEN ATOMS WITH ORGANIC MOLECULES IN THE ATMOSPHERE
The details of all the reactions involved in the production
of oxidant in photochemical smog are still not known. Further
work should be done to elucidate all reactions, and additives
that reduce the conversion rate should be sought.
No mechanism can yet fully explain the oxidation of sulfur
dioxide, the formation of organic aerosols, or the influence of
sulfur dioxide on oxidant production. Aspects of the atmospheric
chemistry of sulfur dioxide require further study, including the
possible reactions of photoexcited sulfur dioxide with hydro-
carbons and the reactions of oxides of sulfur with hydrocarbons
catalyzed by trace-metal contaminants.
METABOLISM OF VPOP IN MAMMALIAN SYSTEMS
Further work is needed for an evaluation of the pathway
and extent of metabolism of peroxides, hydroperoxides , peroxy
acids, volatile nitrates and nitrites, lactones, and ozonides
in man and lower animals.
BIOLOGIC EFFECTS OF VPOP COMPOUNDS IN HUMAN AND OTHER MAMMALIAN
SYSTEMS
Absorption of VPOP
Any discussion of routes and mechanisms of absorption of
vapor-phase organic air pollutants uncovers a paucity of under-
standing of these mechanisms, as well as many unanswered ques-
tions, owing especially to the lack of studies of long-term
subacute and chronic human exposures and related adverse health
effects. So far, these exposures and the resulting absorption
414
-------
effects have been difficult to study and identify properly,
and constitute a subject of research requiring additional
study.
Pathophysiologic Effects
Inadequate attention has been paid to the monitoring of
aldehydes (especially formaldehyde and acrolein) in photo-
chemically polluted atmospheres. More extensive monitoring
is recommended and more careful evaluations of the health
effects of these agents, experimentally and epidemiologically,
are recommended.
Photochemical reactions of VPOP produce irritating effects,
but indexes of such effects have received little attention.
Experimental tests in animals and systematic studies of human
reactions in normal and occupationally polluted atmospheres
should be carried out.
More information must be obtained on the physiologic
effects of hydrocarbon oxidation products, such as hydro-
peroxides, peroxides, and epoxides, ethers, lactones, aldehydes,
ketones, and other species with reactive functional groups.
Studies should be conducted not only on individual products
but also on mixtures of oxidation products to assess potentiating
or other unexpected biologic effects.
Ultrastructural studies should be done with sophisticated
techniques--e.g., scanning electron microscopy; autoradiographic
study of cell turnover in the lung and bronchial tree; bio-
chemical studies of the effects of mucin production, surfactant
production, histamine and serotonin metabolism, and the effects
415
-------
on collagen and elastin; and studies of capillaries using
electron-dense intravascular markers to study altered permea-
bility.
The effects of acute and chronic exposures to single or
multiple pollutants on pulmonary resistance of infection and
invasion by bacteria and viruses must be evaluated more com-
pletely. This will require the use of several animal species
of different ages, as well as a variety of microorganisms.
These studies should include evaluation of the effects of
pollutants on the activity and synthesis of immunoglobulins.
It is important that animals used in these studies
have appropriate anatomic characteristics. For example, in
studying the effects of chronic exposures to air pollutants
on mucin secretion, the animals used should have adequate
glandular structures in the tracheobronchial tree. The
effects of alterations in collagen and elastin after experi-
mental hydrocarbon exposures should be thoroughly studied by a
mult idisciplinary approach. Ultrastructural alterations in
these fibers and chemical changes involving crosslinking and
amino acid composition of the substances in question should
be evaluated in conjunction with physiologic studies of com-
pliance of whole lungs or similar studies involving lung strips
or isolated lung fibers. All these studies should be performed
with appropriate consideration and control of the effects of
aging in these proteins.
416
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Synergism, Antagonism, and Tolerance
Extensive experimental efforts should be made to under-
stand the underlying mechanisms of synergism, antagonism, and
tolerance in humans, rather than test all the possible combina-
tions in all possible ways for all possible effects in different
species. The human health implications of experimental data
should also be evaluated further.
Mutagenic Effects
Industrial workers exposed to relatively high concen-
trations of benzene have shown persistent chromosomal changes.
Workmen exposed to air pollutants suspected of mutagenicity
should be periodically subjected to chromosome analysis of
blood samples or biochemical tests.
Any chemical substance released into the atmosphere
should be evaluated for its mutagenic potential, and combina-
tions of two or more pollutants should be included to detect
possible interactions.
Teratogenic Effects
Numerous pharmacologic, toxicologic, biochemical, and
embryologic aspects need study:
The nature and causes of differences in drug response
between individuals and species must be intensively investigated.
It is necessary to discover, for many chemicals and
in many species, which indexes (e.g., plasma content, tissue
content, surface area) are most closely correlated with in-
tensity of response to chemicals and to document the comparability
of species for these relations.
417
-------
There should be large-scale studies comparing the
relation, or ratio, between maternal and fetal toxic dosages,
in various species, stocks, and strains of animals, and for
many classes of chemicals.
The pharmacologic effects on fetuses and, especially,
embryos should be studied.
Because of its potential importance in terms of
infant mortality and childhood morbidity, a nationwide system
of rapid reporting of all malformations and collateral informa-
tion should be set up.
Experimental animal studies should be carried out
that are adjuncts to and relevant to epidemiologic probes of
the possible harmful effects of vapor-phase organic pollutants
in human embryos.
Carcinogenic Effects
There is no extensive background on in vitro studies of
carcinogenic VPOP. In vitro methods should be expanded as a
means to identify the class of careinogenicity into which en-
vironmental pollutants fall. Human tissues should be used
for in vitro studies whenever possible.
EPIDEMIOLOGIC APPRAISAL OF HUMAN EFFECTS
If the aromatic content of gasoline is increased, three
steps should be undertaken: more careful monitoring of hydro-
carbons and reaction products, the use of physiologic and meta-
bolic indexes in studies of persons occupationally exposed in
filling gasoline tanks and otherwise handling or refining aromatic
418
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fuels, and estimation of alterations in aromatic content of
emission of vehicles with various types o'f control system.
Epidemiologic and experimental studies of concentrations
of nitrogen dioxide to which humans are exposed should be
carried out.
The measurement of PAN, peroxybenzoyInitrate, and
transient reaction products and the detection of human reac-
tions deserve priority, along with defining and detecting
reactions associated with exposures to mutagens, carcinogens,
and teratogens.
INTERACTIONS AND EFFECTS OF VPOP ON TOTAL ENVIRONMENT
Research is required to determine the influence of VPOP
on the formation of atmospheric droplets and, hence, fog and
cloud formation and precipitation patterns.
Research is required to determine the extent of the con-
tributions of VPOP from air to natural waters and their con-
centrations and stabilities therein.
More information is required on the effects of microbial
cometabolism on VPOP (and other organic pollutants), which
can result in a reduction of toxicity, such as for DDT, or an
increase, such as for mercury.
Research is required on the possible use of microbial
systems as monitors for VPOP.
Additional research is required on microbial and plant
metabolism of VPOP, the ability of VPOP to affect such metab-
olism, and the movement of VPOP in aquatic and terrestrial
food chains.
419
-------
Additional research is required to clarify and quantify
the mechanisms of removal of VPOP and organic aerosols from
the atmosphere.
420
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APPENDIX A
COLLECTION AND SAMPLING TECHNIQUES FOR VAPOR-PHASE ORGANIC
AIR POLLUTANTS
Vapor-phase organic air pollutants are generally present at very low
concentrations in large volumes of air. The proper evaluation of these
organic pollutants by instrumental analysis depends directly on their collec-
tion and transport to the laboratory -with a minimum of chemical reactions.
Because vapors mix freely -with the ambient atmosphere, they are easier to
sample than dusts or fumes. But because of their reactive nature and sus-
ceptibility to adsorption, they present many difficult problems. This review
will outline methods of direct collection by plastic bags, glass bottles, valved
gas syringes, and stainless-steel cans; indirect enrichment methods of con-
densation and sorption with and without chemical reaction; and the more
specialized and complex techniques of whole-air liquefaction and reversion
gas chroma.tography.
DIRECT METHODS
Plastic Bags
A popular method of sampling and storing atmospheric gases for analysis
at a central home laboratory uses flexible plastic bags. The advantages of
plastic over rigid containers include the following: the bags are light and
inexpensive, no dilution corrections are necessary when material is removed
from the containers, and reasonably large volumes of air (^150-200 liters)
can be collected. However, caution must be used against their indiscriminate
use, because some substances in some kinds of plastic bags--i.e., styrene--
will be rapidly lost. Consequently, the specific conditions of each type of
chemical compound in a variety of gas mixtures in a specific plastic bag must
be reported.
421
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It should be remembered that no container can prevent vapor-phase
reactions, which result in decreased concentrations of starting materials;
that, if the sample is initially saturated -with water vapor, condensation of
the water will readily remove soluble chemical species, such as formal-
dehyde, organic acids, and alcohols; and that no collection system obviates
precautions against leaks.
The most-studied vapor-phase organic air pollutants include numerous
low-molecular-weight paraffins and olefins, formaldehyde, and acrolein.
Semiquantitative data on stability are available for many other substances,
including aromatic hydrocarbons, phenols, crotonaldehyde, and alcohols.
Data have been reported on the stability of olefins, formaldehyde, and
acrolein in samples of automobile exhaust effluents and diluted irradiated
automobile exhaust stored in plastic bags.
The stability of various organic substances at atmospheric and source
concentrations in plastic containers over storage periods as long as several
1402 72
days has been investigated by Wilson and Buchberg, Baker and Doerr,
34 243
Altshuller et aL , and demons and Altshuller. Baker and Doerr
studied the storage of ethylene, 1-pentene, 2-methylpentane, benzene,
acetone, and butyraldehyde at 30-130 ppm for periods of 16-67 hr in Mylar,
aluminized Mylar, Saran, Scotchpak, and aluminized Scotchpak bags. They
concluded that these organic gases could be stored satisfactorily in the
plastic bags as single components andascomplex mixtures if the film were
impermeable to the studied gases, because diffusion characteristics alone
controlled possible hydrocarbon loss. They recommended that double film
bags be used. The inner film should be impermeable to the gases of interest,
and the outer film to moisture. Aluminized Mylar and aluminized Scotchpak
were shown to be the least permeable to moisture.
422
-------
34
Altshuller et_ al. also have reported that Scotchpak bags were satis-
factory over 24-hr periods for storing synthetic and atmospheric samples
of aliphatic hydrocarbons and acrolein, but not formaldehyde. Mylar bags
were satisfactory for synthetic mixtures of formaldehyde; a 5% loss occurred
over a 24-hr period. However, formaldehyde in atmospheric gases sustained
a 5-10% loss in only 2 hr.
243
Clemons and Altshuller further studied systematic changes in
aliphatic C hydrocarbons during a 10-day period. Paraffinic hydro-
2-5
carbons showed no changes in concentrations in the containers during the
storage time; but slow losses of ethylene and acetylene apparently occurred.
A loss of the major portion of the aromatic hydrocarbons occurred even
after only a few days of storage. Some of the aromatic hydrocarbons later
reappeared in the container. These studies indicated that several precau-
tions are necessary. First, long-term storage of aromatic hydrocarbons
in these containers should be avoided. Second, new bags should be con-
ditioned for several hours with the specific gases of interest in at least
the concentrations of the samples. The greater the losses experienced
with new bags, the more essential repetitive conditioning is before actual
use. With increasing time, the rates of adsorption and desorption become
equal, as the surface-active sites are saturated and equilibrium for the
chemicals being studied is reached.
243
Clemons and Altshuller also reported that a substance with the
same retention time as c>-xylene was very slowly desorbed from the
wallsin Mylar bags. This material was interpreted to be an oxygenated
component or decomposition product of the plasticizer in the Mylar film.
423
-------
34
Altshuller et al. found that the stability of aliphatic and aromatic
hydrocarbons stored in plastic bags made of Tedlar over 6-hr periods
was very good. The concentrations varied by no more than 5%, with
no significant trends. Most of the variability could be attributed to
limitations in analytic precision in the sub-parts-per-million range.
H. Mayrsohn (personal communication), W. A. Lonneman (personal
communication), and P. Groblicki (personal communication) have found
that Tedlar bags are superior to bags of other plastics. Within a 24-hr
period, negligible hydrocarbon-Tedlar interaction occurs (Mayrsohn,
personal communication). However, in a 5-day study, a 15% decrease
in the hydrocarbon content of the bags was measured after several days.
Both wall adsorption of the heavier hydrocarbons (such as the aromatics)
and diffusion losses of the lighter hydrocarbons (like ethylene and
acetylene) through the Tedlar are implicated.
Currently, fluorinated plastic film, such as Teflon and Tedlar, for
constructing plastic bags is gradually replacing all the previously described
types. Unfortunately, although it is generally recognized that Teflon and
Tedlar are superior materials for fabricating sampling bags, because of
their very low surface -wall adsorption characteristics, no systematic
studies that define these superior qualities are yet available.
Glass Bottles
Stainless-steel and glass containers are commonly used for storage
of gases, because of their convenience and ready availability. Specifi-
cally, stainless-steel containers are usually used for sampling of high-
pressure sources, whereas glass containers are used at atmospheric
72
pressure. Baker and Doerr have reported that the storage of a mixture
424
-------
of sulfur dioxide, nitrogen dioxide, and 2-pentene in stainless-steel
cylinders at: atmospheric pressure showed no significant loss at 40 hr.
Comparable storage in glass flasks showed a high loss of sulfur and
nitrogen dioxides, but no significant loss of 2-pentene. These vacuum
glass sample containers are unsuitable for storing other reactive gases,
32
such as hydrogen sulfide, at trace concentrations. Altshuller ^t al. ,
studying raw automobile exhaust, reported that no formaldehyde could
be detected, even when collection was made into dry glass flasks. These
severe losses of formaldehyde from the raw exhaust were probably due
to the transport of formaldehyde to the container walls -with the water-
vapor condensate.
However, glass flasks have been successfully used for sampling
atmospheres for the analyses of such gases as carbon dioxide, carbon
monoxide, hydrogen, nitrogen, oxygen, methane, and a variety of light
1217,1218
hydrocarbons. Stephens and Burleson have collected air
samples both in 20-liter glass carboys and in 300-ml glass sample tubes.
Their system for extensive field collection of ambient air samples in
urban locales involved 12 glass vacuum tubes with stopcocks at each end
packaged in a padded wooden box; the workability of this system was
demonstrated by the analysis of light hydrocarbons in their laboratory
in California, after collection and shipment of air samples from Denver,
New York, and Honolulu. Unfortunately, these rigid containers hold
only small samples, thereby restricting the analytic measurements to
gas-chromatographic (GC) analysis for hydrocarbons. Nevertheless,
1218
Stephens and Burleson could obtain three 100-ml syringe samples
from the original 300-ml glass collection tubes with a positive-pressure
diluent technique, although a twofold loss in sensitivity occurred with
each successive withdrawal.
425
-------
Hydrocarbons are collected for analysis in many laboratories in gas-
tight syringes, enabling the sample to be injected directly into a gas
chromatograph. Because of the limitations on the volume that may be
injected into a GC column, this method is satisfactory only when the
concentrations of the components being analyzed provide an adequate
response to the detector. However, the great sensitivity of the flame-
ionization detector has enabled many investigators to obtain almost routine
sensitivities to a few parts per billion with air samples of 1-10 ml. The
syringes used are normally constructed of stainless steel, glass, and
Teflon.
A recent development in gas syringes promises to establish the syringe
even more firmly as the preferred gas-sampling device. The innovation
consists of a miniature gastight valve integral to a syringe that incorporates
a resilient 0-ring in the Teflon plunger seal. The 0-ring ensures a
leaktight seal against high back pressures and the cold-flow property
of Teflon. This leaktight, valved syringe is commercially available
(Precision Sampling Corporation, Baton Rouge, La. ). It allows the gas
sample not only to be sealed in the syringe from the point of sampling
to the point of analysis, but also to be pressurized. Pressurization
is achieved by pressing the plunger in the syringe barrel before injection,
1054
with the barrel sealed by closing the built-in valve. Rasmus sen has
reported that pressurizing a gas sample before injection greatly improves
the performance of the syringe in injecting a gaseous sample into a gas
chromatograph. M. D. LaHue (personal communication) has used this
new type of syringe to send air samples collected in Brazil and Panama
by airmail to his home laboratory in Colorado for analysis of trace
atmospheric hydrocarbons.
426
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Although glass vessels have been found very satisfactory for storing
trace concentrations of hydrocarbon gases -with slight vessel-wall effects,
a surface-wall reactivity of glass for trace concentrations of the oxygenated
products exists. A comparative study of the adsorptive properties of glass
in altering the true proportions of trace concentrations of such gases as
aldehydes, ketones, alcohols, lactones, epoxides, peroxides, hydroperoxides,
peroxyacids, and ozonides should be investigated, if the chemistry of air
pollutants stored or held static in glass vessels is to be regarded as repre-
sentative of the real atmosphere.
Stainless-Steel Canisters
As previously mentioned, stainless-steel containers were found satis-
factory for the storage of the oxides of sulfur and nitrogen at normal
72
atmospheric pressures. In addition, Baker and Doerr made some
interesting tests on compressing prepared gas mixtures into stainless-
steel tanks to 150 psig. This compression sampling allowed the transfer
of large volumes of gas from the field to the laboratory in containers
of reasonable size. Ketones, aldehydes, aromatics, and saturated and
unsaturated hydrocarbons were tested, as well as nitrogen oxides and
sulfur dioxide. Analyses before and after compression (150 psig) and
with various storage periods were obtained. Acetone and butyraldehyde
were slightly affected; 2-methylpentane, benzene, and 2-pentene showed
no significant change; and nitrogen oxides and sulfur dioxide showed
appreciable losses. Therefore, Baker and Doerr concluded that com-
pression sampling was apparently satisfactory for hydrocarbons, but
not for oxides of nitrogen and sulfur. The stainless-steel containers
used by Baker and Doerr had only the normal manufactured surface
without any modification.
427
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Major technologic steps occurred in regard to the practical utility
of reduced metal-surface reactivity with the advent of the Apollo pro-
gram and its stringent requirements for extreme surface cleanliness
and passivity. The steps were the development and perfection of the
methodology and the necessary solvent chemicals for electropolishing
stainless-steel surfaces (U.S. patent 764462). It is now possible to
use a nontoxic, nonfuming electrolytic solution, such as developed in
Molectric's Summa process, that enables oxygen to combine with the
chrome nickel of the stainless-steel alloy to create a pure chrome
nickel oxide skin on the basic crystalline structure of the metal. The
formation of this oxide not only gives a tremendous passive "wetting
cushion" to the surface of the metal, which aids in retarding trace
gases from reacting with the metal, but also results in a superpolish
o
with a microinch finish as fine as 1 1/2 A. Ordinarily a drawn metal
surface has a microinch R.M.S. (root mean square) finish of #32.
With the appropriate mechanical polish, this surface can be improved
to a #10 R.M.S. The Summa process will lower an already high polish
(#10 R.M.S. ) finish to a #4 or less R.M.S. , which has a very-high-
luster polished surface.
Besides the advantages of better corrosion resistance, a surface
free of imbedded residues from a mechanical polish, and a chemically
pure and surgically clean surface, the chemical purity of the exposed
metal surface is completely compatible with such fuels as hydrogen
peroxide, liquid oxygen, fuming nitric acid, fluorine, etc. This latter
point was not appreciated until a few serious accidents in the Apollo
program showed that impurities imbedded in mechanically polished
and deburred surfaces were excellent catalytic agents able to trigger
428
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explosions in missile and space systems. Interestingly, these same
catalysts can react with trace oxides of sulfur, nitrogen, and other
reactive air pollutants.
With these considerations in mind and with the ready availability of
techniques and solvents for passivating stainless steel, the almost
negligible absorptivity of electropolished stainless-steel surfaces
should be investigated for special cases that require an extremely rugged
and functional collection-storage system.
INDIRECT METHODS
This section reviews methods of collection of complex mixtures of
vapor-phase organic air pollutants by sorption techniques with and with-
out chemical reaction. These substances can be studied best after their
isolation or partial concentration from the air. Prefiltering to remove
particulate matter and adequate metering of the total volume of sampled
air are necessary steps in the analyses. Metering of the air is necessary
to obtain a true value of the total air volume sampled as the filter for re-
moving the particles gradually becomes clogged, which results in a con-
tinuously changing resistance to flow.
Adsorption
At ambient or subambient temperatures, gases adhere variably to solid
surfaces; this phenomenon is called adsorption. In porous solids, both
external and internal surfaces (some possessing a vast network of intensely
minute channels and submicroscopic pores) are exposed. The degree of
adsorption depends on the relationship between pore structure and the
size and shape of the contaminant molecules, as well as on the strengths
of molecular attractive forces. Some of these solids--activated carbon,
429
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silica gel, activated alumina, and the porous organic polymer sorbents
(Chromosorb and Porapaks) used in gas chromatography--are practical
adsorbents.
675
Recently, Kaiser reported a new type of porous carbon black
(Carbosieve) produced by thermal degradation of polymeric polyvinylidene
chloride. This carbon black has both adsorbent and molecular sieve
properties and possesses an extremely nonpolar surface; on this ad-
sorbent, -water is eluted before methane. Graphitized thermal carbon
678
black has also been reported by Kalaschinikova e_t al_. to have
excellent adsorption properties for C alkanes, alkenes, alkadienes,
1-6 382
alkynes, cyclanes, cyclenes, and benzene, and by Eisen ei^ aL , for
C ri-alkenes.
6-10
Polar substances have a strong mutual attraction; water is highly
polar, and polar adsorbents retain water from a humid atmosphere in
preference to electrically nonpolar organic gases. In the sampling of
vapor-phase organic pollutants from ambient air, nonpolar activated carbon
preferentially adsorbs organic material. In fact, previously adsorbed
moisture is displaced from the carbon surface as other organic gases
are adsorbed, making it unnecessary to dehumidify the air before its
passage through the carbon bed. Any prior dehumidification is also
undesirable, inasmuch as alteration or loss of organics may occur in
the dehumidification process.
Chemisorption of low-boiling -point organics like ethylene has been
accomplished by impregnating the activated carbon with bromine, and
stabilized sodium sulfite on activated carbon has been used for formal-
1277
dehyde.
430
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In extensive identification studies of the trace contaminants in en-
closed atmospheres--such as submarines, space simulators, and
manned and unmanned space cabins--activated charcoal has been found
1106
to afford one of the most practical sampling techniques available.
However, correct interpretation of the data from charcoal sampling
depends on a knowledge of the adsorption and desorption efficiency of
each individual component on charcoal. Hence, although inferences
can be drawn, no general quantitative values can be assigned.
1106
Saalfeld e_t aJ. reported several shortcomings of the charcoal
sampling method: Many contaminants in an enclosed atmosphere either
cannot be recovered from charcoal or may be chemically altered by
the adsorption-desorption process. The charcoal itself may pick up
or acquire a background of contaminants during its handling and storage.
The contaminants collected on the charcoal filter usually represent a
long exposure. Thus, contaminants with short retention times are
eluted from the charcoal bed. With increasing exposure time, the
filter is depleted of these contaminants and enriched with contaminants
having a long retention time. The three methods of recovering the
adsorbed contaminants--vacuum-thermal desorption, solvent extraction,
and steam desorption--are strenuous desorption techniques.
Another interesting charcoal-filter collection-enrichment technique
524
has been reported by Grob and Grob; it uses a very-small-capacity
filter. The trap contains 25 mg of wood charcoal with an average
particle size of 0. 08 mm; the air flow is set at 2. 5 ml/min, so that
3
25 m of air are filtered within 8 days. To determine the adsorption
of the substances of interest, a second filter was inserted behind the
first. Carbon disulfide was used to extract the materials from the filters.
431
-------
Both qualitative and quantitative investigations were made on C
524 6-20
volatile organics in city air -with this system. Unfortunately, like
979
the saturation method of frontal analysis, this technique gives ex-
cellent results for only a single major substance, but is inadequate for
studying long-term loading of complex mixtures with a wide range in
volatility, because several phenomena occur. First, the firm bonding
to the active sites by initially well-adsorbed components prevents more
volatile and less well-adsorbed substances from reaching their saturation
equilibrium. Second, some substances, even if present in relatively
high concentrations, may be greatly reduced on the charcoal bed in the
course of prolonged loading. It is interesting to note the similarities
(including fundamental difficulties) between Grob and Grob's more re-
fined carbon disulfide solvent extraction scheme using 25-mg charcoal
524
filters and Turk and d'Angio1 s carbon tetrachloride solvent extracti<
1278
scheme using 325-g charcoal filters.
In general, siliceous and metallic oxides like silica gel and activated
alumina are polar adsorbents that retain atmospheric moisture. However,
as long as the adsorbent does not become saturated with moisture before
sampling is complete, both silica gel and activated alumina can be used
for short-duration sampling from relatively dry atmospheres. Although
silica gel has a lower retentivity for organic gases than does activated
carbon, it possesses the advantages of greater selectivity and easier
desorption.
Absorption
Equipment for absorption has usually consisted of simple bubblers,
bubblers with diffusers, spiral absorbers, and packed towers. Absorption
432
-------
equipment of this type, with gas-liquid contact efficiencies of over 90%,
usually has the disadvantage of relatively low flow capacities. Owing
to the sampling time essential to collect measurable quantities of organic
gases, repeated replacement of evaporated solvent may be necessary.
Also, when trace organic gases have been collected in the form of a very
dilute solution in an organic solvent, the problem of their recovery is
formidable. In general, the problem of sampling trace gases (such as
aliphatic aldehydes, hydrogen sulfide, and mineral oxides) in the
atmosphere requires that the gases be extracted from the atmosphere by
washing the air sample with aqueous reagents. For example, formal-
dehyde is the only organic vapor-phase contaminant currently measured
in the atmosphere that relies on the principle of absorption for later
1170
colorimetric analyses.
Condensation
The collection of vapor-phase organic air pollutants by condensation
at low temperatures has at least two distinct advantages over the other
collection methods: the collected organics are immediately available
for analysis, without requiring either removal of solvents or desorption
from an adsorbent; and condensation is the most reliable method for
preserving the organics without the further occurrence of chemical reac-
tions. The main disadvantage of collection by condensation is that large
quantities of water condense simultaneously with the collected organic
gases.
Simple Cold-Surface Traps. A special problem of condensation of
organic gases in open-bore cold traps is the formation of condensation
mists. Such mists are composed of solid or liquid particles of a few
433
-------
micrometers or less that often pass through the cold traps in sufficient
proportions to reduce significantly the collection efficiency of the equip-
1171
ment. Shepherd et ah have described a simple glass-wool filter
(additional to the prefilter used to remove ambient atmospheric particles
seated between the liner walls of a cold trap to minimize such losses.
For condensing some gases, the refrigerant used must be cold enough
for a sufficient decrease in the vapor pressure of the trapped material
to occur to prevent significant evaporation during the sampling. The
choice of coolent is dictated by the requirement of keeping the vapor
pressure of the condensed gases at less than 1 mm Hg in the cold trap.
Normally, a cryogen colder than -160 C is needed to condense C
2-4
compounds efficiently, whereas a dry ice-acetone bath (-78 C) is usually
appropriate for compounds of C and above. However, the convenient
5
availability of liquid nitrogen and dry ice has resulted in their preferred
usage. Generally, liquid oxygen is a better cryogen than liquid nitrogen
or liquid air, because at low sampling rates there is no condensation of
atmospheric oxygen in the trap. However, there are some safety pre-
cautions necessary in the handling of li.qui.d oxygen that have generally
placed it in disfavor. Liquid argon has restricted usage, largely because
of the difficulty in obtaining it.
One feature of liquid nitrogen and dry ice that has not been very widely
taken advantage of is the spongelike nature of dry ice for holding liquid
nitrogen. Apportioning the appropriate amounts of liquid nitrogen with
powdered or finely crushed dry ice enables a wide range of cryogenic
temperatures to be obtained--between -193 and -80 C. This containment
of the liquid nitrogen -within the granular structure of the dry ice is
especially useful under field conditions, inasmuch as there is no sloshing
of liquid in the Dewar flasks.
434
-------
The method most commonly used to deploy open-bore glass freeze -
out traps involves using them as a series of traps held at progressively
lower temperatures. The stepwise cooling action results in some degree
of fractionation and water condensation before the last trap, which is the
most efficient stage of condensation.
During the early investigatory period in the Los Angeles air pollution
191 529 1170,1171
episodes, CadleetaL , Haagen-Smit, Shepherd and colleagues,
and others reported that many volatile contaminants could be efficiently
collected from the atmosphere with simple cold condensation surfaces.
1171
Shepherd et al. were the first to concentrate air pollutants in a
cold trap and analyze the concentrates by mass spectrometry. The
isolated, frozen concentrate was separated by isothermal distillation or
sublimation at low temperature and high vacuum in the mass spectrometer.
About 60 compounds were identified from the isolation of the vapor-phase
pollutants on a filter at liquid oxygen temperatures.
Simple open-bore cold-surface traps are now seldom used in air
pollution studies, unless there is a need to freeze out the water from
1060,1062
the air to examine the organic materials dissolved in the water.
Packed Cold-Surface Traps. Vapor-phase organic air pollutants from
hydrocarbons are usually identified and quantified by gas chromatography.
An enrichment step before analysis and measurement is required to detect
concentrations of the different types of hydrocarbons at less than 0. 01 ppm
(i. e. , 0. 01 y I/liter), even with the different types of highly sensitive
ionization detectors available. Various organic gases have been concen-
trated in traps containing small volumes of purified charcoal, silica gel,
glass beads, stainless-steel washers, porous polymers, and gas-liquid
435
-------
chromatography partition substrates coated or bonded on refined diato-
maceous earths. The lower limit of measurements of the C hydro-
3-5
carbons can be extended to 0. 1 ppb by concentrating 100 ml of the air
sample in a freezeout trap packed -with the appropriate adsorbent or gas-
liquid partition substrate. Liquid oxygen is used in the freeze trap,
rather than liquid nitrogen, to avoid condensation of oxygen in freezing
the air sample. Dry ice-acetone is commonly used to collect aliphatic
hydrocarbons on silica gel; however, ethylene is not retained. At re-
duced temperatures, neither silica gel or charcoal offers any readily
apparent advantage over gas-liquid partition substrates for concentrating
compounds of C and above. Consequently, most current investigations
3
on the collection of atmospheric hydrocarbons have used short packed
columns similar to those used for the separation of hydrocarbons.
If the concentrations are high enough, the air sample is analyzed directly
by pulling the atmospheric gases continuously through the gas-sampling
valve connected to the inlet system of the chromatograph. If an inter-
mediate step, such as freezeout trapping or collection in a container, is
involved, the stability of the substances retained must be determined.
Losses not only of reactive substances, such as peroxyacylnitrates, but
also of oxygenated hydrocarbons occur in the containers and in the inlet
and column system of the gas chromatograph.
The trapping procedures for concentrating aliphatic, aromatic, and
terpenic hydrocarbons for transfer for analysis on packed, SCOT, and
capillary columns generally use one or a combination of the following:
an external freezeout loop, on-column freezeout, a precolumn inlet
436
-------
freezeout system, and specially designed low-volume injector-needle
freezeout assemblies. The essential details of these trapping techniques
93, 96, 276, 508, 809, 1058, 1217, 1218
have been reported elsewhere.
Equilibration with Partitioning Substrates
825
Mackay e^t aL were among the first to compare the results of
a direct sampling technique for analyzing trace amounts of organics
with the results of using a gas-chromatographic column itself as a
concentration or enrichment device. The most valuable feature of the
direct sampling technique is that all the artifact-introducing processes
that may grossly alter the balance of constituents are bypassed by sampling
the volatiles in their natural representative proportions and subjecting
them to stresses no more severe than the forces encountered in the gas-
chromatographic column. Mackay et aL concluded that an enrichment
column held at 75-100 C cooler than the separation column did not indicate
changes in the apparent composition of the organic volatiles being analyzed,
but afforded increased sensitivity. The low-temperature bath used for
cooling the enrichment column was not used intentionally as a cold trap,
but rather as a means of slowing the passage of the volatiles through the
precolumn so that a longer time for equilibration was available for ab-
sorption of the volatile organics by the partitioning substrate.
348
Recently, Dravnieks and Krotoszynski advanced this type of en-
richment when they reported a high-speed collection system for organic
vapors from the atmosphere. The essential feature of their sampler
was a fluidized bed of Teflon powder coated with Apiezon L, which
collected atmospheric organics through equilibration with the partitioning
437
-------
phase. The fluidized bed was used to produce the best possible contact
•with air. Only Apiezon L, of the many conventional gas-chromatographic
phases tried, permitted recovery of the dissolved organics •without arti-
facts from the decomposition of the trapping phase itself. The bed
temperature was limited to 80 C by incipient thermal decomposition of
the Apiezon L phase. During collection, the bed -was kept at ambient
temperatures; the rate-limiting factor for equilibration -was the diffusion
of the organic material into the Apiezon L partition phase on the Teflon.
Air flow rates of 0. 5-0. 75 liter/sec kept the powder constantly floating,
providing uniform contact with the air. Normally, at such flow rates,
the bed reached equilibrium within 45 min. Organic components dissolved
in the partition phase -were recovered by displacement -with helium at 80 C
and transferred to a thin-wall injector needle held at -196 C. Each collec-
tion yields only one sample for analysis. Quantitation depends on knowledge
of the partition coefficient for each component trapped. Unfortunately,
349
this technique, in its present developmental state, has had a very
limited evaluation for use in air pollution studies.
The primary problem involved in the condensation of air samples
of 1-100 liters or greater is the large amount of water vapor relative
to the organic fraction present in the air. For example, 1 liter of
air at 25 C and 70% relative humidity contains 16. 1 mg of water, where-
as the organic burden in the same volume of air varies between 2 and
200 ng. Because the overloading of open tubular columns occurs with
more than 50 mg of material, the introduction of a few milligrams of
water or more is a very serious problem. In addition, the pumping
units on the ionization sources of most conventional mass spectrometers
-4 -5
are unable to maintain a vacuum of 10 -10 torr if milligram quantities
438
-------
of water are introduced into the mass spectrometer. Conventional
small-bore cold traps that have become almost standard analytic
instruments in trace analysis are also affected by milligram quantities
of water. Even the performance of the most commonly used column
type, a 1/8-in. -O. D. packed column 6 or 12 ft long and using a water-
insoluble silicone phase, is affected by more than 50 mg of water.
These difficulties presented by the large amount of water present
in ambient air relative to the organic species have led to various attempts
to separating trace organics from water vapor. The most common method
is the use of precolumns packed -with desiccants that, interestingly, have
given different investigators varied success and reliability. Unfortunately,
the possibility always exists that the desiccant can irreversibly adsorb
and absorb some of the organics from the air under study, especially
inasmuch as the sorption properties fluctuate as more water is taken
up by the desiccant.
In attempts to circumvent the problems associated with desiccants,
349,812,918, 1145
several investigators have used polymeric adsorbents
of the styrene-divinylbenzene type for separating and concentrating
organic: components from water vapor. The technique involves the
trapping of the organic volatiles in a short precolumn at ambient tempera-
ture while the water vapor is discarded with a sweep gas. The adsorbed
sample constituents are eluted from the precolumn at high temperature
(150-180 C) with or -without reversed flow. There are several advantages
of the technique: the amounts of collected components are proportional
to their concentrations in the atmosphere, sample storage is convenient
and safe, most of the water vapor is rejected, and the organic constituents
adsorbed are readily released on heating. The major disadvantages
439
-------
are interferences from the bleeding of extraneous compounds from the
packing, the chemical instability of the packing, possible chemical
alteration of the sample constituents during the heat-release treatment,
and the limited retention volume before equilibration is reached and the
collected materials begin to pass through the collector.
In an earlier report on the concentration of the trace contaminants
1383
in air on porous polymer beads, Williams and Umstead concentrated
chlorinated and brominated hydrocarbons at ambient temperature on the
same column that was later used for the gas-chromatographic analysis.
They reported that the interference from water was minimized as it was
eluted as an early peak, thus eliminating the need to dry the air samples
before injection onto the column. The original report on the feasibility
of concentrating the organics in air samples on porous polymer beads
605
was probably that of Hollis, -who stated that porous polymer beads
gave low -molecular -weight hydrocarbons retention times that were very
long, compared with those obtained on conventional gas-liquid chromato-
graphic substrates, but allowed the rapid passage of water through the
column.
55
Recently, Aue and Teli reported the sampling of air pollutants
with support-bonded chromatographic phases at ambient temperatures.
The use of very high loading (26%) of the liquid silicone, (C H SiO )n,
18 37 3/2
phase support-bonded to a coarse Chromosorb support permitted maximal
retention and rapid gas flow of air pollutants. The trapped compounds
were solvent-extracted, and the concentrated extracts analyzed by gas
chromatography. Unfortunately, the extraction of minor trace compounds
from the chromatographic phases and the restriction of collecting organics
440
-------
above C limited the amount of information obtained on the ambient
7
air samples reported. Also, no quantitative investigation of the collec-
tion efficiencies or recovery efficiencies with extraction was reported.
Identification of the collected compounds from atmospheric sampling was
beyond the defined scope of the study.
Obviously, the copious water present in air samples must be circum-
vented if accurate, routine collections and analysis of atmospheric vapor-
phase organic pollutants are ever to become a reality. The problem of
interferences from large amounts of water is even more acute in the
analysis of source emissions, such as stack gases and auto exhaust.
In an attempt to solve this problem, C. W. Skarstrom and J. Kertzman
(personal communication) developed a new, self-regenerative air drier
that selectively removes water vapor from saturated air or automobile
exhaust without affecting the hydrocarbon concentration. The drier has
demonstrated efficiencies of removing -water in a saturated ambient air
stream to less than 10 ppm by volume. The b.as ic element is a special
hygroscopic permeable membrane that removes water from the feed
gas. The membrane does not have any of the aging or attrition
characteristics of conventional solid desiccants. The efficiency of
water removal depends on the length of the tube bundle, normally
40-72 in. The present available rates of 0. 5-3. 0 liters/min depend
on the diameters (0. 5-0. 75 in. ) of the tube bundle used. The drier
consists of a single unit that samples continuously; the unit is self-
regenerative.
The very newness of this selective water removal device limits the
quantity of performance data available. However, by providing water
441
-------
reductions of better than 500:1, this concentration process promises to
provide air pollution investigators with a means of obtaining better
accuracy, higher precision, and greater repeatibility of pollution
measurements.
MISCELLANEOUS METHODS
In the category of miscellaneous collection methods are several hybrid
enrichment-analysis systems that deserve to be included here, although
some of their essential points belong in Appendix B.
Reversion Gas Chroma tog raphy
677
Kaiser recently reported automated continuous trace organic
analysis in concentrations of parts per billion to parts per million -with
a reversion gas chromatograph. The novel feature of the technique is
that the air sample under investigation is continuously frozen out on the
head of the column. Removable Peltier cooled copper blocks provide
a chilled enrichment zone. Separation occurs discontinuously by means
of an oven that moves along the length of the column. The oven provides
the necessary temperature field -within -which the appropriate temperature
gradient can be adjusted to the needs of the analysis. Both the upper
limit of the temperature field and the column material determine the
substances that can be eluted per analysis cycle, their rate of elution,
and the coherence of the concentration of the separated compounds.
The procedure can be programed to permit an automatic analysis in
intervals of 10, 15, or 30 min. If the flow rate, freezeout time, number
of temperature fields, and other operational characteristics are changed,
versatile continuous analyses on trace concentrations of light hydrocarbons
442
-------
are possible. Kaiser believes that, because the entire sample volume
passes through the detector in a continuous flow, there are no problems
of sampling, storage, transportation, aging, contamination, quantitative
calibration, etc. However, the reversion procedure is limited by the
following factors: separation efficiency is low; at present, only homologues
of light hydrocarbons can be separated; and direct calibration is difficult
and a known detector constant is necessary for each unknown substance.
The main advantage of the system is that practical ultratrace analyses
with restricted resolution are possible through continuous sample on-
column enrichment, because both nonsystematic and systematic analytic
error is minimized in the sampling procedure.
Automated Airborne Collections
223
Cavanagh reported on the development of instrumentation for
automated airborne collections of atmospheric organic chemicals. The
basic design feature of the sampling system was a concentration procedure
that used gas-chromatographic column packing material to retain organics
selectively while permitting the permanent gases of the atmosphere to
pass through the trap. The broad range--methane to C terpenes--of
10
organics for which the system was developed required that the collection
system be divided into two channels to provide efficient enough collection
of the C organics on one channel and the C organics on the
1-4 5-10
second channel for quantitative analysis.
Two successive stages, each with a thousandfold concentration step,
6
•were used to achieve a 10 -fold total concentration. Liquid argon
(-186 C) was used as the cryogen. Trap design evaluations included
selection and evaluation of packing material for compatibility of the
443
-------
organics of interest with the packing substrate and the efficiency of
collection. Water removal from the atmospheric samples was necessary
before concentration of the C organics. Desiccants were thoroughly
5-10
evaluated on the basis of transmission of terpenes without loss or de-
composition. In addition, methods were developed and tested to remove
particulate material from the incoming air sample before the cryogenic
traps.
Field tests with the prototype sample collector made by Cavanagh
indicated that the principles of the cryogenic technique were valid and
that good collection and measurement of trace C organics could
5-10
be achieved. Unfortunately, in the automatic system later built from
Cavanagh's prototype, the engineering of the automatic cryotrap sequence
in the integrated system has not performed satisfactorily, owing to cold
spots in the transfer lines between the first- and second-stage traps
(B. Tyson, personal communication). The system has also been found
to be too elaborate for routine field use, in that it is very sensitive to
flow fluctuations and requires an intricate array of seven mass flowmeters
with solenoids, two vacuum pumps, and several prefilter -water sorption
and desorption traps.
Cryocondenser-- Whole -Air Liquefaction
The development of a portable air sampling instrument that can collect
and enrich large representative samples of both urban and nonurban
1051
atmospheres has been reported by Rasmussen. The principle
of operation is the cryogenic technique of liquefying air. This simple
and straightforward approach results in several benefits to air pollution
scientists: it allows an economical and practical way of transporting a
444
-------
large volume of the atmosphere from the field sampling site to the
laboratory, where careful, repetitive analysis can be made on the
composition of its trace contaminants; it permits preservation of the
whole composition until the appropriate analyses can be performed;
it offers a simple means of collecting usable samples in remote areas
or under difficult circumstances (industrial, urban, rural, etc.), in
that the necessary equipment includes simply a Dewar flask with liquid
nitrogen and an appropriately calibrated cryocondenser; external
electric lines to service pumps or mass flowmeters with recorders
are obviated; the entire system, including the Dewar flask, is con-
structed of electropolished stainless steel, has no moving parts, and
is extremely rugged; and, at the temperature of the present refrigerant,
liquid nitrogen (-196 C), nearly all the air pollution constituents (except
carbon monoxide) have negligible vapor pressures.
The cryogenic sampler developed, a cryocondenser, consists of a
multiple-column heat exchanger fitted to a reservoir for receiving
liquefied air. The unit requires no external power to operate. Air
is taken in by creating a vacuum in the trap through the condensation
of the incoming air on the walls of the heat exchanger and collection
reservoir. All the air that enters the cryocondenser is condensed;
therefore, the unit can be considered to be 100% efficient. As the air
condenses on the large cryogenic surface provided by the heat exchanger,
the condensed (liquid) air flows to the reservoir can. In this way, an
active cryogenic surface is maintained in the heat exchanger.
The intake rate is controlled with a critical orifice to give a. uniform
806
sampling rate over a sampling period of 10-60 min. The repeatibility
445
-------
of the volume of air collected has been measured under laboratory conditions
at +_!. 2-2. 3%, depending on the pumping capacity of the unit and the orifice
size. Under field conditions, it is more convenient and accurate to use
a battery-operated mass flowmeter with a ratemeter and a digital totalizing
counter. This measuring device eliminates the tedious calculations necessary
for standardizing the volume of air collected under field conditions at different
elevations and changing altitudes experienced during samplings from an aircraft.
At the end of the sampling period, the inlet valve is closed and the cryocondenser
is left in the Dewar flask with liquid nitrogen for transport to the laboratory,
where the liquid air can be distilled precisely with maximal retention of
the collected organics. Samples for gas-chromatographic analysis of the
organic gases, carbon dioxide, water vapor, and other gases in the cryocondenser
can be taken either directly with a valved syringe (Pressure-lok) or indirectly
under conditions of isothermal and isobaric distillation or helium displacement,
so that specific fractions can be trapped for specialized analysis. Syringe
samples enable repetitive and range-finding analyses to be made on the contents
of the cryocondenser, -whereas fractionation and secondary enrichment of the
entire contents of the cryocondenser can provide several hundred nanograms
of material, to permit combined gas-chromatographic and mass-spectrometric
analyses.
The major disadvantages of the system are that generally 24 hr are required
between collection and analysis and that water-soluble organics dissolve in
the water condensed in the unit when it is brought to room temperature.
However, because everything that went into the cryocondenser can be recovered
at cryogenic temperatures either by helium displacement or under vacuum,
the types of analysis are limited only by the perseverance of the investigator
and the amount of material available for analysis.
446
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APPENDIX B
ANALYTIC TECHNIQUES FOR VAPOR-PHASE ORGANIC AIR POLLUTANTS
Understanding and effective regulation of the potential long-term
biologic effects of vapor-phase organic pollutants at trace concentrations
in the atmosphere will be possible only if appropriate analytic methods
are developed. The chemical and physical instrumental methods for
measuring this very broad range of organic chemical species have
generally been restricted to colorimetry for the more reactive aldehydes
and peroxidic species, gas chromatography (GC) with a flame ionization
detector for hydrocarbons, flame photometry for organic sulfur compounds,
electron capture for halogenated and nitrated species, and limited approaches
for measuring the volatile ketones, alcohols, and acids.
The analysis of VPOP in ambient air with such sophisticated instrumenta-
tion techniques as high-resolution GC, mass spectrometry (MS), and com-
bined GC and MS has been successful only for hydrocarbons and halogenated
hydrocarbons. There has not been much success in the analysis of the
oxygenated and nitrated chemical species. This results partly from the
lack of adequate concentration procedures for these more reactive trace
VPOP.
The essential first step in an accurate analysis is to obtain a repre-
sentative and adequate (both in volume and in concentration) sample.
Inasmuch as both GC and MS require discrete samples for analysis,
various preparation steps are necessary to concentrate the atmospheric
contaminants into sample volumes compatible with the particular system's
sample inlet requirements. It is important, when assessing the validity
of reported data on the concentrations of atmospheric contaminants measured
447
-------
in ambient air, that the adequacy of sampling and preparation be con-
sidered, as well as the combined errors of the measurement technique
and the calibration procedure. The purpose of this appendix is to describe
the best approach to the analysis of atmospheric vapor-phase organic
pollutant in trace amounts. A comprehensive review of all current
analytic methods will not be undertaken here.
GAS CHROMATOGRAPHY
In measuring the trace contaminants that constitute the complex mixture
of organic components in polluted air masses, air pollution scientists need
methods that offer high specificity, high accuracy, and speed.
647a
Since the original work of James and Martin, the application of gas-
liquid chromatography to the separation and identification of complex mixtures
of organic materials has increased enormously. A wide variety of column
types and packings are now used, extending from conventional packed
columns to open tubular capillary columns, support-coated capillary columns,
and, more recently, packed capillary columns. However, gas chromatographic
analyses of the hydrocarbon contaminants in polluted atmospheres have re-
mained almost the exclusive realm of packed columns. This is unfortunate,
because the classical packed columns (1/8-1/4 in. O.D.) do not provide
sufficient resolution to separate adequately the broad range of components
detected in urban atmospheres or to make the highly accurate retention-
time measurements needed for identification purposes. Furthermore, the
identification of the numerous compounds measured in the urban atmosphere
by retention time is especially limited in the analyses of the higher -molecular-
weight VPOP. A comparison of column characteristics relevant to VPOP
analyses is given in Table B-l.
448
-------
It is significant that, after more than 10 years of GC investigation
of the VPOP measured in the urban atmosphere, there are no accepted
standard referee methods for the analysis of the primary emissions,
including light hydrocarbons and the intermediate-weight hydrocarbons
(C aliphatics, aromatics, oxygenated and halogenated hydrocarbons,
5-10
etc. ), and the multitude of intermediate and terminal reaction products
from the primary VPOP has not been accurately measured, primarily
because of insufficient sensitivity and the problem of surface adsorption
of trace contaminants currently inherent in the techniques of GC, MS, and
GC and MS combined that are available for the needed sophisticated analyses.
Low-Molecular-Weight Hydrocarbons
A standard referee method for light hydrocarbon analyses is one of the
important analytic tools lacking in air pollution study. At present, none
of the many "operational" GC-column systems have demonstrated sufficient
superiority to be accepted for preferred use by the many investigators per-
forming daily GC analyses of light hydrocarbons in urban atmospheres. It
would be especially desirable to use a method with the advantages of high-
resolution GC. Up to now, the analysis of light hydrocarbons in the
atmosphere has been mostly by packed columns of 1/8 in. O. D. with
a wide variety of support materials and stationary phases. Most of these
column-pa eking material combinations are tailored by the individual to
be so specialized that duplication by other investigators of the GC-column
system is very difficult, if not impossible. Therefore, there has arisen a
•multitude of individual procedures for the analysis of light hydrocarbons.
Of course, the individuality of these measurements has been necessitated
by the specific needs and the equipment available to the analysts.
449
-------
There are too many published papers in this field to report them
individually. However, several of the more popular columns in use
are Porapak N for C and C hydrocarbons, dimethylsulfolane for
1 2
C materials, and the Durapaks n-octane and 3,B'-oxydipropionitrile
3-6
(OPN) on Porasil C for C compounds. W. A. Lonneman (personal
2-6
communication) has been successful in routinely analyzing ethylene,
acetylene, and the C paraffins with a silica gel column, although
2-5
the breakthrough of water onto the column from the preparative sample
freezeout loop has to be carefully guarded against.
Optimal GC separation and measurement of the VPOP in the atmosphere
has been hindered not only by the low resolution inherent in packed columns
but also by the use of isothermal analyses that do not fully utilize all the
available column efficiency for maximal separation. The introduction by
most GC instrument manufacturers of temperature programing capability,
especially subambient oven control, offers a means of significantly in-
creasing the resolution in packed columns. However, packed-column
isothermal analyses of C hydrocarbons have been successfully applied
1-5
to routine air pollution studies. These studies have focused their efforts
on optimal resolution of compounds over a narrow range, such as ethylene
from ethane and acetylene, allene from methylacetylene, and butenes from
butanes. Unfortunately, complicated mixtures of liquid phases, the use
of multiple columns, critical optimal temperature, precise flow character-
istics, and, in many cases, the operation of two or more gas chromatographs
are necessary to obtain these separations. The elaborateness of many of
these GC-column systems precludes their use in mobile field laboratories.
450
-------
629
The present Intersociety Committee method for determining light
hydrocarbons in the atmosphere uses B, 3'-oxydipropionitrile on activated
alumina packed in 10-ft, 1/8-in. -O.D. stainless-steel tubing. Unfortunately,
the necessary resolution of ethane and ethylene is reduced with the use of
concentrated ambient air samples of 100-300 ml, so that these two com-
pounds are not separated sufficiently for accurate measurement. The
resulting poor separation of ethylene from ethane in large air samples is
responsible for the sparse use of this column.
An evaluation of the methods using low-resolution packed columns
indicates that it would be desirable to incorporate the advantages of high-
resolution GC in the analyses of light hydrocarbons. The major advantage
to be gained would be rapid, high-resolution trace analysis of the C
2-5
light-hydrocarbon air pollutant burden under conditions of one analysis.
Specifically, a GC-column method is needed that would resolve, on one
column in one analysis, the following light hydrocarbons: ethane, ethylene,
acetylene, propane, propene, ^-butane, n-butane, 1-butene, _i-pentane,
ii-pentane, and 1-pentene.
The recent development of bonded organic stationary liquid phases
535
to porous silica beads reported by Halasz and Sebastian and commercially
available as Durapak and Spherosils promises to provide thermally stable
column materials with reduced column bleed for lower detection limits.
These materials also offer a way to construct more reproducible columns.
This is especially important, in that the fabrication of column packing is
a highly individualistic art and therefore difficult to duplicate from one
laboratory to another. Present results indicate that, using subambient
temperatures, 1/8-in. packed columns and 1/16-in. packed capillaries
with OPN and ri-octane on Porasil C have sufficient selectivity to separate
451
-------
all the C saturated and unsaturated chemical species relevant to
2-5
air pollution measurements. Certainly, the need for a standard referee
method of analysis of light hydrocarbons is great enough to warrant further
1053
development, with the objective of selecting an optimal column type.
Intermediate-Molecular-Weight Hydrocarbons
The aromatic and aliphatic (C and higher) VPOP compounds have
6
been resolved by a more diverse selection of column types for their
routine analysis, although generally open tubular columns have tended
811
to dominate the analysis. Lonneman et_ al_. have reported separating
aromatic hydrocarbons on a 300-ft, 0. 06-in. -I. D. copper wall-coated
open tubular column (WCOT) with meta-bis-(m-phenoxyphenoxy)benzene
and Apiezon-L at 70 C. This large-bore column is tolerant of larger
sample loads than the more commonly used 0. 01- and 0. 02-in. -I. D.
capillary WCOT columns. Adequate resolution and sensitivity for the
alkylbenzenes associated -with air pollution -were obtained with this column
on a routine analysis time schedule.
843
P. Groblicki (personal communication), Mayrsohn _e_t aL , Z. Tomaras
96
(personal communication), and Belsky and Kaplan have used temperature
programing in conjunction with small-diameter (0.01-in. -I. D.) WCOT to
achieve higher resolution of the atmospheric burden of C organics
6-12
also on a routine basis. However, the restrictive sample capacity of the
0. 01-in. -I.D. capillary columns limits the air sample volume for analysis
to 50 ml, which greatly curtails the lower sensitivity limits obtainable for
1059
trace analysis. Rasmussen and Holdren reported using a 200-ft,
0. 02-in. -I.D. support-coated open tubular (SCOT) column with an OV-1Q1
substrate operated in a subambient temperature program mode for routine
452
-------
analysis of C aliphatic and aromatic organics at trace (0. 1-ppb)
6-12
background concentrations. The SCOT column was chosen because of
its inherent larger sample capacity than 0. 01-in. -I.D. capillary columns,
its proven durability, and its high resolution. This column has the ad-
vantages characteristic of open tubular and packed columns, without the
high pressure drop of packed columns or the low sample capacity of
61
capillary WCOT columns.
1059
One of the remarkable features of the analyses was the maintenance
of the high resolution inherent in SCOT columns during the analysis of
large (1-10 liters) air sample volumes. This was accomplished by trans-
ferring a "slug" sample from an external freezeout loop onto the head of
the column and then zone-freezing it there, before initiating the subambient
temperature program to obtain the increased column efficiency and therefore
the higher resolution inherent in this low-temperature mode of operation.
The use of the external freezeout loop apparently accommodates the excessive
amounts of water in the large volume of air necessary for detection of the
trace constituents. Also, the SCOT columns are more durable than standard
capillary columns in the routine usage necessary for hourly air pollution
analyses.
The use of packed columns for analysis of C aromatics and a
6-9
variety of other atmospheric components has been optimized by E. R.
Stephens and F. R. Burleson (personal communication), who reported
good resolution of benzene and toluene from the xylenes, as well as acetone,
using three 1/8-in. -O. D. columns in series: 10-ft, 5% 1, 2, 3-tris-(2-cyanoethoxy)-
propane; 5-ft, 5% Bentone 34 with 5% dinonylphthalate; and 8-ft, 10% Carbowax
E-600 at 65 C. However, packed columns do not give the analyst the same
453
-------
important advantage that high-resolution columns provide for routine
analysis, namely, an interim solution to the problem of better identifi-
cation of the 50-100 (or more) readily measured hydrocarbons in urban
atmospheres.
High-Molecular-Weight Hydrocarbons
In view of the many studies of the hydrocarbons in urban atmospheres,
it is surprising how little has been published regarding the broad spectrum
676
of organic substances expected in these atmospheres. Kaiser has
postulated that, at the parts-per-billion concentration, the number of po-
tential organic chemicals that may be present in an urban air is greater
5
than 10 . Although higher-molecular-weight organic compounds in
particles have been studied extensively, the wide range of C volatile
10-20
substances present in polluted atmospheres has been practically disregarded.
This is especially true for the ambient air analyses of the tricyclic (C )
14
and tetracyclic (C ) polynuclear aromatics that are not collected in the
18
standard methods used for POM measurement of particulate organic
material. Also, very few analyses of naphthalenes and substituted
naphthalenes in polluted atmospheres have been reported.
1224
Stevens and O'Keeffe, in their report on the modern aspects of
air pollution monitoring, mentioned methane as the only organic compound
being studied per se. Is it to be inferred that routine analyses monitoring
VPOP are not warranted, not possible, or not yet possible? Recently,
instrumental field systems for automated GC analyses of methane, ethylene,
ethane, and acetylene have become commercially available.
19
Altshuller's review of the use of GC in air pollution studies reported
that the majority of papers dealt with low -molecular -weight hydrocarbons,
while recognizing that the higher-molecular-weight organic compounds
454
-------
represented very complex mixtures, possibly involving hundreds of com-
pounds. Altshuller observed that most investigators reported on improve-
ments and modifications of existing techniques rather than devoting their
studies to in-depth, long-term analyses of the components in the atmosphere.
Obviously, the trace character of the materials available for study in
the atmosphere has intensified the difficulties of conducting routine studies.
Nevertheless, there are now appropriate sampling techniques for VPOP for
qualitative and quantitative analyses, commercially available high-resolution
columns, and GC-MS systems with the necessary data-reduction equipment
and interfacing technology to realize the full potential of the separation power
of high-re solution columns working in the nanogram range.
MASS SPECTROM.ETRY
Mass spectrometry, coupled with appropriate auxiliary techniques (such
as GC, computer-assisted data processing, and specific-ion detection), has
the potential to be used to advantage to determine the various relevant
organic air pollutants present in the nanogram to picogram range.. MS is
advantageous for use in gaseous air pollutant analyses for the following
reasons: it has relatively uniform high sensitivity for all chemical species
that can be volatilized (in contrast with the electron-capture GC technique,
5
in -which differences in sensitivity of 10 or greater are observed); it has
excellent selectivity from interfering materials by use of specific-ion
detection or high resolution (as opposed to the nonpositive identification
of chemical species based on relative retention times associated with
GC); and it can identify unexpected compounds easily.
The application of MS to model problems of environmental contamination
3
has been described by Abramson. However, the actual use of the technique
455
-------
in authentic air pollution problems is very limited. This limitation is
directly related to the high cost of the mass spectrometer and the dedicated-
computer system. An additional factor has been the difficulty of adequate
sample preparation of discrete trace gases in the presence of an excessive
overburden of water and the hitherto adequate information content available
in other, less sophisticated and less expensive methods. However, present
concern for more specific information on the types of products formed from
the hydrocarbons reacting in the atmosphere and a need to know more about
the persistence or chemical degradation--in air, -water, soil, and biologic
systems--of the primary emissions and their products require the use of
the highly sophisticated analytic capabilities that MS affords.
Of particular importance in the use of MS is the flexibility in the type
of data that can be obtained from analyses that incorporate the high sensi-
tivity and selectivity of the mass spectrometer. In addition, many extra
capabilities are afforded by the data acquisition and processing systems
now available. For example, computer-reconstructed mass chromatograms
are possible when the total number of ions in each scan is platted against
the scan number. The major peaks resolved at particular scan numbers can
be entered into a library search routine for scanning with the appropriate
search algorithm for identification. In addition, the data processor can
construct a significant peak table, -which is an ordered listing of various
masses -with respect to their relative abundance in each spectrum, for
aiding in the identification process. Other important aspects of the
computer-based data acquisition and processing system are the high volume
of data that can be recorded continuously, the construction of library search
algorithms, and the background subtraction that is available for analyses of
complex systems -with small trace vapor-phase samples.
456
-------
Much lower detection limits are made possible by using single-ion
monitoring, rather than scanning. Focusing the mass spectrometer on
a strong mass signal in the mass spectrum has permitted detection limits
771
of as little as 10 picograms of organic lead compounds in the atmosphere.
To obtain maximal sensitivity, the appropriate molecular ion selected
as the diagnostic species may be enhanced by operating the mass spectrometer
at lower ionizing voltages,to suppress the formation of fragment ions formed
from background molecules, which often interfere with mass-number
characterization.
An alternative approach to elegant chromatographic separation of many
components in a mixture is the use of the superior resolving power of the
high-re solution mass spectrometer to determine the constituents -without
separation of the mixture by GC. An advantage of high-re solution MS is
its potential for determining, from an exact mass, the chemical formula
for the ion.
Very Little has been done with high-re solution MS in determining the
constituents of VPOP. However, an interesting application of high-
resolution MS to VPOP analyses has been accomplished by Schuetzle
1144
et al. A high-re solution MS was interfaced with a digital computer
and adapted for multicomponent analyses of air pollutants. The major
effort of the research was the development of a sophisticated computer
program for identifying the inorganic and organic constituents in airborne
1144
particles. However, Schuetzle et al. were also able to introduce
directly into the mass spectrometer the gaseous pollutants collected on
Chromosorb 102 and desorbed by a thermal-vacuum technique.
457
-------
One of the problems of using a high-re solution instrument is that
data acquisition is complicated by the volume and rate of the incoming
data. Also, data reduction is a difficult mathematical problem on a
small computer, because of the computational precision required.
Nevertheless, the importance of this new method for characterizing
atmospheric VPOP certainly warrants further investigation.
A traditional method of performing high-resolution experiments is
peak-matching by use of an oscilloscope to monitor the output from the
mass spectrometer, which is being slowly alternated (approximately
once every second) between two masses--one a reference mass (typically
a perfluorokerosene) and one the unknown mass. Exact mass is de-
termined by superimposing the position of the unknown peak on the position
of the reference mass on the oscilloscope and reading the difference in
mass from a calibrated dial. Another traditional method for gathering
high-re solution ion data is scanning at 10-100 sec/decade. The latter
method is too insensitive for satisfactory detection of GC effluents con-
taining trace constituents.
High-resolution single-peak monitoring of GC effluents provides much
of the definitive characteristics of a high-re solution analysis, with
sensitivity limits even better than those of low-resolution scanning.
The method is to alternate the two channels of the peak-matching unit
between a reference mass and the unknown molecular ion -with correct
centering. Detection limits below 0. 5 ng have been reported with this
3
technique.
The power inherent in MS as a tool for sophisticated analyses of
environmental contaminants has recently been further increased by the
458
-------
development of a mass spectral search system that can be used by
anyone with a teletype and an acoustic coupler. The program search
1023
is available from the National Institutes of Health at no cost to the user.
This interactive conversational system runs on a DEC PDP-10 time-
sharing computer. Use of the programs and system is simple, and no
knowledge of how the programs work or of program languages is required.
About 9, 000 spectra are in the data base, and there are plans to add
about 5, 000 more from the Aldermaston (England) collection. Experience
•with the system has shown that much information can be learned, even
if the spectra sought are not on file. A manual gives directions on the
use of the system, and a simple phone call brings the system into play.
Programs now available perform the following: search for peaks, search
for molecular weight, search for peaks with molecular formula, search
for peaks with molecular weight, search for complete or partial molecular
formula, search for molecular weight with molecular formula, print out
peaks and intensities, comment and complain, and enter new spectra.
GAS CHRQMATOGRAPHY-MASS SPECTROMETRY
The dominant instrument, both in total use and in projected growth,
for measxiring VPOP is the gas chromatograph. This is no surprise;
since its inception in 1953, chemists all over the world have been quick
to grasp the potential of GC. The intense activity continues today, with
application to pollution analysis and control. The simplicity of the tech-
nique and. its almost unlimited capacity to solve seemingly insoluble
problems have contributed much to its predominant position.
Nevertheless, GC as an analytic technique is severely limited by
the scarcity of standard reference methods to ensure proper standardization.
459
-------
This is especially critical when identification must rely solely on retention
time and detector response for compound identification. The basic nature
of MS removes many of these ambiguities of identification when used as the
GC detector, by providing identification of the components resolved by GC
>
in terms of their mass spectra.
Interfacing gas chromatographs to other instruments, especially
computers and mass spectrometers, has particularly enhanced the gas
chromatograph's present importance and potential for trace atmospheric
analyses. The GC-MS system has found such wide acceptance in trace
analyses that the major interest now in mass spectrometers is for this
combined system. Gas chromatographers experienced in the technique
quickly conclude that a dedicated computer is almost a necessity, so
much so that an integrated GC-MS-computer system is the preferred
analytic instrument.
The successful applications of the GC-MS system have essentially been
in the biomedical and space sciences, the food-flavor industry, pollutant
analyses with respect to pesticides, and in connection with the high-
molecular -weight polycyclic organic materials recoverable from high-volume
glass-fiber (Hi-Vol) filters. Unfortunately, pollution analyses of VPOP
with GC-MS systems have been very few.
The most noteworthy contribution to VPOP analyses with GC-MS
524
systems is that of Grob and Grob, who reported using capillary
columns in a GC-MS investigation of C organic compounds present
6-20
in the urban atmosphere of Zurich, Switzerland. Most of the 108 substances
identified were aliphatic and aromatic hydrocarbons. Benzaldehyde and
several of its alkyl derivatives, which may be oxidation products of
aromatic hydrocarbons, were also found.
460
-------
To identify the trace components eluted from their high-re solution
capillary columns, Grob and Grob reported that GC-MS was indispensable.
The two most important considerations in their analyses were the avail-
ability of only a limited quantity of material for analysis and the necessity
of preventing the separated substances from mixing again before reaching
the ion source in the mass spectrometer; the higher the GC resolution of
the complete mixture, the more important the latter is.
The means of interfacing GC to MS is one of the most difficult tech-
nical problems encountered. Essentially, the column effluent at
atmospheric pressure must be delivered to the MS source, which is at
high vacuum. The interfacing device must ensure transfer of the column
eluates quantitatively and without degradation of the GC resolution. There-
fore, the purpose of the interface is to conserve the sample--minimal loss
of sample with maximal removal of the carrier gas and no significant loss
of sample components because of interaction of the sample and the inter-
facing separator. Specifically, the interface must enrich the sample:
carrier gas ratio -while reducing the gas flow to the ion source. The
-5
pressure at the ionization source must be maintained at 10 torr or
lower to ensure optimal operation and to prevent burnout of the filaments
in electron impact sources. Chemical ionization or high-pressure MS is
not limited by this high-vacuum-source problem; however, it also is in-
fluenced by inherent interface-related losses of substances or reduced GC
resolution.
To overcome these limitations, the column flow rates used by Grob and
524
Grob were optimized so that the total column effluent was delivered to
the ion source of the mass spectrometer for improved resolution and sensi-
1146
tivity. The technical details are described by Schulze and Kaiser.
461
-------
From the standpoint of conservation and precise quantitative control of
the sample, the best interface is the avoidance of any interface separators
or splitting devices. Direct coupling ensures that the GC resolution is
fully retained; all the effluent is available to the MS, enabling it to be a
more sensitive detector than the FID (flame ionization detector).
576
Henderson and Steel also reported recently that GC-MS instruments
have been used predominantly with packed columns and that, with few ex-
ceptions, the advantages of coupling high-resolution columns to a mass
spectrometer have not been realized. They concluded that the drawback
to using high-resolution columns in GC-MS systems involves the limitations
of the interface, which result in loss of resolution and sensitivity. Ryhage
„ 1105
and Wikstrom recently reviewed the problems of interfacing GC with
MS for a wide variety of separators.
576
To overcome the limitation of the separator, Henderson and Steel
developed a system that allows the introduction of the total GC effluent,
up to 20 ml/min, directly into the ion source of the mass spectrometer.
The system has been used effectively with high-resolution columns on a
wide range of sample types, including fixed gases and C hydrocarbons.
10-40
The direct coupling of the GC column effluent to the ion source of the mass
spectrometer is made possible by high-speed differential pumping of the
source and analyzer using all stainless-steel diffusion pumps.
The effects of flow rate on ion source and analyzer pressures, overall
sensitivity, and resolving power of the system were evaluated and reported.
The sensitivity of the GC-MS varied by only a factor of 2 over a helium flow
range of 1. 1-7. 2 ml/min, measured using cholesterol trimethylsilyl ether
-11
at a sample flow rate of 10 g/ sec. The resolution was not significantly
changed when the separation of a doublet at nominal mass 156 (separation,
462
-------
136. 38 millimass units) was monitored as a static system, and then a
dynamic system with helium flows of 2. 5 and 7. 2 ml/min.
The concept of a differentially pumped mass spectrometer removes
the need to maintain the ion source at an extremely low pressure to
524 1146
preserve resolution. Grob and Grob, Schulze and Kaiser, and
576
Henderson and Steel have provided the logical extension of this con-
cept by using diffusion pumps of much larger capacity to handle the GC
effluent while maintaining low source pressures. This application should
significantly advance the use of GC-MS instruments for pollution analysis,
especially the hitherto forgotten sector of C VPOP.
10-22
The preceding discussions have dealt almost exclusively with the universally
used electron-impact (El) MS ionization source. The exciting new technique
931
in MS is the development of the chemical ionization (CI) source.
127
Bonelli and Story reviewed the features of the El with the CI source
and concluded that CI offers the advantages of simpler cracking patterns,
405
intense quasimolecular ions, and easy-to-interpret spectra. Fales
recently reviewed the newer ionization techniques available to MS and
concluded that CI is possibly the most significant new source for general
MS work.
The CI principle of operation is a form of high pressure MS (0. 5-1. 5
torr) in which the sample components are reacted with reactant ions.
Methane is the most common source of the reactant ions, which are formed
by a combination of electron impact and ion-molecular reactions. The
reaction of these reactant ions with the material to be chemically ionized
occurs by ion-molecule reaction. However, it is possible to use many
different gases as reactants and thus produce spectra from reactant ions
of different energies.
463
-------
In interfacing GC to MS using a CI source, the same two problems
that confront the use of EI-MS systems must be overcome: high gas
flow rates and low sample:carrier gas ratio. However, in CI-MS, the
low samplercarrier (reactant) gas ratio is an asset, because, with a
CI source, the reactant gas: sample gas ratio must be at least 100:1.
It is therefore advantageous to consider not using a separator or splitter
to interface the GC effluent to the MS. The reality of directly introducing
the total GC effluent to the CI source promises to extend the operational
sensitivity of this system to the subnanogram range for VPOP analyses.
464
-------
TABLE B-l
Comparison of Column Characteristics for VPOP Analyses
Characteristic
Resolution
Numbers of
Packed,
1/8-1/4 in.
Low
High
Capillary,
0. 01 in.
Very high
Low
SCOT,
0. 02 in.
High
Low
Packed Capillary,
0. 02-0. 03 in.
Moderate
Moderate
separating
phase/supports
available
Ease of
fabrication
Prior use in
VPOP studies
Durability
Sample
Capacity
Cost
Good
Large
Very good
Large
Low
Poor
Small
Poor
Very small
'High
Poor
Small
Very good
Small
High
Good
Small
Good
Moderate
Moderate
465
-------
APPENDIX C
AIRBORNE CONTAMINANTS*
Threshold limit values refer to airborne concentrations
of substances and represent conditions under which it is be-
lieved that nearly all workers may be repeatedly exposed day
after day without adverse effect. Because of wide variation
in individual susceptibility, however, a small percentage of
workers may experience discomfort from some substances at con-
centrations at or below the threshold limit, a smaller percentage
may be affected more seriously by aggravation of a pre-existing
condition or by development of an occupational illness.
* * *
Threshold limit values refer to time-weighted concentrations
for a 7 or 8-hour workday and 40-hour workweek. They should be
used as guides in the control of health hazards and should not
be used as fine lines between safe and dangerous concentrations..
Threshold limits are based on the best available information
from industrial experience, from experimental human and animal
studies, and, when possible, from a combination of the three.
The basis on which the values are established may differ from
substance to substance; protection against impairment of health
may be a guiding factor for some, whereas reasonable freedom
from irritation, narcosis, nuisance or other forms of stress
may form the basis for others.
Excerpted from American Conference of Governmental Industrial
Hygienists, pp. 1-3.
466
-------
The amount and nature of the information available for
establishing a TLV varies from substance to substance; con-
sequently, the precision of the estimated TLV is also subject
to variation and the latest Documentat ion should be consulted
in order to assess the extent of the data available for a given
substance.
* * *
In spite of the fact that serious injury is not believed
likely as a result of exposure to the threshold limit concen-
trations, the best practice is to maintain concentrations of
all atmospheric contaminants as low as is practical.
These limits are intended for use in the practice of
industrial hygiene and should be interpreted and applied only
by a person trained in this discipline. They are not intended
for use, or for modification for use, (1) as a relative index
of hazard or toxicity, (2) in the evaluation or air pollution
nuisances, (3) in estimating the toxic potential of continuous,
uninterrupted exposures, (4) as proof or disproof of an existing
disease or physical condition, or (5) for adoption by countries
whose working conditions differ from those in the United States
of America and where substances and processes differ-.
467
-------
APPENDIX D
TOXICITY DATA ON OCCUPATIONAL EXPOSURE
TO SELECTED SUBSTANCES*
ACETALDEHYDE
Most of the subjects of Silverman, Schulte, and First-'--'-'"
experienced some eye irritation at 50 ppm, but were willing to
work an 8-hr day in the presence of 200 ppm.
ACETIC ACID
Vigliani and ZurIo1319,1320 reported that workers exposed
for 7-12 years to 60 ppm and for 1 hr to 100-260 ppm, had no
injury except slight irritation of the respiratory tract, stomach,
and skin. Parmeggiani and Sassi (cited in Patty, ^ p. 1779)
found conjunctivitis, bronchitis, pharyngitis, and erosion of
exposed teeth, apparently in the same workers.
ACETONE
Vigliani and Zurlo-'-319 found chronic respiratory tract
irritation and dizziness in workers inhaling 1,000 ppm 3 hr/day.
F. L. Oglesby et al. (unpublished data) concluded, as a result
of extensive studies on thousands of workers exposed to acetone
in the manufacture of cellulose acetate yarn, that 200-400 ppm
was detectable only on immediate contact, that 700 ppm cannot
be detected after a brief period, and that 2,500-3,000 ppm caused,
at most, only minor irritation of the eyes and nose.
Adapted from American Conference of Governmental Industrial
Hygienists.40
468
-------
ACETYLENE
The inhalation of 100,000 ppm has a slight intoxicating
effect on man; marked intoxication occurs at 200,000 ppm,
incoordination at 300,000 ppm, and unconsciousness within 5
min of exposure to 350,000 ppm.631
ACROLE1N
Prentiss reported that concentrations as low as 0.25
ppm may cause some irritation, and 1 ppm is almost intolerable
and can cause lacrimation and marked eye, nose, and throat irrita-
tion within 5 min.
BENZENE
Bowditch and Elkins concluded that, of 11 deaths caused
by exposure to benzene, three resulted from concentrations of
200 ppm or more, four from concentrations of more than 100 but
less than 200 ppm, and three from concentrations estimated to
be below 100 ppm (the concentration in one case varied).
Greenburg et a_1.516 described nine cases of benzene poisoning,
with one death, in the rotogravure printing industry; of 48 air
analyses, 20 showed less than 100 ppm. Savilahti found
that 107 to 147 workers in a shoe factory had blood abnormalities;
the source of benzene was cement, and the concentrations were
reported as 318-470 ppm; one death occurred. Winslow re-
ported blood changes in workers where concentrations of benzene
vapor below 100 ppm were found. Heimann and Ford found one
death and three cases of blood changes where air analysis for
benzene showed a concentration of 105 ppm. Wilson reported
three fatal cases in a plant where the average concentration of
469
-------
benzene vapor was 100 ppm. Hardy and Elkins^^^ reported one
death and several cases of blood changes in a plant where re-
peated air analyses indicated benzene vapor concentrations of
191
about 60 ppm. Blaney found little evidence of benzene
intoxication in a group of 90 workers regularly exposed to
25 ppm for about 13 years. Pagnotto et a_l. found that
rubber spreaders were exposed to benzene vapor at 6-25 ppm;
some blood studies showed abnormalities, but no apparent
correlation with exposure; none of this group developed serious
blood dyscrasias.
1,3-BUTADIENE
Two human volunteers breathed 8,000 ppm for 8 hr with no
effects other than slight irritation of the eyes and upper
respiratory tract. '^
n-BUTANOL
Tabershaw et a_l. ^49 reported eye irritation in workmen
exposed to concentrations above 50 ppm, but no systemic effects
below 100 ppm. Nelson et al. reported mild irritation at
25 ppm, which became objectionable and was followed by headaches
at 50 ppm.
BUTANONE (METHYLETHYLKETONE)
Elkins, ' k reporting on industrial exposures, noted that
human exposures to 700 ppm in the air were without evidence of
permanent ill effects. Smith and Mayers1-*-"' reported that derma-
toses were common among workers handling butanone; some workers
exposed to 300-600 ppm complained of numbness of the fingers
and arms.
470
-------
tert-BUTYL ALCOHOL
Contact with human skin produced only slight erythema and
. . 988
hyperemia.
CROTONALDEHYDE
1188
Skog reported that crotonaldehyde produced symptoms similar
to those of acrolein. L. D. Pagnotto (personal communication)
reported a case of sensitization in a laboratory worker who
handled, small amounts of cr o t onald ehyd e . Rhinehart found
that exposure to 45 ppm was very disagreeable, and conjunctival
irritation was observed; at 15 ppm, the odor was strong, but
not intolerable, and no eye irritation was observed after brief
exposures.
CYCLOHEXANE
Gerarde reported that 300 ppm was detectable by odor
and somewhat irritating to the eyes and mucous membranes.
CYCLOHEXANOL
Browning reported one case of suspected intoxication--
characterized by vomiting, coated tongue, and slight tremors--
in a worker engaged in spraying leather with a preparation that
contained butylacetate and cyclohexanol.
1, 2-D;[CHLOROPROPANE (PROPYLENE BICHLORIDE)
1,2-Dichloropropane causes mild irritation on the open
f\ *3 r\
skin and some eye irritation.
471
-------
ETHYLBENZENE
Experimental exposure of six men showed that a concentra-
tion of 1,000 ppm was very irritating to the eyes, but that
the irritation decreased on continued exposure, until, after a
minute or two, it was scarcely noticeable; at 2,000 ppm, one
observer stayed in the atmosphere 5 min and found that irritation
to the eyes and throat gradually disappeared, but that vertigo
developed.476
ETHYLENE DIBROMIDE (1,2-DIBROMOETHANE)
-j o T
Kochmann reported a case of subacute poisoning due to
the accidental breathing of vapors in a plant; he said that a
concentration of 50 ppm could be dangerous to man. Olmsteady^
described a fatality caused by the ingestion of several capsules
containing a total of 4.5 ml of ethylene dibromide; autopsy
findings were hepatic necrosis and renal damage.
ETHYLENE OXIDE
Joyner, in a study of 37 chemical operators who worked
in ethylene oxide production for over 10 years, failed to de-
tect any evidence of ill effects due to exposure to this gas; in
general, long-term exposure appeared to be to 5-10 ppm, with
very few tests indicating concentrations above 50 ppm.
FORMALDEHYDE
u A A u ^578 (p. 128) . _,, . 386 (pp. 116, 231)
Henderson and Haggard VK ' and Elkins rr
reported that formaldehyde irritates the eyes, respiratory tract,
and skin. Henderson and Haggard noted that persons may become
more susceptible on repeated exposure. Elkins also reported
472
-------
complaints from persons exposed to a maximal concentration of
5-6 ppm; eye irritation was noted in "unacclimated" persons ex-
posed to much lower concentrations. Morrill and Bourne and
Seferianl-33 reported irritation in the form of itching eyes,
dry and sore throats, disturbed sleep, and unusual thirst on
awakening in a few workers exposed to 1-2 ppm. Shipkowi t z , *-*-' 3
in an environmental study of formaldehyde vapor emissions in
the permanent-press fabrics industry (eight plants), based on
workers' complaints, revealed formaldehyde concentrations ranging
generally from 0.3 to 2.7 ppm (sewing area), with an average of
0.68 ppm; complaints consisted of annoying odor, constant
prickling irritation of the mucous membranes, and disturbed
sleep. Harris-^? reported that the only positive finding among
25 men engaged in the manufacture of urea-formaldehyde and
phenol-formaldehyde resins, in concentrations well below 10 ppm,
was dermatitis in four of the men.
FORMIC ACID
Fahy and Elkins (unpublished data) reported that workers
exposed to formic acid and acetic acid, averaging 15 ppm, in
a textile plant complained of nausea; such concentrations were
judged to be very irritating.
HEPTANE
Patty and Yant reported that 1,000 ppm caused slight
dizziness in man after exposure of 6 min; higher concentrations
for shorter peiods resulted in marked vertigo, incoordination ,
and hilarity; brief exposures (4 min) to 5,000 ppm produced
473
-------
complaints of nausea, loss of appetite, and a gasoline taste
that persisted for several hours after exposure.
HEX AN IS
Patty and Yant^ -* reported that 10 min of exposure to
5,000 ppm resulted in dizziness and a sensation of giddiness.
Drinker et a_1.3->l found slight nausea, headache, and eye and
throat irritation after exposure to 1,400-1,500 ppm. Nelson
et _a_1.9ou found no irritation at 500 ppm in unacclimated
subjects.
2-HEXANONE (METHYL-2-BUTYLKETONE)
I I o Q
Schrenk e t a_l. exposed human volunteers for several
minutes to 1,OOU ppTn, which produced moderate eye and nasal
irritat ion.
ISOVALERALDEHYDE (2-METHYLBUTYRALDEHYDE)
Wilkinson-'- 381 reported that exposure of chemists to high
concentrations resulted in nausea, vomiting, headache, and
weakness.
METHANOL
ft 7 7
McNally stated that occupational methanol poisoning
has often caused death or blindness; several cases resulted
from work in confined spaces, e.g., varnishing beer vats where
the concentration was 500-6,000 ppm. Browning166 stated that
cases of chronic poisoning from repeated exposure to methanol
vapor were manifested by conjunctivitis, headache, giddiness,
r Q n
insomnia, gastric disturbances, and failure of vision. Henson-5
reported that workers exposed to 300 ppm during the operation
of duplicating machines complained of headaches.
474
-------
METHYLBROMIDE
Von Oettingen1324 recorded 47 fatal and 174 nonfatal cases
of methyIbromide intoxication. Hine^95 reviewed 10 cases (two
fatal) where the concentration was 100 ppm. Ingram"^' re-
ported that tests made in date-processing and -packing houses,
where a number of employees had been stricken, showed concen-
1357
trations ranging from 100 ppm to over 1,000 ppm. Watrous
described nausea, vomiting, headache, and other symptoms of
mild systemic poisoning in workers exposed for 2 weeks to con-
centrations below 35 ppm.
2-METHYL-1-BUTENE-3-ONE (METHYLISOPROPENYLKETONE)
Human experience has shown this ketone to be a definite
skin irritant; several minutes may elapse before blistering
and pain develop (Biochemical Research Laboratory, Dow Chemical
Co., unpublished data).
PENTANE
Human exposures for 10 min to 5,000 ppm did not cause
mucous membrane irritation or other symptoms.1015
2-PENTANONE (METHYL-N-PROPYLKETONE)
Yant, Patty, and Schrenkl^2! reported that 1,500 ppm had
a strong odor and was markedly irritating to the eyes and nasal
passages of humans.
PHENOL
Intermittent industrial exposure (5-10 min/hr) inside a
conditioning room for phenol-impregnated asbestos resulted in
475
-------
marked irritation of the nose, throat, and eyes; the average
phenol concentration in the room was 48 ppm, although formaldehyde
(8 ppm) .was also found; workers at the same plant, continuously
exposed during winding operations (average concentration, 4 ppm),
experienced no respiratory irritation (Connecticut Bureau of
Industrial Hygiene, unpublished data).
PROPANE
1 / 1 O
Wolf and Menne reported the case of a man who was
exposed to propane (concentration unknown) from a leaking
tank in an automobile; he exhibited colicky pains, stupefaction
disorientation and excitement, narrowing of the pupils, and
marked salivation; he recovered, but suffered from retrograde
525
amnesia. Gueffroy reported a study of five women workers
who were exposed to propane that escaped through improper pipe
fittings; headache, numbness, a "chilly feeling," and vomiting
were reported.
TOLUENE
Longley and co-workers°^° reported that 26 men were over-
come by toluene vapor aboard ship; there were no deaths or
serious aftereffects. Wilsonl^04 found that, among workers
exposed to toluene at less than 200 ppm, there were some com-
plaints of headache, lassitude, and nausea, but that physical
findings were essentially negative. Knox and Nelson722 described
an instance of permanent encephalopathy involving a man who in-
haled toluene regularly for over 14 years. Powars reported
six cases of aplastic anemia, one of them fatal, among glue-
1^25
sniffers. Von Oettingen and co-workers found that human
476
-------
subjects exposed to 200 ppm suffered slight but definite
991
changes in muscular coordination. Ogata et al. found that
experimental human subjects exposed to 200 ppm for 7 hr showed
prolongation of reaction time, decreased pulse rate, and de-
creased systolic blood pressure.
TRIMETHYLBENZENE
Battig et a_l. described human exposure to two isomers
of trimethyIbenzene—mesitylene and pseudocumene; workers ex-
posed for a number of years to vapor concentrations of 10-60
ppm had symptoms of nervousness, tension, and anxiety and
asthmatic bronchitis; in addition, the peripheral blood showed
a tendency to hypochromic anemia and a deviation from normal
in coagulability.
m-XYLENE
Gerarde listed headache, fatigue, lassitude, irrita-
bility, and gastrointestinal disturbances--such as nausea,
anorexia, and flatulence--as the most common symptoms among
workers exposed to xylene. Browning recorded reports of
gastrointestinal and neurologic disturbances and injury to
heart, liver, kidneys, and nervous system among workers exposed
to xylene; in addition, she noted a number of reports of blood
dyscrasias, some of them fatal, associated with xylene exposures
Q O /
DeOliveira described the death from aplastic anemia of a
lithographer who used xylene for several years. Goldie re-
ported a patient who had an apparent epileptiform seizure after
relatively brief exposure to xylene vapor.
477
-------
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/1-75-005
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Vapor-Phase Organic Pollutants - Volatile Hydrocarbons
and Oxidation Products
5. REPORT DATE
October 19Z5
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Panel on Vapor-Phase Organic Pollutants
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
National Academy of Sciences
National Research Council
Committee on Medical and Biologic Effects of Environmen-
tal Pollutants, Washington, B.C.
10. PROGRAM ELEMENT NO.
1AA001
11. CONTRACT/GnANT-*K).
68-
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Protection Agency
Special Studies Staff
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report concerns vapor-phase substances likely to be produced as
community pollutants in sufficient amounts to affect health and well-being.
Sources of vapor-phase organic pollutants are listed, including collection and sampling
techniques and. analytical methods. Possible mechanisms of formation of oxygenated
organic hydrocarbon compounds in the atmosphere and of atmospheric reactions of oxides
of nitrogen and sulfur are studied.
Toxicologic, pathophysiologic, and epidemiologic information on vapor.-phase
organic.pollutants is reviewed, their metabolism, and their effects on the total
environment. Special attention is given to oxidized compounds, formaldehyde, ozone, anc
benzene.
The report stresses the importance of oxidation reactions in the vapor-phase and
the human health hazards produced from the more or less transient products of oxidatior
The review of metabolism indicates that, although vapor-phase hydrocarbon pollutants
are modified usually by enzymatic oxidation within mammalian systems from nonpolar to
polar compound.3 (which are then excreted by the kidney) , this sometimes occurs with
the production of toxic intermediates. These reactions occur mostly in the liver and
to a lesser extent in the kidney, intestine, and lung.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Air Pollution
Vapor phases
oxidation
Organic compounds
QUA
06T
07D
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report/
Unclassified
21. NO. OF PAGES
660
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73;i
660
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