EPA-600/1-77-013
February 1977
NITROGEN OXIDES
by
Subcommittee on Nitrogen Oxides
Committee on Medical and Biologic Effects of
Environmental Pollutants
National Research Council
National Academy of Sciences
Washington, D.C.
Contract No. 68-02-1226
Project Officer
Orin Stopinski
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
I
JL 0*417.
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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 that is the subject of this report was approved by the
Governing Board of the National Research Council, whose members are
drawn from the Councils of the National Academy of Sciences, the National
Academy of Engineering, and the Institute of Medicine. The members of
the Committee responsible for the report were chosed for their special
competences and with regard for apropriate balance.
This report has been reviewed by a group other than the authors
according to procedures approved by a Report Review Committee consisting
of members of the National Academy of Sciences, the National Academy of
Engineering, and the Institute of Medicine.
ii
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FOREWORD
The many benefits of our modern, developing, industrial society are
accompanied by certain hazards. Careful assessment of the relative risk
of existing and new man-made environmental hazards is necessary for the
establishment of sound regulatory policy. These regulations serve to
enhance the quality of our environment in order to promote the public
health and welfare and the productive capacity of our Nation's population.
The Health Effects Research Laboratory, Research Triangle Park,
conducts a coordinated environmental health research program in toxicology,
epidemiology, and clinical studies using human volunteer subjects. These
studies address problems in air pollution, non-ionizing radiation,
environmental carcinogenesis and the toxicology of pesticides as well as
other chemical pollutants. The Laboratory develops and revises air quality
criteria documents on pollutants for which national ambient air quality
standards exist or are proposed, provides the data for registration of new
pesticides or proposed suspension of those already in use, conducts research
on hazardous and toxic materials, and is preparing the health basis for
non-ionizing radiation standards. Direct support to the regulatory function
of the Agency is provided in the form of expert testimony and preparation of
affidavits as well as expert advice to the Administrator to assure the
adequacy of health care and surveillance of persons having suffered imminent
and substantial endangerment of their health.
To aid the Health Effects Research Laboratory to fulfill the functions
listed above, the National Academy of Sciences (NAS) under EPA Contract
No. 68-02-1226 prepares evaluative reports of current knowledge of selected
atmospheric pollutants. These documents serve as background material for
the preparation or revision of criteria documents, scientific and technical
assessment reports, partial bases for EPA decisions and recommendations
for research needs. "Nitrogen Oxide-" is one of these reports.
John H. Knelson, M.D.
Director
Health Effects Research Laboratory
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ABSTRACT
This report is a review of current knowledge of the environmental
and health basis for control of manmade sources of nitrogen oxide
emissions. The literature review covered the period through 1974.
The principal subject areas considered in the report include: sources
and control of atmospheric nitrogen oxides; analytical methodology;
concentrations and chemical reactions in the atmosphere; and the
effects of nitrogen oxides on human health, materials, vegetation,
light transmission, and natural ecosystems. Emphasis is primarily
on nitric oxide (NO) and nitrogen dioxide (N02)» designated
by the composite formula NOx for nitrogen oxides. The major manmade
source is the combustion of fossil fuel. Highest atmospheric
concentrations are found in heavily populated, industrialized urban
areas. Both acute and chronic health effects resulting from short-
term and long-term exposures, are discussed in the report. Effects
range from slight increases in airway resistance to death depending upon
exposure concentrations.
IV
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SUBCOMMITTEE ON NITROGEN OXIDES
T. TIMOTHY CROCKER, University of California College of Medicine, Irvine,
California, Chairman
CRAIG T. BOWMAN, United Aircraft Research Laboratories, East Hartford,
Connecticut
JACK G. CALVERT, Ohio State University, Columbus, Ohio
RICHARD EHRLICH, IIT Research Institute, Chicago, Illinois
ELLIOT GOLDSTEIN, University of California, Davis, California
DAVID C. MACLEAN, Boyce Thompson Institute for Plant Research, Yonkers,
New York
CARL M. SHY, University of North Carolina, Chapel Hill, North Carolina
LESTER F. WOLTERINK, Michigan State University, East Lansing, Michigan
Resource Persons
MARTIN ALEXANDER, Cornell University, Ithaca, New York
DAVID VINCENT BATES, University of British Columbia, Vancouver, British
Coluir.* '.a, Canada
CHARLES R. FRINK, Connecticut Agricultural Experiment Station, New Haven,
Connecticut
EVALDO KOTHNY, Air and Industrial Laboratory, Department of Public Health,
Berkeley, California
VICTOR S. SALVIN, University of North Carolina, Greensboro, North Carolina
JEROME F. THOMAS, University of California, Berkeley, California
GEORGE TURNER, Beckman Instruments, Fullerton, California
JOHN REDMOND, JR., Division of Medical Sciences, National Research Council,
Washington, D.C., Staff Officer
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COMMITTEE ON MEDICAL AND BIOLOGIC EFFECTS OF ENVIRONMENTAL POLLUTANTS
HERSCHEL E. GRIFFIN, Graduate School of Public Health, University of Pittsburgh,
Pittsburgh, Pennsylvania, Chairman
RONALD F. COBURN, University of Pennsylvania School of Medicine, Philadelphia,
Pennsylvania
T. TIMOTHY CROCKER, University of California College of Medicine, Irvine,
California
CLEMENT A. FINCH, University of Washington School of Medicine, Seattle,
Washington
SHELDON K. FRIEDLANDER, California Institute of Technology, Pasadena, California
ROBERT I. HENKIN, Georgetown University Hospital, Washington, D.C.
IAN T.T. HIGGINS, School of Public Health, University of Michigan, Ann Arbor,
Michigan
JOE W. HIGHTOWER, Department of Chemical Engineering, Rice University, Houston,
Texas
HENRY KAMIN, Duke University Medical Center, Durham, North Carolina
ORVILLE A. LEVANDER, Agricultural Research Center, Beltsville, Maryland
DWIGHT F. METZLER, Kansas State Department of Health and Environment, Topeka,
Kansas
I. HERBERT SCHEINBERG, Albert Einstein College of Medicine, Bronx, New York
RALPH G. SMITH, School of Public Health, University of Michigan, Ann Arbor,
Michigan
ROGER P. SMITH, Dartmouth Medical School, Hanover, New Hampshire
T.D. BOAZ, JR., Division of Medical Sciences, National Research Council,
Washington, D.C., Executive Director
vi
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CONTENTS
1 Introduction
2 Nitrogen Oxides—Their Properties and Effects on Atmospheric
Light Transmission
3 Sources and Control of Atmospheric Nitrogen Oxides
4 Analytical Methodology for the Determination of Nitrogen Oxides
in Air
5 Atmospheric Levels of Nitrogen Oxides
6 Chemical Interactions of Nitrogen Oxides in the Atmosphere
7 Effects of Nitrogen Oxides on Natural Ecosystems
8 Effects of Nitrogen Oxides on Materials
9 Effects of Nitrogen Oxides on Vegetation
10 Health Effects of Oxides of Nitrogen
11 Summary, Conclusions, and Recoiomendations for Future Research
References
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ACKNOWLEDGMENTS
This document was prepared by the Subcommittee on Nitrogen Oxides.
Dr. T. Timothy Crocker served as Chairman and Dr. Carl M. Shy, as Vice
Chairman, Although the initial drafts of the various sections were pre-
pared by individuals, the entire document was extensively reviewed by the
entire subcommittee and represents a group effort.
The original draft of the Introduction was written by Dr. Carl M.
Shy. Chapter 2, describing the properties of nitrogen oxides and their
effects on atmospheric light transmission, was the responsibility of
Dr. Jack G, Calvert. Dr. Craig T. Bowman prepared Chapter 3, which focuses
on the sources and control of atmospheric nitrogen oxides. The author of
Chapter 4, concerning the analytical methodology for the determination of
nitrogen oxides in air, was Dr, Evaldo L. Kothny. Atmospheric levels of
nitrogen dioxide are described in Chapter 5. This was the responsibility
of Mr. George Turner. Chapter 6, in which the chemical interactions of
nitrogen oxides in the atmosphere are discussed, was prepared by Dr, Jack
G. Calvert with the assistance of Dr. Jerome F. Thomas.
Dr. Martin Alexander prepared Chapter 7, which concerns effects on
natural ecosystems. Dr. Charles R, Frink provided Dr. Alexander with
material on soils; Dr. Boyd R. Strain on plants; Dr, Charles R. Goldman
on aquatic life; and Dr. Tony J, Peterle on animals. Effects on materials
are covered in Chapter 8, which was authored by Dr, Victor S, Salvin and
Dr. Norman Bornstein, with assistance from Dr. Craig T. Bowman, Dr. David
C. MacLean provided the material for Chapter 9, which deals with effects
viii
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on vegetation. Chapter 10, on health effects, was prepared by Drs, T,
Timothy Crocker, Elliot Goldstein, Richard Ehrlich, and Carl M, Shy, The
Summary, Conclusions, and Recommendations were a subcommittee effort,
The preparation of the document was assisted by the comments of
anonymous reviewers designated by both the Assembly of Life Sciences and
the Report Review Committee, The Committee on Medical and Biologic Effects
of Environmental Pollutants was very helpful. The subcommittee is partic-
ularly indebted to Dr. Ralph G, Smith who served as Associate Editor.
Dr. Robert J.M. Horton of the Environmental Protection Agency gave
invaluable assistance by providing the subcommittee with various documents
and translations. Informational assistance was obtained from the National
Research Council Advisory Center on Toxicology, The National Academy of
Sciences Library, the National Library of Medicine, the National Agricultural
Library, The Library of Congress, the Department of Commerce Library, and
the Air Pollution Technical Information Center.
Initially, the staff officer for the Subcommittee on Nitrogen Oxides
was Dr. Elizabeth Force. After her departure from the National Research
Council, Mr. John Redmond, Jr, was appointed staff officer and continued
in that capacity through completion of this report. The document was edited
by Mrs. Frances M. Peter.
IX
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CHAPTER 1
INTRODUCTION
Nitrogen oxide formation is an inherent consequence of fossil fuel
combustion. Over the next several decades, such combustion will continue
to be our major source of energy for electricity generation and motor vehicle
propulsion. Unlike many other atmospheric pollutants, more nitrogen oxides
are formed at higher combustion efficiencies. Fixation of atmospheric ni-
trogen and oxygen, and oxidation of nitrogen compounds in fuel are the two
processes by which fuel combustion results in nitrogen oxide formation.
Nitric oxide (NO) is the dominant oxide released initially; however, the
photochemical interaction between nitrogen oxides, hydrocarbons, and various
intermediary compounds generated in a sunlight-irradiated atmosphere results
in conversion of nitric oxide to nitrogen dioxide (N0?) and ultimately to
further oxidation products. Among the various atmospheric oxides of nitro-
gen, nitric oxide and nitrogen dioxide, designated by the composite formula,
NO , are the most important in relation to photochemical reactions, and their
X
known effects on materials, vegetation, and health.
In this monograph, a subcommittee of the National Academy of Sciences,
National Research Council has reviewed and summarized a large amount of
literature published since the 1971 report of the U.S. Environmental Pro-
563
tection Agency entitled Air Quality Criteria for Nitrogen Oxides. The
majority of the subcommittee's work was performed in 1974. Consequently, data
from 1975 and 1976 publications could not be included.
The purpose of the monograph is twofold: one, to provide the U.S.
Environmental Protection Agency with an assessment of current literature in
preparation for that agency's own reevaluation of the scientific basis for the
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1-2
nitrogen dioxide air quality standard; and, second, to present an independent
assessment of the environmental and health bases for control of manmade sources
of nitrogen oxide emissions.
Chapters 2 and 3 provide a straightforward presentation of the physical
and chemical properties and the sources and control of atmospheric emissions
563
of NO . Since the 1971 publication, Air Quality Criteria for Nitrogen Oxides,
X """ " L ~ T .---—-
considerable advances have been made in methodologies to monitor atmospheric
NO . Existing analytical methodologies are reviewed and evaluated in Chapter 4.
X
The conclusions reached in this chapter may elicit controversy because of dis-
agreements concerning the merits of several basic methods of NO monitoring.
X
Chapter 5 contains considerable data on atmospheric concentrations of
nitrogen oxides. The tables in this chapter will be useful because they com-
pile, in one publication, monitoring data that exclude results obtained by
the discredited Jacobs-Hochheiser method. Their estimates of atmospheric NO
concentrations in the United States are therefore more accurate than were
possible in the 1971 air quality criteria document.
In Chapter 6, atmospheric reactions of nitrogen oxides with gaseous
organic molecules are discussed. These reactions produce a variety of short-
lived intermediary and end products which cannot as yet be identified by
conventional analytical techniques. An attempt is made to identify these
secondary section products by relating chemical and kinetic laboratory models
to atmospheric observations. While it is impossible to make quantitative
predictions concerning concentrations of these reaction products, enough
information exists to identify the main qualitative features of the reaction
model. Attention is focused on new avenues of research on the atmospheric
chemistry, and the environmental and health effects of the atmospheric reac-
tion products of nitrogen oxides.
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1-3
Chapters 7 to 10 deal with the environmental and health effects of
nitrogen oxides. Existing evidence implicates nitrogen dioxide as the most
reactive and important of the atmospheric nitrogen oxides. However, some of
the intermediary and end products of the photochemical conversion of NO ,
X
identified in Chapter 6, are potentially as important in their environmental
and health consequences. By analogy with the emerging concern over reaction
products of sulfur dioxide (SO ), we may anticipate an increasing focus of
research on various NO reaction products. Unfortunately, insufficient data
x
on these compounds prevented the reviewing subcommittee from making more than
a passing reference to them in the chapters concerning environmental and
health effects.
This monograph brings together an array of information, not previously
reviewed in one report, on material and health effects of nitrogen dioxide.
The subcommittee has attempted to place these data in the perspective of the
U.S. air quality standard for nitrogen oxides. While finding no basis in
this evidence for altering the existing standard, the subcommittee reemphasizes
the need for a standard controlling short-term nitrogen dioxide exposures of
24 hr or less. This need is demonstrated in the body of evidence showing acute
effects of single or repeated nitrogen dioxide exposures of six or less hours.
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CHAPTER 2
NITROGEN OXIDES—THEIR PROPERTIES AND
EFFECTS ON ATMOSPHERIC LIGHT TRANSMISSION
Nitrogen and oxygen combine to form several stable oxides of different
molecular composition. The two oxides identified as important pollutants
in the lower atmosphere are nitric oxide (NO) and nitrogen dioxide (NO ).
Their interconvertibility in photochemical smog has led to their combined
designation as NO . Nitrous oxide (N?0) occurs in significant concentra-
X ^
tions even in the natural unpolluted atmosphere. It arises from natural
biological processes that occur in the soil, and is not classified as an
air pollutant. Other oxides of nitrogen, which can occur in polluted
atmospheres at very low concentrations are: symmetrical nitrogen trioxide
(NO ), unsymmetrical nitrogen trioxide (0-0-N-O), dinitrogen trioxide
(NO), dinitrogen tetroxide (NO), and dinitrogen pentoxide (NO).
OCCURRENCE
Significant concentrations of nitric oxide (several hundred ppm) and
smaller concentrations of nitrogen dioxide are formed at the high temper-
atures accompanying the burning of fossil fuels in mixtures with air.
These pollutants are emitted to the atmosphere from auto exhausts, power
plant and furnace stacks, incinerators, and vents from certain chemical
processes.
Natural biological reactions also generate large quantities of nitric
oxide and nitrogen dioxide. However, since the sources of these natural
reactions are diffuse and the rate of natural removal processes significant,
very low ambient levels of NO result.
x
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2-2
Most of the NO produced in combustion processes is initially in the
x
form of nitric oxide. This is subsequently oxidized in the atmosphere to
the more toxic and irritating compound, nitrogen dioxide.
Nitrous oxide is not considered an air contaminant since there is no
evidence involving it in the series of complex chemical reactions producing
photochemical smog. Nitrous oxide is a product of natural chemical reac-
tions that occur in the soil. Its concentration in the lower atmosphere
?/m3
470
o
ranges from 0.47-0.55 mg/m (0.25-0.29 ppm) with only slight seasonal and
geographical variations.
The nitrogen oxides5symmetrical nitrogen trioxide, dinitrogen tri-
oxide, dinitrogen tetroxide, and dinitrogen pentoxide have not been
identified as trace components of the polluted atmosphere, although their
presence at trace levels is inferred from the thermodynamics and kinetics
of their reactions as studied in the laboratory. Table 2-1 shows the
theoretical concentrations of the different oxides and acids of nitrogen
which would be present at equilibrium with molecules of nitrogen, oxygen,
and water present in air at 1 atm, 25°C, and 50% RH. Thermodynamic
equilibrium is not maintained between most of the components in a polluted,
sunlight-irradiated atmosphere. The concentrations of the oxides and acids
of nitrogen are controlled largely by the reaction rates and are usually
much greater than those expected at equilibrium. Compare column 2 with
column 1 in Table 2-1. Theoretically dinitrogen pentoxide, symmetrical
nitrogen trioxide, nitrous acid (HONO), and possibly other reactive tran-
sient species, are involved significantly in the reactions producing photo-
chemical smog.
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2-3
TABLE 2-1
Theoretical Concentrations of Nitrogen Oxides and Nitrogen
Acids which would be Present at Equilibrium with Molecular
Nitrogen, Molecular Oxygen, and Water in Air at 25°C, 1 Atm, 50% RH
Compound
Concentrations in Hypothetical Atmosphere, ppm
In Typical Sunlight-Irradiated,
At Equilibrium Smoggy Atmosphere3-
2.06 x 105
2.06 x 105
7.69 x 105
1.56 x 104
7.69 x 10-
1.56 x 10
NO,
1.91 x 10
-4
10'
-1
NO
2.69 x 10"
10'
,-1
NO,
3.88 x 10
-16
ID'8 - 10-9
N2°3
N2°4
N2°5
HONO(cis)
2.96 x 10~20
2.48 x 10~13
~17
3.16 x 10
7.02 x 10'
-9
10~8 - 10~9
10~7 - 10~8
10~3 - 10~5
10"
HONO(trans) 1.60 x 10
-8
10
,-3
HONO.
1.33 x 10
-3
10~2 - 10~3
—Theoretical estimates made using computer simulations of the chemical
reactions rates in a synthetic smog mixture. From Demerjian et al.
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2-4
THE PROPERTIES OF THE NITROGEN OXIDES
Nitric Oxide
Because nitric oxide does not absorb visible light, it: is colorless.
The first electronic absorption band of nitric oxide occurs at wavelengths
o
less than 2,300 A (Figure 2-1). Nitric oxide is odorless and only slightly
soluble in water (0.006 g/100 g of water at 25°C). It is the major oxide
of nitrogen formed during high temperature combustion processes resulting
primarily from the interaction of the nitrogen content of the fuel with
the nitrogen and oxygen present in the air during combustion. The net re-
sult is the near establishment of the nitrogen-oxygen-nitric oxide and
nitrogen-oxygen-nitrogen dioxide equilibria at the high temperatures of the
flame.
N2 + 02 t 2NO (1)
2NO + 0 J 2NO (2)
Nitric oxide formation is favored at high temperatures. The concentrations
of nitric oxide and nitrogen dioxide are limited by the thermodynamic and
kinetic properties of nitrogen, oxygen, nitric oxide, and nitrogen dioxide
molecules, and nitrogen and oxygen atoms, the flame temperature, the NO ,
nitrogen, and oxygen molecule concentrations, and the rate gases are trans-
ported through the different temperature zones in the combustion chamber.
Several hundred to several thousand ppm of nitric oxide may be formed through
Reaction (1) during combustion. The effects of increased temperature on the
equilibrium concentrations of nitric oxide and nitrogen dioxide formed in
heated air are illustrated in Table 2-2.
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2-5
1500
1700 1900
Wavelength, A
2100
2300
FIGURE 2-1. Absorption spectrum of nitric oxide; the form of the absorption
law employed is I = I exp(-apl), where £ is in atm at 0°C.
° 347
Data from McNesby and Okabe.
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2-6
TABLE 2-2
Theoretical Equilibrium Concentrations of Nitric Oxide
and Nitrogen Dioxide in air (50% RH) at Various Temperatures
Q
Concentration, mg/m (ppm)
Temperature °K (°C) NO N£2_
298 (24.85) 3.29 x 1CT10 3.53 x 10~4
(2.63 x 10~10) (1.88 x 10~4)
500 (226.85) 8.18 x 10~4 7.26 x 10~2
(6.54 x 10-4) (3.86 x 10"2)
1,000 (726.85) 43 3.38
(34.4) (1.80)
1,500 (1,226.85) 1,620 12.35
(1,296) ( 6.57)
2,000 (1,726.85) 9,946.25 23.88
(7,957) (12.70)
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2-7
Oxygen, as a reactant in combustion processes, is present in the flame
region well below its concentration in air. In addition, the products of
the combustion [carbon dioxide (CCM, water, etc.] dilute the nitrogen and
remaining oxygen molecules. These factors cause the equilibrium concentra-
tions of nitric oxide formed in a combustion at atmospheric pressure to be
somewhat lower than those shown at each temperature in Table 2-2. Nitric
oxide and nitrogen dioxide gases produced in a flame-maintained equilibrium
would revert largely to nitrogen and oxygen gases when cooled to ambient
atmospheric temperatures. However, most combustion equipment quickly channels
the thermal energy in the product gases to perform such work as turning a
turbine to generate electricity, moving a piston to ultimately propel an
automobile, etc.
The chemical equilibrium between products is not maintained during the
rapid cooling of the gases that occurs in these circumstances. The product
nitric oxide and nitrogen dioxide gases are "frozen out" at concentrations
typical of those present at equilibrium near the flame temperature. Nitrogen
contributing to NO formed in combustion can come from the molecular nitrogen
X
of the air or from the nitrogen-containing organic compounds in fossil fuels.
Nitrogen Dioxide
Nitrogen dioxide gas absorbs light over a wide range of the visible
wavelengths (Figure 2-2) causing the characteristic light yellowish-orange
to reddish brown colors of gaseous nitrogen dioxide seen at relatively low
and high concentrations, respectively. Nitrogen dioxide has a very pungent
odor, a high oxidation rate, and is extremely corrosive. It may be physiolog-
ically irritating and toxic (see Chapter 11).
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2-8
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2-9
Nitrogen dioxide may react with water to generate nitric acid (HONO-)
and nitrous acid;
2NO 4- HO ~t HONO + HONO (3)
and with nitric oxide and water to give nitrous acid:
N02 + NO + H20 J 2HONO (4)
The homogeneous gas phase reactions of nitrogen dioxide and nitric oxide
with water, forming nitric and nitrous acids, appear to be very slow at
ambient temperatures. Therefore, the possible significance of Reactions
314
(3) and (4) as NO removal paths remains uncertain.
X
The major source of nitrogen dioxide in the atmosphere is the oxida-
tion of the primary air pollutant, nitric oxide, through one of several
pathways. During the first few minutes a nitric oxide-rich exhaust gas is
mixed with air, oxidation may proceed significantly through the elementary
Reaction (5) :
2NO + 02 -> 2N02 (5)
The rate of nitrogen dioxide formation in Reaction (5) is given by the fol-
lowing rate law:
The dependence of the rate on the square of the nitric oxide concentration
results in a marked change in the rate with change in nitric oxide concen-
tration. Thus, when the concentration of nitric oxide in exhaust gases is
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2-10
3
1,250 mg/m (1,000 ppm) , the rate of nitrogen dioxide formation can be large;
3 —1
it maximizes at about 80.8 mg/m (43 ppm) min during the dilution caused
467
by exhaust gases mixing with about 35% air by volume at 25°C. When nitric
oxide-rich exhaust gas is mixed with air, about 25% of the nitric oxide may
be converted to nitrogen dioxide through Reaction (5) . Once the nitric
oxide concentration has reached its normal, low ambient level [less than
o
1.88 mg/m (1 ppm)], Reaction (5) is very slow [less than
(1.5 x 10 ppm) min~ ]. However, in a sunlight-irradiated, polluted atmo-
sphere, nitric oxide may be oxidized to nitrogen dioxide rather quickly by
mechanisms involving the hydrocarbons, aldehydes, carbon monoxide, and other
compounds. The oxidizing agents in this case are believed to be the tran-
sient hydroperoxy (HO ) , alkylperoxy (RO ) , and acylperoxy (RCOO_) free
radicals.
NO + HO -> N02 + HO (6)
NO + RO -v N02 + RO (7)
NO + RCOO -> NO + RC09 (8)
2 ^- ^
Here R represents methyl (CH3) , ethyl (C2H5) , and higher alkyl groups. The
hydroperoxy, alkylperoxy, and acylperoxy radicals are formed in chain reac-
tions initiated by several active free radicals — i.e., hydroxy (HO), singlet
D oxygen atoms [0(4))], ozone (0 ) — radicals interacting with the hydrocarbons, alde-
~~ ~~ 3
hydes, and carbon monoxide. (See the more detailed discussion in Chapter 6.)
Nitrogen dioxide photolysis in sunlight is believed to be largely
responsible for the generation of ozone in the sunlight-irradiated, polluted
105,314,467
atmosphere. When a quantum of sunlight of wavelength less than
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2-11
4,300 A is absorbed by a. nitrogen dioxide molecule, the excited molecule
formed is of sufficient energy to dissociate into ground state oxygen atoms
and nitric oxide:
N02 + hv (A<4,300 A) -> 0 + NO (9)
Oxygen atoms in air react predominantly with molecular oxygen to form ozone:
0 + 02 (-HM) -> 03 (+M) (10)
The M represents a third molecular species (nitrogen, oxygen, water, etc.)
that removes a fraction of the energy released during the interaction of
the oxygen atom with the oxygen molecule. This energy removal stabilizes
the ozone product. At the usual levels of common impurities present in
urban air, ozone reacts most rapidly with nitric oxide to regenerate nitrogen
dioxide:
0 + NO ^ NO +0 (11)
J £ £.
The combination of the results of Reactions (9), (10) , and (11) causes
generation of a small concentration of ozone directly related to the ratio
of the concentration of nitrogen dioxide to the concentration of nitric
oxide and the intensity of the sunlight absorbed by the nitrogen dioxide.
As the intermediates hydroperoxy, alkylperoxy, and acylperoxy radicals
form during the complex interactions of the hydroxy radical, oxygen atom,
ozone, etc., with hydrocarbons, aldehydes, and carbon monoxide, they react
in part by Reactions (6), (7), and (8) and drive nitric oxide to nitrogen
dioxide. This in turn builds the ozone concentration through the rapid
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2-12
interplay of Reactions (9), (10), and (11). (See the more detailed
discussion in Chapter 6.)
Nitrous Oxide
Nitrous oxide is a colorless gas with a slightly sweet taste and odor
at high concentrations. It has been used for years as an anesthetic in
medicine and dentistry. Inhalation of small quantities in air often produces
a type of hysteria. This phenomenon is the source of its trivial name
"laughing gas." Nitrous oxide in the atmosphere is believed to be created
by decomposition of nitrogen compounds in the soil by anaerobic bacterial
470
action. The possible homogeneous formation of nitrous oxide in atmosperic
reactions, e.g., 0(1D) + N (+M) -> N20 (+M), appears to be negligible. The
mole fraction of nitrous oxide present in the atmosphere decreases markedly
470
above the tropopause, presumably as a result of photochemical dissoci-
ation of the nitrous oxide molecule.
o
N20 + hv (A<2,200 A) -> 0(1D) + NZ (12)
Singlet-I) oxygen atoms are electronically excited oxygen atoms. Absorbed
o
light of wavelengths less than 2,200 A, present in solar radiation above
the tropopause, can initiate Reaction (12).
The Unsymmetrical Nitrogen Trioxide
This oxide of nitrogen may be an intermediate in the kinetically third-
order Reaction (5) which leads to oxidation of nitric oxide to nitrogen
dioxide.
NO + 0 J 0-0-N-O (13)
0-0-N-O + NO + 2NO (14)
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2-13
The involvement of such an intermediate in equilibrium with oxygen and
nitric oxide in a rate-determining step [Reaction (14)] would be consistent
with the observed third order kinetics of the oxygen and nitric oxide ob-
served in Reaction (5). No direct evidence supports the existence of the
properties of this hypothetical transient species. Its significance as a
participant in atmospheric reactions is unknown.
The Symmetrical Nitrogen Trioxide (N03)
0
Symmetrical nitrogen trioxide is a well established reactive transient.
It is more important to air pollution than unsymmetrical nitrogen trioxide.
Its well characterized absorption in the visible region leads to the blue
color associated with this species at high concentrations. Much reliable
9 f\f\
laboratory evidence, both kinetic and spectroscopic, identifies symmetrical
nitrogen trioxide as an important transient in systems containing nitrogen
dioxide-ozone, nitrogen dioxide-oxygen atom, and dinitrogen pentoxide.
Symmetrical nitrogen trioxide can be formed as follows:
03 + N02 -»• N03 + 02 (15)
0 + N02 (-H4) + N03 (+M) (16)
N205 (+M) + N03 + N02 (+M) (17)
Symmetrical nitrogen trioxide is highly reactive toward nitric oxide and
nitrogen dioxide:
NO + NO •*• 2NO (18)
-J £,
-------
2-14
NO + N02 (+M) •+ N205 (+M) (19)
It can be expected to attain only a very low concentration in the pol-
luted atmosphere (^10 ppm) . Its rate of reaction with acetaldehyde
369
(CH CHO) and propene (C H ) have been determined. These reactions
•j 36
must occur in smog mixtures. The computer simulation of the rates of
photochemical smog formation suggests that the hydrogen-atom abstraction
reactions of symmetrical nitrogen trioxide such as:
NO + CH CHO -»- HONO + CH CO (20)
are probably much less important than the analogous reaction involving the
105
hydroxyl radical. However, the possible participation of symmetrical
nitrogen trioxide in other types of reactions not now recognized (addition
to olefinic hydrocarbons, etc.) remains open to question.
Dinitrogen Trioxide (or Nitrogen Sesquioxide)
The association of nitric oxide and nitrogen dioxide molecules to form
dinitrogen trioxide and the dissociation of dinitrogen trioxide both occur
rapidly establishing the equilibrium:
NO + N02 J N203 (21)
Dinitrogen trioxide is the anhydride of nitrous acid and reacts readily
with water, at least in the condensed phase, to generate nitrous acid:
N2°3 + H2° "" 2HON°
The equilibrium concentration of dinitrogen trioxide expected in the pol
uted atmosphere is very low compared with the common nitric oxide and
-------
2-15
nitrogen dioxide concentrations encountered (Table 2-3). No important role
for dinitrogen trioxide in atmospheric chemistry has been recognized to date.
Dinitrogen Tetroxide
In contrast to the brownish-red colored nitrogen dioxide gas, the dini-
trogen tetroxide dimer of nitrogen dioxide is colorless. It absorbs only
o
light outside the visible range (X<4,000 A) (Figure 2-1). Dinitrogen te-
troxide, commonly called nitrogen tetroxide, is formed by the association
of nitrogen dioxide molecules. It also readily dissociates to establish an
equilibrium:
2N02 t N204 (23)
The fraction of the nitrogen dioxide expected to associate at equilibrium
is very small for the ambient concentrations and temperatures common to
the urban atmosphere (Table 2-3). These low concentrations have never been
observed directly, nor is there speculation suggesting that dinitrogen te-
troxide offers any significant contribution to the chemical changes observed
in the atmosphere.
Dinitrogen Pentoxide
The pure compound, dinitrogen pentoxide, commonly referred to as nitrogen
pentoxide, occurs as a colorless gas at reduced pressures near 25°C. It is
unstable toward decomposition [Reaction (17)]. In the atmosphere, it forms
through the reaction of symmetrical nitrogen trioxide with nitrogen dioxide:
(19)
-------
2-16
TABLE 2-3
Theoretical Concentrations of Dinitrogen Trioxide
and Dinitrogen Tetroxide
Levels of Gaseous Nitric Oxide
in Equilibrium with
and Nitrogen Dioxide
Various
in Air at 25°
C
Concentration, ppm
NO
0.050
0.10
0.50
1.00
NO
0.050
0.10
0.50
1.00
N003
1.3 x 10~9
5.2 x 10~9
1.3 x 10~7
5.2 x 10~7
N-,0,
1.7 x
6.8 x
1.7 x
6.8 x
io-8
io-8
io-6
10~6
-------
2-17
The dinitrogen pentoxide association product of nitrogen trioxide and nitrogen
dioxide is more favored under atmospheric conditions than the dinitrogen te-
troxide and dinitrogen trioxide association products of nitrogen dioxide-
nitrogen dioxide and nitrogen dioxide-nitric oxide, respectively. Atmospheric
simulations predict that about 10~ ppm of dinitrogen pentoxide
470
may accumulate in photochemical smog (Table 2-1). Actual ambient levels
of dinitrogen pentoxide have never been observed directly. Dinitrogen
pentoxide is the anhydride of nitric acid:
H20 + N205 -*• 2HON02 (24)
368,470
The homogeneous rate of this reaction in the gas phase is uncertain.
The extent to which atmospheric dinitrogen pentoxide reacts with water to
form nitric acid either homogeneously or heterogeneously, e.g., on a moist
aerosol surface, has not been determined. The heterogeneous reaction pathway
may be significant to nitric acid and nitrate salt formation in photochemical
smog.
Some important physical properties of the major oxides of nitrogen
are summarized in Table 2-4. Where no solubility constants are given, the
species react with water. These reactions do not follow Henry's law which
states that the weight of a gas dissolved by a liquid is proportional to
the pressure of the gas. The actual amount of gas in solution at a given
point in time is not only a function of the gas pressure and temperature,
but also of the diffusion rate, the pH of the solution, and the interfacial
surface area.
-------
2-18
"I,
i
CN
W
CO
01
T:
"r-
c
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rH CM CN
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1
-------
2-19
EFFECTS ON ATMOSPHERIC LIGHT TRANSMISSION
Visibility reduction, common to urban air pollution, is caused by the
scattering and absorption of light by particles or gases in the atmosphere
and depends in a complicated way on the concentration and properties of the
555
gases and particles present.
Nitrogen dioxide absorbs light over the entire visible spectrum. This
absorption is strongest at the shorter wavelengths—violet, blue, and green
(Figure 2-2) reducing the brightness and contrast of distant objects and
causing the horizon sky and white objects to appear pale yellow to reddish-
brown. A token amount of light is attenuated by the molecular scattering
effect of nitrogen dioxide.
Figure 2-3 shows the calculated percent of visible light transmitted
as a function of wavelength for different atmospheric nitrogen dioxide con-
centrations and viewing distances in an aerosol-free atmosphere. Each
o
curve is based on the concentration [mg/m (ppm)]-distance (km) product;
3 3
thus, 9.02 mg/m (4.8 ppm)-km could be 1.88 mg/m (1 ppm) at 4.8 km, or
o
9.02 mg/m (4.8 ppm) at 1 km, or any combination of concentration and dis-
2
tance whose product is 9.02 mg/m (4.8 ppm). A more normal picture of
conditions actually existing in the atmosphere, with both nitrogen dioxide
and aerosols present, is shown in Figure 2-4. These data are given in
72
terms of calculated attenuation coefficients. Here, the effect of visible
light absorption is accompanied by transmitted light attenuation because
of light scattering by the aerosol. The photochemical system may involve
NO , hydrocarbons, sulfur dioxide, and other molecules in the formation of
X.
visibility-reducing aerosols.
-------
2-20
100
6*5
z
o
5
\r>
Z
1
3^00 4.000 4,500
VIOLET BLUE
5000 5,500 6000 6,500 7,000
GREEN YELLOW ORANGE RED
WAVELENGTH. A
FIGURE 2-3. Transmittance of visible light at different N0r concentrations
and viewing distances. Data from State of California.2
-------
2-21
u
r<
e
U-
LU
LU
O
O
Z
UJ
<
1.0
0.8
0.6
0.5
0.4
O 0.3
0.2
0.1
0.08
0.06
as
I
I
as = Aerosol Scattering Coefficient
CT\ = Extinction Coefficient for Given Wavelength
c = Concentration of NC>2
I
4000 4500 5000 5500 6000 6500
Violet Blue Green Yellow Orange Red
WAVELENGTH, A
7000
FIGURE 2-4 Aerosol attenuation in spectral luminance of horizon sky and distant objects at different
NO2 and aerosol concentrations. Data from State of California.72
-------
2-22
The additional presence of particulate matter masks the coloration
effect of nitrogen dioxide, but causes markedly reduced visibility, con-
trast, and brightness of distant objects. Particulate matter and aerosols
in the atmosphere are primary contaminants originating from urban sources
(i.e., industrial combustion and vehicular transportation), natural sources
(i.e., the sea, soil, and fog), and through photochemical reactions.
Light scattering associated with the presence of aerosols is thought to be
the primary cause of visibility reduction in photochemical smog and absorp-
84
tion of light by nitrogen dioxide a minor contributor.
SUMMARY
Among the various oxides of nitrogen present in the sunlight-irradiated,
polluted atmospheres, nitric oxide and nitrogen dioxide, designated by the
composite formula NO , play the most important role in chemical and photo-
X
chemical changes. A major source of these oxides is the combustion of
fossil fuels, which releases a predominance of nitric oxide. The rapid
cooling of the gases in the combustion chamber prevents the return of nitric
oxide to molecular nitrogen and oxygen to the extent required to establish
equilibrium at ambient temperatures. A fraction of the nitric oxide is
converted to nitrogen dioxide by reaction with oxygen during the exhaust
dilution process; however, the major pathway leading to formation of nitrogen
dioxide from nitric oxide is the photochemical interaction between NO ,
hydrocarbons, and various other compounds and intermediate free radicals
generated in the sunlight-irradiated polluted atmosphere.
-------
2-23
The extent to which nitrogen dioxide reduces visibility and colors the
horizon sky depends on the concentration of the pollutant, the viewing dis-
tance, and the accompanying aerosol concentration. The presence of photo-
chemical aerosol or other particulate matter suppresses the coloration
effect of NO and increases the visibility-reduction effect.
-------
CHAPTER 3
SOURCES AND CONTROL OF ATMOSPHERIC NITROGEN OXIDES
Manmade sources account for only a small fraction of the total global
nitrogen oxide emissions (see Table 3-1); however, in localized urban atmo-
spheres, these sources can be significant.
In this chapter both natural and manmade sources of nitrogen oxide are
discussed, and the principles governing nitrogen oxide formation and control
of manmade sources are reviewed.
NATURAL SOURCES OF NITROGEN OXIDES
The most abundant nitrogen oxide in the atmosphere, nitrous oxide (N_0),
has an estimated annual global emission of 54 x 10 kg. It is not classified
as a significant atmospheric pollutant since ambient levels are assumed to
be harmless and since it does not appear to interact with the nitrogen di-
oxide (NO ) photolytic cycle.
Bacterial action on nitrogen compounds under anaerobic conditions is
thought to be the major natural source of nitric oxide. The nitric oxide
formed in this way enters the photolytic cycle and eventually is converted
to nitrogen dioxide and nitrate aerosols. The estimated annual global
nitric oxide emission from biologic sources is 45 x 10 kg. Uncontrolled
9
combustion, such as forest fires, contributes an estimated 0.2-1.1 x 10 kg
of nitrogen dioxide annually to nitrogen oxide emissions in the United
80,560
States. This represents 1 to 5% of the total nationwide emissions
from combustion sources. Atmospheric nitric oxide formed by lightning is
442
thought to be negligiblec
-------
3-2
TABLE 3-1
Estimated Annual Global
Emissions of Nitrogen Oxides—
Estimated emission,
Compound Source 10 kg/yr
NO Manmade 4.8—
x
Nitric oxide Biological action 45
Nitrous oxide Biological action 54
a
Trom Robinson and Robbins.
441
"Expressed as nitrogen dioxide (NO^)
-------
3-3
Ammonia (NH ) plays a significant role in the atmospheric nitrogen
3 442
cycle and may be a precursor to formation of nitrate aerosols. The
principal natural source of ammonia emissions in the atmosphere is thought
442
to be the bacterial breakdown of amino acids in organic material.
MANMADE SOURCES OF NO
1 " " " " " ' "X
Combustion provides the major manmade source of NO emissions. Minor
contributions are made by industrial processes and agricultural operations,
which produce not only NO but also nitrates and ammonia, a potential source
x
of nitrate aerosols.
An inventory of estimated nationwide NO emissions for 1970 is pre-
x
sented in Table 3-2. Although useful in summarizing available source data,
these inventories can be misleading since the relative importance of a
particular source depends on many factors, including land use, climate,
and topography.
Manmade sources of NO emissions fall into two major categories — mobile
X
Q
and stationary. In 1970, an estimated 106 x 10 kg/yr of NO , or about 51%
X
of the total NO emissions, were emitted by mobile sources. Combustion in
x
stationary sources, including waste disposal, produced approximately 95 x
Q
10 kg/yr of NO , or 46% of the total. Industrial processes and agri-
x
Q
cultural operations accounted for the remaining 5.6 x 10 kg of NO .
Mobile Combustion Sources
Motor vehicles are the major mobile source of NO emissions, accounting
X
for approximately 40% of all manmade NO emissions in the United States.
X
The importance of this source varies greatly with location. For example,
motor vehicles contribute an estimated 90% of the annual NO emissions in
x
-------
3-4
TABLE 3-2
o
Estimated Annual NO Emissions in the United States in 197CP
Source
Mobile
Motor vehicles
Gasoline
Diesel
Aircraft
Railroads
Marine use
Nonhighway use
Stationary
Electric utilities
Industrial combustion
Commercial
Residential
Solid waste disposal
Agricultural burning
Industrial process loss
Miscellaneous
Estimated emission,
108 kg/yr^-
106.1
82.5
70.7
11.8
3.3
1.3
1.5
17.5
90.9
42.7
41.1
2.0
5.1
3.6
2.5
1.8
1.3
Total, (%)
51.4
40.0
34.3
5.7
1.6
0.6
0.7
8.5
44.1
20.7
19.9
1.0
2.5
1.7
1.2
0.9
0.6
Total
206.2
a
""From Cavender et al.
80
—Expressed as nitrogen dioxide.
-------
3-5
71
Sacramento, California; an estimated 56% in San Francisco; and an estimated
406
8% in Northwest Indiana.
Q
In 1970, aircraft contributed an estimated 3.3 x 10 kg of NO annually,
X
about 1.5% of the total manmade NO emissions. Although aircraft are not a
X
major contributor nationally, they are the major NO source in the vicinity
171 x
of many airports; furthermore, at high altitudes they could become a
269
major source of NO in the stratosphere.
x
The relative importance of different stationary and mobile NO sources
x
varies greatly from one location to another. Therefore, nationwide emission
inventories have limited value in assessing local problems. The importance
of various NO sources to a particular region should be defined by making a
X
local source inventory in order to evaluate effectively the control strategies
for that region.
NO emissions from most mobile sources in the United States have shown
x
a continuing upward trend from 1940 - 1970 (Table 3-3). During this period,
NO emissions from motor vehicles increased nearly 300%—from an estimated
X
8 8
27 x 10 kg/yr to 83 x 10 kg/yr. This increase is attributed to the addi-
tional numbers of motor vehicles in operation and their increased use.
Nationwide data (Table 3-3) do not necessarily reflect local trends.
For example, in Los Angeles County, NO emissions from motor vehicles in-
x 324
creased more than sixfold between 1940 and 1970. Standards for NO
X
emissions from motor vehicles, first adopted in 1971, have been updated
several times since then. By 1980, if implementation of existing standards
proceeds on schedule, NO emissions from motor vehicles should be reduced
X R 386
to 30 or 40% of the 1970 figures, or approximately 25 to 33 x 10 kg/yr.
-------
3-6
TABLE 3-3
Nationwide NO Emission Trends, 1940 - 197CF
x
(108 kg/yr)^
Source
Mobile
Motor vehicles
Aircraft
Railroads
Marine use
Nonhighway use
Stationary
Electric utilities
Industrial combustion
Commercial
Residential
Solid waste disposal
Agricultural burning
Industrial process losses
Miscellaneous
1940
29.2
26.6
neg.£
0.1
0.9
1.6
32.1
5.4
17.6
0.5
8.6
1.2
1.8
0.3
6.8
1950
47.2
40.8
0.1
1.9
1.1
3.3
39.3
11.1
18.4
1.0
8.8
1.7
2.0
0.5
3.6
1960
72.5
66.0
0.1
1.4
0.7
4.3
46.8
20.8
16.2
1.8
8.0
2.2
2.4
0.7
2.3
1970
106.1
82.5
3.3
1.3
1.5
17.5
90.9
42.7
41.1
2.0
5.1
3.6
2.5
1.8
1.3
Total
71.4
94.3
126.9
206.2
80
—From Cavender et al.
—Expressed as nitrogen dioxide.
Negligible: < 0.1 x 10 kg/yr.
-------
3-7
Stationary Combustion Sources
Electric utilities are the most significant stationary source of
atmospheric NO emissions in the United States, accounting for approximately
X
21% of manmade NO emissions. The amount of NO emitted by electricity
generating plants varies with location and season. Electric utilities
account for an estimated 9.5% of the annual NO emissions in Los Angeles
324 x 406
County and an estimated 26% in Northwest Indiana. Seasonal emission
71
variations of up to 60% have been reported, reflecting fluctuations in
the demand for electricity to meet changing heating and cooling requirements.
Industrial combustion provides an estimated 20% of annual U.S. NOX emissions.
Oil and gas pipelines, petroleum processing, and metallurgical refining are
the principal sources of NO emissions in this category.
X
NO emissions from stationary sources continued their upward trend in
X
the United States from 1940 - 1970 (Table 3-3). During this period, NO
X
emissions from electric utilities increased nearly 800%—from an estimated
8 8
5.4 x 10 kg/yr to 43 x 10 kg/yr. This increase was attributed to greater
electric power consumption. Again, nationwide trends do not necessarily
reflect local trends. For example, in Los Angeles County, NO emissions
X
from electric utilities decreased approximately 20% from 1960 to 1970 while
consumption of electric power increased approximately 150%. This decrease
resulted from the implementation of NO emission standards for stationary
X
sources and a changeover to natural gas from fuel oil by the utility
companies. During 1970 to 1980, U.S. consumption of electric power is
expected to double and the availability of fuels, such as natural gas and
low-nitrogen-content oil, to decrease. Since these fuels are used to re-
duce NO emissions, implementation of existing standards will become more
X
-------
3-8
difficult. Because of these factors, a slowdown in the upward nationwide
trend of NO emissions is unlikely and local downward trends, such as in
x
Los Angeles County, will be difficult to maintain. The upward trend in
NO emissions from industrial sources also is expected to continue during
x
1970 to 1980 due to increased industrial output and fuel problems similar
to those affecting the electric utilities.
Noncombustion Sources
Industrial process loss refers to nitrogen oxide emissions from noncom-
bustion industrial sources. Although nationally insignificant, these
emissions can be important near metal processing plants using nitric acid
and in the vicinity of facilities manufacturing chemical products such as
nitric acid, explosives, and nitrate fertilizers. In 1970, an estimated
80
Q
1 x 10 kg/yr of NO were emitted by nitric acid manufacturing facilities,
X
making them the most significant noncombustion NO emission source. Of
X
secondary importance is the explosives industry which, in 1970, emitted
Q
an estimated 0.25 x 10 kg/yr of NO during the manufacture of trinitro-
80 X
toluene (TNT) and nitrocellulose.
As stated above, ammonia may play a role in the formation of nitrate
aerosols in the atmosphere. Most ammonia produced from noncombustion sources
is emitted during the manufacture of ammonia- and nitrogen-base fertilizers.
On a local level, agricultural operations are another noncombustion source
of nitrogen oxides; i.e., the nitrates in dust from feed lots is a sig-
621
nificant local source of airborne nitrates.
Limitations of Nitrogen Oxide Emission Inventories
The nitrogen oxide emission data discussed in the previous sections were
obtained from the published emission inventories referenced above. These
-------
3-9
inventories are useful summaries of available source data, but have the
following limitations which should be considered during data interpre-
tation:
• Since most emission inventories cover large geographic areas, they
do not accurately reflect the distribution of sources on a local
level.
• These inventories usually span an extended period. Therefore, the
time-dependent nature of nitrogen oxide emissions from many sources
is not reflected, e.g., the intermittent nature of the emissions
from batch processes such as solid waste disposal, the diurnal
variations of the emissions from commuter automobile traffic,
and the seasonal variations of the emissions from stationary sources
due to varying heating and cooling demands. During certain periods,
the importance of these sources may be greater or less than indicated
by a time-averaged emission inventory.
• Many emission inventories are estimates based on tabulated emission
574
factors and on a compilation of the number and size of various
201
source types. Since published emission factors may not be
accurate for all cases, the inventories using these estimates may
be in error.
• Emission inventories generally do not discriminate between nitric
oxide and nitrogen dioxide, but report a combined NO emission.
X
Such discrimination is desirable because each oxide interacts dif-
ferently with the atmospheric photolytic cycle and because the
ratio of nitric oxide to nitrogen dioxide can influence the selec-
tion of techniques for reducing the concentration of nitrogen
oxides in the effluent gases.
-------
3-10
PRINCIPLES OF NITROGEN OXIDE FORMATION AND CONTROL
Formation in Combustion Sources
Nitric oxide is the major oxide of nitrogen produced in the combustion
process. Under certain conditions during combustion, significant amounts
462
of nitrogen dioxide may also be produced; but, for the most part, nitrogen
dioxide in the effluent gases results from oxidation of nitric oxide after
completion of combustion. The two principal sources of nitric oxide in the
combustion of conventional fuels are oxidation of atmospheric (molecular)
nitrogen and oxidation of nitrogen-containing compounds in the fuel (fuel
nitrogen). In most combustion systems, the first process is the dominant
source of nitric oxide. However, in combustion systems such as electric
utility boilers which use distillate or crude oil, fuel nitrogen can be a
significant source of nitric oxide. The nitrogen content of fossil fuels
can vary considerably, depending on geographic origin of the fuel and on
fuel processing techniques. Typical nitrogen contents of distillate oils,
shale oil, and coal are given in Table 3-4.
The mechanism of nitric oxide formation from atmospheric nitrogen has
been studied extensively. It is generally accepted that in combustion of
fuel-air mixtures the principal reactions governing formation of nitric
oxide from molecular nitrogen are:
0 + N2 t NO + N (1)
N + 0 t NO + 0 (2)
206,619
Experimental studies have shown that the nitric oxide formation is
much slower than the combustion process so that most of the nitric oxide is
formed after completion of combustion. The nitric oxide formation process
-------
3-11
TABLE 3-4
Typical Nitrogen Content of Fossil Fuels3-
Fuel Average Nitrogen, wt% Range, wt%
Distillate oil
Crude 0.49 0.01 - 1.00
Asphaltenes 2.35 2.20-2.50
Heavy distillate 1.40 0.60-2.20
Light distillate 0.07 0 0.60
Crude shale oil 2.6 2.0-3.2
Coal 1.7 0.6 - 2.5
a 90 7R^ 116
-From Ball and Rail, Kirner, ° and Dinneen et al.
-------
3-12
can therefore be decoupled from the combustion process. A simplified
537
expression for the initial nitric oxide formation rate may be written
dX /dt = 1.5 x 1017T~^ X^ X exp(-68,000/T) ppm NO/sec (3)
NO ° N
where X and X^ are the equilibrium oxygen and nitrogen mole fractions at
°2 2
the temperature, T (°K), of the postcombustion gases. The strong dependence
of the nitric oxide formation rate on temperature is evident from Equation
(3) . Elevated temperatures and high oxygen concentrations in the postcom-
bustion gases result in relatively high nitric oxide formation rates. In
certain situations, these rates exceed the rate predicted by Equation (3).
Near the combustion zone, the concentration of oxygen atoms generally exceeds
the equilibrium values in the postcombustion gases, resulting in an acceler-
ation of the nitric oxide formation rate via Reaction (1). In this situation,
observed nitric oxide formation rates exceed that given by Equation (3). Cal-
culation of nitric oxide formation rates must consider the coupling of the
55
nitric oxide formation reactions with the combustion process.
In fuel-rich flames, nitric oxide formation rates significantly in excess
141,258
of those predicted by Equation (3) have been observed. While this may
be due partially to excess oxygen atom concentration near the flame, Reactions
(1) and (2) do not suffice as an explanation. Other reaction paths forming
nitric oxide must therefore be postulated. For example, these paths may in-
volve reactions between fuel fragments and molecular nitrogen to form nitrogen
atoms, which in turn form nitric oxide [Reaction (2)]. Nitric oxide concen-
trations in the exhaust gases of most combustion devices are considerably
higher than would be predicted from chemical equilibrium at exhaust gas temper-
atures, indicating that nitric oxide formed in the combustion process is re-
moved only very slowly as the combustion gases are cooled. In most combustion
devices, the perturbations of this nitric oxide formation process discussed
-------
3-13
above are insignificant, and the rate nitric oxide forms from atmospheric
nitrogen may be estimated using Equation (3).
Recent experiments involving nitric oxide emitted from stationary
332,479
combustion devices revealed that organic nitrogen compounds in
fossil fuels (fuel nitrogen) were an important nitric oxide source.
Although the combustion of many organic nitrogen compounds has been studied
extensively, scant information on the mechanism of nitric oxide formation
from these compounds has been obtained. Existing data indicate
that oxidation of many organic compounds is rapid occuring on a time
scale comparable to that of the combustion process. Conversion of fuel
nitrogen to nitric oxide can occur at temperatures much lower than those
required to oxidize atmospheric nitrogen to nitric oxide. In lean mixtures,
measured nitric oxide concentrations in the postcombustion gases indicate
a nearly complete conversion of the organic nitrogen compound to nitric
oxide. Although the mechanism by which fuel nitrogen converts to nitric
oxide is uncertain, several empirical correlations are available for
142,148
estimating nitric oxide yields.
Control Methods for Combustion Sources
There are two basic approaches to control NO emissions from mobile
X
and stationary combustion sources. One is to modify the combustion process
either through changes in operating conditions, the fuel, or the design of
the device to decrease nitric oxide formation. A second approach is chemical
or physical removal of nitrogen oxides from the effluent gas.
Mobile Sources. NO emissions from mobile sources result almost ex-
~ - - ~ -~ -- - - - - x
clusively from oxidation of atmospheric nitrogen during combustion. Control
-------
3-14
methods either prevent the formation of nitric oxide in the combustion
chamber or remove it from the effluent gas. Since elevated temperatures
in the presence of oxygen favor nitric oxide formation [Equation (3)],
modifications to reduce this formation rate generally involve reductions
of peak temperatures and oxygen concentrations in the combustion chamber.
NO emissions from conventional spark ignition internal combustion
X
engines have been effectively decreased by spark retardation, lean combus-
tion, low-compression ratio, water injection, and exhaust gas recirculation.
However, these modifications may reduce performance and increase carbon
monoxide and hydrocarbon emissions. Fuels such as alcohol, hydrogen, or
fuel blends may also reduce NO emissions from conventional engines. They
X
not only can provide lower combustion temperatures but also may permit
operation at lean mixtures, both factors contributing to decreased nitric
oxide formation in the engine. Stratified-charge engines, diesel engines,
gas turbines, and Wankel rotary engines are alternative power sources
capable of reducing NO emissions from mobile sources.
X
Several catalytic methods are used to remove nitric oxide from the
effluent gases from mobile sources. A major disadvantage of these methods
is deterioration of the catalyst material. NO reductions obtained by
X
these control techniques may be limited by considerations of fuel economy.
Comprehensive discussions and evaluations of the various techniques
for controlling NO emissions from mobile sources are presented in a 1973
X
report by the Committee on Motor Vehicle Emissions, National Academy of
386
Sciences, and a 1970 publication of the National Air Pollution Control
558
Administration.
-------
3-15
Stationary Sources. Oxidation of atmospheric nitrogen accounts for
most of the NO emitted from stationary sources. In those sources using
crude oil or coal, fuel nitrogen can also oxidize producing significant
amounts of nitric oxide. In selecting methods to reduce nitric oxide
formation in the combustion chamber, the nitric oxide source should be
considered since mechanisms leading to oxidation of atmospheric nitrogen
and fuel nitrogen differ.
Operating conditions in stationary sources have been modified to
reduce NO emissions through the introduction of low-excess-air firing,
X
staged combustion, over-fire air, flue gas recirculation, and water injection.
Variations in burner design, i.e., burner repositioning and tangential,
rather than horizontal, firing have also reduced NO emissions. Premixing
X
of fuel and air combined with off-stoichiometric combustion has reduced
NO emissions from gas-fired combustion devices. All of these techniques
X
cause peak temperature reduction in the combustion chamber, thereby de-
creasing the rate at which nitric oxide is formed from atmospheric nitrogen.
Temperature reduction does not significantly affect the rate
fuel nitrogen oxidizes to form nitric oxide; however, the burning of a
mixture which is fuel rich in the primary combustion, can result in
reduced nitric oxide emissions. The two control methods with the potential
to reduce NO emissions from fuel nitrogen are therefore low—excess-air
x
firing and staged combustion.
A change in fuel type may also result in reduced NO emissions. For
x
example, substitution of natural gas for fuel oil, reduction of the organic
nitrogen content of coal or oil, fuel processing, vaporization of liquid
fuels and gasification of coal, and catalytic processing of fuels for
-------
3-16
selective removal of organic nitrogen compounds are all potential NO
X
control techniques.
Combustion flue gas treatment processes (aqueous scrubbing, selective
catalytic reduction or decomposition of NO , various adsorption processes,
x 33,387,559
and afterburning) also reduce NO emissions from stationary sources.
X
Control Methods for Noncombustion Sources
Control of nitrogen oxide emissions from industrial process losses
generally involves removal of the emissions from the effluent gas. Since
the concentration of these oxides and the ratio of nitrogen dioxide to
nitric oxide can differ markedly from composition of flue gas from combus-
tion sources, the treatment methods for each effluent are also different.
The reader is referred to a 1970 publication of the National Air Pollution
Control Administration entitled "Control Techniques for Nitrogen Oxide
Emissions from Stationary Sources" which contains a comprehensive discussion
559
of these various treatments.
SUMMARY AND RECOMMENDATIONS
Annual global emissions of nitrogen oxides from manmade sources are
substantially less than those from natural sources; however, manmade sources
play a very significant role in atmospheric pollution in localized areas.
The principal manmade source of nitrogen oxides is combustion. Although
industrial process losses contribute only a small amount to the total
manmade nitrogen oxide emissions, they can be important locally.
Existing methods for obtaining emission inventories have limited
application in local evaluations of NO sources since the methods generally
X
use data from large geographic areas covering extended periods. Further,
-------
3-17
inaccuracies result from differences between actual NO emissions from
x
specific sources and tabulated emission factors. Source inventories should
discriminate between nitric oxide and nitrogen dioxide emissions since each
interacts with the atmospheric photolytic cycle in different ways.
Nitrogen oxides are produced in combustion of conventional fuels
through oxidation of both atmospheric (molecular) nitrogen and nitrogen
compounds in the fuel (fuel nitrogen) via different mechanisms. Nitric
oxide is formed from atmospheric nitrogen in the high-temperature regions
in the combustion chamber; fuel nitrogen oxidizes at lower temperatures and
can be a major source of nitrogen oxide emissions in some combustion devices.
Two basic approaches to control NO emissions from combustion sources
are modification of the combustion process and exhaust gas treatment. Present
understanding of the principles of nitric oxide formation in combustion is
sufficient for development of techniques to reduce these emissions. However,
implementation of these techniques may be hampered by excessive cost, various
operational problems, and loss in combustion efficiency.
Treatment of effluent gas is the principal technique for reducing ni-
trogen oxide emissions from noncombustion sources.
-------
CHAPTER 4
ANALYTICAL METHODOLOGY FOR THE DETERMINATION
OF NITROGEN OXIDES IN AIR
The 1967 Amendments to the Clean Air Act required that: "From time
to time, but as soon as practicable, [the Administrator of EPA] shall
develop and issue to the States such criteria of air quality, ... [that ]
reflect the latest scientific knowledge useful in indicating the kind and
552
extent of all identifiable effects on health...." This statement was
responsible for the publication of several reviews of methodologies for
134,561,563
measuring the oxides of nitrogen. Unfortunately, since most
epidemiological studies do not provide enough information to compare
older and newer methods, assessment of one method over another is dif-
ficult, if not impossible.
Chemical methods for measuring pollutants have been used for almost
100 years. One example is the Griess-Ilsovay Method in which nitrous
acid (HNO ) is reacted with an aromatic amine to form diazonium salts.
These salts are then coupled with an organic substance to produce an azo-
dye.
These chemical methods can be divided into two general categories:
sampling and analysis. Since these categories are interchangeable in many
methods described in the literature, countless variations in effective
methods are possible. Chemical methods can be further classified as
direct and indirect. In direct chemical methods a single solution is
used to sample the gas and measure the produced color. In indirect
-------
4-2
chemical methods, a separate sampling solution is required and the color
is developed later by addition of certain chemicals.
Physical methods have been developed recently, especially as applied
to ambient air measurements. Both chemical and physical methods are dis-
cussed below.
MANUAL METHODS FOR CHEMICAL ANALYSIS OF NITROGEN DIOXIDE
Direct Methods
Absorbing Solutions. An early (1939) chemical method applied to
186
atmospheric levels required that the contaminant be sampled in an impinger
containing a reagent mixture composed of 0.17% sulfanilic acid (I^NCgH.SO H)
and 0.033% 1-naphthylamine (C H NH ) in 14% acetic acid (CH COOH), This
resembles the method proposed originally by Ilosvay (quoted by Treadwell and
542
Hall in 1935). To avoid the handling of the highly carcinogenic and
67
volatile naphthylamines, other coupling substances were subsequently
57
substituted. For instance, in 1939 Bratton and Marshall proposed the
use of 1-(N-1-naphthyl) ethylenediamine dihydrochloride (C H NHCH CH NH '2HC1)
which is nonvolatile and readily soluble in water. With minor modifications
in the concentrations of the components, this mixture is the same as the
451
Griess-Saltzman reagent previously used extensively in automatic colori-
metric analyzers and designated as a Tentative Method by the Intersociety
251
Committee on Methods for Ambient Air Sampling and Analysis, and a
18
Standard Method by the American Society for Testing and Materials.
A later modification of the same reagent proved more economical and faster
452
in response. To eliminate the corrosive acetic acid, several formula-
296,297,375
tions appeared with less corrosive organic acids or with isopropyl
-------
4-3
611
alcohol [ (CH ) CHOH]. However, the presence of acetic acid seems to
•J ^
inhibit secondary reactions, leading to higher sensitivity.
Studies initiated in 1965 by the Air and Industrial Hygiene Labora-
tories, California Department of Public Health, showed that possibilities
for formulating similar reagents are seemingly endless, provided the fun-
damental components are present: diazotizer, coupler, buffer, and a
297,298,299,374,375
surfactant. The reagent inducing the shortest
296
response time in a microcell is formulated as follows: 0.12% 2-amino-p_-
benzenedisulfonic acid [C,H^NH^(SO H) J or sulfanilamide (H NC H,SO NH );
63232 26422
0.18% sulfuric acid (H SO ) 0.025% l~(N-l-naphthyl)-4-(acetyl) ethylene-
diamine-p-toluene sulfonate [c H NHCH CH NH (OCCH )0 SC H CH ]. This
10 7 222 33643
formulation reduces the response time 10 to 20 times faster than earlier
formulations. A similar speed could be obtained with 2,5-dichlorosulfanilic
acid (C,,H0C10NH0SOQH) and l-amino-7-naphthalenesulfonic acid (C,nH.NH S00H).
0 / i. t. j 1_(J b 2 3
During sampling, some of the incoming nitrogen dioxide (NO ) is converted
to nitric oxide (NO). Nitric oxide in the mixture affects the overall
recovery, otherwise it is not an interferent. Aging effects limited earlier
attempts to sample dilute ambient levels of nitrogen dioxide for an ex-
tended time and to store the exposed reagent. Some interfering gases,
such as sulfur dioxide (SCO, produce noticeable bleaching effects during
storage, which make the direct absorbing azo dye reagents useless. To
451
prevent this, the addition of acetone (CH COCH ) has been suggested.
62,519 J 3
Controversial effects of this addition have not been reevaluated.
Several simple methods are available to measure higher concentrations
392
of nitrogen oxides. With the syringe technique, a gas sample is drawn
into a syringe containing oxygen and Griess-Saltzman reagent. Shaking
-------
4-4
promotes oxidation of nitric oxide to nitrogen dioxide, absorption, and
254
color development. The phenoldisulfonic acid [C H OH(SO E) ] method is
63 32
also suitable for these higher concentrations. Possibly most of the other
methods described for nitrate on page 19 of this chapter can also be used
after adequate allquoting of the absorbing solutions.
Stoichiometric Factor. In the determination of gaseous nitrogen
dioxide with azo-dye reagents, the nitrogen in nitrogen dioxide reacts
to produce a colored specimen. This conversion is not quantitative. The
factor introduced to express the conversion efficiency of the reaction is
called the Stoichiometric factor.
Under strict sets of chemical and physical conditions, the Stoichio-
metric factor for each method considered remains constant and can be estab-
lished by a previous calibration with known concentrations of nitrogen
dioxide.
Methods using azo-dye reagents can also be calibrated with secondary
calibration standards, such as with nitrite solutions, once the Stoichio-
metric factor is known. Secondary calibration standards are preferred in
routine work over the more cumbersome gas calibration techniques.
The stoichiometry depends on the geometry of the sampling device, the
chemical characteristics of the components in the reagent mixture, the
presence of impurities, the temperature during sampling, light exposure
298,451,452
during or after sampling, and aging.
Authors who suggested that the Stoichiometric factor differs greatly
62,
from the established value of 0.72 either used doubtful diluting devices,
205,208,395,519
differed markedly in their experimental designs to prove
-------
4-5
247 155,392
new theories, or sampled higher concentrations. A carefully
461
designed experiment using preconditioned permeation tubes produced a
factor of 0.764, which is close to values found working under standardized
451,452,455,478
procedures at atmospheric concentrations of nitrogen dioxide.
The Air and Industrial Hygiene Laboratory, of the California Department of
3
Health, evaluated the factor in 1969 by generating a 28 yg/m (0.15 ppra)
455
nitrogen dioxide stream using the asbestos plug dilutor. The nitrogen
294
dioxide was then absorbed into alkaline-potassium permanganate (KMnO )
4
and the absolute amount of nitrate generated was determined by UV-spectro-
29,34,81
photometry after separation of the manganese dioxide (MnO ). The
extensive study produced the factor 0.735 which nearly coincided with the
values reported earlier. Experimentally weak designs lead to equivocal
291
results, such as the variable factor which was analyzed and explained
478
later.
Methodology for Establishing Nitrogen Dioxide Concentrations. The
principal function of a chemical method is to determine concentrations.
To establish values with precision and accuracy, the method must with-
stand certain tests. It must be sufficiently rugged to give valid results
under differing ambient conditions. Most azo-dye reagents stand up well
under tests of temperature, variations in chemical concentrations, and
long-term storage, but they are sensitive to strong lights, especially
at low nitrogen dioxide concentrations. For calibration purposes, light
shielding can be arranged so that azo-dye formulations can effectively
standardize pure gas streams containing nitrogen dioxide generated by
252,405,444,455
diluting techniques. This standardization process is
251,451
facilitated with aqueous solutions of nitrite. Thus, a dilute gas
-------
4-6
stream can be fed into a manifold from which different sampling devices
and instruments can simultaneously draw samples of gas. One port of the
manifold is used for analyzing the gas with a reference method for nitric
250,251
oxide or nitrogen dioxide.
Indirect Methods
Absorbing Solutions. The use of alkaline absorbing solutions has been
259
described by Jacobs and Hochheiser:
To avoid the bleaching effect of sulfur dioxide also present in
atmospheric samples when using azo-dye reagents directly, the pollutant
gases are first sampled into 0.1 N^ sodium hydroxide (NaOH) solution con-
taining a surfactant such as butanol which improves the gas transfer
efficiency. Dispersion is facilitated by use of a fritted sparger.
Sulfite formed during the absorption of sulfur dioxide into the sodium
hydroxide solution is oxidized to sulfate by the addition of hydrogen
peroxide (HO) which does not affect the nitrite formed by reaction of
nitrogen dioxide with the alkaline solution. The nitrite is stable for
48 hr, which allows sufficient time for transporting the aqueous sample
to a centrally located laboratory. The nitrite in solution can be
analyzed by addition of azo-dye forming chemicals and acidifying the
solution with phosphoric acid (H PO,).
Stoichiometric Factor. The term "stoichiometric factor" used in
conjunction with alkaline absorbers has the same significance as in direct
single-solution azo-dye forming reagents. However, to avoid confusion,
the product of sampling efficiency and stoichiometric factor was used in
-------
4-7
some of the reported investigations and called empirical conversion
427,566
factor. This is clearly not a precise use of terms.
The sampling efficiency can be determined by sampling a pure stream
of a dilute mixture of a known nitrogen dioxide concentration or with a
train composed of several identical collecting devices. With the first
scheme, only an empirical conversion factor can be calculated by simple
azo-dye colorimetry. To establish percentage recovery of sampling, the
nitrate formed in solution must also be determined. With the second
alternative, carryover of nitrogen dioxide to the subsequent bubblers can
be established, and sampling efficiency can be calculated by assuming that
no secondary reactions (e.g., reduction to nitric oxide) has taken place
during collection in the first bubbler. Since losses through secondary
reactions are unlikely when alkaline absorbing solutions are used, a
close approximation can be expected.
The sampling efficiency obtained in controlled, clean laboratory
229
conditions may be greater than can be achieved in the field. Sampling
566
efficiency is also affected by such variables as: flowrate, porosity
364,365,566 364,365 294
of frits, liquid level, container material, incoming
164,412,485,486,527,569
pollutant concentration, and contaminants present
364,365
at sampling location. Because of these variables, the term
"alkaline absorber" will denote a specific combination of absorber and
absorbing solution.
The stoichiometric factor in alkaline absorbers is variable. One of
the first methods, which was developed in 1958, was originally designed
259 238
for intermittent 40 min sampling. In 1965 this was modified to a
batch-type 24-hr operation. This was adopted with further modifications
-------
4-8
by the National Air Sampling Network and in 1971 was promulgated as a
566
reference method in the Federal Register. Originally, the absorption
efficiency was found to be better than 90% and a stoichiometric factor
410
of 52% to 65% was assumed. During operation of the Network the factor
varied from 0.45 to 0.85. In 1966 the average for 11 locations was 0.62
364,365
± 0.07.
246
A detailed study by Huygen and Steerman revealed that the factor
varies with incoming nitrogen dioxide concentration, decreasing in the
presence of sulfur dioxide and increasing in the presence of nitric
229,300 427
oxide. In 1972, Purdue et^ al. postulated that the stoichiometric
factor may not deviate greatly from unity and that the product with the
sampling efficiency is equal to the so-called empirical conversion factor
566
used in the reference method described above. This postulation does
not stand up well when compared to other investigations using similar
87,246,385
reagents. Variations encountered during sampling with alkaline
solutions were affected by the presence of even minute amounts of hydrogen-
87,246,
donors which greatly increased the solubility of nitrogen dioxide.
256,318,385
Many air contaminants may be hydrogen-donors. This provides
one explanation for the variability of the stoichiometric factor with
364,365
location.
Data collected on continuous recording instruments for ambient levels
of nitrogen dioxide were compared with data collected with alkaline absorbers.
349,526
Results were quite different for field as compared to laboratory conditions.
(Kinosian 1971, personal communication.) This difference may result from
the delay in response to nitrogen dioxide variations found in continuous
analyzers which prevents the recording of the entire area integrated by
calculation. The calculation may be affected by a negative error of up to
-------
4-9
20%. The integration of all the errors resulting from different variables
which affect conversion factors and absorption efficiencies in unmodified
alkaline absorbers are unpredictable and deviate by a factor of 2 or greater.
To reduce the sampling error caused by a single alkaline absorber, a method
486
using two absorbers in series was adopted in 1970 for an epidemiological
study. Results from different methods using alkaline absorbers are com-
pared in Table 4-1.
Nitrite Reagents. After collection of the nitrogen dioxide in the
alkaline absorber, the solution is acidified and the nitrite formed during
collection is measured after reaction with an azo-dye forming mixture.
Methods to determine nitrites in aqueous solutions are countless. In the
594
classicial book Organic Analytical Reagents, written in 1948, Welcher
lists over 100 possibilities which, when added to those published in the
last 25 years, show that methods to determine nitrite are not restricted
to those few proposed in those papers (loc. cit.) for use with alkaline
absorbers. The amount of nitrite in the absorbing solution is established
by reference to a calibration curve prepared from nitrite standards.
Jacobs-Hochheiser Modifications. A method used to establish an air
quality standard should be scientifically and practically tested before
adoption. Unfortunately, many established methods have not been subjected
to such exhaustive tests. Pressures of time have resulted in the use of
methods which have not even been tested against all possible naturally
occurring variables. Because some methods satisfactorily produced a great
mass of information during an initial test phase, their optimal performance
has been often taken for granted.
-------
4-10
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259
One such case is the Jacobs-Hochheiser Method for collecting 40
min samples in a fritted glass collector using a 0.1 N. sodium hydroxide
solution with some butanol added to improve the gas-liquid transfer. The
sampling efficiency of the first bubbler was calculated by analyzing the
total nitrogen dioxide collected in two absorbers in series. The stoichio-
410
metric factor was taken from a previous similar work. Later, Hochheiser
238
and Ludmann modified the method to a 24-hr batch sampler. Confusion
arose because the findings of one method were applied to another. Although
there were only minor differences, equivalency was assumed without thorough
experimental confirmation.
During development of the Federal National Air Sampling Network (NASN),
238
the Hochheiser Method was adapted to the entirely different NASN gas
364,365
samplers. Absorbing efficiency (60-70%) was checked by four orifice
bubblers in series and stoichiometry was arbitrarily assumed to be 50%.
349
Meadows and Stalker omitted the butanol in the absorbing solution and
placed six orifice bubblers in series. They determined the sampling
efficiency to be poor and highly variable with flow rate. Fritted tips
were not used because they tend to clog and are difficult to clean.
Nonuniformity of pore sizes also increased the variability coefficient.
566
The method described in the Federal Register (glass frits and plastic
212,486
container) and the method used in the Chattanooga study (two bubblers
in series) are entirely different methods that have departed from Jacobs-
47,212,427
Hochheiser in their design and performance. The only similarity
retained is the final measurement of the nitrite content in the absorbing
solution.
-------
4-15
As a result of the Air Quality Act of 1967 (Public Law 88-206), and
a request by the City of Chattanooga, Tennessee, an air quality survey was
initiated on October 1, 1967, covering an interstate area extending from
Chattanooga to Rossville, Georgia, and over parts of Hamilton, Walker, and
219
Catoosa counties in Tennessee. This area was selected because its
manufacturing plants produce a large amount of NO . Data collected by the
X
continuous colorimetric Saltzman monitoring method from October 1967 to
November 1968 were used to establish a frequency distribution of NO con-
219 X
centration.
From November 1968 to April 1969, the National Air Pollution Control
Administration (NAPCA) and the U.. S. Army made an epidemiological study
485,486,563
covering the same geographical area. They adopted a modification
of the alkaline absorber method first used by the National Air Sampling
364,365,412,427
Network, and later used in the first Chattanooga study.
Since the method used in the first study was subject to question at extreme
47
concentrations, comparison of the modification with its prototype was not
229
practicable. Approximate comparisons can be attempted, however, by
combining statistical values of recovery information from two alkaline
47,485,486
bubblers in series using permeation tubes with the frequency
distribution of the atmospheric concentration obtained with monitoring
561
instruments during the first two studies and from April 1972 to December
100
1973 in the same area of Tennessee. Data resulting from this comparison
could provide a basis on which the collected concentration values for ni-
trogen dioxide can be saved, confirmed, and adjusted. The true values
will probably be not more than 20% lower than the concentration reported
in the original study.
-------
4-16
Although these corrections are not extreme, they suggest the need to
replace data obtained with the modified Jacobs-Hochheiser two-tube col-
lecting method.
MANUAL METHODS FOR CHEMICAL ANALYSIS OF NITRIC OXIDE
Direct Methods
There are only a few manual methods for the direct measurement of
nitric oxide in gaseous mixtures. Ferrous sulfate (FeSO ) can be used to
4 3 295,400,401
determine nitric oxide at concentrations >30 ppm (56.4 mg/m ).
112,113,525 30,31,112,113,413
Direct instrumental measurement in the UV and IR
can be made only at high concentrations or by using long optical paths such
390
as those used when monitoring from aircraft. Mass spectrometry and gas
chromatography are batch instrumental methods.
Indirect Methods
Indirect methods to determine nitric oxide concentrations involve
selective oxidation of nitric oxide to nitrogen dioxide. Separation of
coexisting nitrogen dioxide in the mixture before oxidation of the nitric
oxide is necessary if both gases are to be determined separately. If not
separated, the result would be an unresolvable mixture (NO ).
X
Selective Absorbers for Nitrogen Dioxide. The first attempt to
separate nitrogen dioxide at low levels involved the passing of effluent
373
gas through an azo-dye forming reagent contained in a fritted glass bubbler.
In this method, however, some of the nitrogen dioxide converts to nitric
oxide, thereby increasing the nitric oxide concentration.
-------
4-17
The conversion percentage of nitrogen dioxide to nitric oxide varies
with the chemicals used and their concentration in the particular azo-dye
247
reagent formulation. Generation of 7 to 16% nitric oxide can occur with
the Griess-Saltzman reagent.
Widely used absorbers for nitrogen dioxide in the late 1960's were
ascarite (asbestos -supported sodium hydroxide) and soda lime. Although this
worked well with dilute air, or nitrogen streams of either nitric oxide or
nitrogen dioxide alone, their absorption efficiency varied with mixtures of
319
the two gases. During the absorbing process of nitrogen dioxide, some
nitric oxide is lost as with absorbers using dilute alkali.
Most recently, a tube containing granules impregnated with tri-
250,319
ethanolamine has been used for nitrogen dioxide absorption. In this
process, only 2 to 4% of the incoming nitrogen dioxide is converted to nitric
oxide.
Oxidizers for Converting Nitric Oxide to Nitrogen Dioxide. The oldest
oxidizers for the conversion of nitric oxide to nitrogen dioxide used either
539 270,437,601
manganese or chromium compounds at their highest valence state.
Nitric oxide can be oxidized in the gas phase by slowly introducing a small
454,564
excess over the stoichiometric amount of ozone (0 ) or chlorine
535 3 209
dioxide (CIO ) . Other oxidizers include manganese dioxide anc*
2 241
iodine pentoxide (^O,-) . A chromic oxide (CrO ) oxidizer, supported on
inert granules made by soaking firebrick in a chromic oxide solution and
drying in an oven [in absence of any other inorganic acid (H-PO ; H^SO ; etc.)]>
has been shown to be insensitive to small humidity changes and gives
practically quantitative (>99%) conversion of nitric oxide to nitrogen
250,316,317
dioxide between 20 and 80% RH.
-------
4-18
ANALYSIS OF NITRATE
Oxidizers for Total Oxidation of Nitrogen Oxides to Nitrate.
Methods which measure nitrate after conversion of higher than ambient
concentrations of nitrogen oxides, such as in source analysis, are both
cumbersome and time-consuming. They require large batch sampling and the
difficult procedure by which gas phase nitrogen oxides are quantitatively
transferred to an aqueous solution; however, the analysis of the formed
nitrate poses no particular problems.
To ensure complete conversion of nitrogen oxides to nitrate, the method
254
using phenoldisulfonic acid needs a long reaction time which permits slow
thermal oxidation of nitric oxide to nitrogen dioxide. A strong oxidizing
17,254
absorbing solution, such as hydrogen peroxide in dilute sulfuric acid,
aids in the absorption of nitrogen dioxide. However, dilute sulfuric acid
is not the ideal absorbent since nitrogen dioxide has a. lower solubility
492
in dilute acid than it does in distilled water. Nitrogen dioxide dis-
solves most easily in alkaline solutions containing any one of many hydrogen
87,246,256,318,385
donors, alkalies, and strong oxidizers such as hydrogen
293,302 283,294
peroxide, or alkalies and permanganate. By this latter
methodology, the nitric oxide in a gas mixture can be oxidized as well as
absorbed. A potassium carbonate-(K CO ) impregnated paper can be used for
166
dry absorption of nitrogen dioxide.
Analysis of Nitrate from Particulate Matter
Although most of the nitration methods described below were originally
designed to analyze nitrate in natural waters, they have also been used to
226
analyze nitrate in aqueous extracts of particulate matter and in solutions
283,294
obtained through absorption of streams of nitrogen oxides in air.
-------
4-19
255,263,327,443
The oldest nitration procedures use brucine (C H N 0 )
17,36,124,254,528 23 26 2 4
and phenoldisulfonic acid. Newer procedures extensively
used to analyze nitrate in atmospheric particulate matter extracts involve
the nitration of xylenols [(CH ) C H OH] and separation of the nitro-
3263 20,63,210,242,249,524,609
derivative by extraction or distillation. Nitration
597 306,493
of chromotropic acid [C H, (OH) (SO H) ] and coumarin (C H,02) analogs
have also been reported. The advantage of coumarin analogs over other
phenolic compounds is their easier nitration rate at lower concentrations
of sulfuric acid (Kothny, unpublished). Small amounts of nitrate can be
determined by the quenching of the flourescence after nitration of
26
fluorescein (C H 0 ). Finally, nitrate analysis can be accomplished
ZO 12 5 282,435
through reduction with Devarda alloy to ammonia (NH ), or reduction
86 3 367,521,606
of nitrate to nitrite with the help of zinc (Zn), cadmium (Cd),
380
or hydrazine (NH NH ) . Automation instituted by the NASN has improved
the hydrazine reduction process by curtailing the unwanted effects resulting
366
from its sensitivity to motion. The addition of antimony sulfate
[SbfSO.) ] eliminates the chloride interference found in most nitration
2 43
597
methods. The brucine procedure circumvents the effect of chlorides by
255
adding an excess of sodium chloride before nitration.
Nitrate analysis by ion-selective electrodes has several disadvan-
tages: potential drifts caused by agitation speed, necessity of frequent
restandardization, interferences caused by nonspecificity of the electrodes
which respond to other ions in the aqueous extracts, and nonstoichiometric
111,121,182,188
absorption of the gases in the collecting reagent. In
atmospheric analysis, the electrode has no advantage over direct UV
15
determination of either nitrite formed in an alkaline absorbent (with
81
the inherent weakness of all alkaline absorbers) or nitrate obtained after
-------
4-20
283
oxidation and absorption of nitrogen oxides in alkaline permanganate
294
and separation of the manganese with hydrogen peroxide. Microscopic
44
techniques also allow analysis of individual nitrate particles.
PRIMARY REFERENCES FOR NITROGEN DIOXIDE AND NITRIC OXIDE
211
In 1973 Hauser et al. defined the only purpose of a reference
method as the provision of the best measurement of a pollutant in an
environment. Questions of cost, field worthiness, and convenience should
be of secondary importance.
There are two classifications of reference methods—primary and
secondary. In primary reference methods, accurate data can be produced
through repetition of chemical or physical operations and calibration can
be done by weighing a standard substance. A secondary reference method
cannot give reliable data without a concurrent standardization or com-
parison with a primary reference method.
Since a primary reference method may only be used to calibrate other
methods or instruments by monitoring a single diluted pollutant gas, it
need not be absolutely free of interferences, as long as it gives highly
accurate information. Conversely, a secondary reference method used for
intermittent or continuous sampling of real gas mixtures should be free
of interferences in order to provide specific information with reasonable
accuracy and precision over an extended time.
The advent of permeation tubes has elevated many secondary methods
to the primary reference category.
-------
4-21
Reference Methods
The Griess-Saltzman Method for determining nitrogen dioxide is a
18,
primary method which has been thoroughly tested and used for many decades.
38,251 515
In 1973 Stevens ejt al. criticized the variability of its
response to mixtures containing simultaneously-introduced ozone, as com-
pared to results obtained from sequential introduction of ozone used in
the initial testing of the method. Despite criticisms of the instability
291
of its conversion factor, the manual Griess-Saltzman Method is one of
the most specific and reproducible primary methods available. If it is
251,451
accurately followed the factor is constant and interferences do
not affect its use in standardizing calibrating gases. At present, this
is the only method collaboratively tested and endorsed by the American
152
Society for Testing and Materials.
Standard Gas Sources
Primary standard gas sources were developed to obviate the need for
a primary reference method to test approximate or unknown gas mixtures or
193
the purity of standard gases in cylinders. The first attempt to develop
such a source involved the careful dilution of pure or concentrated mixtures
62,
of nitrogen dioxide and dinitrogen tetroxide (NO ) by inert gases or air.
205,208,252,291,395,444,451,455,519 2 4
A diluting device with turning stopcocks with a precisely known volume
62,519
of the bore enabled calculation of precise mixing ratios. With this
device, several European scientists discovered independently that the
ratio of gaseous nitrogen dioxide to azo-dye formed after absorption was
unity. Shortly after, a departure from unity to lower values was reported
205,395
for higher levels of nitrogen dioxide.
-------
4-22
Through volatilization of liquid nitrogen dioxide into a high velocity
534
air stream, Thomas and Amtower generated known concentrations of nitrogen
dioxide by adjusting the weight loss rate and measuring the air volume.
455
In 1965 Saltzman and Wartburg developed a procedure consisting of
a one step dilution of a 0.4% nitrogen dioxide mixture with purified air by
an asbestos plug with a precisely known leakage rate. This concentrated
mixture could be analyzed with a gravimetric procedure by absorption into
ascarite (asbestos + NaOH) and anhydrone [anhydrous magnesium perchlorate,
The development and study of permeation tubes, correctly used and
405,461,485,486
carefully calibrated, enabled creation of dilute mixtures.
The prior usage of the tubes (e.g., exposure to atmospheric humidity) may,
however, affect rate and stability of permeation.
Dilute nitric oxide streams can be generated by permeation of com-
pressed nitric oxide through membranes. Calibration is still made with
444 250
carefully prepared gas mixtures, and dilution techniques. In 1963,
227
Hersch and Deuringer proposed that electrolytic generation of nitric
oxide is a precise source of this gas; but this has not yet been exploited.
Precise concentrations of nitric oxide for primary calibrating purposes
may also be generated by nitrogen dioxide permeation tubes followed by a
highly efficient catalyst (i.e., pyrolyzed sugar in glass wool) that con-
58
verts nitrogen dioxide to nitric oxide. Photolysis of known concentrations
of nitrogen dioxide in pure nitrogen gas and rapid dilution by purified air
197
is another source of low concentrations of nitric oxide.
-------
4-23
SECONDARY REFERENCES FOR NITROGEN DIOXIDE AND NITRIC OXIDE
12
National Bureau of Standards Publication 351 stresses that the
criteria for air monitoring instrumentation should be simplicity, repro-
ducibility, and low cost.
It also emphasizes that a new instrument or method may require 2 to 4
years to progress from the drawing board to actual implementation. We
cannot, therefore, be overly optimistic about adopting any new development
in a short time because of the necessary exhaustive testing, cost analyses
(of calibration, maintenance, data reduction, salaries, etc.), and comparison
with other techniques such as manual methods.
A constantly upgraded list of commercially available instruments is
134
published by the University of California. It records manufacturers'
parameters, a glossary, and a summary of available calibration procedures,
methods, and policies. An evaluation of procedures for determining
performance of air monitors, routine calibration, and their limitations
was published in 1974 by Mueller and his associates in A Guide for the
378
Evaluation of Atmospheric Analyzers.
Traditional Monitoring Instrumentation
Continuous instrumentation based on the Griess-Ilosvay Reaction for
243
nitric oxide after oxidation by permanganate, and instruments using the
similar Griess-Saltzman reagent are still in operation. These instruments
require constant calibration and replenishment of reagent. Some minor
296,297,298,299,
changes have included refinements in reagent formulation,
374,375,451,452,611 539,563 209,250,
operational modes, and oxidizer design.
270,316,317,437,535,539,601
-------
4-24
When permanganate is used as an oxidizer, it tends to dry out and
492
accumulate crystals. A dilute sulfuric acid solution can minimize these
problems. Substitution of permanganate solutions by solid chromic oxide
supported on firebrick, creates problems in regulation of incoming relative
humidity of the sampled atmosphere. Sodium acetate (NaC H 0 -3H 0 + NaC HO )
£„ j £. £. £* J L,
250
humidifier used for regulating the RH in the manual procedure cannot be
applied in continuous instrumentation; therefore, use of an aqueous humidifier,
followed by heating of the chromic oxide oxidizer to 10 to 15°C above ambient
316,317
temperature, controls the range of optimal performance. Instruments
using this principle of measurement differ in design of flow systems and
contact columns and range from large stationary installations to smaller
418,533
semiportable units. Not all contact columns absorb gases efficiently,
and the conversion factor in liquid azo-dye continuous colorimetric instru-
ments varies from 0.5 to 1.0 depending on the design and construction
484,533,535
material of these columns. Error-free static calibration of
these instruments is impossible; therefore, dynamic calibration with known
gas mixtures is preferable.
An Intersociety Committee report details procedures for the calibration
of continuous colorimetric monitoring instruments for nitrogen oxides using
252
liquid azo-dye forming reagents. A summary of the required performance
565
criteria with definitions was published in the Federal Register.
377
Mueller et al. considered interferences in the colorimetric method to
377
be negligible at atmospheric concentrations, although many parameters
and operational conditions of real atmospheric sampling have not been
thoroughly evaluated.
-------
4-25
Chemiluminescent Instrumentation
A reproducible reaction for nitric oxide is the chemiluminescent
150
oxidation with ozone. This reaction is almost specific for nitric
515,516
oxide and can be applied to air pollution measurement. Its principal
drawbacks, like those of the other continuous analyzers, are the needs for
99
frequent calibration with complex instrumentation and electromechanical
maintenance. To adapt this reaction for nitrogen dioxide analysis, a
highly effective thermostat-equipped converter for nitrogen dioxide to
58,240
nitric oxide must be incorporated into the sampling line. This con-
verter must be checked frequently to ensure its continued efficiency.
453,515,516
Different materials have been evaluated as working catalysts.
Ammonia and other nitrogen—containing compounds may be oxidized in certain
572
converters, but other converters discriminate against such compounds.
Many substances produce chemiluminescent reactions with ozone, such as
ethylene (CH CH ), triethylamine [(C H ) N], carbonyl compounds (R CO),
2 5 3 303,389,421,506 *
mercaptans (RSH), and other sulfur compounds. For separation
515,516
of these interferences, optical filters can be used. Nitrogen
dioxide is electronically estimated from the difference between the signals
obtained with and without passing air through the converter. Error is
possible through the batch sampling and the consequent uneven intake.
515,516
Breakdown of valves and pumps is possible. Other deficiencies of
570
this method are chronicled in the Federal Register.
564
The chemiluminescent titrator described in the Federal Register
150
is an outgrowth of a technique described by Fontijn et al. in 1970. In
this technique the fast gas phase reaction between nitric oxide and ozone
267,
produces stoichiometric conversion of the nitric oxide to nitrogen dioxide.
268,454
-------
4-26
The method has been used to measure ozone indirectly through reaction with
454 99
nitric oxide and may be used to calibrate chemiluminescent instruments.
However, it is complicated and its instrumentation is delicate.
To assume that calibration of nitric oxide in cylinders serves as a
secondary calibration standard is highly idealistic. Oxygen traces in the
nitrogen used for dilutions generate nitrogen dioxide over a span of several
days. Once stabilized, a slow reaction occurs over several weeks or months,
especially when using common steel cylinders, causing disappearance of
nitrogen dioxide (Air and Industrial Hygiene Laboratory, Department of
Health, State of California, unpublished). Newly prepared nitric oxide
cylinders may therefore contain both oxides of nitrogen.
99
In a further evaluation of this problem, Decker et al. suggest bi-
monthly controls. Chemiluminiscent instruments need one or two primary
methods for calibration such as neutral buffered potassium iodide (KI) for
253 251
ozone or the manual Griess-Saltzman for nitrogen dioxide. Since
many calibrating steps result in a deterioration in overall precision, a
more direct approach is desirable; for example, calibration of chemiluminescent
instruments with dilute nitric oxide as indicated under Standard Gas Sources.
Since 1975, the Bureau of Standards has offered stable, certified mixtures
of nitric oxide in nitrogen for this purpose.
A chemiluminescent method for both nitrogen dioxide and nitric oxide
analysis at atmospheric levels is the reaction with atomic oxygen obtained
45,351
by thermal decomposition of ozone. The resulting signal has to be
corrected for nitric oxide in order to obtain nitrogen dioxide concentration.
A more specific, but indirect method, is photolysis of nitrogen dioxide with
UV light after removal of atmospheric ozone. During this photolysis an equiv-
alent amount of ozone is produced which is measured by a chemiluminescent
197,340
reaction.
-------
4-27
Batch Methods
Batch methods are used for grab, or manual, sampling on a predetermined
time basis, e.g., a few minutes, 2 hr, 24 hr, etc. Instrumental methods
in which gas is analyzed intermittently, can also be considered as batch
methods. In principle, all instrumental methods can be adapted to inter-
mittent data by electronically-operated integrators. Most of these methods
require constant standardization.
Volumetric wet chemical analysis of nitric oxide is performed in nitric
123
acid plants. Other wet chemical methods were explained above.
High concentrations of nitrogen oxides are suitable to several instru-
mental procedures after thermal oxidation of nitric oxide to nitrogen
dioxide. The sampling and preparation of the gas before measurement has
been audited for the following methods, e.g., Nondispersive Ultraviolet
112,113 112,113,413
(NDUV), Nondispersive Infrared (NDIR), phenoldisulfonic
112,113 112,113,393
acid (PDS), and spectrophotometry.
Gas chromatographic methods can be used at higher than atmospheric
concentrations only. Nitrogen dioxide and nitric oxide are collected
separately on cold traps containing the polyethylene glycol, "Carbowax
1500," and porous polystyrene, "Poropak Q," respectively. A thermal con-
106
ductivity cell acts as a detector after separating the gases on Poropak
49,106
Q contained in a stainless steel column.
Heated iodine pentoxide oxidizes nitric and nitrous oxide (NO) to
106,241 2
nitrogen dioxide. For low nitrous oxide concentrations, gas chroma-
59,106,576
tography is the best, if not only, available method, but, as
previously stated, this oxide is not an important air pollutant. Nitric
284 59
oxide can be separated on molecular sieve 5A or silica gel in the
-------
4-28
absence of oxygen, and detected at very low levels (0.01 yl) with the aid
284
of an argon ionization cell. For low levels of nitrogen dioxide, no
59
satisfactory gas chromatographic method has been found; the lower limit
3 3?1
is 5 ± 2 ppm (9,400 ± 3,800 yg/m ) using an electron-capture detector.
Miscellaneous Methods
To determine health hazards in industrial situations, indicator tubes
312,313,
containing substances sensitive to nitrogen dioxide are commonly used.
362,363
They are accurate within 20 to 30%. Continuous batch type analyses
205
have also been made with the Auto Analyzer. The determination of nitrogen
120,125,515
oxides with surface-specific electrochemical sensors is limited
since it does not reach the lower atmospheric levels.
A field method for analysis of atmospheric concentrations of nitrogen
239
dioxide employs a visual color comparator. This is useful for levels of
3
nitrogen dioxide above 0.05 ppm (94 yg/m ) whereas common indicator tubes
are sensitive for levels of 5 ppm (9,400 yg/m3) or more. This method is
accurate within 15 to 20%.
10
Electrochemical methods are based on galvanic cells such as the Mast
Nitrogen Dioxide Analyzer and the Hersch Cell. Water interference in
nitrogen dioxide analysis could be eliminated if the gases could be dried
without loss of nitrogen oxides. These two methods are in good agreement
203
with other methods (loc. cit.) when tested with combustion gases, but
are unsuitable for ambient levels. Instrumental methods using fluorescent
549
excitation of nitrogen oxides are under development. Other miscellaneous
instruments and principles of lesser importance have been described by the
134
Lawrence Instrumentation Group in Berkeley.
-------
4-29
SOURCE MONITORS FOR NITROGEN OXIDES
The two main sources of manmade oxides of nitrogen are combustion of
fossil fuels and the internal combustion engine. Instrument systems
designed to monitor stack gas emissions of nitrogen oxides have been studied
348,497
intensively by the Environmental Protection Agency and others. The
handling of the hot, wet, corrosive, and often dirty sample, and its intro-
duction to the analyzer in a condition permitting correlation between the
analytical result and the original sample composition are most difficult.
Additionally, the sample must be representative of all the gases passing
out of the stack, and the total flow of gases must be measured, or otherwise
determined to calculate the total quantity of nitrogen oxides being dis-
charged to the atmosphere. This applies to both instrumental and manual
wet chemical methods.
Two analytical approaches are currently in use. Conventionally, a
gas sample is withdrawn from the source via a high temperature probe. It
is then filtered, and possibly chilled, or dried by some other means,
depending on the requirement of the analyzer. The other approach uses an
413 393
in situ monitoring instrument, measuring absorption of infrared, visible,
112,113 .. ._
or ultraviolet radiation traversing through the gas. Since the in situ
instruments require that optical components not be too heavily loaded with
particulates, means to compensate for fouling must be implemented.
Nitrogen dioxide absorbs radiation in the visible and UV spectral
regions, and can be quantitatively determined using these wavelengths.
Nitric oxide absorbs weakly in the ultraviolet region; in the infrared, its
absorption ability increases. Since this absorption is overlapped by the
-------
4-30
spectrum of water vapor, correction or compensation for large percentages
of water, typically present in combustion gases, is necessary.
Nitric oxide may be determined indirectly by oxidation to nitrogen
dioxide, followed by UV determination. Since the concentration of nitrogen
dioxide in combustion gases is typically less than 5% of total NO , infrared
X
determination of nitric oxide is frequently acceptable. The chemiluminescent
analyzer can determine both nitric oxide and nitrogen dioxide, operating
on either a wet or dried sample. An electrochemical cell, appropriately
120,125
sensitized, can give quantitative measurement of NO . Galvanic
x
cells such as the Hersch cell, have been used for monitoring nitrogen
203
dioxide.
Nitric oxide and nitrogen dioxide in internal combustion engine
112,113
emissions are measured almost exclusively by the chemiluminescent analyzer.
Several hundreds of these instruments are in operation. The chemiluminescent
analyzer is preferred because of its specificity, speed of response, wide
dynamic range, and relative simplicity. Analysis may be made on raw exhaust,
where concentrations of nitrogen dioxide, carbon monoxide (CO), molecular
oxygen (0 ), and unburned hydrocarbons are correlated directly with the
mode of engine operation. In some tests, diluent air is added to the engine
emission from which a sample is collected over a certain period in a plastic
bag for later analysis. This dilution technique, which reduces the concen-
tration of nitrogen oxides to a few ppm, retards the thermal oxidation of
nitric oxide to nitrogen dioxide to a few percent before analysis.
Optical instruments based on correlation spectroscopy have been
30,31,600
mounted on aircraft for surveying emissions on a regional scale.
-------
4-31
Miscellaneous Considerations in the Choice of Methods
In choosing the method to be used, one must consider the cost and
bulkiness of each method on a sliding scale. Regarding field methods, for
instance, distinction must be made between a mobile laboratory and a
portable, suitcase-sized unit.
Equivalency testing of methods would be best done in a well-equipped
laboratory having a programmable manifold from which mixtures of varied con-
centration and composition of gases could be tapped.
12 516
Altshuller and Stevens and Hodgeson noted that target criteria
for air monitoring instrumentation should be simplicity, reproducibility,
and low cost. Other goals are accuracy within certain limits in the
presence of interferences, ease of calibration, and simple maintenance.
Few methods in use meet all these criteria.
The most ambiguous goal is "low cost" since the relativity of that
term defies definition. Regional surveys with the alkaline plate method
166,587
would provide the lowest cost empirical method available; however,
this method is of unknown accuracy, reproducibility, and sensitivity to
interferents. There is no calibration necessary. The plates are simple
to make, analyze, install, and maintain. The estimated datum cost per
unit per month, including personnel and overhead, is about $15.00, com-
pared with $20.00 for a single determination or calibration of a gas
mixture in a laboratory using the Griess-Saltzman manual method. On the
other hand, a 24-hr method would cost $400 a month and a continuous
monitor about $600 per channel. The information provided from these three
methods is quite different. The higher expense is justifiable in an area
with a progressive worsening of the air quality discovered in surveys of
large regions by empirical methods such as the alkaline plate.
-------
4-32
The need would then be defined for each area of use. Regionally, a
tested method giving a monthly average is still needed, or an existing
method such as the alkaline plate should be calibrated side by side with a
monitoring station, to provide a better understanding of the mechanism
and insight into data interpretation.
Secondly, several 24-hr average nitrogen dioxide concentration
methods are collaboratively tested. Four methods, tested side by side,
all indicated good precision (<±4%). The two methods using sodium hydroxide
and arsenite demonstrated lower sampling efficiency and more interference
from nitric oxide than the two methods using triethanolamine [(HOCH CH ) N]
211 223
as an alkalyzer. Methods selected from this and future evaluations
should always be used with strict adherence to details to avoid cases
similar to those in which the term "Jacobs-Hochheiser" was erroneously
applied.
A precise method for establishing primary nitrogen dioxide concen-
251
trations is the Griess-Saltzman Method, to which Griess gave the
principle and Saltzman optimized the conditions. This method must be
used without departure from the original design. The most important com-
ponents of this method are the bubbler (with a specified porosity glass
frit) and the reagent formulation. There is no need to improve this
calibrating method if a standard factor can be universally agreed upon.
For short-term field use, however, an improvement in the light stability
of the reagent formulation is desirable.
Quantitation of nitrate in particulate matter can be achieved by
several methods. The results of each method are uncertain since the
efficiency of all the different steps, such as sampling, aqueous extraction,
-------
4-33
nitration, extraction or distillation of the products, and colorimetric
determination, is unknown. Filter preparation to reduce blanks is very
important. Better and more uniform filter material is needed. A good
study could determine the efficiency of the first two steps, whereas an
intercomparison of methods for extracts from real samples would determine
which is the most precise, the most accurate, or both. Automation of
such analyses would eliminate the high cost of handling and processing;
miniaturization would permit the study of short-term ambient variation on
a continuous basis.
Instrumental methods appeared rap'idly after the development of
chemiluminescent reactions. Many initial inconveniences and instrumental
variations were eliminated or reduced as a result of a strong advance
in electronics. The instruments are less subject to flow rate variations
and do not need reagents to operate. However, in chemiluminescent methods
applied to nitrogen dioxide, reliability is in demand. Inconveniences
are the batch sampling procedure for nitric oxide and NO with a further
x
subtraction of the signals for obtaining the nitrogen dioxide concentration.
The reducing catalyst for nitrogen dioxide has an unknown lifetime in
real situations and the instruments need frequent calibration with
calibrating gases or permeation tubes, which are delicate and need working
thermostats with supporting gases. A better approach might be photo-
fragmentation by UV irradiation to ozone and chemiluminescent measurement
197,340
of that ozone. It is not known if this principle worked trouble-
free in other situations besides those studied initially. This method
can be used as a continuous monitoring method for nitrogen dioxide up to
1,880 yg/m3 (1,000 ppb).
-------
4-34
Inconveniences cited for continuous colorimetric azo-dye forming
instruments can be summarized as follows: variability of the flowrates
of intake air and reagent, long response time, negative ozone interference
at high ratios of ozone to nitrogen dioxide, and reagent replenishment.
Ozone interference on the continuous colorimetric instruments occurs only
occasionally in areas of high pollution.
The azo-dye method has given us the first accurate information about
nitrogen oxides and should not be disregarded despite its weaknesses.
Many instruments having weaknesses continue to be used and have provided
the majority of valuable information. These older instruments could be
improved by replacing the inefficient liquid permanganate oxidizer for
nitric oxide with the efficient chromic acid (CrO ) solid oxidizer and a
316,317 3
humidifier. This would prevent the appearance of negative nitric
oxide values on the recorder. More compact instruments using the same
principle have been marketed, but further size reduction is still desirable
to facilitate transportation and lessen monitoring space requirements.
SUMMARY
This review of available methodology for oxides of nitrogen summarizes
the development of procedures designed to establish an air quality standard.
Only traditionally accepted terms have been used. "Reference" denotes
only a calibrating method. Primary reference methods serve as calibrating
methods; secondary reference methods are reliable, accurate, and mostly
interference-free measuring methods. Weighable primary standards, or
secondary standards measurable by a primary reference method, can both be
used for zalibration. Permeation tubes are primary standards, whereas a
-------
4-35
cylinder with a dilute gas mixture, previously calibrated by measurement
with a primary reference method, is a secondary standard.
The 100-year-old chemical methodology for nitrogen oxides measurement
involves the transfer of the nitrogen from nitrogen oxides directly into
a measurable compound. This methodology, and many of its variations, has
generated most of the data over the last 40 years and is still being used
locally and internationally. The manual Griess-Saltzman Method (not to
be confused with the Griess-Saltzman reagent or principle) has been used
to calibrate continuous analyzers since the 1950's. It is based on
the direct reaction of nitrogen dioxide with a single reagent mixture to
form a colored azo-dye. The Griess-Saltzman reagent and several modifications
thereof form the basis for many continuous analyzers. Nitric oxide cannot
be measured directly with this technique and must be oxidized to nitrogen
dioxide with catalyst.
A recent development is the technique based on chemiluminescence
which measures the light emitted by reaction of two gases. Improve-
ments to this technique have come from both the official and the private
sector. Nitric oxide is measured through reaction with ozone produced
within the instrument by irradiation of air with a shorfwave UV lamp.
Measurement of nitrogen dioxide by chemiluminescence requires a previous
reduction step to nitric oxide over a heated catalyst. Photofragmentation
of nitrogen dioxide by ultraviolet light and measurement of the ozone
produced does not involve contact with a catalyst but does need nitric
oxide as a supporting gas.
Precisely known concentrations of dilute gases are required. These
concentrations can be obtained by dynamic dilution of more concentrated
-------
4-36
gas mixtures or with permeation tubes having precisely known leakage
rates. In most wet chemical methods an alkaline nitrite or a nitrate is
used as a primary standard.
Alkaline methodology has also been used to sample and measure
nitrogen dioxide. The departure from original sampling conditions,
absorbing reagent formulations, and assumption of constant performance
generated equivocal results. A variety of new methods called Jacobs-
Hochheiser were created but have not been completely evaluated. In these
absorbing solutions nitrite is analyzed applying the Griess principle of
azo-dye formation. More recent methods without these shortcomings have
been developed and evaluated.
Most methods transforming nitrogen oxides into nitrate are most
satisfactory at higher than atmospheric levels of nitrogen oxides. Higher
levels of nitrogen oxides can also be analyzed with the same methods used
in atmospheric analysis after proper aliquoting or dilution. Additionally,
some optical methods, currently used for higher concentrations, cannot be
applied to atmospheric analysis except when measuring from aircraft.
CONCLUSIONS AND RECOMMENDATIONS
Methods should be recommended only after careful evaluation of perfor-
mance through collaborative testing by a balanced team of scientific groups
and representatives of official and private agencies. These recommendations
should consider the short- and long-term budgetary outlays of those involved.
The user must be satisfied with their accuracy (equivalency) and precision
(reproducibility) under real sampling conditions. Simple means of calibration
should be available. Very few of the methods in use would meet all these
parameters.
-------
4-37
Some wet chemical methods may be reproducible if the methods are
strictly followed. As a primary reference method, the manual Griess-
Saltzman Method (not to be confused with the Griess-Saltzman reagent or
principle) is recommended. The primary standard for calibration of this
method is sodium nitrite.
Physical methods of analysis need frequent, regular calibrations,
preferably in well-equipped laboratories. The most direct chemilumi-
nescent method for nitrogen dioxide involves photofragmentation to ozone,
then chemiluminescent measurement of this ozone. Less direct is the dif-
ferential measurement of nitrogen oxides (less nitric oxide) by chemilumi-
nescence after reduction over a heated catalyst. This second method
requires close control because the lifetime and memory effects of the
reducing catalyst under real situations are not known. More experience
with this method is required before it can be recommended for nitrogen dioxide.
Calibration of chemiluminescent methods with permeation tubes instead of
secondary standards changes their category to primary reference methods.
Despite recent developments of chemiluminescent instrumentation, improvements
in reliability and simpler quality control checks are desirable.
To calibrate analytical methods using nitric oxide as standard gas,
primary reference methods are preferred.
During environmental monitoring, the position of either the make-up
atmosphere inlet or the sampling probe (or both) is of great importance
and should be established by a team effort. A real time measurement of
nitric oxides can be provided by chemiluminescent instrumentation; however,
due to the uncertainties of their performance, the manual Griess-Saltzman
Method should be used concurrently at the same manifold as a back-up control.
-------
4-38
Future development should produce size and cost reduction of large ,
stationary instrumentation without impairing performance. A new regional
survey method for background level control over large areas is also desirable
and could reduce the monetary burden of local agencies. Further needs
include continuous particulate nitrate and gas phase nitric acid monitors.
Finally, simplicity and a more precise terminology in the editing of
documents would preclude misinterpretations.
-------
CHAPTER 5
ATMOSPHERIC LEVELS OF NITROGEN OXIDES
The distribution of nitrogen oxides is by no means uniform. Localized
concentrations often exceed the "average" concentration by a factor as high
563
as 100. These concentrations are located in urban areas having heavy
automotive traffic, and in those major industrial areas whose nitric acid
plants and uncontrolled stationary combustion sources produce oxides of
nitrogen. The effect of these sources on actual pollutant levels is largely
determined by the movement of the air mass into which the pollutants are
emitted.
The most complete and authentic source of data on nitrogen oxide con-
centrations is the National Aerometric Data Bank of the Environmental Protec-
tion Agency (EPA). This data bank receives inputs from the National Air
Surveillance Network (NASN) as well as from other state and local sources.
NASN is comprised of approximately 100 sites which monitor nitrogen dioxide
(NO ) and sulfur dioxide (S0~). Until recently, it also included six Con-
tinuous Air Monitoring Project (CAMP) stations. Additional data have been
219
provided by more localized studies, such as the Chattanooga Study, and the
70
California Air Resources Board, which recently issued a 10-year summary of data
gathered through their large network of monitoring stations.
Current techniques for measuring oxides of nitrogen are not entirely
satisfactory (See Chapter 4). As continuous instrumental methods are
refined, and advances in the technology of preparing stable calibration
mixtures are made, a more adequate data base will be developed. Until
-------
5-2
such a data base is available, however, each analytical method used should
be carefully evaluated in order to draw accurate conclusions from the data.
The EPA formerly designated the modified Jacobs-Hochheiser method as
the Reference Method for nitrogen dioxide. This method has subsequently
been proven inaccurate under certain conditions (See Chapter 4). Since
data from this method must be used only with extreme care, they have not
been included in this chapter.
The balance of the existing data was generated primarily by the con-
554
tinuous colorimetric technique, a modification of the Saltzman method.
This technique, which uses either the Griess-Saltzman reagent or the Lyshkow
145
modified Griess-Saltzman reagent, is inaccurate at low concentrations,
and is subject to negative interferences from large concentrations of
569
ozone. Nevertheless, results from this method compare favorably with
those of the chemiluminescent method, which does not suffer ozone inter-
87
ference, and with the so-called "Arsenite" method of Christie et al. The
EPA now regards these three methods as the most promising, and is subjecting
them to rigorous evaluation.
GLOBAL DISTRIBUTION OF NITROGEN OXIDES
Ninety percent of nitrogen oxides in the earth's atmosphere is produced
by natural bacterial action (See Chapter 3).
Many studies have been made to determine global background levels of
nitrogen oxides. This task is difficult, and the results become less
204
precise, in areas having many manmade sources. For 1968, Hamilton reported
3
average concentrations of nitric oxide (NO) at 3.4 ug/m (2.7 ppb) and ni-
3
trogen dioxide at 7.7 yg/m (4.1 ppb) at Pike's Peak, Colorado. Earlier
-------
5-3
observations by Ripperton et al. at an altitude of 1,573 meters in the
438
Appalachian area of North Carolina had indicated similar concentrations—
3 3
nitric oxide at 3.4 yg/m (2.6 ppb) and nitrogen dioxide at 8.6 yg/m (4.6
ppb). At the same time, these authors observed mixing effects of small
scale turbulence, and of larger scale mixing caused by large vertical
movements of air masses, such as those accompanying a cold front. Observa-
273
tions in Hawaii, showing average nitrogen dioxide concentrations of
3 323
2.4 yg/m (1.3 ppb), and in Panama, showing total NO values from
X
0.9 to 3.6 ppb, suggest that background concentrations in continental
states are higher than those in less industrialized areas.
On the basis of observed background levels and the emission rates of
440
nitrogen oxides, Robinson and Robbins estimated the average residence
times for nitric oxide and nitrogen dioxide at 4 and 3 days, respectively.
Scavenging processes that limit residence times and, hence, the build up of
nitrogen oxides, include photochemical reaction, oxidation of nitric oxide
to nitrogen dioxide, nitrate formation, and probably other mechanisms.
The stratosphere, roughly between 11 and 35 km in altitude, is currently
drawing considerable interest. Using airborne and balloonborne spectroscopy,
3
Ackerman et al. observed nitric oxide and nitrogen dioxide levels of
1 ppb at 16 km altitude, increasing to 5 ppb each additional 30 km altitude.
The supersonic Concorde 001 was used to collect the airborne data. The
observations of nitric oxide and nitrogen dioxide were not made concurrently.
Using the same aircraft and a fast Fourier interferometer spectrometer,
140
Farmer et^ al^. computer concurrent values of 0.2 to 0.4 ppb nitric oxide
and 1.8 ppb nitrogen dioxide at approximately 16 km, giving a nitrogen
-------
5-4
dioxide to nitric oxide ratio of approximately 4.5. These observations
were made at sunset in June 1973. For additional data on the contribution
of high-flying aircraft to nitrogen oxide levels in the stratosphere, see
6
the report of the Advisory Group for Aerospace and Development.
Concentration of nitrous oxide (N~0) has also been determined in the
140
experiments of Farmer ej^ a^. A relatively constant value of 0.2 ppm
nitrous oxide was observed between 12 and 20 km.
Nitrous oxide concentrations are significant as the probable source of
nitric oxide in the lower stratosphere, and the effect which supersonic
394
aircraft might have on nitrogen dioxide concentration.
The two primary sources of oxides of nitrogen generated by man are
the internal combustion engine and fossil fuel burning power plants (See
Chapter 3).
568
Five CAMP stations recorded a gradual increase in average NO con-
X
centration (Figure 5-1). The same trend is indicated in limited data
399
available from European cities (Table 5-1). Although the trends are the
same, the trend lines vary slightly from location to location.
TIME RELATED VARIATIONS IN NITROGEN OXIDE CONCENTRATIONS
Diurnal Patterns
Geographic and meteorological factors can combine to amplify the effect
of manmade emissions. In the calm air mass of the Los Angeles basin, both
horizontal and vertical movement of the air mass are minimal. Under these
conditions, nitrogen oxides build up, and nitric oxide converts to the more
harmful nitrogen dioxide (Figure 5-2). Figure 5-3 contrasts the patterns
of nitrogen dioxide concentrations found on days of low and high ventilation
399
and solar radiation in Vienna, Austria.
-------
5-5
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D DATA SATISFYING IMADB MINIMUM SAMPLING CRITERION
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*NOTE CHANGE IN ORDINATE SCALE FOR THESE DATA
CHICAGO CAMP
O
CINCINNATI CAMP
7
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PHILADELPHIA CAMP
ST. LOUIS CAMP
'62 '63 '64 '65 '66 '67 '68 '69 '70 '71
YEAR
FIGURE 5-1. Trend lines for NOX annual averages in five CAMP cities.
From U.S. Environmental Protection Agency, 1973.
568
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FIGURE 5-2. Variation in nitric oxide and nitrogen dioxide concentration,
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LOW WIND - HIGH IRRADIATION
HIGH WIND - LOW IRRADIATION
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TIME OF DAY, hour
FIGURE 5-3. Diurnal variation of nitrogen dioxide in Vienna, Austria, 1966.
From North Atlantic Treaty Organization, Committee on the
Challenges of Modern Society, 1973.3"
-------
5-9
Because a large portion of urban nitrogen oxides is generated by human
activity, variations in oxide concentrations correlate directly with such
activities. The major variable is vehicular traffic. Periods of heavy
traffic, such as morning and evening rush hours, produce correspondingly
high concentrations of nitric oxide. During slack periods, such agents as
breezes and sunlight disperse, convert, or otherwise reduce these high
concentrations. These competing factors create typical diurnal patterns
(Figure 5-2).
Figure 5-2 data were collected from a relatively stable air mass in
Orange County, California, 48 km southeast of Los Angeles, on October 16,
1974. The day's intense sunshine was followed by low clouds in the evening
and throughout the night. The high temperature was approximately 32°C.
Vehicular traffic in Orange County increases sharply beginning at 6 a.m.,
accompanied by a rapid nitric oxide buildup. This nitric oxide converts
quickly to nitrogen dioxide in bright sunlight. A local influence—perhaps
a short-lived breeze or a brief period of cloudiness—can cause a minimum
nitrogen dioxide concentration at noon. The late afternoon levels of
3
75.2-94 ug/m (40-50 ppb) nitrogen dioxide, with nitric oxide levels of
3
approximately 12.5 yg/m (10 ppb), are typical of a sunny day. In the early
evening,concentrations of both pollutants build up. Nitric oxide concen-
trations increase appreciably immediately after sunset.
Certain elements of this diurnal pattern are shared by other urban
population centers. Nitrogen dioxide data from Rome, Italy; Vienna, Austria;
398
and Rotterdam, The Netherlands all show the morning and evening maxima,
with tendencies toward lower concentrations between these periods.
-------
5-10
Seasonal Patterns
Patterns associated with the seasonal variation of temperature and
prevailing winds may also be traced. Increased use of heating fuels
during the winter months increases the contribution from this source.
Since the rate at which nitric oxide converts to nitrogen dioxide is re-
lated to the intensity of solar radiation, conversion is most rapid during
warm summer days. Figure 5-4 shows the seasonal variation of nitric oxide
in the Los Angeles area, and Figure 5-5 shows the variations of nitrogen
dioxide.
Each "typical" pattern can be disturbed by factors localized in space
or time. A cloudy day will prolong the duration of high nitric oxide con-
centrations. A strong wind will quickly disperse all pollutants and provide
a clean atmosphere. Geographical features, particularily hills and valleys,
affect both temperature profiles and air movement, and can have a major
effect on pollutant levels.
Annual Trends
The most complete continuous data base used to determine long-term
trends of oxides of nitrogen on a national scale has been generated by the
Federal Government's Continuous Air Monitoring Program (CAMP). One CAMP
station is currently located in the main business district of each of the
following cities: Chicago, Cincinnati, Denver, Philadelphia, St. Louis,
and Washington, D.C. Since the Washington, B.C. station was relocated in
1969, its data are not continuous. For the other five cities, the CAMP
567
program has produced continuous data for 12 years.
All sampling methods and operating modes were chosen to facilitate
comparison of data between cities. Nitric oxide and nitrogen dioxide are
-------
5-11
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100
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1971
1972
JAM/MARCH APRIL/JUNE JULY/SEPT OCT/DEC
FIGURE 5-4. Quarterly mean of hourly average concentration of nitric oxide at downtown Los Angeles, California.
From State of California, 1974.70
-------
5-12
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JAN/MARCH APRIL/JUNE JULY/SEPT OCT/DEC
FIGURE 5-5. Quarterly mean of hourly average concentration of nitrogen dioxide at downtown Los Angeles, California
From State of California, 1974.70
-------
5-13
determined individually by the continuous Saltzman colorimetric method.
In addition to oxides of nitrogen, the CAMP stations determine carbon
monoxide, sulfur dioxide, total hydrocarbons, and methane.
568
Tables 5-2, 5-3, and 5-4 compare the average concentrations of nitric
oxide, nitrogen dioxide, and NO for two 5-year periods, 1962-1966 and
X
1967-1971. The data indicate a general trend toward higher concentrations.
Nitric oxide increased by an average of 13%, nitrogen dioxide by 6%, and
the combined NO by 9% over the two 5-year periods. The tables also list
X
the average annual second highest observed values to indicate changes in
extreme concentrations. In Chicago and Cincinnati nitric oxide concentra-
tions increased considerably (32 and 36%, respectively), while in Denver,
Philadelphia, and St. Louis, only minor changes were recorded. The second
highest values for nitrogen dioxide do not parallel those for nitric oxide,
and show only a 1% increase in the two averages. This lack of correlation
between nitric oxide and nitrogen dioxide values could result from differing
solar radiation or variation in reactive hydrocarbons, both of which affect
the nitric oxide to nitrogen dioxide conversion rate.
Tables 5-5 and 5-6 give the year to year arithmetic mean and maximum
values of nitric oxide and nitrogen dioxide for six CAMP stations, including
Washington, D.C. These data are from the National Aerometic Data Bank. The
general upward trend of most of these data is evident in Figures 5-6 and 5-7,
568
where computed regression lines have been included. Exceptions to the
general trend are the nitrogen dioxide levels for Denver and St. Louis which
are fairly stable.
Mathematical Models for Predicting Pollutant Concentrations
Marked progress is being made in the mathematical simulation of environ-
mental pollution phenomena. Simulations range from the relatively simple
-------
5-14
307
models developed by Larsen on the basis of empirical observations to
sophisticated systems such as the Implementation Planning Program model
545
developed for EPA by TRW Systems. The TRW model uses individual sources
of pollutants as the basic input to a diffusion model. Concentration of
pollutants expected at several selected receptor points are calculated.
The program is designed to facilitate calculation of the effect of various
control strategies, differing pollutant standards, and the cost of the
different approaches.
Larsen's model, which predicts pollutant concentrations on the basis
of previously observed average values and an assumed lognormal distribution
(found to be true for data from eight cities, for seven pollutants, over a
period of several years) continues to produce good forecasts of pollutant
levels. This model has determined pollutant reductions required at the
32 419
source in order to meet air quality standards. A recent paper confirms
the validity of Larsen1s approach, while at the same time questioning the
stringency of the vehicle emission standards considered necessary by EPA to
meet the air quality standards.
Several mathematical models which have been developed for specific
151
regions are capable of broader application. The Fortak model has been
used in city planning, and for determining the potential effect of proposed
major commercial or industrial installations on an existing pollutant pro-
290
file. It has been applied successfully in Duesseldorf and Stockholm. Kohn
has studied the St. Louis, Missouri area and developed a model of air
200
quality, and Haagenson et al. have modeled pollutant plume behavior in
the same area to determine locations of mobile monitoring stations.
328
MacCracken et al. used a multi-box approach and a mass consistent wind
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TABLE 5-4
5-yr Average NOX Concentrations at CAMP Stations,
Measured by the Continuous Saltzman Colorimetric Method
o
Average concentration, yg/m 25 °C
Station
Chicago
Cincinnati
Denver
Philadelphia
St. Louis
CAMP average
1962-1966
208.7
105.8
110.9
122.9
98.3
129.3
1967-1971
226.6
113.6
122.3
143.0
101.8
141.5
Change , %
+ 8
+ 7
+10
+16
+ 3
+ 9
568
Trom U.S. Environmental Protection Agency, 1973.
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O DATA SATISFYING IMADB MINIMUM SAMPLING CRITERION
O INVALID AVERAGE (BASED ON INCOMPLETE DATA)
*NOTE CHANGE IN ORDINATE SCALE FOR THESE DATA
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CHICAGO CAMP
I
CINCINNATI CAMP
DENVER CAMP
O O -
PHILADELPHIA CAMP
'62 '63 '64 '65 '66 '67 '68 '69 '70 '71
YEAR
FIGURE 5-6. Trend lines for nitric oxide annual averages
in five CAMP Cities. From U.S. Environmental
Protection Agency, 1973.568
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FIGURE 5-7. Trend lines for nitrogen dioxide annual averages in five CAMP cities.
From U.S. Environmental Protection Agency, 1973,
568
-------
5-23
field submodel to simulate the San Francisco Bay region. Their model
furnished frequency distribution data compatible with observed data.
EFFECTS OF METEOROLOGICAL FACTORS
The study of air pollution involves the generation of pollutants and
their transport to receptors, usually at or close to ground level. This
simplistic relationship immediately brings to mind the importance of the
transport mechanism in determining the final effect of a given pollutant
loading. Increasingly taller industrial smoke stacks are visible reminders
that wide range dispersal is one means of "solving" pollution problems.
When one considers the vast quantities of nitrogen oxides generated and
disposed of by natural processess (see Chapter 3), the relative ineffective-
ness of man's efforts becomes quite apparent.
Movements of the air mass into which pollutants are emitted are governed
largely by meteorological factors, with some influence from local topographic
features. The velocity and direction of surface winds determine the fraction
of a given emission mass to be received at each receptor point. Vertical
movement of the air mass is also important. Episodes of severe pollution
are usually characterized by stagnant air masses, with inversion of the
normal profile of temperature decrease with altitude. Such inversion tends
to inhibit vertical movement of air.
Temperature and the amount of solar radiation reaching the earth's
surface have particular influence on the oxidation of nitric oxide to ni-
trogen dioxide, and subsequent photochemical reactions. Figures 5-4 and 5-5
show seasonal nitric oxide and nitrogen dioxide variation for the Los Angeles
area. Although fall and winter levels of nitric oxide are typically much
-------
5-24
higher than summer levels, nitrogen dioxide concentrations are only marginally
higher. An added indication of the effect of solar radiation is the incidence
of photochemical smog in this region, which is most pronounced in the summer
and early fall (the seasons of highest radiation).
151
Fortak's paper on mathematical modeling includes a diagram which
indicates the interrelationship of many meteorological factors in their
effect on pollutant dispersion (Figure 5-8). Meteorological factors add
greatly to the complexity of air pollutant problems. Fortak discusses
atmospheric motions in some detail, pointing out the great difficulty of
forecasting any movements of air masses other than large scale, synoptic
movements.
OBSERVED URBAN NO CONCENTRATIONS
x
Table 5-7 gives the mean hourly average concentration and the maximum
observed hourly concentration of nitrogen dioxide for representative U.S.
cities. These data were obtained from the National Aerometric Data Bank,
with some augmentation of the California data from the California Air
70
Resources Board (ARE) Ten-year Summary. In all cases the indicated
method of analysis is the instrumental colorimetric method, using either
the Griess-Saltzman reagent or the Lishkow reagent. As indicated earlier,
data from this method of analysis are the best comprehensive data currently
available. Major revision will probably not be required.
The California ARB data include determination of nitric oxide and
total NO (see Tables 5-8 and 5-9). Most of the nitric oxide was determined
X
with the parallel mode of operation, in which nitrogen dioxide is measured
in one system and NO in a second. In the NO system, nitric oxide is
x x
oxidized to nitrogen dioxide prior to the determination. A few district
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Weather Situation
Air Mass
Pressure Distribution
Temperature
Cloudiness
Lapse Rate
Sources
Sedimentation
Topography 1 Roughness
Properties of the Ground
Wind Speed
Radiation
Turbulence
Transport
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FIGURE 5-8. Meterological influences on transport and turbulent diffusion of air pollution components in the atmosphere.
From Fortak, 1973.151
-------
5-31
stations use the series mode of operation in which nitric oxide is oxidized
and measured after the initial measurement of nitrogen dioxide. Table 5-10
shows the frequency distribution of daily maximum hourly concentrations of
nitrogen dioxide for a number of California monitoring stations. These
data are also from the ARE Ten-year Summary.
Annual average concentrations of nitrogen dioxide for five European
cities indicate a trend toward higher concentrations (Table 5-1). Munich
data were collected near streets with high traffic density, and are not
representative of typical ambient levels for the city.
These data are supplemented by those for CAMP stations (Tables 5-5 and
5-6), whose general upward trend in total oxides of nitrogen was discussed
above.
In a 1972 program to recheck nitrogen dioxide concentrations in the 47
"Priority I" Air Quality Control Regions, the EPA used the arsenite chemical
and chemiluminescent instrumental methods of analyses. Simultaneous measur-
ments were made by the original reference method for comparison purposes,
with the understanding that uncertain results should be expected. Regions
were classified "Priority I" if their average annual concentration of ni-
569
trogen dioxide exceeded 0.0585 ppm. Results for the 47 regions (Table 5-11)
cannot be used to compare the two analytical methods, because sampling
locations and analysis periods were different.
The effects of a localized source of oxides of nitrogen, and other
219
pollutants, are illustrated by the Chattanooga study. This study was
undertaken to determine pollutant levels and their possible effects on
materials and vegetation in the area. The major pollutant source in this
area is the Volunteer Army Ammunition Plant at Tyner, Tennessee, where
-------
5-32
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TABLE 5-11
Nitrogen Dioxide Concentrations by Various Methods,
1972, for Air Quality Control Regions (AQCR)
Originally Classified Priority 1—
Nitrogen Dioxide Average Concentra-
tion for Period of Operation,
?, 25°C
Region
FRMJ1 Arsenite£ Chemiluminescent
Atlanta
Baltimore
Boston
Chattanooga ,
Chicago!
Cincinnati
Cleveland
Columbus
Corpus Christi-Victoria
Dallas-Fort Worth
Dayton
Denver!'.?.
Detroit-Port Huron
Dubuque
Florida, Southeast (Miami)JL
Florida, West Central (Tampa)
Four Corners
Genesee-Finger Lakes (Rochester)
Hampton Roads (Norfolk)
Hartford-New Haven-Springfield
Houston-Galveston
Indianapolis
Los Angeles
Louisville
Massachusetts, Central (Worcester)
Memphis
Michigan, Central (Grand Rapids)
Minneapolis-St. Paul
National Capital!
New York-New Jersey-Connecticut
Niagara Frontier (Buffalo)
Omaha-Council Bluffs
Pennsylvania, Central (Johnston)
Pennsylvania, South Central (Lancaster)
Pennsylvania, Southwest (Pittsburgh)
Pennsylvania-Upper Delaware Valley,
Northeast (Reading)
183
159
132
125
238
156
126
149
85
145
158
106
180
70
120
156
47
98
123
125
137
107
252
184
120
148
127
57
146
182
76
113
—
132
177
158
80
96
74
53
117
73
57
68
43
76
64
42
80
30
55
56
30
48
52
82
64
61
182
87
71
64
59
31
88
100
32
60
25
60
78
52
62
64
38
121
61
53
52
43
47
53
110
60
23
53
52
26
39
73
66
56
118
68
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31
44
47
64
65
49
30
64
36
64
60
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5-43
TABLE 5-11 - continued
Region
Nitrogen Dioxide Average Concentra-
tion for Period of Operation,
yg/m3, 25°C
Arsenite£. Chemiluminescent
Philadelphia
Phoenix-Tucson
Providence
Puget Sound (Seattle)
San Diego
San Francisco Bay Area.2.
St. Louis
State Capital (Richmond)
Toledo
Wasatch Front (Salt Lake City)
Wisconsin, Southeast (Milwaukee)
197
159
98
134
136
193
123
171
139
159
124
83
80
45
47
63
85
79
58
54
62
76
84
69
51
76
84
58
37
38
114
~~0n basis of earlier Federal Reference Method (FRM) determinations.
From U.S. Environmental Protection Agency, 1973.569
—Federal Reference Method.
c^
Arsenite data are corrected to reflect 85% collection efficiency. Avail-
able data indicate that there is 95% confidence that the corrected measure-
ments are within ±10% of actual NO,, concentrations.
—All measurements at same site. In other AQCR's, all measurements were not
made at the same site.
e
—Originally classified priority III.
r
—City names in parentheses are for identification only.
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5-44
high levels of nitrogen oxides emissions result from the production of
trinitrotoluene.
Observation sites included areas where high levels of pollutants were
expected and control areas where near normal levels were anticipated. Nitric
oxide and nitrogen dioxide were determined with the Saltzman method. Tables
5-12 and 5-13 show average hourly concentrations and maxima of nitric oxide
and nitrogen dioxide for the various sites. Stations 5 and 27 were in
downtown Chattanooga; Station 7 was in Rossville, Georgia. Stations 15, 17,
19, 20, 21, 161, and 201 were near the ammunition plant.
SALTS OF NITROGEN IN THE ATMOSPHERE
NASN has collected data on suspended particulate nitrate levels since
1958. Selected data from this source are presented in Table 5-14. Some
earlier data were obtained using nitration of 2,4-xylenol [(CH-)-C,H_OH]
as the analytical method, after collecting particulates on a high volume
sampler. These data are underlined in the table. The remaining data were
obtained using high volume sampling, diazotization, coupling with N_-(l-
naphthyl)ethylenediamine dihydrochloride (C H NHCH CH NH -2HC1), and
colorimetric determination.
Nitrates in Chattanooga Study *
The Chattanooga study was designed to gather air quality information in
the Chattanooga, Tennessee and Rossville, Georgia interstate areas, particularly
as affected by the Volunteer Army Ammunition Plant at Tyner, Tennessee. The
study included determinations of suspended nitrates, sulfates, and ammonium
particulates (Table 5-15).
-------
5-45
Stations 17, 19, 20, and 21 were closest to the ammunition plant.
Nitrate concentrations at each station exceeded the NASN maximum station
o
average for 1965 which was 13.5 yg/m . The reading at Station 19 was 48.9
3
Mg/m , more than three times higher than that average.
Sampling means at five stations were selected for their ability to
measure respirable particles (those under 5 ym diam) separately from the
total sample collected by the high volume sampler (Table 5-16).
The data represent seasonal averages. Time periods during which
observations were made were different for the various stations.
Individual analyses indicated the smaller, respirable portion contained
higher concentrations of nitrate, sulfate, and ammonia than did the total
sample of suspended particulates (Table 5-17).
Nitrate was analyzed by hydrazine reduction and diazotization; sulfate
by the methylthymol blue method; and ammonia by the indophenol method.
Nitrates in California South Coast Air Basin Study
A recent study by the Air and Industrial Hygiene Laboratory of the
California State Department of Health has determined nitrate and sulfate
concentrations found in aerosols throughout California. A paper by Appel
21
et a_l. summarizes the data. The study attempts to define mechanisms
which form the compounds which account for the concentrations, cation
moieties, and particle sizes observed. Table 5-18 gives 24-hr average
values for eight locations. Nitrate determination was made with a micro
version of the 2,4-xylenol procedure.
The study indicated that most of the particulate nitrate and sulfate
was in the form of ammonium salts. Nitrate concentrations tend to increase
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TABLE 5-17
Analysis of Selected Dust Samples —
Station
WDEF-TV
Post Office
Rossville
Chattanooga StudyjJL
T
Total, pg/m
Respirable
Dust
48.8
44.7
42.7
Suspended
Particulate
106.2
110.0
117.9
Total, %
Nitrate
2.4
3.3
2.2
4.2
2.3
2.8
Sulfate
8.7
18.6
8.7
20.4
7.7
19.7
Ammonia
1.0
2.9
1.1
4.0
0.6
2.3
219
—From Helms et al., 1970.
-------
5-54
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5-55
during west to east moves across the basin while average particle size
decreased with this movement. Maximum concentrations of nitrate tend to
correspond with maximum NO values.
x
SUMMARY
Despite the loss of data resulting from disqualification of the modified
Jacobs-Hochheiser method, there is a sizeable body of reliable information
on nitric oxide and nitrogen dioxide concentrations. Most of these data
were generated by the continuous colorimetric Saltzman method. This method
has correlated quite well with the instrumental chemiluminescence method,
which is not subject to major interference, and with the 24-hr arsenite
87
method of Christie et al.
Data on nitrogen dioxide and nitric oxide levels have been collected
by the NASN, CAMP stations, and the California Air Resources Board network.
In general, these data provide mean hourly average and maximum hourly
average concentrations. EPA data on nitrogen dioxide led to reclassification
o
of 43 regions from Priority I to Priority III [below 110 Mg/m (58.5ppm)
annual arithmetic mean].
In contrast to pollutants showing a decreasing or stable trend, total
emissions of oxides of nitrogen continue to increase. This is reflected
in concentrations observed at the CAMP stations, where a general upward
trend is evident. At five CAMP stations, the average NO concentration
x
for 1967 through 1971 was 9% higher than for 1962 through 1966.
Nitrate concentrations have been observed by the NASN and the 1967-1968
Chattanooga study. The Chattanooga study recorded nitrate levels up to 10
times the national average.
-------
5-56
Current concern with pollution of the stratosphere has resulted in
recent determinations of its nitric oxide and nitrogen dioxide concentra-
tions. Nitric oxide in the stratosphere probably results from oxidation
of nitrous oxide, which is present in the stratosphere at 0.20 ppm.
Several mathematical models are of value in predicting maximum pol-
lutant levels on a short-term basis, in deploying mobile monitors, and in
estimating the impact to be expected from major new pollutant sources.
CONCLUSIONS
An appreciable amount of data must be discarded because of questionable
accuracy of the methods used to measure nitric oxide and NO . However,
x
sufficient accredited data exist to support the following conclusions:
1. Highest average concentrations of oxides of nitrogen are found
in heavily populated, industrialized urban areas. (Localized
sources such as munitions plants are excepted.)
2. These concentrations show a generally increasing trend between
1962 and 1971.
3. The current national primary air quality standard of 100 yg/m
(0.05 ppm) nitrogen dioxide is consistently exceeded in the Los
Angeles area and, if present trends continue, will soon be exceeded
in Chicago and Philadelphia areas.
4. Physico-chemical models of pollutant distribution, based on ana-
lytical data, are of value in predicting maximum concentration
values of NO which will be reached under a given set of conditions
x
for localized areas. Data from these models can be used to give
warning of impending "alert" conditions.
-------
5-57
5. Relationships between nitrogen dioxide and nitrate concentrations,
and the types of nitrate formed in different areas, are currently
under study.
RECOMMENDATIONS
1. Continuing increase in NO levels in regions of high population
X
density necessitates the speedy and precise determination of the
short- and long-term effects of this pollutant. This information
is basic in determining whether more, or fewer, monitoring stations
are required.
2. Marked variations in local concentrations over short periods create
the necessity of continuous monitoring in regions of high NO
X
concentrations.
3. Stratospheric data on nitric oxide, nitrogen dioxide, and nitrous
oxide concentrations are incomplete, and chemical interactions
occurring in this region are not precisely known. Therefore,
additional study is required. Effects at the surface of the earth
of stratospheric pollutants also require further investigation.
4. The relationship between nitrogen dioxide and nitrate concentra-
tions under various meterological and geographical conditions
requires further study. The health hazard of nitrogen dioxide and
nitrates, and synergistic effects between these pollutants and
sulfates in the atmosphere, need further evaluation.
-------
CHAPTER 6
CHEMICAL INTERACTIONS OF
NITROGEN OXIDES IN THE ATMOSPHERE
Solar radiation triggers a series of reactions in the atmosphere
between gaseous organic molecules and nitrogen oxides. This produces a
wide variety of secondary pollutants. The totality of primary and sec-
ondary pollutants involved in these photochemical reactions is known as
photochemical smog. The major chemical characteristic of this mixture
is its oxidative nature in contradistinction to sulfur oxide pollutants
which are of a reductive nature.
Secondary pollutants, photochemically produced in the atmosphere from
primary pollutants, create unique analytical problems that cannot be solved
by conventional techniques: there are no point sources to sample; many
significant intermediate products are present in such small concentrations
that their presence can only be hypothesized; and many intermediate prod-
ucts are very transient. Despite these problems these intermediates must
be qualitatively and quantitatively identified so hazards can be evaluated,
limits established, and controls developed.
This chapter identifies the intermediates primarily by chemical and
kinetic modeling, relating laboratory studies to atmospheric observations.
Starting with the relatively innocuous materials, nitric oxide and simple
hydrocarbons, information is advanced on the complex photochemical inter-
actions eventually explaining the diurnal genesis of smog and the potential
sinks and end products. The oxidizing intermediates, i.e., nitrogen dioxide
-------
6-2
(NO ) and peroxyacylnitrates (PAN's), responsible for eye irritation,
adverse respiratory effects, plant damage, and browning of the atmosphere,
presumably end as nitric acid incorporated into aqueous aerosol agglomer-
ations as nitrate salts. Many of the organic molecules present as primary
pollutants or their oxidation products polymerize as they mature forming
organic aerosols which reduce visibility and also brown the atmosphere.
These are also eventually incorporated into aqueous aerosol agglomerations
and scrubbed from the atmosphere. Tables in this chapter indicate the
intermediates, their calculated concentrations, and their theoretical first
half-lives to facilitate future evaluations of possible health effects.
105
Demerjian et^ aJ^. developed a chemical reaction mechanism after a
thorough study and evaluation of all available, related kinetic and mech-
anism studies. This mechanism was tested and improved through the use of
a great variety of smog chamber data from many research groups. This was
74,75,102-104,135,136,162,217,396,472,589,598
not the only effort in this area,
but represents the most complete chemical system developed to date. Any
such complex chemical scheme, no matter how complete and accurate its
reaction details, must be cautiously used. No model today can legitimately
predict quantitatively the chemical and physical changes occuring in the
atmosphere, since certain key information related to many reactants in the
system is still lacking. However, the main features of this model should
be qualitatively correct and should provide a reasonable basis for the
consideration of detailed atmospheric interactions.
-------
6-3
CHEMICAL INTERACTIONS IN THE SUNLIGHT-IRRADIATED, NOx-HYDROCARBON-
POLLUTED ENVIRONMENT
556
Data obtained on a smoggy day in Los Angeles, indicate that the level
of nitrogen oxides rises during peak traffic hours (Figure 6-1). Nitric
oxide concentration declines as that for nitrogen dioxide rises to its
maximum, suggesting a conversion of nitric oxide to nitrogen dioxide.
After most nitric oxide has disappeared, the ozone (0 ) concentration
rises to a maximum near the middle of the day, then falls off during the
afternoon. The dependence of these product rates on the intensity of the
solar radiation, and the extent and nature of the contamination present,
suggested to early workers that the observed changes resulted largely from
sunlight's action on the components of the polluted atmospheres. Thus,
the unpleasant character of these atmospheres was designated as "photo-
chemical smog."
The major portion of the total oxides of nitrogen emitted by combus-
tion sources is nitric oxide. The rate nitric oxide is converted to nitrogen
dioxide through thermal oxidation by the oxygen in air:
2NO + 0 + 2N02 (1)
is proportional to the square of the nitric oxide concentration; it is
therefore very sensitive to changes in nitric oxide concentration. Reaction
(1) can be important (conversion rate: 8% per min) in generating a small
level of nitrogen dioxide (up to 25% of total NO ) during the initial stage
x
of dilution with air. Nitric oxide concentration is then commonly at a
o
level of 625 mg/m (500 ppm). Reaction (1) is much too slow, however, to
account for any significant fraction of the nitric oxide to nitrogen dioxide
-------
6-4
E
o.
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-------
6-5
conversion observed in the real atmosphere for typical ambient levels of
3
nitric oxide 0.06-0.63 mg/m (0.05-0.5 ppm). A much more imaginative reac-
tion scheme must be developed to account for the data in Figure 6-1.
Most of the chemistry that occurs in a sunlight-irradiated urban
atmosphere involves the interaction of a variety of unstable, excited mol-
ecules and molecular fragments (free radicals) which have only a transitory
existence. These species include: the electronically excited forms of
molecular oxygen, singlet-delta oxygen [0 ( A )], and singlet-sigma oxygen
2 g
[0 ( £ )]; the unexcited and first excited electronic states of the oxygen
^* o
atom, triplet-]? oxygen atoms [0( P) ] and singlet-I) oxygen atoms [0( D) ],
respectively; ozone; symmetrical nitrogen trioxide (NO ) ; dinitrogen pent-
oxide (NO); hydroxyl radicals (HO); hydroperoxyl radicals (HO ) ; formyl
radicals (HCO); alkyloxyl radicals (RO); alkylperoxyl radicals (RO );
0 2
acylperoxyl radicals (RC!0 ); and other less important species. R in the
formulas represents a methyl (CH ), ethyl (C H ), or another, more complex
hydrocarbon radical. The paths by which these intermediates are formed and
destroyed are important keys in explaining the chemical changes that occur
in the polluted atmosphere.
Since sunlight triggers the phenomenon of photochemical smog formation,
it is important to recognize those impurities that will absorb light energy.
In some cases, these impurities decompose or become activated for reaction.
A dominant sunlight absorber in the urban atmosphere is the brown gas, ni-
o
trogen dioxide. Light absorption at wavelengths <4,300 A can cause the
rupture of one of the nitrogen-oxygen (N-0) bonds in the nitrogen dioxide
(0-N-O) imlecule and generate the reactive ground state oxygen atom, the
-------
6-6
triplet-P^ oxygen atom, and a nitric oxide molecule. The efficiency of this
process is wavelength-dependent1
NO + sunlight (2,900-4,300 A) > 0(3!P) + NO (2)
The highly reactive triplet-P^ oxygen atom formed in air collides frequently
with oxygen molecules. During such encounters ozone may be formed!
0(3P) + 0 + M •+ 03 + M (3)
M in this equation represents a nitrogen, oxygen, or other third molecule
that absorbs the excess vibrational energy released thereby stablizing
the ozone produced. For most concentration conditions common in polluted
atmospheres, the very reactive ozone molecules regenerate nitrogen dioxide
by reaction with nitric oxide!
0 + NO -> N02 + 02 (4)
Alternatively, ozone may react with nitrogen dioxide to create a new tran-
sient species, symmetrical nitrogen trioxide:
03 + N02 -> N03 + 02 (5)
The nitrate species forms dinitrogen pentoxide, the reactive anhydride of
nitric acid, by reaction with nitrogen dioxide:
NO + NO + NO (6)
Dinitrogen pentoxide may redissociate to form symmetrical nitrogen trioxide
and nitrogen dioxide or possibly react with water to form nitric acid
(HON02):
-------
6-7
NO H, NO + NO
25 3 2
N 0 + H00 ->- 2HON00
252 2
(7)
(8)
The two electronically excited and chemically reactive singlet molecular
oxygen species, singlet-delta oxygen and singlet-sigma oxygen, are produced
in the atmosphere by at least three different mechanisms—
(1) by direct absorption of sunlight:
0 + sunlight
•12,700 A
• 7,600 A
(9a)
(9b)
(2) by electronic energy transfer from electronically excited nitrogen
dioxide molecules (NO *) formed when nitrogen dioxide absorbs sunlight at
o
wavelengths >4,000 A. (This does not provide sufficient energy to dissociate
the nitrogen dioxide):
NO,
N0
2 + 02( A
(10)
(3) and by ozone phytolysis in sunlight:
o
2,900-3,060 A
2,900-3,500 A
4,500-7,000 A
0 + sunlight
02( A ) + 0(1D)
1 3
0 + 0( D) or 0( P)
0(
(Ha)
(lib)
(He)
The singlet-]} oxygen atom is much more reactive than the ground state
triplet-!^ oxygen atom. For example, it reacts efficiently during collision
with a water molecule to form an important transient species in smog, the
hydroxyl radical:
-------
6-8
0( D) + H 0 -> 2HO (12)
~ 2
This radical is also formed through the sunlight photodecomposition of
nitrous acid (HONO):
HONO + sunlight (2,900-4,000 A) -> HO + NO (13)
The hydroxyl radical can reassociate with nitrogen dioxide to produce
nitric acid:
HO + NCL+ M -> HONO + M (14)
or form nitrous acid by reacting with nitric oxide:
HO + NO + M -> HONO + M (15)
A careful review of the net results of Reactions (1) through (15)
reveals that these reactions alone cannot explain the rapid conversion of
nitric oxide to nitrogen dioxide observed in the real atmosphere (Figure
6-1). In fact, if these reactions alone occurred the original supply of
nitrogen dioxide in our atmosphere would be slightly depleted as irradi-
ation with sunlight occurred, and a small and near constant level of ozone
would be created in a few minutes. The key to the observed nitric oxide
to nitrogen dioxide conversion lies in a sequence of reactions between
the transient species which have been generated and other reactive molecules
such as carbon monoxide (CO), the hydrocarbons, and the aldehydes present
in the polluted atmosphere.
One such rational sequence of reactions was first delineated indepen-
218
dently by two groups of scientists, Heicklen and coworkers of Aerospace
-------
6-9
507
Laboratories and Pennsylvania State University, and Stedman and coworkers
at the Ford Motor Company. They noted that a reaction "chain" involving
the hydroxyl radical and carbon monoxide may be important in driving nitric
oxide to nitrogen dioxide in the atmosphere:
HO + CO -> H + CO (16)
2
H + O+M + HO +M (17)
HO + NO -> NO + HO (18)
^. £-
HO + CO -> H + CO , etc. (16)
The hydrogen atom (H) produced in Reaction (16) forms a new transient species,
the hydroperoxyl radical [Reaction (17)] which may oxidize nitric oxide to
nitrogen dioxide [Reaction (18)]. But since a hydroxyl radical is regener-
ated in Reaction (18), many cycles of sequence (16), (17), and (18) may
occur and many molecules of nitric oxide may be oxidized to nitrogen di-
oxide for each hydroxyl radical formed. In principle, therefore, the nitric
oxide to nitrogen dioxide conversion can be accelerated through such a
series of reactions.
Several other molecules present in the polluted atmosphere can play
a similar potentially important role for carbon monoxide. The aldehydes
and hydrocarbons may participate in the sequence of reactions reforming
hydroperoxyl radicals from hydroxyl radicals. Following is the reaction
path involving formaldehyde (CH 0):
HO -' CH 0 -> H 0 + HCO (19)
-------
6-10
HCO + 0 -> HO + CO (20)
2 2
HO + NO -> HO + NO (18)
2 2
The aldehydes in the polluted atmosphere can also function to generate the
76
hydroperoxyl radical. For example, formaldehyde is decomposed by sunlight:
-> H + HCO (21a)
o
CH 0 + sunlight (3,700-2,900 A)
-> H + CO (21b)
2
Both the formyl radical and the hydrogen atom produced in Reaction (21a)
will react in air to form the hydroperoxyl radical:
H+0 +M + HO +M (17)
2 2
HCO + 0 -> HO + CO (20)
2 2
Direct clues to additional steps in the mechanism of smog formation are
provided by both smog chamber studies of hydrocarbon-nitric oxide-nitrogen
dioxide mixtures and atmospheric studies of changes in the relative compo-
sition of hydrocarbon pollutants as the day progresses. For example,
511
Stephens and Burleson found that the complex mixture of hydrocarbon
pollutants sampled in the early morning atmosphere contained a much greater
fraction of olefinic hydrocarbons than a similar sample taken in the late
afternoon. Similar direct observations were made on the composition of
products of trapped auto exhaust before and after irradiation with ultra-
510,511
violet lamps in the laboratory. These results clearly imply that
the chemical reactions initiated in the complex nitric oxide-nitrogen
-------
6-11
dioxide-hydrocarbon mixtures by the action of sunlight remove the olefinic
hydrocarbons at a much higher rate than the paraffinic hydrocarbons.
A great variety of excellent research in both industrial and govern-
mental laboratories has helped establish various reactivity scales for
hydrocarbons, based on the relative ability of the hydrocarbons to gen-
14,66,114,466
erate nitrogen dioxide from nitric oxide in chamber experiments.
550,602
From these experiments and many others, scientists have concluded
that one or more intermediate species present in smog must react with the
olefins and ultimately create hydroperoxyl radicals and organic peroxy
radicals which stimulate the observed conversion of nitric oxide to nitrogen
dioxide. In the lower atmosphere, the olefins are transparent to sunlight;
therefore, they are not acted upon by the sunlight. However, when an olefin
is added to an irradiated mix of nitrogen oxides in air, its removal can
be relatively rapid when attacked by various reactive intermediates. The
olefin propene (C H ) may undergo the following primary reactions:
J o
0 + CH CH = CH -* C H 6 (22a)
+ HO + CH2 = CHCH2 (22b)
0 + CH CH = CH -> [CH CHCH 000] (23)
HO + CH CH = CH -> HO + CH = CHCH (24a)
-> CH CHCH OH (24b)
-y CH CHOHCH (24c)
HO + CH CH = CH -> CH C"HCH 0 H (25a)
2 3 2 322
-------
6-12
HO + CH CH = CH -> H 0 + CH = CHCH
23 2222 2
NO + CH CH = CH -> HONO + CH = CHCH
32 2 22 2
0 ( A ) + CH CH = CH -> CH = CHCH^O^H
2 g 3 2 2 22
(25b)
(26)
(27)
A great variety of chemical reactions follow the formation of the initial
reaction products (10) through (15). For simplicity, consider only the
net effect of the reaction sequences. These reactions ultimately generate
some number (a) of hydroperoxyl radicals, some (3) of alkylperoxy or
acylperoxy radicals, and a variety of different molecules which are largely
aldehydes, ketones, and acids.
HO
or other intermediate
in the mixture
+ RH
olefinic, paraffinic,
or aromatic hydro-
carbon or aldehyde
aH02 + $R02 or (RC02) + other products
(28)
The hydroperoxyl radicals resulting from hydroxyl-olefin interactions
[Reaction (28)] may react in Reaction (18) to cause nitric oxide conversion
to nitrogen dioxide. Presumably the alkylperoxy and acylperoxy radicals
formed in Reaction (28) may oxidize nitric oxide as well in Reactions (29)
and (30). These reactions are analogous to the Reaction (18) involving
hydroperoxyl radicals:
RO + NO -> RO + NO
2 2
0 0
RC02 + NO -> RCO + N02
(29)
(30)
-------
6-13
0
The peroxyacylnltrates, of which peroxyacetylnitrate (CH CO NO ), or
PAN, is the most common, will form in the atmosphere when acylperoxy radi-
8 "8
cals associate with nitrogen dioxide (_R-C-0 N0~) :
0 0
II 'I
RCO + NO -> RCO NO (31)
Speculation and uncertainty exist concerning the importance of the
various atmospheric intermediates in the attack on the olefin hydrocarbons
314
in smog. Leighton discussed- the probable importance in photochemical
smog of oxygen atom and ozone molecule reactions with the olefinic hydro-
carbons. The limited data available in 1961 precluded his evaluation of
these reactions compared to those for transients such as hydroxyl, hydro-
peroxyl, nitrate, alkyloxy, and alkylperoxy radicals, and singlet oxygen
[00( A ) and On( £ )]. Since 1961 several investigators have provided
2 8 2 8
further insight into the potential significance of these possible reactions.
It is now generally accepted that the theoretically calculated combined
rate of attack of triplet-P_ oxygen atoms and ozone molecules on olefinic
hydrocarbon molecules in the photooxidation of nitric oxide-olefin systems
in air may be significantly less than the experimentally observed rate of
13,115,465,509 218
olefin loss in such systems. Recently Heicklen et al. and
507
Stedman et al. have presented evidence that the hydroxyl radical plays
a major role in both the nitric oxide-carbon monoxide- and the nitric oxide-
olefin-photooxidation chains in photochemical smog simulation studies.
35,154,271,604 280 304
Also Bayes, Khan, Kummler, and their research groups have
stimulated interest in reactions of singlet-delta oxygen in the atmosphere.
They suggested that this species participates in the olefinic hydrocarbon
-------
6-14
removal reactions in photochemical smog. The extent of this involvement
514 603
remains untested. Stephens and Price, Wilson et_ al_., and Louw
325
et al., have speculated on the role of nitrate and dinitrogen pentoxide
in the chemistry of polluted air.
A detailed chemical model of the polluted atmosphere, in conjunction
with available computer techniques, can simulate smog formation. This
process has the potential to answer key questions concerning the detailed
mechanism of smog formation and, in particular, to evaluate the various
intermediates involved in smog formation.
SIMULATION OF CHEMICAL CHANGES IN A TYPICAL NITRIC OXIDE-NITROGEN DIQXIDE-
HYDROCARBON-CARBON MONOXIDE-ALDEHYDE-POLLUTED ATMOSPHERE
In the simulations of the polluted atmosphere-like systems described,
it is assumed that there is no atmospheric dilution of products and reac-
tants and that the sunlight is of fixed intensity. All photochemical rates
have been estimated for a solar zenith angle of 40°, a value near the
average encountered during a typical day in the United States. Estimates
314
of the actinic irradiance were taken from Leighton, and are presumed to
be applicable to representative atmospheric conditions near sea level on a
clear day. Modifications incorporating diffusion and regular changes of
solar zenith angle can be made readily, but offer a degree of sophistication
that current chemical knowledge does not warrant.
This model will not provide product rates for positions near point or
line sources such as smoke stacks or freeways where large fluctuations in
236
the input pollutant levels occur. Nevertheless, the "box" models should
give reasonably good answers to chemical questions related both to those
-------
6-15
urban atmospheres somewhat removed from major pollutant sources as well as
for conditions of strong inversion. Such chemical schemes must be coupled
to atmospheric diffusion and local emission models to achieve the full pre-
136,162,472
dictive potential necessary for planning local air pollution control.
Sophisticated kinetic detail might be employed with our present mech-
anism to attempt simulating chemical events that take place in simulated
NO -hydrocarbon polluted atmospheres. A simple mixture of the important
x
classes of reactants has been chosen to illustrate the chemistry of these
systems. To the mixture of nitric oxide at 0.075 ppm and nitrogen dioxide
at 0.025 ppm, is added a background level of methane (CH,), 1.5 ppm; a
typical level of carbon monoxide, 10 ppm; typical levels of total olefinic
hydrocarbon, represented by trans-2-butene (C,H0) at 0.10 ppm; and aldehydes,
4 o
formaldehyde at 0.10 ppm, and acetaldehyde (CH_CHO), at 0.06 ppm (repre-
sentative of all of the higher aldehydes). The effects of adding 0.10
ppm of butane (C/H,~) , representing the saturated paraffinic hydrocarbons,
will be considered later. The relative humidity is assumed to be at 50%
(25° C). Computer simulation enables calculation of the theoretical time-
dependence of products expected when this mixture is irradiated in sunlight
(zenith angle = 40°) (Figure 6-2). Calculations have been made using two
different assumptions concerning the rates of Reactions (8), (32), and (33).
NO + H 0 + 2HONO (8)
2522
NO + NO + H 0 -> 2HONO (32)
2 2
2HONO -> NO + NO + H 0 (33)
22
-------
6-16
20
16
£
.c
Q.
C
o
"•p
£
+j
c
0)
o
c
o
o
12
N 2O 5 + H 2O - 2HONO2 (a)
NO+N02+ H 2O- 2HONO (b)
2HONO - NO+ NO 2 + H 2 O (c)
Solid lines: ka =k^=kc =O
Dotted: max. possible
experimental values
for ka,kt>,kc
20
40 60 80
Irradiation Time ( min.)
100
120
FIGURE 6-2. Theoretical rates of product formation in a sunlight irradiated
(z = 40°), simulated nitric oxide-hydrocarbon-aldehyde-polluted
atmosphere; initial concentrations (ppm) : [nitric oxide]0 = 0.075;
[nitrogen dioxide]0 = 0.025; [carbon monoxide]0 = 10.0; [methane]0 =
1.5; [trans-2-butene]0 - 0.10; [formaldehyde]0 = 0.10; [acetaldehyde]
= 0.06; relative humidity, 50% (25°C).105
-------
6-17
The degree to which these reactions are involved in the real atmosphere is
not yet known. Certainly the magnitude of the homogeneous component of
these rates remains unclear. Thus the simulations have been carried out
using the two possible extreme views concerning the rates of these reactions.
In both extremes (Figure 6-2) it is assumed that there is no prior estab-
lishment of the equilibrium level of nitrous acid before sunlight irradi-
ation. The data represented by the dashed curves in Figure 6-2 were
calculated using the "high" values for the rate constants, k and k ,
590 32 33
estimated by Wayne and Yost, who employed conditions of high surface-to-
volume ratio . The solid curves were calculated using the value, k = 1.0
8
x 10 ppm min , which was derived from "best fits" in the simulation
of several smog chamber studies; this value is about 25 times lower than a
260
previously published estimate of this constant. On the other hand, in
calculating the product concentrations in Figure 6-2, it has been assumed
that Reactions (8), (32), and (33) do not occur at all; that is, the rate
constants k , k , and k are all equal to 0.
o 32 -JJ
Surprisingly, comparisons of the two sets of data in Figure 6-2 indi-
cate that the choice of rate constants for the Reactions (8), (32), and
(33) produces results with only minor differences. For example, solid
curves show a slightly higher concentration of nitrogen dioxide and ozone
for longer periods. This results from neglecting, in the calculation, the
rate dinitrogen pentoxide is removed by water. It is evident that the
rapid occurrence of Reactions (32) and (33) is not a critical factor in
photooxidations of hydrocarbons at the levels encountered in real NO -
x
hydrocarlJn polluted atmospheres.
-------
6-18
The relative significance of the different intermediates involved
in the attack on the hydrocarbons and aldehydes in this simulated pol-
luted atmosphere can be examined in Figure 6-2. Since the reactant con-
centrations in these simulations are comparable with those in real atmo-
spheres, the results are of direct interest to the atmospheric scientist.
The computer-estimated concentrations of the various intermediates and
products at several time intervals during the sunlight exposure are shown
in Table 6-1. These concentrations are very small compared to the stable
product molecules, but their reactivity is so great that they can participate
in many chemical reactions at significant rates. The rates at which given
intermediates react with certain molecules in the atmosphere (olefinic
hydrocarbons in this case) create the conditions governing the nature and
extent of atmospheric reactions. Thus, the product of the concentration
of a given intermediate at a specific time, times the concentration of the
olefin at that same instant, times the rate constant for the given reac-
tion [analogous to those shown in Reactions (22) to (27)], gives the
instantaneous rate of reaction of the given intermediate with the olefin.
(See data summaries in Table 6-2.)
The data predict the great importance of the hydroxyl radical species
in the reaction with the olefin. Only over long periods do the ozone and
hydroperoxyl radicals contribute rates comparable to those for the hydroxyl
radical. The attack of triplet-P^ oxygen atoms is significant only at short
times, but even here its rate is much less than that of the hydroxyl radical.
The forced conversion of the methoxy radical (CH 0) to formaldehyde occurs for
these c> nditions in which the ratio of oxygen concentration to olefin con-
centration is high. There is a marked decrease in methoxy radical attack
-------
6-19
TABLE 6-1
Estimated Concentrations of Intermediates
._ - •--._•-...• _L . _.— i_ __L- - .---i.._ ' . - - ---_.- r --•--_- «3
in Simulated Smog Mixture as Function of Time-
Species
Estimated Concentrations, ppm
2 min
30 min
60 min
HO
HO,
1.7 x 10'
,-7
2.1 x 10
-4
0.88 x 10
,-7
3.2 x 10
-4
0.72 x 10~7
3.7 x 10~
NO,
0(3P)
0.03 x 10 l
3.8 x 10~9
2.2 x 10"
8.9 x 10
,-9
7.4 x 10'
9.5 x 10"
,-6
0.30 x 10
3.9 x 10~6
-15
3.0 x 10
-15
5.1 x 10
-6
5.0 x 10'
,-15
5.2 x 10
-6
°3
NO
0.0085
0.0669
0.0838
0.0163
0.1390
0.0092
NO
0.0324
0.0720
0.0668
C H
4 8
0.0960
0.0595
0.0358
[0 ][NO]
[NO,,]
0.0175
0.0190
0.0192
a.
"The simulated auto exhaust polluted atmosphere has the following initial
concentrations (ppm) of trace contaminants: [NO]0 = 0.075; [N02]° = 0.025;
[trans-2-C,Hg]° = 0.10; [CO]0 = 10; [CH20]° = 0.10; [CH~CHO]° = 0.060; [CH,]°
1.5; relative humidity, 50% (25° C); z = 40°; the rate constants for the
Reactions (8), (32), and (33) are assumed to be equal to zero.
-------
6-20
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6-21
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-------
6-22
when olefin concentration is very high in comparison to its relatively
104
important position observed in previous studies. The theoretical
fraction of olefin attack resulting from nitrate radical and singlet-
delta oxygen is negligible for these conditions. In Table 6-2 the numbers
in parentheses were calculated assuming the rate constants k0, k , and
o 32
k equal to 0 as in the simulation shown by the solid lines in Figure
6-2. The other numbers were calculated using the finite values for these
rate constants as in the simulation shown by the dashed lines in Figure
6-2. There is no significant change in the rates obtained for the attack
on the olefin by the various species when these alternative assumptions
are used.
The paths of destruction of the aldehydes are also interesting to
consider for this case. Table 6-3 summarizes the rates of attack of the
various intermediates on formaldehyde. There is even more selectivity
shown between species here than seen in Table 6-2 for the: olefin. The
hydroxyl radical attack represents about 99.94% of the total rate of
aldehyde reaction which occurs in short times, and this order of preference
continues through the 2-hr irradiation period. The attack on acetaldehyde,
or the higher aldehydes if present, is similar to that on formaldehyde and
need not be considered in detail here. The estimated rate of formaldehyde
-4 -1
loss through photodecomposition (7.3 x 10 ppm min at 2 min) is about
twice the rate due to radical attack alone. The lifetime of the aldehydes
at a concentration of 0.1 ppm is relatively long (about 62 min) in these
systems.
Questions have been raised concerning which reactions trigger the
process of nitrogen dioxide conversion in sunlight-irradiated polluted
-------
6-23
TABLE 6-3
Rate of Attack of Various Reactive Intermediate
Species on Formaldehyde in a Sunlight-Irradiated,
Simulated Auto-Exhaust-Polluted Atmosphere3.*^.
Time,
min
2
10
30
60
90
120
-Initial
Rate of Attack (ppm min x 10 )
o(3p)
0.0010
(0.0010)
0.0019
(0.0019)
0.0023
(0.0024)
0.0019
(0.0023)
0.0013
(0.0019)
0.0008
(0.0014)
concentrations
HO
3.86
(3.66)
3.31
(2.93)
2.34
(2.05)
1.69
(1.66)
1.27
(1.45)
1.05
(0.96)
(ppm) :
H02
0.0046
(0.0045)
0.0063
(0.0057)
0.0077
(0.0071)
0.0075
(0.0073)
0.0064
(0.0066)
0.0056
(0.0046)
[NO]0 = 0.075;
CH3
0.0005
(0.0005)
0.0004
(0.0004)
0.0003
(0.0003)
0.0002
(0.0002)
0.0002
(0.0002)
0.0001
(0.0001)
[N0~]° = 0.025;
N°3
0.0000
(0.0000)
0.0004
(0.0004)
0.0029
(0.0034)
0.0058
(0.0092)
0.0307
(0.0133)
0.0078
(0.0151)
[trans-C/.Hc
0.10; [CO]0 = 10; [CH20]° = 0.10; [CH3CHO]° = 0.06"; [CH,]° = 1.5; relative
humidity = 50% (25° C); z = 40°.
—Results from two simulations are shown; one was made assuming finite
literature values for the kg, k-^j and k-jo rate constants; the other
(results shown in parentheses) was made assuming kg, k32> and koo
equal 0.
-------
6-24
314
atmospheres. The simulation under discussion shows the hydroxyl radical
to be the most important transient species from the standpoint of initiating
the olefin oxidation; however, any reactant which creates either the hydroxyl
or hydroperoxyl radical will effectively generate hydroxyl radicals, since,
for these conditions, the reaction H0? + NO -> HO + NO is the major fate of
hydroperoxyl radicals. To understand this system, it is important to in-
vestigate the relative importance of the several possible sources of both
these radicals for the present system.
The cases representing the two possible extremes for the real atmo-
sphere are again encountered. At one extreme, there is no initial nitrous
acid formation; that is, the rate constants k „ and k equal 0. For assumed
impurity levels there will be only the following initial sources of stimulus
to chemical change:
o -4 -1
NO + hv •* 0( P) + NO; Initial rate = 120 x 10 ppm min (2)
-4 -1
H CO + hv -*• HCO + H; Initial rate = 2.2 x 10 ppm min (21a)
-4 -1
CH CHO + hy -*• CH + HCO; Initial rate = 1.5 x 10 ppm min (34)
Although the rate of oxygen atom formation is by far the largest for the
intermediates formed here, very little impetus to the rate of photooxidation
of the olefin or the conversion of nitric oxide to nitrogen dioxide is
given by Reaction (2). The occurrence of this reaction establishes an
appreciable initial rate of ozone formation. However this rate is not seen
since both ozone destruction and the very fast regeneration of nitrogen
dioxide occur through Reaction (4):
-------
6-25
03 + NO -> 02 + N02 (4)
The extent of chemical change in nitrogen dioxide and ozone is limited very
quickly. If only nitric oxide and nitrogen dioxide were present as impu-
rities in this system, it would be expected that the concentration of
nitrogen dioxide would fall somewhat as Reaction (2) occurs, and that of
ozone would rise to a relatively small steady-state value of about 5.1 x
_3
10 ppm in about 2 min. Reaction (2) results in too small a rate of
-4 -1
oxygen atom attack on the olefin (0.13 x 10 ppm min ), to explain the
rates of chemical change observed in Figure 6-2.
Thus, the major part of the initial impetus to oxidize nitric oxide
and olefin in this system must come from the aldehyde photolyses. Since
most formyl radicals react to form hydroperoxyl radicals under these con-
ditions, the total initial rate of hydroperoxyl radical generation from the
aldehyde photolyses is expected to be about 5.5 x 10 ppm min . From
the initial reactant concentrations, about 94.8% of the hydroperoxyl
radicals oxidize nitric oxide and about 5% react with olefin:
HO + NO + N0o + HO (18)
2 2
HO + CH CH = CHCH -+ CH^CH(00H)CHCH (35)
23 3323
In Reaction (34), an additional reactant, the methyl radical, is formed,
most of which will then form the methylperoxyl radical (CH,^):^^ + 02^-
CH,.09). Thus, the rate of formation of the methylperoxyl radical is:
4 -1
Ratenv _ - 1.5 x 10 ppm min . For these conditions nearly all of
CH~0~
the methylperoxyl radicals will react to oxidize nitric oxide:
-------
6-26
CH 0 + NO -> CH.O + N00 (36)
32 32
Thus the initial rate of nitrogen dioxide formation by way of Reactions
-4 -1
(18) and (36) will be about 7 x 10 ppm rain . This rate of formation
is much less than the initial rate of destruction of nitrogen dioxide by
-4 -1
photolysis (120 x 10 ppm min ). Nitrogen dioxide increase in the smog
system is therefore caused primarily by hydroxyl-olefin reactions which
generate the chain processes that indirectly allow each hydroxyl radical
to oxidize several nitric oxide molecules.
The rates at which hydroxyl and hydroperoxyl free radicals are gener-
ated from the several major sources in the system at various times through-
out photooxidation are shown in Figure 6-2. Most hydroxyl radicals are
formed by the following reactions:
(A) HONO + _hv -> HO + NO (13)
(B) 0(1D) + H20 -»• 2HO (12)
(C) H02 + NO -> HO + N02 (18)
(D) H202 + hv + 2HO (37)
(E) CH CHO H -> CH CHO + HO (38)
-j £- 3
Compare the rates of hydroxyl radical generation from each of these steps
as functions of the irradiation time in Table 6-4.
At very short periods, the photolysis of nitrous acid is not signif-
icant because by setting the rate constant k = 0, its presence is excluded
from consideration. However, even in this case nitrous acid is formed in
Reactions (14) and (39) as the run progresses,
-------
6-27
TABLE 6-4
Comparison of the Theoretical Rates of the
Hydroxyl Radical Forming Reactions in a Simulated,
Sunlight-Irradiated, Auto-Exhaust-Polluted Atmosphere—
Time,
min
0.05
-1 4
Rates of HO Formation (ppm min x 10 )
Reaction Reaction Reaction Reaction
0.0088
0.0015
28.3
0.0003
Reaction
1.5
0.50 0.0907
0.0108
29.3
0.0029
1.6
2.0
0.32
0.023
27.8
0.013
1.6
10.0
30.0
60.0
90.0
0.64
0.29
0.12
0.07
0.082
0.23
0.38
0.48
18.5
10.0
5.9
4.0
0.080
0.33
0.79
1.2
1.7
1.5
0.9
0.5
120.0 0.05
0.55
1.2
1.6
0.2
—The simulated auto-exhaust-polluted atmosphere has the following initial
concentrations (ppm) of trace contaminants: [NO]0 = 0.075; [N02]° = 0.025;
[trans-2-C/jHg]0 = 0.10; [CO]0 = 10; [ClUO]0 = 0.10; [CHgCHO]0 = 0.060;
[CH4]° = 1.5; relative humidity, 50% (25° C); z = 40°. The rate constants
for the Reactions (8), (32), and (33) are assumed to be equal to 0 as in
the data for the solid line shown in Figure 6-2 for this same system.
—See page 26 for Reactions (A) through (E).
-------
6-28
HO + NO -* MONO + 0 , (39)
and its photolysis contributes a maximum of about 3% to the total rate of
hydroxyl radical generation at 10 min. Ozone is absent at the start of
the run; hence, the singlet-D oxygen atom, its photolysis product, is also
absent, and Reaction (12) is not important. However, late in the photo-
oxidation, as ozone builds up, the singlet-D^ oxygen atom contribute sig-
nificantly to the hydroxyl radical formation rate. Hydrogen peroxide
photolysis, also unimportant initially, provides steadily increasing amounts
of the hydroxyl radical as hydrogen peroxide builds up; in fact, after most
of the butene has been oxidized at 120 min, Reaction (37) becomes the major
source of hydroxyl radicals in the mixture with Reaction (38) providing a
steady but rather minor source of hydroxyl radicals. The radical CH CHO H
is one of the products of the reaction sequence which follows hydroperoxyl
addition to the butene.
The highest rate of hydroxyl radical generation occurs through reaction
of the hydroperoxyl radical with nitric oxide [Reaction (18); see Table
-4 -1
6-4]. In fact, the magnitude of this rate (29.3 x 10 ppm min at 0.5
min) is much greater than the rate at which radicals are made in sunlight-
-4 -1
initiated steps (7.0 x 10 ppm min ); from hydrogen and formyl radicals
-4
formed by aldehyde photolyses (5.5 x 10 ppm); and by way of methyl
formation from acetaldehyde photolysis (1.5 x 10 ppm min ) [Reaction
(34)]:
(02) (NO) (0 )
+ CH302 + CH30 ^ H02 + CH 0
-------
6-29
Hydroperoxyl radicals must be regenerated in a chain reaction to provide
the observed rate of Reaction (18). This chain process can be understood
by comparing the rates of hydroperoxyl formation from the various sources
of this radical:
(A) CH 0 + hv -* HCO + H; H + 0 + M -> HO + M (21a)
HCO + 02 -y H02 + CO
(NO) (02)
-> HC007 -> N0? + HCO -> HO + CO
^ £- £- 2.
-+ (HCOO NO ) -> HONO + CO
(B) CH CHO + h\> -> CH + HCO (34)
•J -3
HCO + 0 [as in (A) ] ->- aHO + products
(C) HO + CO -> CO + H; H + 0 + M + HO + M (16)
(D) CH CH((5)CHO -»• CH-CHO + HCO (40)
HCO + 0 [as in (A)] -> aHO + products
(E) CH 0 + 0 ^ HO + CH 0 (41)
J ^ £• £~
(F) CH CH(6)6 + 02 ->• CH3C02 + H02 (42)
-------
6-30
The rates of hydroperoxyl radical formation from each source are compared
in Table 6-5. This comparison indicates that the photolyses of both
formaldehyde and acetaldehyde continue as significant sources of the hydro-
peroxyl radical through hydrogen and formyl radical formation (see columns
A and B in Table 6-5). Acetaldehyde photolysis also forms hydroperoxyl
radicals through these methyl radical reactions:
(02) (NO)
CH -> CH 0 -> CH 0
3 32 3
The methoxy radical generates the hydroperoxyl radical in Reaction (41);
however, this reaction provides only a small part, 1.5 x 10 ppm min ,
of the total rate after a short period (Table 6-5, Column E).
The reaction of the hydroxyl radical with carbon monoxide is a third
source of the hydroperoxyl radical [Reactions (16) and (17)]. This may be
the major hydroperoxyl regeneration step in the chain oxidation of nitric
218,507
oxide to nitrogen dioxide:
HO + CO -> H + CO (16)
H + 0 + M -> HO + M (17)
•*— £.
HO + NO -> HO + N02 (18)
Although important in this regard, it is not the main source of the hydroxyl
radical to hydroperoxyl radical conversion in this simulated polluted
atmosphere.
In sequence (40), the hydroperoxyl radical is produced in the decompo-
sition of the theoretical intermediate radical, CH CH(6)CHO, formed in the
reaction sequence which follows the hydrogen atom abstraction from butene
-------
6-31
TABLE 6-5
Comparison of the Theoretical Rates of the
Hydroxyl-Forming Reactions in a Simulated,
Sunlight-Irradiated, Auto-Exhaust-Polluted Atmosphere—
Time,
min
0.05
0.5
Reaction Reaction Reaction Reaction Reaction
(A)- (B)- (C)- (D)- (E)-
4.0 1.5 4.2 5.1 15.8
4.0 1.5 4.4 5.2 16.5
Reaction
(F)b
0.05
0.1
2.0
4.1
1.6
4.3
4.9
15.9
0.2
10.0
30.0
4.3
4.3
1.8
2.2
3.2
2.2
3.0
1.5
11.8
8.8
0.6
1.1
60.0
4.2
2.5
1.8
0.7
6.9
1.1
90.0
3.8
2.5
1.7
0.4
5.4
0.8
120.0
3.4
2.4
1.3
0.2
3.9
0.5
Ct
—The simulated auto-exhaust-polluted atmosphere has the following initial
concentrations (ppm) of trace contaminants: [NO]0 = 0.075; [NC^]0 = 0.025;
[trans-2-G,Hs]° = 0.10; [CO]0 = 10; [CH-O]0 = 0.10; [CH^CHO]0 = 0.060;
[CH4]° = 1.5; relative humidity, 50% (25° C); z = 40°. The rate constants
for the Reactions (8), (32), and (33) are assumed to 0 as in the data shown
in the solid curves in Figure 6-2 for this same system.
—See pagf 29 for Reactions (A) through (E).
-------
6-32
by the hydroxyl radical (or other radicals that abstract hydrogen atoms to
a much smaller extent) .
The rates in Column E, Table 6-5, show the largest source of hydro-
peroxyl radicals in this system to be the methoxy radical [Reaction (41) ] .
Shortly after photooxidation of the butene-containing mixture begins, the
metlivisy radical is formed from methyl radicals derived from sevemi renc-
tion paths. Acetaldehyde photolysis provides a relatively minor source.
The methyl radical is also formed in the reaction sequence which follows
hydroxyl radical addition to the butene. Specifically, its formation is
postulated in the following step:
CH3CH(OH) -> CH3 + HC02H (43)
Another source of methyl radicals is the attack by hydroxyl radicals on
acetaldehyde:
(HO) (02) (NO)
CH3CHO -> CH3CO -> CH3C002 -* CH3C02 -> CH3 + C02
When the ratio of nitric oxide to nitrogen dioxide concentration is rela-
tively high, the nitric oxide oxidized by acetylperoxyl radicals (CH COO )
yields primarily the acetate (CH CO ) radical; peroxyacetyl nitrate is the
-3 £•
favored product only at low ratios of nitric oxide to nitrogen dioxide con-
centrations over the long run.
In theory, the reaction sequence which follows ozone attack on the
• •
olefin leads to the acetylhydroperoxyl radical [CH~CH(0)0], Its reaction
with the oxygen molecule is the fifth source of the hydroperoxyl radical
(Reaction (42)]; this contribution to the rate, shown in Column F, Table
-------
6-33
6-5, is negligible at short times, but as ozone builds up over long expo-
sures, it makes a finite contribution.
These considerations illustrate the complex interplay among the various
reactants in this simulated smog-forming atmosphere which stimulates the
generation of the important hydroxyl and hydroperoxyl radicals. In the
selected reactant conditions, the olefin is the major source of conversion
of hydroxyl to hydroperoxyl radicals. Both the abstraction of hydrogen
atoms from the olefin by the hydroxyl radical, and the addition of the hydroxyl
radical to olefin ultimately generate a hydroperoxyl radical with moderate
efficiency. In this reaction sequence at least one nitric oxide molecule
is also oxidized to nitrogen dioxide by various alkylperoxy radicals. Thus,
during short exposures (Figure 6-2), when each hydroxyl radical abstracts
hydrogen atoms from butene, at least one molecule of nitric oxide is con-
verted to nitrogen dioxide and one hydroperoxyl radical is formed. For
each addition of a hydroxyl radical to butene, nitrogen dioxide and another
hydroperoxyl radical are formed from an average of three nitric oxide mol-
ecules. Most of the hydroperoxyl radicals are rapidly reconverted to
hydroxyl radicals through the reaction: HO + NO -> HO + NO . Therefore,
the chain sequence of hydroxyl radical attack on butene (C H ) continually
4 o
degrades the olefin until the chain is terminated. The rate of hydroperoxyl
formation in the primary photolytic processes (7.0 x 10 ppm min at
2 min) and the total rate of attack by both hydroxyl and hydroperoxyl
-4 -1
radicals on this (19.8 x 10 ppm min at 2 min) suggests a short chain
reaction (about 2.8 cycles in length at 2 min) involving hydroxyl and
hydropercxyl radical oxidation of the olefin. Of course, the chains
stop whenever a hydroperoxyl or hydroxyl radical is removed through
-------
6-34
reaction with another radical or an odd electron molecule such as nitric
oxide or nitrogen dioxide. The dominant chain-ending steps for the con-
centration conditions selected for this system are:
+ H^ + 02 (44)
HO + NO + M -> HONO + M (15)
HO + NO + M -> HONO + M (14)
CH0 + H0 -* CH0H + 0 (45)
The rates radicals are removed by reactions in the Figure 6-2 simulation
-1 -4
at 2 min, are as follows (ppm min ): Rate = 4.6 x 10 ; Rate = 1.2 x
-4 -4 -4
10 ; Rate,, = 0.8 x 10 ; Rate = 0.07 x 10 . These account for hydro-
-4 -1
peroxyl and hydroxyl radical removal rates of 6.6 x 10 ppm min . This
-4 -1
should match the rate of primary radical production (7.0 x 10 ppm min )
if all radical sources and termination reactions are included.
The origin of the rapid rise in the concentration of nitrogen dioxide
in the smog simulation is shown in Figure 6-2. The nitrogen dioxide oxida-
tion rate in these experiments also reflects the occurrence of these chain
processes. In the simulations the observed rate of nitrogen dioxide forma-
-4 -1
tion at 2 min is about 52 x 10 ppm min . The true rate of formation of
nitrogen dioxide at 2 min is much greater (208 x 10 ppm min ) since
-4 -1
nitrogen dioxide is destroyed in Reaction (2) at 156 x 10 ppm min by
the action of sunlight. The major processes forming nitrogen dioxide at
the 2 min exposure are:
-------
6-35
0 + NO -»• NO +0 (4)
HO + NO -> HO + NO (18)
RO + NO -> RO + NO (29)
The simulation predicts that the rate nitrogen dioxide forms in Reaction
-4
(4) at 2 min is about 131 x 10 ppm, while that of Reaction (18) is 28 x
-4 -1
10 ppm min . Therefore, the several reactions of the various acyl and
alkylperoxy radicals [RO- in Reaction (29)] must account for about (208-
-4 -4 -1
131-28) x 10 = 49 x 10 ppm min . This expectation is consistent with
the rates of alkyl and acylperoxyl radical generation at 2 min. The hydroxyl
radical attack on olefin and acetaldehyde, and the hydroperoxyl radical
attack on olefin are the only important sources of alkyl and acylperoxyl
radicals for these conditions. The simulations show that at 2 min irradiation
-4
the hydroxyl radical reacts with olefin to add at a rate of 12.3 x 10 ppm,
-4 -1
and to abstract hydrogen atoms at a rate of 4.9 x 10 ppm min ; the
-4
hydroxyl radical abstracts hydrogen atoms from acetaldehyde at 2.4 x 10
-1 -4
ppm min ; and the hydroperoxyl radical adds to olefin at 1.6 x 10 ppm
min . Each hydroxyl radical addition to olefin forms about three alkyl-
peroxyl radicals; each hydroxyl radical that attacks acetaldehyde creates
about two acylperoxyl radicals; each hydroxyl radical that abstracts
hydrogen atoms from olefin and each hydroperoxyl radical which adds to
olefin usually form one alkylperoxyl radical. Therefore, the total rate
of alkyl and acylperoxyl radical formation can be given by:
Rate =_ (3 x 12.3 + 2 x 2.4 x 4.9 + 1.6) x 10~4 = 48 x 10~4 ppm min"1.
-------
6-36
This agrees with the rate of nitrogen dioxide formation attributed to nitric
oxide oxidation by alkyl and acylperoxyl species in Reaction (29) (^49 x
A —1
10~ ppm min ).
Nitrogen dioxide destruction by photolysis and its subsequent refor-
mation by reaction with nitric oxide and ozone [Reaction (4)] can by itself
only lead to a net decrease in the nitrogen dioxide equal to the ozone level
developed.
NO + hv -> 0 + NO (2)
2
0+0+M + O+M (3)
0 + NO -> 0 + NO (4)
Therefore, the nitrogen dioxide increase observed in irradiated smog results
from both hydroperoxyl and alkyl and acylperoxyl radical Reactions (18) and
(29), and under the conditions at the 2 min point of the photochemical smog
simulation (Figure 6-2), the alkyl or acylperoxyl radical contributes to
the observed rate of nitrogen dioxide formation about 1.8 times more than
the hydroperoxyl radical. Both types of radicals, however, are very impor-
tant in this conversion.
Since the ozone-olefin and oxygen-atom-olefin reactions contribute
little to the rate at which hydroperoxyl and hydroxyl radicals form early
in the run, it is incorrect to assume, as many investigators have, that
the original driving force for the nitric oxide photooxidation must result
from radicals produced in the ozone and oxygen atom reactions with olefin;
obviously this is not the case when aldehydes are initially present.
-------
6-37
In the alternate hypothesis of nitrous acid preequilibration in the
simulated polluted atmosphere, nitrous acid is assumed to have been formed
_0
at its equilibrium value, [HONO] = 6.1 x 10 ppm, before sunlight ir-
radiation of the mixture. All other compounds remain at their previous
concentrations; Reactions (32) and (33), in which nitrous acid is generated
and destroyed, respectively, are assumed to have their literature values.
The initial rate of radical generation in this system includes the same
rates of hydroperoxyl and methylperoxyl formation from formaldehyde and
acetaldehyde photolyses as before, but there is an additional source of
hydroxyl radical formation from the nitrous acid photolysis:
HONO + hv -> HO + NO (13)
This rate is about 7.3 x 10 ppm min for zenith angle = 40°. Compare
this with the rate of hydroperoxyl formation from all of the aldehyde
-4 -1
photolysis processes; Rate = 7.0 x 10 ppm min . The nitrous acid
H0«
in the atmosphere can significantly boost the initial rate of olefin photo-
oxidation and nitric oxide to nitrogen dioxide conversion, approximately
doubling these initial rates for the simulations if nitrous acid pre-
equilibrium were allowed before irradiation. Assuming preequilibration of
nitrous acid and allowing the finite values for the rate constants k ,
8
k „, and k as before, it can be predicted from simulations not shown
here that the maximum in the nitrogen dioxide concentration would occur at
22 min, compared with 24 min to reach the maximum concentration for the
same mixture but without nitrous acid initially. (See the dashed curves,
Figure 6-2.) Assuming the rate constants k0 = k = 0, it takes 31
o jz
min to reach this maximum when there is no nitrous acid initially present.
(See the solid curves, Figure 6-2.)
-------
6-38
Nitrous acid is not necessary for smog formation; but its presence
can enhance the initial rate of the smog-forming reactions. Its presence
is invoked to rationalize nitric oxide photooxidation in moist atmospheres
containing carbon monoxide as the only other oxidizable component. The
levels of nitrous acid present in the real atmospheres must be experimentally
determined along with which of the two extremes considered above best
represents the real situation.
The large ozone concentration formed in these simulated polluted
atmospheres is significant in view of the low concentration limits
specified for ozone by the U.S. Environmental Protection Agency: a max-
imum concentration of 0.08 ppm/l-hr period, not to be exceeded more than
566
one time/yr. In theory, the ozone concentration ultimately formed in
these systems is largely controlled by the magnitude of the ratio of
nitrogen dioxide to nitric oxide concentration and the sunlight intensity.
The product of the concentrations of nitric oxide and ozone to nitrogen
dioxide ratio is virtually a constant for the zenith angle = 40° simula-
tions at the three times shown (see Table 6-1). The concentrations of
hydrocarbons (olefins, paraffins, and aromatics), aldehydes, and carbon
monoxide, however, all influence the final nitrogen dioxide to nitric oxide
concentration ratio, determining ozone concentrations in a complex and
interrelated way.
-------
6-39
THE EFFECTS OF THE SATURATED HYDROCARBONS ON THE CHEMISTRY OF THE SIMULATED.
POLLUTED ATMOSPHERE
The "reactive" paraffinic hydrocarbons have been excluded previously
in an attempt to simplify the simulated system. If butane or similar
paraffins were present, hydroxyl radicals would attack them at a moderate
rate to abstract hydrogen atoms; the secondary butyl radical (CH^-CH-CH^CH^
is the major organic product of this interaction with normal butane
n-c4H10 + HO -* H20 + ^ec-C^H (46)
The main sequence of reactions for the higher alkyl radicals in oxygen
leads to the generation of carbonyl compounds and hydroperoxyl radicals;
thus, for secondary butyl radicals, the major sequence at early stages of
this reaction, for the concentrations of mixture components chosen in the
simulation, would be the following:
(Q2)
. , , N -> CH COG H + HO
(0 ) (NO) 325 2
sec-C H -> sec-C,H 0 -> sec-C,H 0
4 9 492 4 9
-> CH CHO + C2H5
(o2)
(0) (NO)
-> CH + CH 0
3 2
(02) (NO) (0 )
CH 0 -> CH 0 -> CH 0 + HO
32 3 22
-------
6-40
Using the rate constant estimates and the initial concentration conditions,
about one hydroperoxyl radical is formed for every hydroxyl radical which
attacks butane and about 1.5 nitrogen dioxide molecules are generated from
nitric oxide. Butane, in the simulated atmosphere at the level of the
olefin, 0.10 ppm, would be attacked by the hydroxyl radical at about
-4 -1
1.2 x 10 ppm min at 2 min. This increases the rate of nitrogen di-
oxide formation in the system by only 2%. Paraffin hydrocarbons are
added to the real atmosphere at levels near those of olefins, during
510
the early morning auto-exhaust-pollution. This causes only minor
disturbances in the reaction scheme outlined for the simpler, simulated
polluted atmospheres. (See page 51 for further considerations of the
olefin-free system containing NO and paraffin hydrocarbons.)
X
THE EFFECTS OF VARIATION IN THE INITIAL ALDEHYDE CONCENTRATIONS IN THE
SIMULATED POLLUTED ATMOSPHERE
77
Calvert and McQuigg have investigated the influence of the initial
aldehyde concentrations on the smog chemistry of the simulated polluted
atmosphere. Figure 6-3 plots product-time data for simulated smog mixtures
containing aldehydes initially (solid curves) and without initial aldehydes
(dashed curves); each of the other components is initially at the same con-
centration in both cases. As the aldehydes are removed completely for the
initial conditions, a small induction period appears in the rates of olefin
removal, and nitrogen dioxide, ozone, PAN, and nitric acid formation. The
final levels of the dashed and solid curves indicate higher concentrations
of ozone, PAN, and nitric acid when aldehydes are present initially.
-------
6-41
20 i-
E
.c
Q.
Q.
C
o
"•p
SS
4->
C
0)
o
c
o
o
[CH 2 Ol°=0.10ppm
CH 3 CHOl° = 0.06
Dotted jfRCHO 1°=0.0
60 80
Time (min.)
FIGURE 6-3. The effect of initial aldehyde concentration on the theoretical
rates of product formation in a sunlight irradiated (z = 40°)
simulated auto exhaust polluted atmosphere; composition same
as that in Figure 6-2 with the exception of the absence of
73
initial aldehydes in the dashed curves.
-------
6-42
THE EFFECTS OF VARIATION OF THE OLEFINIC HYDROCARBON CONCENTRATIONS ON THE
PRODUCT-TIME DATA IN A SIMULATED POLLUTED ATMOSPHERE
Observing the relationship of certain smog manifestations with the
138
concentrations of hydrocarbons and the oxides of nitrogen,, Faith et al.
derived the relation of eye irritation to NO and hydrocarbon levels in
x
smog chamber studies (Figure 6-4). Figure 6-5 shows the now famous rela-
tionship between the maximum daily 1-hr average oxidant levels and the 6-9
a.m. average concentration of nonmethane hydrocarbons for several cities
464
(Schuck et_ aJL. ). The maximum observed oxidant values produce a curve
that falls with decrease in hydrocarbon level; but the shape of the curve
at low hydrocarbon values is obscure. A search for similar predicted
correlations with the simulated atmosphere should be conducted cautiously
because of the serious limitations imposed by our present state of knowledge.
The predicted time dependence of ozone as a function of olefin (trans-
77
2-butene) level (Calvert and McQuigg ) is summarized in Figure 6-6. In
this series of experiments the initial concentrations of the oxides of
nitrogen are fixed: [nitric oxide]0 = 0,075 ppm and [nitrogen dioxide]0 =
0.025 ppm; and aldehydes absent initially. The relationship between ozone
and olefin is obviously complex; but, in general, some decrease in ozone
concentration can be expected for lowered initial olefin concentration. The
final ozone concentration reached in these systems can be well above the initial
total NO present. Figure 6-7 effectively illustrates the 8-hr integral of the
X
ozone concentration versus time, and the concentration of peroxyacetylnitrates
77
versus time data as a function of the olefin concentration. Data are given for
with aldehydes present initially (dashed curves) and with no aldehydes present
-------
6-43
C
o
0)
>»
LU
fa
"»;
FIGURE 6-4.' Relation of eye irritation to hydrocarbon and oxides of
nitrogen levels in smog chamber experiments from Faith
138
et al.
-------
6-44
E
a
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c
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CO
0
<
CO
a
E
3
E
'x
CO
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0.25
0.20
0.15
0.10
0.05
0
Los Angeles
Los Angeles ^^^*
Washington ± ^^ Denver
^ A Los Angeles
^•^ Philadelphia
Los Angeles
Philadelphia ^ ^
Philadelphia **
Washington^* AA
Washington^* Philadelphia
Washington A A A A / A
Washington
AA
A A
A A
A 1* fc£ AAA*AA±AAA*AAAA AA
A
A A
0 0.5 1.0 1.5 2.0 2.5
6-9 a.m. Average Nonmethane Hydrocarbon Concentration (ppmC)
FIGURE 6-5. Relationship between the maximum daily 1-hr average oxidant
levels and the 6-9 a.m. average concentration of nonmethane
hydrocarbons derived from the data from several cities; from
464
Schuck et al.
-------
6-45
NOJ0= 0.025 ppm
NO]° = 0.075
[RCHO]° = 0.0
[CO]0 = 10.0
246
Irradiation Time (h)
FIGURE 6-<".
The theoretical time dependence of O- as a function of olefinic
hydrocarbon (trans-2-butene) level in a sunlight irradiated
(z = 40°), simulated auto exhaust polluted atmosphere; aldehydes
are assumed absent initially in this case; from Calvert and
73
McQuigg.
-------
6-46
160 -
120 -
480
(ppm - min.)
[CH 2 O]=0.10 ppm
fCH 3 CHO]=0.06
[NO 2K 0.025 ppm
fNOl°= 0.075
n*1^ ^-w ~i « •• f* f\
[CO] ° =10.0
i CO ° =10.0
iiini iiiiiiMiiiiiiiiiiiiiiiiiiii i nun 111
\ 480
X f TO oldf value if
480
TO 3]df value if
0
0.08 ppm for 8 h
i.
16
12
8
480
I [PANldf
J 0
(ppm - min.)
4
10
20 30
fC 4 H 8J ° (pphm)
40
FIGURE 6-7.
The theoretical effect of the reactive hydrocarbon concentra-
tion (trans-2-butene) on the 8-hr integral of the [ozone] and
[peroxyacylnitrate] versus time data derived from a simulated
sunlight irradiated (z = 40°), auto exhaust polluted atmosphere;
73
data from Calvert and McQuigg.
-------
6-47
initially (solid curves). The variation of integrated oxidant and peroxy-
acetylnitrate levels with the initial olefinic hydrocarbon concentration,
so important in the determination of standards, are altered dramatically
by the initial presence of aldehydes. A reference value of the / [0 ]dt
data is represented by the horizontal dashed line drawn at 38.6 ppm-min,
which is the value of this integral if the maximum allowable 1-hr ozone
concentration average in the Environmental Protection Agency (EPA) ambient
air quality standards were maintained for 8 hr. These data indicate that
this standard could not be met if the aldehydes remained high (formaldehyde
concentration = 0.10 ppm, acetaldehyde concentration = 0.06 ppm) even if
nearly all of the olefinic hydrocarbon were removed. The increase in the
integrated [PAN concentration]-time levels results from the presence of
aldehydes, particularly acetaldehyde. Both ozone and PAN data for runs
with and without aldehydes (Figure 6-7) converge at high olefin levels and
the integrals approach plateaus. The curves representing the "true"
relationship between nonmethane hydrocarbons and maximum 1-hr oxidant at
low hydrocarbon levels could be influenced by the aldehydes, a variable
not commonly measured.
THE EFFECTS OF VARIATION OF THE CONCENTRATIONS OF THE NITROGEN OXIDES ON
THE PRODUCT-TIME CURVES IN A SIMULATED POLLUTED ATMOSPHERE
For years smog chamber experiments have indicated that an inhibiting
effect on product rates and certain smog manifestations can be expected
at very high nitric oxide concentrations. For example, in runs at a
fixed hydrocarbon concentration of about 4.5 ppm, the expected eye irri-
tation passes through a maximum level, then decreases again as NO is
X
-------
6-48
increased (Figure 6-4). The same inhibiting effect of high nitric oxide
concentrations has been observed in product rate data determined by Tuesday
550
and coworkers in smog chamber experiments using olefin-NO mixtures.
X
Predictions can be made from the simple model.
Figure 6-8 shows the time dependence of expected ozone concentration
for runs at fixed hydrocarbon levels: [butene]0 = 0.10 ppm; [aldehydes]0
= 0; [carbon monoxide] = 10 ppm; [methane]0 = 1.5 ppm; 50% relative
humidity; the ratio of the initial concentrations of nitric oxide to
nitrogen dioxide was held constant at 3.0. Concurrent with the progres-
sion from the very low level of an initial nitric oxide concentration
of 0.15 to 15 pphm, is a gradual increase in oxidant level at 2 hr of
irradiation; but further increases of the initial nitric oxide concen-
tration to 30, 60, and 120 pphm cause a suppression in the ozone con-
centration-time curves. In Figure 6-9, the 2-hr integrals of the con-
centrations of ozone- and PAN-time data are given as a function of the
initial nitric oxide concentration. The maximum in the integrals occurs
near the stoichiometric mixture of one olefin to one NO molecule.
x
These data do not mean that unrestricted emissions of NO would solve
x
the smog problem; however, they do imply that smog formation would be
delayed. At some point downwind, the turbulent mixing, diffusion, and
dilution of the NO -containing mixture will cause a reduction in the NO
x x
level which will be loaded for smog formation. The recent experience in
the Los Angeles area probably relates to this phenomenon. Ozone levels
are lower in downtown Los Angeles compared to previous years, probably as
-------
6-49
20
15
N
(pphm)
10
[C4 Hs]° = °-10 ppm
[RCHO]° = 0.0
[CO]° =10 ppm
[NO]
(pphm)
V
15
Irradiation Time (min.)
FIGURE 6-8. The theoretical effect of variation of the concentrations or
the nitrogen oxides on the ozone product-time curves in a
simulated polluted atmosphere; the initial concentrations of
the reactants are as shown, and [nitric oxide]"/[nitrogen
73
dioxide]0 is constant at 3.0. From Calvert and McQuigg.
-------
6-50
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6-51
a result of an increase in nitric oxide levels; yet, very high levels of
ozone and other smog products are seen in the Riverside area after the
Los Angeles mixture has been modified and diluted during transport to the
Riverside area by prevailing winds.
THE EFFECTS OF VARIATION IN THE INITIAL CARBON MONOXIDE CONCENTRATION ON
THE PRODUCT-TIME CURVES IN A SIMULATED POLLUTED ATMOSPHERE
Relatively low levels of carbon monoxide in simulated NO -hydrocarbon-
X
atmospheres produce no dramatic effects in the rates of product formation.
Figure 6-10 shows that increases in initial carbon monoxide concentration
from 10 to 50 ppm decreases the ozone and PAN concentrations for a given
sunlight exposure time. Hydrocarbons and aldehydes are attacked pri-
marily by the hydroxyl radicals which are generated in the smog system.
The reaction of hydroxyl radicals with carbon monoxide competes with
the reactions of the olefins and aldehydes at higher carbon monoxide
concentrations, and an increasing share of the chain regeneration of
the hydroperoxyl radicals from the hydroxyl radicals is then born by
the following sequence:
HO + CO -> H + CO (16)
H + 0 + M ->- HO + M (17)
HO + NO -> HO + NO (18)
£. z,
Reduced attack by hydroxyl radicals on aldehyde and olefinic hydrocarbon
results in lower alkylperoxy and acetylperoxy radical formation. Thus ,at
higher concentrations of carbon monoxide,there is a reduction in the nitric
-------
6-5?
20 i-
rc H
L 4 8j
CH2OJ° =0.10
CH 3 CHOJ ° - 0.06
FIGURE 6-10.
60 80
Time (min.)
The theoretical effects of variation of the initial concen-
tration of the carbon monoxide on the ozone and peroxyacyl-
nitrate formation in a sunlight irradiated (z = 40°), simu-
lated NO -hydrocarbon polluted atmosphere.
X
From Calvert and
McQuigg.
73
-------
6-53
oxide conversion to nitrogen dioxide associated with the alternative chain
processes involving the hydrocarbons and aldehydes. As a result the ozone
concentrations are slightly lowered. The lowered rate of acetylperoxyl
0
II
radical (CH CO ) generation is principally caused by the less important
-3 ^
attack by hydroxyl radicals on acetaldehyde and olefin. This is reflected
in a lowered PAN concentration since PAN comes largely from the following
sequence:
CH CHO + HO -> CH CO + HO (47)
0
CH3CO + 02 ~> CH3C02 (48)
0 0
If II
CHC0 + N0 -> CHC0N0 (49)
At extremely high levels of carbon monoxide (2,000 ppm) , PAN formation
is practically eliminated since attack of hydroxyl radicals on acetal-
dehyde and olefin no longer competes with that of carbon monoxide
[Reaction (16)]. It is not suggested that PAN formation be reduced in our
polluted atmospheres by removing controls on carbon monoxide emissions
since the toxic properties of carbon monoxide outweigh the usefulness gained
by PAN reduction.
There is one other unexpected aspect of the chemistry of carbon mon-
oxide-containing atmospheres. If the hydrocarbon and aldehyde impurities
were entirely removed from the atmosphere and carbon monoxide allowed to
rise along with the oxides of nitrogen, then carbon monoxide could act as
an effective reactant, pumping nitric oxide to nitrogen dioxide, and con-
75
tributing to ozone levels.
-------
6-54
Figure 6-11A plots the experimentally-determined concentration/time
from a smog chamber study of W.E. Wilson, Jr. and D.F. Miller (personal
communication). The concentrations in the chamber at the start of the
irradiation were: [nitric oxide]0 = 51 pphm; [nitrogen dioxide]0 = 10
pphm; and [carbon monoxide]0 = 100 ppm. The ozone concentration climbs
to a level of about 13 pphm after 5 hr of irradiation. Figure 6-11B
shows the computer simulation of these product rates based on the mechanism
105
of photochemical smog formation described by Demerjian et^ al. Figure
6-11C shows other expected products for which analyses were not made.
The driving force for this reaction is the generation of hydroxyl radicals
from nitrous acid photolysis:
0
HONO + sunlight (2,900-4,000 A) -> HO + NO (13)
If the reactions of nitrous acid formation and destruction,
HO + NO + NO -> 2HONO (32)
2HONO -> HO + NO + NO , (33)
occur in the atmosphere at rates comparable to those observed in chambers,
then the carbon monoxide-effect can be significant for relatively low
75
ambient levels of NO and carbon monoxide (see Table 6-6). Ozone concen-
x
trations of about 7 pphm, approaching the 1-hr maximum level of 0.08
ppm, are expected after about 2.5 hr of irradiation. Further study is
necessary to define the rates of nitrous acid formation in the real atmo-
spheres before a conclusion can be reached concerning this potentially
important carbon monoxide effect.
-------
6-55
FIGURE 11A.
Initial concentrations, [nitric
oxide]0 = 51 pphm; [nitrogen
dioxide]0 = 10 pphm; [carbon
monoxide]0 = 100 ppm; relative
humidity, about 13% at 32.78° C.
FIGURE 11B.
Computer simulation for the
experimental conditions
employed in Figure 6-11A.
FIGURE 11C.
Computer simulation of the
expected time dependence of the
minor products for the con-
ditions employed in Figure 6-11A.
60 120 180
IRRflDIflTION TIME.MIN
300
FIGURE 6-11.
The photooxidation of nitric oxide in carbon monoxide-containing
mixtures; comparison of experimental and computer simulated chem-
ical changes in nitric oxide-nitrogen dioxide-carbon monoxide
mixtures irradiated in moist air; experimental analysis for these
products was not made. From W.E. Wilson, Jr., and D.F. Miller,
personal communication. Data are unpublished results from studies
conducted in a 17.2 m^ smog chamber at Battelle-Columbus
Laboratories, 1970.
-------
6-56
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fo-58
In theory, the same influence noted for carbon monoxide can be expected
in mixtures of the reactive paraffin hydrocarbon with NO that is free from
X
olefinic hydrocarbons, provided that the rates of Reactions (32) and (33)
in the real atmosphere are comparable to those in chambers. Because the
values of the rate constants for the homogeneous and heterogeneous experi-
mental olefin-free reactions are uncertain, the key questions for these
systems cannot be definitively answered. However, these unexpected effects
should be considered in the development of more detailed air quality
standards.
Because of the technological difficulties in removing NO and carbon
A.
monoxide from auto exhaust and NO from stack gases, ozone levels may
X
continue to plague many urban areas, although a near total removal of the
reactive hydrocarbons might be effected.
THE THEORETICAL MASS BALANCE OF THE NITROGEN-CONTAINING COMPOUNDS FORMED IN
THE SIMULATED POLLUTED ATMOSPHERE
The nature of the nitrogen-containing products formed in photochemical
smog has been a matter of considerable interest among scientists. Note the
distribution of the products predicted by the present model. Figure 6-12
plots the percentage of total nitrogen present in selected compounds at
various sunlight irradiation times in the synthetic NO -hydrocarbon-aldehyde
x
polluted atmosphere. Nitric oxide conversion to nitrogen dioxide is fol-
lowed by a continuing transformation of the nitric oxide and nitrogen diox-
ide into two major products—nitric acid and PAN. A much smaller quantity
of methyl nitrate builds up as the reactions continue. The percentage of
the total nitrogen contained in the reactive transients, symmetrical nitrogen
trioxide and dinitrogen pentoxide, is negligible. Nitrous acid and methyl
-------
6-59
100
80
60 -
% Total N
in Compd.
40
20 -
[c4Hg]°=0.10ppm
[l\IO2J°=0.025
[NO] °= 0.075
[CO]°=10.0
[RCHO]°=0.0
MONO.
PAN
NO
FIGURE 6-12.
iiiiiiiiiiiiiiiiiimiiiiiiiiiiiiiiiiiiiiiiiimiiiiiiiiiiiiiiiiiiiQiii
60 120
Irradiation Time (min.)
Theoretical distribution of the nitrogen-containing products
formed in the simulated, sunlight-irradiated (z = 40°), NO -
hydrocarbon-aldehyde polluted atmosphere; the initial X
73
composition of the mixture is the same as in Figure 6-2.
-------
6-60
nitrite are formed but their concentrations do not climb appreciably since
both compounds are rapidly dissociated by sunlight.
o
HONO + sunlight (2,900-4,000 A) -> HO + NO (13)
0 '
CH ONO + sunlight (2 ^'9 00^4,000 A) -> CH 0 + NO (50)
The nitric acid, PAN, and methyl nitrate absorb sunlight only weakly.
Since other reactions which remove them are not fast, they accumulate. In
the real urban atmosphere nitric acid does not build up as expected from
the present reaction scheme; but presumably, the nitric acid that is formed
reacts with certain basic impurities in the atmosphere: ammonia, the oxides
of the metals, etc.
Because quantitative treatment of such heterogeneous reactions is not
possible at this time, such potentially important steps have not been
included in the present simulation. However, they must occur in the real
atmosphere, and nitric acid may be converted efficiently to ammonium
nitrate if- the concentration of ammonia impurity is sufficiently high.
Ammonium nitrate comprises approximately 10-15% of the total airborne
particles in composite samples collected in the Los Angeles area from 1971-
183
1972.
The presence of nitrates in food and water and the possible direct
deposition of the soluble ammonium nitrate salt in the respiratory system
23,183
have been of concern because of possible biological effects. The
formation of the large amounts of nitrate predicted by the model has never
been observed in studies attempting nitrogen balances in the urban atmo-
sphere. Perhaps this indicates the incompleteness of the mechanism, or,
-------
6-61
possibly, that the rate constant for nitric acid formation, derived from
the limited published data, is slightly larger than the true value:
HO + NO + M -* HONO + M (15)
It is also possible that there are some as yet unrecognized analytical
problems associated with nitric acid and nitrate determinations in the
urban atmospheres.
FORMATION OF NITRATE SALTS IN THE ATMOSPHERE
Nitrate salts may be formed in the atmosphere through a variety of
reaction paths. One path, similar to the mechanism forming sulfuric acid
73,77,557
(H SO ) and sulfate salts, is not an important source of nitric
acid and nitrate salt. The low vapor pressure of pure sulfuric acid results
in a rapid homogeneous nucleation at partial pressures of sulfuric acid in
-8 -10 119 281
the range 10 to 10 torr, according to Doyle and Kiang et al.
The hydration of sulfuric acid droplets is thermodynamically favorable over
a wide range of relative humidities, and the aerosol particles of the sul-
furic acid solution grow as they take on water. This process continues
until the vapor pressure of the water over the sulfuric acid solution
equals the partial pressure of water in the atmosphere. In comparison,
the vapor pressure of nitric acid is very high, and homogeneous nucleation
of nitric acid aerosol formation in a moist atmosphere is not expected to
281
occur under normal atmospheric conditions. However, nitric acid does
form some mixed compounds or complexes of considerable stability in sulfuric
174
acid solutions and may become incorporated within sulfuric acid droplets
79,433
at the lower temperatures of the upper atmosphere. To assess the
importance of this possible removal pathway for nitric acid requires the
measurement of the nitric acid vapor pressures over solutions of sulfuric
79
acid-water-nitric acid at various compositions and temperatures.
-------
6-62
Presumably such a chemical entrapment of nitric acid would be followed by
neutralization of the aerosol with atmospheric ammonia (NH ), ultimately
producing ammonium nitrate (NH,NO.) and ammonium sulfate [(NH.)0SO.].
43 424
The direct homogeneous capture of gaseous nitric acid by gaseous
ammonia (NH + HONO -> NH NO ) may be a significant source of ammonium
J £• t J
nitrate salt in the atmosphere if the ammonia levels in the polluted atmos-
92
phere are sufficiently high. Countess and Heicklen have found that the
analogous homogeneous reaction between ammonia and hydrogen chloride (HCl)
-2 -1 -1
gases occurs with a rate constant of 2.8 x 10 ppm min . Assuming
the ammonia-nitric acid reaction to have a rate constant of this magnitude
and concentrations of 1 pphm, the homogeneous rate of ammonium nitrate
formation is estimated to be 2.8 x 10 ppm min . This rate is a sig-
nificant fraction of the theoretical rate of nitric acid formation in
Reactions (14) and (8). This nitric acid removal path is worthy of
further quantitative consideration and study. The rate constant of the
nitric acid-ammonia reaction must be measured, and the ambient levels and
the diurnal pattern of gaseous ammonia present in the urban atmosphere
must be established before any realistic modeling of this reaction can
be made.
Most of the reaction paths for nitrate salt formation considered thus
far involve the reactions of gaseous nitric acid. It is»therefore,important
to consider the probable sources of this compound in a smoggy atmosphere.
The analysis of the systems points to two major homogeneous sources of
nitric acid in photochemical smog mixtures:
HO + NO + M -> HONO + M (14)
2 2
NO + H 0 ->- 2HONO (8)
252 2
-------
6-63
The rate nitric acid forms through abstraction of hydrogen from molecules
and radicals by the intermediate, symmetrical nitrogen trioxide present in
photochemical smog is much less important than Reactions (14) and (8) for
normal atmospheric conditions.
Rate data for Reactions (14) and (8) are not quantitatively estab-
lished, but they are quantitatively useful in estimating the approximate
rates and the relative importance of the two reactions in a simulated
sunlight-irradiated, NO -hydrocarbon-polluted atmosphere. The best current
X
estimates of the homogeneous rate constant for Reaction (14) vary within
4 4-1-1
the range 1.5 x 10 to 0.46 x 10 ppm min , expresed as apparent second
order reactions with M=l atm of nitrogen gas. These values have been
105 546
estimated by Demerjian et al. and Tsang. To date, the best kinetic
368
data for the homogeneous Reaction (8) are those of Morris and Niki
which give the rate constant kQ < 1.9 x 10 ppm min . These rate
o
data and concentrations of the intermediates and reactants for a simulated,
typical NO -hydrocarbon-polluted atmosphere yield the following rates of
X
nitric acid formation:
Rate ^. 2.0 x 10 ppm min (using rate data of ref. 508)
14 ~
-5 -1
Rate £o 6.6 x 10 ppm min (using rate data of ref. 105)
14 ~
-5 -1
Rate < 1.6 x 10 ppm min
8 —
The estimated rate of Reaction (8) is probably much higher than the
actual homogeneous rate since it is very difficult to extract the rate
constant k from experiments in which a large heterogeneous component of
o
-------
6-64
the reaction dominates the rate. Thus, a lower value (k0 = 5 x 10 ppm
o
min ) was required to match chamber data in some of the simulation studies
105
of Demerjian e_t^ al. The nitric acid formed homogeneously in the smoggy
atmosphere probably results largely from Reaction (14). The rate of ni-
trogen dioxide conversion to nitric acid in this reaction amounts to
about 2.4-7.9% per hr at the 30 min irradiation point picked for the
183
simulation. This range is slightly higher, but not out of line with the
formation rates of the aerosol containing nitrate ions observed in Los
Angeles smog (Whitby, personal communication). Nitric acid conversion
to aerosol containing nitrate ions may not be complete on the time scale
selected since capture mechanisms ultimately require ammonia (or another
basic compound) as a neutralizing agent, which may be in short supply.
The typical homogeneous rate of nitric acid formation given above
should, therefore, represent a maximum rate of nitrate salt formation
from these particular reactions.
The nature of the nitrogen-containing products expected in smog simu-
lations ,which neglect the heterogeneous removal processes, reveal that
nitric acid, peroxyacetylnitrate, and methyl nitrate are the major sumps
73
for the nitrogen oxides in smog. As described previously, the conversion
of nitric oxide to nitrogen dioxide is followed by a continuing transfor-
mation of nitric oxide and nitrogen dioxide into two major products—nitric
acid and peroxyacetylnitrate.
105
The Demerjian et al. estimate for kn. has been used in these
— — 14
simulations. Thus, the rate of nitric acid formation may be lower
somewhat if the equally reliable estimate of Tsang is employed for
-------
6-65
508
the rate constant k . It follows that the amount of peroxyacetyl-
14
nitrate expected may exceed the nitric acid formed.
The simulation predicts that a large fraction of the NO ultimately
X
ends up as peroxyacetylnitrate. It is known that peroxyacetylnitrate
119
hydrolyzes in solution to form nitrite ions. Therefore, the hetero-
geneous removal of peroxyacetylnitrate may follow encounters with an
absorption by the aerosol droplet, and nitrite salts may be formed from
the peroxyacetylnitrate if the solution's pH remains sufficiently high
through ammonia molecule capture by the aerosol.
The extent to which nitrate and nitrite salts result from gaseous
nitrogen dioxide and nitric oxide absorption into aerosol droplets is not
yet known. But the following reactions can lead to nitrate in principle,
if sufficient ammonia, or other basic compounds, neutralize the acids
formed upon formation of the solution:
2NO + HO ^ H+ + NO ~ + HONO (Aqueous Solution)
NO + NO + HO J 2HONO (Aqueous Solution)
HONO + OH~ + HO + NO (Aqueous Solution)
^ £.
2NO + 0 (in aerosol solution) -»• 2NO
22 3
NO ~ + 0 (in aerosol solution) -»• NO ~ + 0
23 32
These potential heterogeneous reactions forming nitrate may be sufficiently
rapid to account for all of the observed nitrate formation according to
232,233
Hidy. However, this conclusion remains open to question at this
-------
6-66
incomplete stage of knowledge concerning these mechanisms. A major problem
with the former "standard" method for nitrogen dioxide analysis in the
atmosphere (Jacobs-Hochheiser method) is the inefficient collection of
nitrogen dioxide by the basic solution used in the bubblers in the collec-
tion train. Although the nitrogen dioxide should dissolve readily in the
highly basic bubbler solution, the rate of solution is also a function of
the interfacial area. In the acidic or near neutral aerosol droplets
finely dispersed in the atmosphere, however, solution is slow and insig-
nificant. Further study of this system is necessary to evaluate its
significance in nitrate salt formation in smog and health-related problems.
107
De Pena et al. have observed in the laboratory that ammonium
nitrate is generated by a complex reaction between ammonia and ozone.
There is evidence of heterogeneity associated with the rate-determining
step in the overall reaction. It is not clear if any fraction of the
rate is truly homogeneous in character.
A variety of additional homogeneous and heterogeneous reactions have
been considered in simulations of reactions in atmospheres containing
73,183
sulfur dioxide (SO ). Space limitations prevent their detailed
considerations. However, such reactions are expected to lead to sulfuric
acid, sulfate salts, and other sulfur containing compounds which presumably
557
contribute to aerosol formation.
There are many uncertainties concerning the mechanism generating
sulfate and nitrate salts in the sunlight-irradiated, NO -sulfur dioxide-
X
hydrocarbon polluted atmosphere. However, theoretical homogeneous rates
of transformation of NO to nitric acid and sulfur dioxide to
x
-------
6-67
sulfur trioxide and sulfuric acid are significant. Free radicals generated
in the smog reactions probably are the important reactants that promote
these changes in the real atmosphere.
Several heterogeneous paths may independently effect these trans-
formations, although their importance is also uncertain. The heterogeneous
reactions leading to the accumulation of acids in the aerosol particles
and the subsequent neutralization reactions must be significant in the
overall conversion of these acids to the salts.
Extensive fundamental research related to these processes is neces-
sary to establish the relative importance of the different possible mech-
anisms for the atmospheric conversion of sulfur dioxide and NO to sulfate
and nitrate salts and to develop realistic control procedures. An aerosol
21
characterization study by Appel et al. relates analytical data to possible
formation mechanisms.
HEALTH EFFECTS RELATED TO REACTIVE INTERMEDIATES FORMED IN THE SUNLIGHT-
IRRADIATED. URBAN ATMOSPHERE
Since the early days of research in photochemical smog mechanisms,
there has been interest and speculation concerning the influence on
biological systems of the transients in smog (such as triplet-]? oxygen
265
atoms, etc.). For example, many years ago Johnston, and Leighton and
315
Perkins discussed this possibility in relation to eye irritation. The
probability that these species dominate the chemistry within the atmosphere
suggests that they might react equally efficiently with various biological
systems.
These intermediates probably do not act directly on lung tissue
since most of the driving force for radical formation in the atmosphere
-------
6-68
comes from the sunlight. Therefore, when a portion of the atmosphere is
inhaled into the darkness of the lung, the intermediates should die too
quickly to act on the lung tissue. But, more detailed investigation
indicates that the lifetime of some intermediates is not so short after
all. The present knowledge of the reactions of the transients and their
rate constants encourages reconsideration of the possibility of this
direct influence of the intermediates.
Inhalation of urban air removes this air from the sunlight turning
off most of the radical forming steps.
The transients then decay by the various paths characteristic of
these species and the other contaminant molecules present. The time
required for each transient species in the simulated atmosphere to decay
to one-half of its initial concentration when removed from the sunlight
into a dark region,such as the lung,is compared in Table 6-7. (No dilution
effects were considered in deriving the data of Table 6-8.) Calculations
have been made for the concentration conditions which are present at 2,
30, and 60 min sunlight irradiation of our polluted atmosphere employed
previously (Figure 6-2, solid curves). Compare these times with the period
_2
for the inhalation cycle, about 3.5 x 10 min. If the half-life of the
species is very short with respect to this time, then there should be few
species left as the air reaches the lung.
As expected, the ozone has a half-life as long as 4 min when nitric
oxide and olefinic hydrocarbon concentrations decrease after 60 min of
irradiation. It lives a slightly shorter time (t, /„ ^_ 0.4 min) when a
sample is taken earlier during sunlight exposure. These times are suf-
ficient to allow levels of ozone in the lung to approach those of the
-------
6-69
atmosphere itself. Furthermore, the ozone-olefin reaction will continue
to occur in the dark, generating within the lung the reactive ozonide, the
peroxy diradical (the so-called zwitterion), and various other free radical
intermediates unique to this pollutant.
The life of the hydroperoxyl radical may be comparable to the breathing
cycle (Table 6-7). This species should be considered as a possible source
of health effects. The conclusions concerning the possible roles of
singlet-delta oxygen and symmetrical nitrogen trioxide are not as clear
cut, since the half-times are a factor of 10 to 100 times shorter than
that of the inhalation cycle. These species might seem unimportant;
however, look at the singlet-delta oxygen system in more detail. Firestone
146
and Calvert have estimated the time dependence of singlet-delta oxygen
in a model lung by computer simulation of a breathing cycle and the kinetics
of transport and decay of singlet-delta oxygen. The model involves the
initial impurity concentrations of the simulated atmosphere employed in
Figure 6-2. The level of singlet-delta oxygen at the steady state in this
irradiated atmosphere is about 6.2 x 10 ppm. Two different mathe-
matically simplified models of the air-flow time pattern have been
employed in these calculations: a linear lung expansion model and a
sinusoidal variation in the lung volume with time. During the inhalation
3 3
cycle of about 0.035 min, 500 cm of air is brought into a 500 cm
volume of retained gases in the lung. Perfect mixing of this "new" air
occurs during the turbulent inhalation and it is saturated with water
3 3
vapor. Then 500 cm of the 1,000 cm total lung gases is exhaled in a
cycle of equal duration (Figure 6-13).
-------
6-70
TABLE 6-7
Theoretical First Halflives for Some Reactive
Chemical Species in Photochemical Smog When Moved
from the Irradiated, Simulated Polluted Atmosphere to a Dark Volume—
Time into Simulated Irradiation for which con-
centration conditions were chosen» ppm
Species 2 min 30 min 60 min
°(I^ -12 -12 -1
8.9 x 10 8.9 x 10 xz 8.9 x 10 1
0(^ -1 -1 -7
1.7 x 10 ' 1.7 x 10 ' 1.7 x 10 '
HO
3.9 x 10~5 5.0 x 10~5 6.0 x 10~5
N03 3
7.1 x 10~A 1.1 x 10 1.5 x 10
0?(lA ) -444
8 9.0 x 10 9.0 x 10 9.0 x 10
H°2 -2 -2 l
4.2 x 10 9.5 x 10 1.1 x 10 -1
3 4.3 x ID"1 2.1 4.2
a
"Composition of simulated polluted atmosphere same as that in Figure 6-2.
-------
6-71
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-------
6-72
In Figure 6-13, the cross-hatched area is bounded by the concentration
curves obtained using the two different breathing models„ The linear model,
which more closely simulates the deep breathing pattern, shows the highest
maximum that one would expect. The concentration of singlet-delta oxygen
which is expected to be reached during the inhalation is significant; this
amounts to 2.6% of the original level of singlet-delta oxygen in the irra-
diated air. The gradual decline in the concentration of singlet-delta
oxygen after the maximum is reached comes from the increasing extent of
dilution of the incoming air as the lung expands. As exhalation starts,
there is no further supply of singlet-delta oxygen available and the decay
rate alone determines the rapid fall observed for this period. Symmetrical
nitrogen trioxide will exhibit a similar pattern since its lifetime for
these conditions is about the same as that for singlet-delta oxygen.
The expected maximum concentrations for the hydroxyl radical are
significantly lower than the conditions chosen here. The lifetime data
for triplet-P^ oxygen atoms and singlet-D^ oxygen atoms suggest that few of
these species will live long enough to populate our lungs.
We should remain suspicious that some of the transients in photo-
chemical smog may themselves introduce health related problems. Until
more direct evidence is available to evaluate this possibility, no firm
decision seems possible.
SUMMARY STATEMENTS
Rational hypotheses have been developed from present knowledge of
chemical kinetics allowing mathematical descriptions of the relationships
between photochemical intermediates such as ozone, PAN's, and hydroxyl
-------
6-73
radicals. Such relationships will be useful in the development of air
quality control planning.
Typical polluted atmospheres used in chemical and kinetic models have
indicated concentrations, measured in ppb, of photochemical intermediates
such as hydroperoxyl radicals and symmetrical nitrogen trioxide which have
a long enough life to allow transport to the respiratory system. The
significance must still be determined.
Because of the many interdependent factors involved in photochemical
intermediates and the high uncertainty surrounding some aspects of the
reaction mechanism and rates, it is not possible to set numerical standards
for individual pollutants without further definition of the important
interrelationships between pollutant molecules.
CONCLUSIONS
Solar radiation induces a number of reactions in the atmosphere between
gaseous organic molecules and nitrogen oxides producing a variety of so-
called secondary pollutants. These secondary pollutants are present in
extremely small concentrations and many are very transient. Because of
these two factors it is very difficult, if not impossible, to identify
these pollutants by conventional analytical techniques.
Starting with simple hydrocarbons and nitric oxide, attempts have been made
to identify intermediate and end products of the photochemical reactions
by relating laboratory studies involving chemical and kinetic modeling to
atmospheric observation. Since certain key information is still lacking,
it is not yet legitimately possible to make quantitative predictions.
-------
6-74
Enough information is available to indicate that the main features of the
model are correct, placing considerable reliability on the qualitative
predictions.
Based on calculated concentrations and theoretical first half-lives,
it is possible to predict photochemical intermediates that might persist
long enough to be transported into the respiratory system. The health
significance must still be determined.
It is not possible to set numerical standards on secondary pollutants;
however, sufficient information is available concerning the different
relationships to assist in the development of air quality controls.
With respect to heterogeneous reactions, the extent to which
nitric and nitrous acids and nitrate and nitrite salts form
as a result of gaseous nitrogen dioxide and nitric oxide absorption into
aerosol droplets is not now known. It is known that the rate of absorption
of acidic gases is a function of the partial pressure of the gases, rate
of diffusion of the gases into solution, the pH of the solution and the
interfacial area. It is still necessary to evaluate these parameters
relative to nitrate salt formation in smog and in related potential health
problems.
RECOMMENDATIONS FOR FUTURE STUDY
Efforts should be made to determine the effect on health of photo-
chemical intermediates such as hydroxyl, hydroperoxyl, and symmetrical ni-
trogen trioxide which can be predicted in typical polluted atmospheres and
which have a life long enough to allow transport to the respiratory system.
-------
6-75
These intermediates must always be produced in a reaction vessel as one of
an array of intermediates. They cannot be studied as independent entities.
Further smog chamber studies of hydrocarbon-nitric oxide-nitrogen
dioxide mixtures should be made in which intermediate and final products
are related to the composition of hydrocarbon pollutants found in atmo-
spheric samples.
It is necessary to continue kinetic studies of all possible photo-
chemical reactions that can be induced by sunlight and of transients that
are encountered within the troposphere.
More detailed information, such as turbulent diffusion, local emissions,
and solar zenith angle changes, must be included as well as chemical reactions
in the development of useful simulations of actual urban atmospheric reactions.
Future studies should continue to investigate the relative importance
of the various reactive species in the removal of olefin hydrocarbons from
the atmosphere.
More meaningful reactivity scales should be developed for hydrocarbons
based on their ability to generate nitrogen dioxide from nitric oxide in
chamber studies.
Further elucidation is required concerning the modes of formation of
nitrate salts in the atmosphere in both homogeneous and heterogeneous reac-
tions (primarily in gas-gaseous liquid phases) and their relation to potential
adverse health effects.
Ultramicroanalytical techniques should be developed to analyze suspect
trace intermediate products and short-lived products formed in chamber
studies and in the real atmosphere.
-------
6-76
The inhibiting effect of very high nitric oxide concentrations on
product rates and certain smog manifestations such as eye irritation re-
quires further study.
-------
CHAPTER 7
EFFECTS OF NITROGEN OXIDES ON NATURAL ECOSYSTEMS
It is difficult to assess the complex cause and effect relationships of
any pollutant with a single organism. When attempting to assess such rela-
tionships with populations, communities, and ecosystems, the problems
increase still further. The determination of the effects of pollutants on
natural communities is additionally complicated by the presence of multiple
contaminants that might promote synergistic or antagonistic effects.
The constant fluctuation in the number of organisms in natural sys-
tems results from both the normal interactions of the physicial and biotic
environment, and either purposeful or inadvertant interference by man.
Since man-produced pollutants are now distributed globally, they should
be considered in the study of any natural community, regardless of the
distance from man's influence.
ANTICIPATED EFFECTS OF NO ON NATURAL PLANT COMMUNITIES
AND ECOSYSTEMS x
Ecosystems are integrated units of organisms and the abiotic physico-
chemical factors with which the organisms interact. After centuries of stable
annual climatic and geochemical conditions, ecosystems become self-perpetuating,
403
or climax, units.
Climatic, physicochemical, or biological changes, regardless of their
source or nature, will affect the nature of the ecosystem. Some ecosystems are
durable and relatively stable when subjected to a given environmental change;
496
others become unstable given the same change. W.H. Smith examined
these differences and divided these air pollutant relationships into three
classes: low, intermediate, and high dosage.
-------
7-2
EFFECTS OF NO ON ANIMAL COMMUNITIES
x
It is difficult to disassociate the effects of NO on animals from the
x
effects of other air pollutants. However, the meager information that does
exist fails to indicate that current levels of nitrogen oxides have much
influence on functioning animal communities. Another difficulty encoun-
tered in the assessment of NO effects on natural animal communities is
x
the unreliability of the cause and effect relationships predicted in laboratory
studies. Extrapolation from one species to another is also untrustworthy
since susceptibility can vary widely among species.
339
McArn et aJ. demonstrated that granule-lad en microphages
developed in the lung tissues of English sparrows (Passer domesticus)
from high pollution urban centers, but not in the lungs of those from wind-
swept, unpolluted environments. Because the sparrow is a relatively short-
lived species, potential chronic effects could not be observed. The longer-
lived domestic pigeon or rock dove (Columba livia) can also be used as a
model in similar studies. But because pigeons are intimately associated
with urban pollution problems, pest control programs might eliminate
a study population at a crucial time.
EFFECTS OF NO ON PLANT COMMUNITIES
x
In most terrestrial ecosystems, the majority of both long- and short-
term effects of pollutants on ecosystems are caused by the sensitivity of
various species to environmental change. Additional long-term effects
491, 607
are created by pollutant action on reproductive capacity and genetics.
117 133
Dowdy and Elton provided evidence that the basic biotic
responses of an ecosystem to disturbance may be simply the replacement
of unsuited species by those newly favored in a changing environment.
-------
7-3
404
Odum listed the general effects of environmental perturbation on
ecosystems as: reduction in standing crop; reduction in productivity;
differential kill; food chain disruption; succession setback; and changes
in nutrient cycling rates.
608
Woodwell added that any chronic pollutant affecting ecosystem
structure reduces the recovery potential of the site. Changes in the
plant community, such as size, rate of energy fixation, or species
complement, affect the structure of the animal and microbial com-
munities and, therefore, the ecosystems. These changes alter behavior
608
patterns and disrupt the competitive relationships among species.
Structural changes that reduce environmental or biological variability
in plant communities have been shown to cause new species to assume
52, 97, 272, 277, 342, 343, 355, 356, 496, 544, 608
dominance.
Very little is known about the specific effects of NO on native plant
X
species. As pointed out in Chapter 9, however, NO shares the following
X
characteristics of other pollutants on crop species: differential suscep-
tibility of species; differential effects due to diurnal and age conditions;
and generally unpredictable synergistic effects when considered simul-
taneously with other environmental factors.
Although NO compounds have seldom been demonstrated to cause
X
ecosystem, or even species, damage, visible damage may constitute only
543
a small part of the actual damage to plant communities. It is dif-
ficult to single out the effect of any particular environmental factor on
plant communities because plants are influenced by many factors simul-
225
taneously. Hepting stated, "Since the rate of growth of forest stands
and the vigor and appearance of individual trees are influenced by so many
-------
'-4
site factors, including soil type, moisture, temperature, drainage,
competition, etc. , alien impacts not identifiable -with known diseases or
insects can go unrecognized unless very severe damage is done."
Reductions in plant vigor and survival rates have recently been
225
attributed to air pollution. A well-publicized example is the ozone
(O ) damage to ponderosa pine forests in Southern California -which had
3 355
been previously ascribed to pathogenic action.
Secondary or synergistic effects of air pollutants on plant species
275
and communities are known. In Chapter 9, the synergistic action of
NO with sulfur dioxide (SO ) and ozone is reviewed. Studies on crop
x 2
species showed gas mixtures to be damaging at concentrations well below
the threshold causing injury when gases were applied alone. In natural
ecosystems environmental factors as diverse as insects, bacteria, fungi,
soil, water stress, etc., are candidates for synergistic impacts and should
501, 502,544
be considered when determining the effects of pollutants.
608
Woodwell established that the effects of all environmental per-
403
turbants are similar. Odum reviewed those effects, but they were
422
perhaps most succinctly summarized by Platt. Environmental changes
affect single species and populations in different ways depending on geno-
typic variation, life cycle stage, and microhabitat condition. Among the
ecological effects of a given pollutant are the physiological tolerances
to other environmental stresses such as heat, moisture, and light; com-
petitive capacity; and susceptibility to parasites and other disease
organisms. Physiological phenomena such as growth, flowering, photo-
synthesis, and respiration also result. In plant communities or eco-
systems, environmental changes, including chemical changes, affect
-------
7-5
energy flow, productivity, succession, community structure and
composition, and such interspecific characteristics as competition and
various aspects of symbiosis.
Although the responses of communities and ecosystems to pollutants
are very difficult to examine, some published studies indicate progress
in this area. The proceedings of the first two U.S. National Symposia
391,468
on Radioecology, published in 1963 and 1969, are major con-
tributions to the literature on this subject. In 1973, the excellent work of
355
Miller and his colleagues provided a model demonstrating the expected
effects of pollutants on plant communities.
The impact of air pollutants on plant communities and ecosystems
409,496, 596
has been studied extensively. Each investigator restated
422
the 1963 conclusions of Platt and called for immediate and intensive
research on the response of ecosystems to acute and chronic environ-
mental pollution. They all included NO among the pollutants definitely
X
requiring further research.
52,
There are several possible techniques for studying ecosystems.
53, 170
Specific models for studies on radioactivity appear in the afore-
391,468
mentioned proceedings of the U.S. National Symposia on Radioecology.
356
Models for ozone have been described by Miller and Yoshiyama.
EFFECTS OF NITROGEN OXIDES ON MICROBIAL PROCESSES IN SOILS
AND WATERS
Microorganisms are essential to the biosphere and to the function of
diverse natural ecosystems. They are chiefly responsible for destruction
by decomposition of dead plant and animal tissues. Their metabolism re-
generates the carbon dioxide (CO ) essential to plant life. Microfloras are
-------
7-6
the major agents for destruction of synthetic chemicals introduced into soils
and waters. Marine algae are essential for the generation of the oxygen re-
quired to sustain life in all higher animals. In soil, the bacteria, fungi, and
actinomycetes convert compounds of nitrogen, sulfur, and phosphorus to the
inorganic state, thereby providing plants with the required inorganic nutrients.
Biological nitrogen fixation and nitrification are affected solely by these micro-
scopic organisms, which also maintain soil structure and form the humus
important to abundant plant growth. In addition, many of the pathogens that are
constantly discharged into soils and waterways are eliminated by microbial
actions.
Since microorganisms are critical to the balance of ecosystems,
any disturbance in their activities could have serious consequences on a
local, regional, or global scale. The potential impact on microorganisms
by substances as widespread and pervasive as the nitrogen oxides must
therefore be assessed. Surprisingly, this subject has been neglected to
date. The few data are based on NO concentrations in excess of those
x
found in the atmosphere.
In addition to nitric oxide (NO) and nitrogen dioxide (NO ), the effects
of nitrous oxide (N_ O) must also be considered in any effective assessment
of the responses of natural ecosystems to the nitrogen oxides.
Nothing is known about the influence of NO on microbial activities
2C
in soils, waters, or other ecosystems in which microorganisms multiply,
and the published reports deal only with individual species in vitro.
Furthermore, generalizations are impossible because some reports show
effects where other studies show no effect at low NO concentrations.
x
For example, 0. 002% nitric oxide, the lowest concentration tested by
301
Krasna and Rittenberg, inhibited hydrogenase activity of the bacterium
-------
7-7
Proteus vulgar is by 87%, whereas high nitric oxide concentrations failed
to kill significant numbers of bacteria when tested in the absence of oxygen
477
but in highly artificial test conditions. Conversely, a 70 ppm concen-
tration of nitrogen dioxide stimulates luminescence by an unnamed bac-
495
terium, although at a concentration of 100 ppm, it was toxic. Nitro-
gen dioxide also affects the survival of airborne microorganisms. Thus,
a 1. 5 ppm concentration of nitrogen dioxide was lethal for the bacterium
605
Rhizobium meliloti in aerosols. Airborne Venezuelan equine enceph-
alomyelitis virus was inactivated at a 5 ppm nitrogen dioxide concentration,
but not at 0. 5 ppm. At 10 ppm, the highest concentration tested, the
viability of airborne spores of the bacterium Bacillus subtilis, was not
131
reduced.
The antibacterial effects of radiation are modified by nitrogen oxides.
For example, a 0. 5% concentration of nitric oxide sensitized wet spores of
Bacillus megaterium to radiation, but the sensitivity of the spores
449
decreased at higher nitric oxide concentrations. Nitrogen fixation
by Clostridium pasteurianum ceases at a 0. 1% concentration of nitric
53
oxide. Nitrogen-fixing enzyme preparations are also sensitive to
nitric oxide; it has been reported that concentrations of 0. 039% and
322a,438a
0. 0025% have depressed the reduction of molecular nitrogen
(N ) to ammonia (NH ).
The nitrite formed from the oxides in natural ecosystems has long
been known to be an antimicrobial agent. As an illustration, Clostridium
perfringens may fail to grow in laboratory media containing an 80 ppm
436
concentration of sodium nitrite (NaNO, ). Considerable information is
also available on the inhibition of toxin production by the bacterial agent
of botulism, Clostridium botulinum, but the levels needed for toxicity are
-------
7-8
considerably higher than those found in soils and waters. The inhibition
439
is affected appreciably by pH and salt concentration. Nitrite that
473
occasionally accumulates in soils is toxic to fungi in that environment.
Recent evidence indicates that blue-green algae are inhibited mark-
edly by nitrite (NO~), one of the products of NO with water. Thus, the
^ X
rate of photosynthesis at pH 6. 0 by blue-green algae was reduced from 75
_o
to 100% by 10 M_ nitrite. By contrast, photosynthesis by green, yellow,
and red algae and respiration of several bacteria were inhibited from 0
to 20% by the same nitrite concentration. Thus, blue-green algae are
apparently uniquely sensitive to the nitrite formed from nitrogen oxides,
an inhibition which might be of considerable importance in those natural
ecosystems where these algae are abundant (R. S. Wodzinski, D. P. Labeda,
and M. Alexander, unpublished observations).
Nitrous oxide has been the subject of some attention, but again the
interest has been on individual microorganisms in vitro. For example,
0. 25 atm of nitrous oxide, a very high level, inhibits nitrogen fixation but
434
not the assimilation of ammonium-nitrogen by Azotobacter vinelandii.
Nitrous oxide is also a specific inhibitor of molecular nitrogen utilization
by two other types of nitrogen-fixing bacteria, Clostridium and Bacillus
56
polymyxa; but the concentrations required for effect are probably
greater than those in the atmosphere.
Some microorganisms are more sensitive than others to nitrous
oxide. Only one of three strains of Escherichia coli survived after ex-
163
posure to a solution through which nitrous oxide had been bubbled.
The growth of two fungi and only one of two species of Clostridium, but
none of the other bacteria, -was inhibited in an atmosphere with 90 psig
216
of nitrous oxide. Nitrous oxide also supresses gaseous hydrogen
-------
7-9
308
accumulation in anaerobic soils. The applicability of these findings
to problems of air pollution is minimal because of the high gas levels
used.
Therefore, the knowledge concerning the potential impact of nitrogen
oxides on microbial processes in soils and -waters is sparse. Although ambient
concentrations probably do not significantly affect biological processes in
natural ecosystems, it is not possible to support this view with experimental
data.
REACTIONS OF NITROGEN OXIDES WITH SOILS
In studies considering the possible use of nitrogen dioxide as a
9
fertilizer, liquid nitrogen dioxide was injected into 1 kg soil samples in
amounts equivalent to 100 to 1,000 kg of nitrogen/ha. Losses as gaseous
nitrogen dioxide or nitric oxide were less than 1%. In acid soils, the ni-
trogen dioxide was rapidly oxidized to nitric acid (HNO ). In highly buf-
fered calcareous soils, however, some nitrite was observed temporarily.
Soils adsorb large amounts of nitrogen dioxide (and probably nitric
oxide) -when the gas is injected into the soil. However, the ability of soils
to scavenge small amounts of NO from the air is probably more relevant
X
to the study of air pollution.
2
Abeles et al. measured the uptake of nitrogen dioxide by 250 g of
soil in petri dishes contained in 10 liter desiccators. Resulting data in-
dicated that the concentration of nitrogen dioxide was reduced from 100 to
3 ppm in 24 hr. Since autoclaving the soil altered the final concentration
only slightly, the authors concluded that the nitrogen dioxide was probably
removed by some chemical reaction. From their data, they calculated that
soil in the United States could remove 5. 4 x Iff" kg of nitrogen dioxide
-------
7-10
annually--about 20 times the estimated annual U.S. production of
nitrogen dioxide. At present, emissions are estimated to be 207 x 10°
kg/year (Table 3-2). This figure suggests that the production estimate of
Abeles and his colleagues is high. Extrapolation of data from petri dish
experiments to the entire U.S. land mass is risky. Their report does not
state that soil actually can serve as a sink for the present NO production.
425,426 x
Prather and his associates measured the sorption of nitro-
gen dioxide from dry (humidity <5%) and moist air (humidity> 95%) by cal-
careous and noncalcareous soils. Under dry conditions, sorption of
nitrogen dioxide from air streams containing 0. 1 to 0. 5% nitrogen dioxide
by volume reached an equilibrium value within 2. 5 min. The amount
sorbed was related to the surface area of the soil and amounted to as much
as 1% by weight of the soil. Moist air increased the sorption capacity by
as much as 10-fold and resulted in a measurable decrease in titratable
basicity of the soils. The authors assumed that sorbed nitrogen dioxide
9
probably oxidized to nitrate as reported in 1955 by Aldrich and Buchanan.
W. C. Chiorse and M. Alexander (unpublished data) showed that the sorbed
nitrogen dioxide was converted chemically to nitrite and nitrate, but the
nitrite was readily oxidized to the nitrate by soil microorganisms.
Soils serve as both a sink and a source of nitric oxide. However,
when nitric oxide enters the atmosphere, it is ultimately converted to
nitrogen dioxide which can react with soils. We are therefore concerned
here only with soil as a sink for nitric oxide.
The clay mineral montmorillonite adsorbed nitric oxide by chemical
reactions when the cation-exchange complex was saturated with such tran-
sition metal cations as iron, cobalt, or nickel. Only physical adsorption
was observed when the clay was saturated with alkali metal or alkaline
-------
7-11
372
earth metals. In the latter case, exposure to air resulted in rapid
oxidation to nitrite.
425,426 359
Prather et_ a_l. and Miyamoto et aL examined the reac-
tions of iii ric oxide with both dry and moist calcareous soils. As -with
nitrogen dioxide, sorption under dry conditions was related to the specific
surface area of the soil, but the nitric oxide sorption capacity was about
half that for nitrogen dioxide. Sorption increased with increasing moisture
in the soil or in the air + nitric oxide stream, up to the titratable bas; ity
of the soil. Under these extreme conditions, where the soil sorbed as
much as 7% by weight nitric oxide, the soil pH was reduced to the 2. 0 to
3. 5 range. Most of the nitric oxide sorbed by the soil was converted to
nitrate.
Aside from the above, much remains to be learned about the reac-
tions of nitric oxide and nitrogen dioxide with soil.
EFFECTS ON AQUATIC ECOSYSTEMS
Although there are no data, nitrogen oxides may have important
environmental consequences on individual species and food chains in
aquatic ecosystems. The data cited above on the sensitivity of blue-
green algae to the nitrite formed from NO may be of some significance.
X
CONCLUSIONS
The influence of NO on natural plant and animal communities is an
x
area in which data are limited. Although no specific information is avail-
able for ecosystem responses to NO , research on crop plants indicates
X
that NO compounds have the characteristic effects of other air pollutants
on some species. Consequently we anticipate: differential species sensi-
tivity to NO ; complications due to synergistic or antagonistic interactions
-------
7-12
between NO , other air pollutants, and natural environmental stresses;
x
and secondary ecosystem responses caused by changing symbiotic and
competitive interactions as species respond differentially.
Nitric oxide, nitrogen dioxide, and nitrous oxide all affect the growth
or survival of individual microorganisms when tested in artificial media;
but the effect of nitrogen oxides on microorganisms or microbial processes
at common atmospheric concentrations are unknown. No attention has been
given to the effect of ambient NO concentrations on populations or activ-
X
ities in both natural habitats and in vitro. Although it is likely that little
suppression of heterotrophs arises from the presence of these gases, a
definitive conclusion is impossible without direct experimentation.
The algae that are especially important to primary production in fresh
and marine waters, the algae and lichens that are significant in the weathering
of rocks and in certain soil processes, and the activity of microorganisms
colonizing leaves and causing plant disease are affected by low concentra-
tions of some air pollutants, but their sensitivity to ambient NO concentra-
x
tions has yet to be tested.
Both nitric oxide and nitrogen dioxide react readily with soils, and
generally are converted to nitrate. Sorption of large amounts of NO lowers
x
soil pH. The addition of lime to the soil can correct this acidity. Since
the sorption of NO from airstreams containing the low concentrations
x
typically found in polluted air has not been reported, the scavenging role
of soil in removing NO from the atmosphere cannot be evaluated.
X
RECOMMENDATIONS
Research on several major types of animal/plant communities and
ecosystems is necessary to determine the effects of NO at ambient con-
centrations on species and systems.
-------
7-13
Data should be collected in order to determine the effects of NO , at
x
ambient concentrations on microorganisms and microbial processes in soils
and water. Particular attention should be given to the effects of NO on
x
microbial breakdown of organic matter and plant remains, biological nitrogen
fixation, nitrification, and processes effected by algae.
In view of the indications that the nitrite that would be formed from
NO has a deleterious effect on blue-green algae, additional evaluations are
X
necessary to determine whether this effect is of ecological significance and
what might be the consequences of such an inhibition in aquatic and terres-
trial ecosystems.
More studies are also needed to provide the basis for determining the
reactions of soil with NOX at low concentrations and to determine whether
soil serves as a sink at these concentrations.
Further, it is important that special attention be given to the degree
of contamination of natural ecosystems by atmospheric nitrogen oxides.
The effects of air to water transfer of these oxides should also be studied.
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CHAPTER 8
EFFECTS OF NITROGEN OXIDES ON MATERIALS
Field studies and laboratory research have demonstrated that nitrogen
oxides can have deleterious effects on textile dyes, natural and synthetic
fibers, metals, and various rubber products. The cost of this pollution
damage can be considerable, especially at the consumer level.
Of the various oxides of nitrogen in the atmosphere, the most damaging
to materials are nitrogen dioxide (NO ) and airborne nitrates. Nitrogen
oxides also play an indirect role in material damage by atmospheric pol-
lutants. Participation of nitric oxide (NO) and nitrogen dioxide in the
atmospheric photolytic cycle results in the formation of ozone (0 ) and
in the photooxidation of sulfur dioxide (SO ) in the presence of reactive
hydrocarbons to produce sulfuric acid (H SO ) aerosols. These photochemical
2 4 457,556,557
reaction products significantly damage a wide range of materials.
EFFECTS OF NITROGEN OXIDES ON TEXTILES
Awareness of Problem - Communication
The atmospheric contaminants known to affect fabrics and their dyes
are nitrogen oxides, sulfur dioxide, and ozone. These effects are
known to fabric manufacturers and usually are considered in the manufacturing
process. Manufacturers of such end-use products as carpets and apparel
may be aware of the problem; however, they generally react only to com-
plaints. Unfortunately, the number of consumer complaints made at the
retail level is small.
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8-2
The following damage to textiles from air pollutants is of concern to
both consumer and producer:
• Soiling of fabrics due to particulates
• Premature degradation of cotton and nylon due to sulfur oxides and
nitrogen oxides
• Fading of dyes on cellulose acetate due to nitrogen oxides
• Yellowing of white fabrics due to nitrogen oxides
• Fading of dyes on cotton and viscose due to nitrogen oxides
• Fading of dyes on wool due to sulfur oxides
• Fading of dyes on acetate due to ozone
• Fading of dyes on nylon due to ozone and nitrogen oxides
• Color destruction on permanent press garments due to ozone and
nitrogen oxides
Since manifestation of fabric deterioration and color failure can
require up to one year, retailers resist assuming responsibility for
such damage when charged by the consumer. This is especially true in
cases of fading due to light. Consequently, such deterioration of light-
fastness is considered a shared responsibility.
Of major concern to manufacturers is the fading of fabrics or garments
in warehouses or on retail shelves. This deterioration generates com-
plaints to the fiber producer, the fabric manufacturer, or the dyers and
finishers. Such damage can be of major economic importance when garments
affected number in the thousands.
Adequate technical information and testing procedures for the manu-
facture of lightfast fabrics are available. Therefore, the cause of
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3-3
fading, whether it be discoloration of whites or fading of dyes, can be
determined. It is possible to avoid the manufacture of such unsaleable
products which are a source of expensive complaints.
As the textile industry has become consolidated into fewer and larger
companies, better quality control has become possible. Fiber producers
have set quality standards which they have used in the promotion of their
brand names. Competitors have consequently become aware of deficiencies
of one fiber as compared with another. In setting up quality control pro-
cedures, manufacturers have recognized atmospheric contaminants as important
factors to be considered.
Test procedures developed by the American Association of Textile
16a
Chemists and Colorists (AATCC) and the American Society of Textile
18a
Manufacturers, and bulletins issued by manufacturers of dyes all
emphasize the effects of air pollution on colorfastness of dyes. The
L-22 standards are voluntary standards set by apparel manufacturers for
fabric shrinkage and abrasion resistance as well as colorfastness to
light, washing, and atmospheric pollutants as measured by the AATCC
tests for degree of change. Tags on garments and home furnishings do
not consistently refer to damage due to atmospheric contaminants, although
the fabric's resistance to light exposures and washing are specified.
Many home economists and other consumer-oriented individuals have
examined consumer attitudes toward textile damage due to air pollution.
Fading caused by atmospheric pollutants ranked as a major consumer
305
complaint at a large Pittsburgh department store. Statistical analysis
showed such fading as a long-standing source of damage claims received by
264
dry cleaners. Studies have specified oxides of nitrogen, ozone, and
acids derived from sulfur dioxide as the pollutants responsible for this
345
color loss.
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8-4
From the retailers point of view, colorfastness is a principal consumer
446,494
requirement. Standards and test methods for effects of atmospheric
pollutants on colorfastness are consequently of considerable importance
505
to fabric production. Testing laboratories on the retail level enable
167
better merchandising resulting in consumer satisfaction.
Several department stores, such as Macy's, and major chains, including
Sears Roebuck & Company and J. C. Penney, have their own laboratories.
Among the standard test methods adopted by these laboratories are those
determining colorfastness of fabrics exposed to atmospheric contaminants.
The Detroit Dry Cleaning and Laundry Institute maintains extensive
files on textile performance. A large portion of their complaints stem
350
from atmospheric fume fading.
In the complaints mentioned above, blue was the color most affected.
350
Data compiled in 1962 indicated the majority of complaints were received
during October, November, and December—the months garments were removed
from storage.
Cost of Air Pollution Effects on Textiles
The following data must be obtained in order to make sound economic
estimates:
• The incidence of air pollutant damage to specific fibers or end
uses.
• The magnitude of the damage compared to total fiber used in the
specific example.
• The acceleration of depreciation resulting in decreased wear life
(due to air pollution).
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3-5
• The expected wear life of the article in areas of negligible
pollution effect and the calculation of reduction in wear life
from the standard.
• The dollar value of the damaged textile product.
• The percentage of the fabric used in air pollution areas: urban
versus rural.
Consideration of all these factors should provide a logical basis
on which costs can be calculated for textile products used in their unmod-
ified forms, i.e..without the fiber modifications or protective additives.
A separate series of estimates must be made for textiles intentionally
modified to reduce the effects of air pollution. The textile industry is
aware of the vulnerability of dyes and fibers to such specific air pollutants
as sulfur oxides and nitrogen oxides, and has sought to eliminate claims
for damage due to air pollution by (a) instituting research programs to
seek remedial measures, (b) modifying processes or substituting dyes and
inhibiting chemicals, and (c) setting up test methods and other quality
controls in an attempt to meet consumer demands. The extra costs incurred
in the implementation of these measures are offset by the decreased cost
of handling consumer complaints.
The current trend involves modification of the yarn or the manufacturing
process to minimize a deficiency or to gain a new useful or marketable
property. Manmade fibers can be physically altered, chemically modified,
or cospun with additives to meet desired criteria. Natural fibers such
as cotton and wool cannot be physically changed with the exception of the
addition of resins during the finishing process to control shrinkage and
shape retention.
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8-6
The manufacturers of cellulose acetate fibers, both secondary acetate
and triacetate, have been faced with fading of the blue disperse dyes.
This fading has been attributed to nitrogen dioxide. In the early 1940's,
this problem restricted the marketing of acetate fibers for such end uses
as linings, draperies, rugs, and apparel. By instituting research on dyes
and inhibitors, the manufacturers found ways to control this fading prob-
lem. The costs of these controls may be legitimately passed on to the
consumer since such controls are part of the manufacturing process and
their cost is included in the retail price.
Such costs to the consumer include:
• Research costs to improve product.
• Added costs caused by more expensive chemicals or dyes,or extra
processing.
• Costs of quality control tests.
The remedial measures may not be completely effective, and wear life may
not be optimized. Distinction should be made between gradual degradation
and "catastrophic" losses in which textiles lose color within three to six
weeks or become weakened to one-tenth of their expected wear life. In the cases
of sudden and unexpected behavior, the consumer suffers an immediate loss
necessitating replacement at his own or the supplier's expense.
There is no excuse for offering merchandise known to suffer cata-
strophic changes, and such products are usually withdrawn from the market.
An expensive alternative may then be necessary. For example, draperies
and bedspreads made of cellulose acetate require high resistance to light
and to atmospheric pollutants. Since this cannot be guaranteed with con-
ventionally dyed acetate, acetate fiber cospun with colorfast pigment dyes
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3-7
has been substituted. These yarns, Celaperm by Celanese Fibers Company
and Chromspun by Eastman Kodak, have saved this market for
acetate fibers, but at added cost of manufacture. In some cases, the
effects of air pollution can be prevented by replacing a vulnerable fiber
by a resistant one. Drapery fabric that is vulnerable to degradation by
sulfur oxides can be replaced by glass or polyester fabrics that retain
their strength in a sulfur oxide rich environment.
Such alternatives are not always used since the consumer may wish to
risk air pollution effects in order to obtain a particular color or
texture available only in cotton, or other nonresistant fabric. In indus-
try, on the other hand, where cost considerations outweigh appearance,
fibers are commonly substituted to avoid damage from air pollution.
The incidence and magnitude of damage reported in this chapter were
derived from the published literature and from direct contacts with fiber
manufacturers, fabric mills, dye and textile chemical companies, and from
manufacturers of apparel, home furnishings, and industrial textile articles.
Another very useful source of information was the Research Committee of
the American Association of Textile Chemists and Colorists. These sources
also provided information on remedial measures used to counteract the
known cases of air pollution effects on textiles.
A realistic approach to the economics of air pollutant damage to
textiles must apportion costs as follows:
1. Modification of fiber or process. Since textiles are manufactured
throughout the United States, these costs should cover the entire
production rather than be limited to metropolitan air pollution
areas.
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8-8
2. Variable destructive agents in addition to air pollution. The
effect of light in degradation of fibers or abrasion during
washing are additional factors which are active at the same time
as air pollutants. Damage caused solely by air pollutants has
been ascertained by comparing high pollution with standard
nonpollution areas.
3. Effects of concentration of pollutants. The costs to the nation
of the effects of air pollutants were estimated by V.S. Salvin
551a
in a study conducted for the Environmental Protection Agency.
He selected and analyzed nine cases of fabric damage as the
basis for determining costs. In all cases, the effects of small
quantities of atmospheric pollutants were recognizable indicating
that nitrogen oxides, sulfur oxides, and ozone will affect dyes
and fibers at concentrations as low as 5 ppb, which are typical
in nonpollution areas. When studying the effects of air pol-
lutants, the effects in normal environments should be compared
with those effects from high pollutant concentrations. Tests
designed by the American Association of Textile Chemists and
Colorists indicate the relationships between results obtained
in a laboratory simulator and results obtained through equivalent
exposures during actual use at various locations. Values obtained
give averages. However, service tests, or tests of actual use,
also show that changes can occur within three months in areas
of high pollution, whereas 12 to 15 months may be required in
areas of low air pollutant concentrations.
-------
8-9
Costs of Nitrogen Oxide Fading of Dyed Acetate Fabrics
Disperse dyes are used for dyeing cellulose acetate. Blue dyes are
especially vulnerable to fading by nitrogen oxides; certain widely used
reds and oranges are also susceptible. Some disperse dyes, known to be
resistant, are not always used because of high cost and difficulty in
application. The quality dyes are termed Class A; conventional dyes are
designated as Class B.
Blue dyes are used in approximately 70% of all dyeings not only to
obtain shades of blue but also green, brown, grey, beige, and purple.
Small amounts of blue are even used to obtain certain oranges and yellows.
The Textile Organon estimates that approximately 228 million kg of
acetate are dyed annually in the United States. Of this total, approximately
25%, or 57 million kg, are Class A dyed, giving reasonable resistance to air
pollution damage but at higher processing costs. The remaining 171 million
kg are dyed with Class B dyes, which are susceptible to air pollution-
induced fading.
The estimated annual expenditures associated with the prevention of
nitrogen oxide-induced fading of dyes on acetate include:
• The increased cost of Class A dyeings whose complicated structures
resist nitrogen oxide-induced fading.
Of the 57 million kg of acetate used in Class A dyeings or
pigment spun, 70% or 40 million kg use blue dyes. According to
the Bureau of the Census data quoted in Textile Organon, approxi-
mately 4.6 m per 2.2 kg of fiber may be made into fabric, resulting
in 402 million meters of fabric for Class A dyeings. Considering
an increased cost of 4.4<; per meter by using more expensive dyes
-------
8-10
and refined dyeing procedures, the increased cost for Class A
dyeings using blues which are resistant to nitrogen dioxide is
$17.6 million.
• The cost of inhibitors applied to reduce nitrogen oxide-fading.
Of the 171 million kg of acetate used in Class B dyeings,
70% or 120 million kg use blue dyes. Converted to meters, 1.21
billion meters of fabric that use blue dyes are produced. At
a cost of 1.09C per meter, the use of inhibitors increases total
costs by $13.2 million.
• Costs of research.
Costs of research conducted for improving the resistance of
fibers and dyes to nitrogen dioxide are estimated to be $.5 million.
• Costs of product quality testing and quality control.
Testing and quality control costs are estimated to be $.5
million based on 250,000 batches processed at a $2 testing cost
per batch.
• Loss due to fading at the manufacture and retail level.
Losses due to fading at the manufacture and retail level are
estimated at $1 million. Current technology can now prevent losses
before consumer sale.
• Costs to consumers resulting from reduction in wear life of apparel
or home furnishings due to limited protection by inhibitors.
Of the 120 million kg containing blue dyes of Class B, 60
million kg are in air pollution environments. Of the 60 million
kg, a 22% deduction is made for blends and fashion shades where
slight loss of the color will not lead to complaints. This results
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8-11
in 46 million kg that are subject to fading in relation to
inhibitor protection. This protection is only partial since
inhibitors are consumed by sulfur dioxide in the air and the
nitrogen dioxide absorbing property of the diphenylethylenediamine
[C.HCNHCH0CH^NHC.H_] inhibitor structure. Some shades lose their
6 5 22 b _>
protection from air pollutants at the end of one year; objectionable
change despite the use of inhibitor is noted at 18 months. Since
the consumer expects a wear life of 2 years for apparel and 5
years for home furnishings, this fading represents a value loss.
The color change may not be noticed by less critical con-
sumers who will continue to use the garment or the furnishing
without complaining or discarding. However, a premature loss of
wear life of 25% is not uncommon for many acetate fabrics. Garments
or furnishings may also fail from light exposure, excessive'wear,
etc. Forty—six million kg represents a value of $400 million in
end products retail. Assuming 10% loss in fabric wear life due to
color failure, only half of which is severe enough to cause dis-
carding, a value of about 5%, or $20 million, represents consumer
loss.
The total cost in prevention of nitrogen oxide damage to acetate dyed
fabrics and in estimated damage is $52.8 million.
Costs of Nitrogen Oxide Fading of Cotton Dyes
Total annual consumer consumption of cotton is approximately 1.4
billion kg. Consumer products made of cotton or cotton blends are estimated
to include 50% of the approximately $40 billion annual apparel market,
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8-12
resulting in a $20 billion market of products containing cotton. Many
dyes used on cotton products are susceptible to nitrogen oxide-induced
color fading.. Below are estimates of dollar losses from nitrogen oxide
fading of three common cotton dyes. The following standard formula was
used:
Total losses (TL) = Size of market (constant at $20 billion)
x percentage of goods used in air pollution areas (constant
at an estimated 50%) x estimated percentage of the dyes in
the specific class which is sensitive to nitrogen oxide
fading x percentage yearly loss in wear life.
Sulfur Dyes. Sulfur dyes are used in approximately 5% of all cotton
products. Twenty percent of these dyes are sensitive to nitrogen oxide.
A yearly wear life loss of 5% is assumed.
Substituting into the formula:
TL = ($20 billion) x (.5) x (.05) x (.20) x (.05)
TL = $5 million, the annual cost of nitrogen oxide fading of
sulfur dyes.
The analysis is conducted similarly for the following dye classes:
Direct Dyes
TL = ($20 billion) x (.5) x (.3) x (.25) x (.02)
TL = $15 million
Reactive Dyes
TL = ($20 billion) x (.5) x (.05) x (.08) x (.05)
TL = $2 million
Vat dyes on cotton are not reported here because they are not
vulnerable to fading from nitrogen oxides.
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8-13
The costs of research, quality control, and more elaborate processing
techniques are not appreciable factors for the above dyes. Inhibitors are
not used. The total loss caused by nitrogen oxide fading of cotton dyes
is $22.05 million.
Costs of Nitrogen Oxide Fading of Dyes on Viscose Rayon
Nitrogen oxide acts primarily on direct dyes used on rayon. The
viscose rayon used in apparel and home furnishings totals approximately
228 million kg annually. This figure includes fabrics made of 100% rayon
as well as blends with nylon, polyester, or acetate. Direct dyes are most
commonly used; reactive dyes and sulfur dyes are not used. The end uses,
the percentage of vulnerable shades used, and the percentage of fabrics
used in air pollution areas are essentially the same as for nitrogen oxide
fading on cotton. Losses from direct dyes are reduced somewhat since cer-
tain widely used blue dyes are not susceptible to oxides of nitrogen although
possibly 60% of the shades using blue are susceptible. The premature loss
of wear life end use rayon products, such as apparel, can be 20-25%.
Since other wear factors, such as light and abrasion, contribute to this
loss, the percentage due to air pollution is probably around 10%. This
can be compared to a yearly wear life loss of 5% for the nitrogen oxide
fading of dyes on viscose. Following are estimates of the losses caused
by the effects of nitrogen oxides on rayon dyed with direct dyestuffs in end
uses:
Value of rayon goods - $2.4 billion
50% of fabric exposed in air pollution areas - $1.2 billion
60% of blue shades that are vulnerable - $720 million
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8-14
of $720 million is dyed with sensitive dyes - $432 million
5% loss in wear life due to objectionable color fading - $21.6 million
Similar calculations can be made for other fabrics for which nitrogen
oxides fading effects are clearly established; for example, yellowing of
white cotton fabrics, fading on nylon carpets.
The losses incurred by oxides of nitrogen deterioration of nylon and
cotton are not as clearly established since acids derived from sulfur di-
oxide are a major co-contributor.
Essentials of Fading Phenomena
To ascribe fading of dyes to a particular atmospheric contaminant ,
the following three criteria must be observed:
1. The pollutant responsible for change must be isolated.
2. It must be demonstrated that the pollutant can react with the
dye resulting in destruction of the dye.
3. The means by which the pollutant comes in contact with the dye
in sufficient quatity to cause fading under exposure conditions
must be proved.
Effect of Oxides of Nitrogen on Dyes - Contribution of Fiber
The classic case of dye fading due to atmospheric pollution is that
of blue dyes on cellulose acetate as initially described by Rowe and
448
Chamberlain. Cellulose acetate is dyed with dyes of low water solubility
that are absorbed by this fiber through a mechanism of solid solution. The
blue dyes which were in widespread use are of anthraquinone structure as
illustrated in Disperse Blue 3, a l-alkylamino-4-alkylaminoanthraquinone:
-------
8-15
NHCH
NHCH CH OH
Dyes of this structure are moderate in cost, dye rapidly, and have easy
leveling properties.
Fading of acetate dyed blue, or shades in which blue is a component,
results in pronounced reddening. Since this change occurred in rooms
448
heated by gas heaters, it was termed "gas fading." Rowe and Chamberlain
demonstrated that the causative factor in the combustion gases was nitrogen
dioxide. A concentration in air of 2 ppm was found sufficient to cause
fading.
The fading caused by the action of nitrogen oxides on anthraquinone
blue dyes of the Disperse Blue 3 structure results from both nitrosation
93
and oxidation of the vulnerable amino group.
Although cellulose acetate is an excellent absorber of gaseous nitrogen
459
dioxide in a closed system containing fiber and injected oxides of nitrogen,
polyester and polyacrylic fibers have a low absorption rate; and nylon, cel-
lulosics, and wool have an intermediate rate. Acetate and triacetate re-
lease their absorbed nitrogen oxides on heating. Nylon and wool hold the
nitrogen oxides by chemical combination but release them by hydrolysis in
water. The nitrogen oxides are retained within the cellulose.
The reaction of nitrogen oxides with cellulose acetate is most pro-
nounced in the anthraquinone blues, although fading also occurs in anthra-
quinone reds such as Disperse Red 11 and 55. Fading also occurs with
certain azo dyes, in particular, Disperse Orange 3, and with the yellows
25
of diphenylamine structure. These fading tendencies are noted in the
shade books published by dye suppliers.
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8-16
Although polyester, which Is dyed with disperse dyes, absorbs
nitrogen oxides at a low rate, slight fading changes are recorded in the
DuPont shade books on disperse dyes. The rate of fading accelerates when
temperature and humidity are increased as demonstrated in the noticeable
fading that occurs with Disperse Blue 81 in the presence of combustion
gases during the curing of resins in finishing processes.
The fading of disperse dyes on nylon due to nitrogen oxides has re-
sulted in consumer complaints. Nylon contains amino groups that act as
acceptors for the nitrogen oxides partially neutralizing their action as
nitrosating or oxidation agents. Excess nitrogen oxides act on nylon
causing a yellowing indicative of nitrosation and diazotization. However,
susceptibility of specific dyes to nylon gas fading is noted in the dye bul-
17 Oa
letin: Disperse Dyes on Nylon Piece Goods - General Aniline. This
tendency is great in nylon carpets in which the nylon is textured by a
process that increases fiber absorption of nitrogen oxides.
Fading of Dyes on Cellulose Acetate
The fading of dyes on cellulose acetate fabrics presents serious
marketing problems especially in those end uses where color retention is
critical, e.g., suit linings and draperies. Test procedures have been
established to predict performance of dyed fabrics. These test procedures
471
showed excellent correlation with service trials. Seibert examined the
behavior of dyed acetate material under different conditions and recommended
the use of more resistant structures where available. He also suggested
the use of inhibitors to get better end use performance.
190
Greenspan and Spoerri studied the factors influencing the action
of nitrogen oxides on the vulnerable amino anthraquinone dyes. The work of
-------
8-17
these early investigators was instrumental in developing the standard test
method later recommended by the American Association of Textile Chemists
and Colorists (AATCC) for evaluating the resistance of dyed textiles to
color change caused by oxides of nitrogen. This test method is described
in the AATCC Manual as "Colorfastness to Oxides of Nitrogen in the Atmo-
16a
sphere." Basically, in this method a test specimen is exposed to combustion
gases containing a 1 to 2 ppm concentration of nitrogen oxides. These
combustion gases are derived from a gas burner using natural gas or butane
as fuel. Specimens remain in the chamber until the control sample shows a
change corresponding to a fading standard. This exposure is considered
equivalent to a 6-month exposure in populated southern New Jersey.
The results of various test methods can vary with temperature and
432
humidity. Ray tested for dye-fastness to burnt gas fumes under controlled
conditions of temperature and humidity as well as with chemically generated
nitrogen oxides with and without the sulfur dioxide. Concentration and
time of exposure are the most critical factors requiring control in test
methods. The effects of temperature and humidity on acetate fabrics are
of less importance.
429
Rabe and Dietrich conducted comparative tests to determine effects
of gas fading using equipment suggested by the International Standards
Organization and the German Fastness Commission. In the German test
method, the nitrogen oxides are generated by the addition of phosphoric acid
to a dilute sodium nitrite solution. This procedure is carried out in a
closed system in which the dyed fabric is exposed to the generated nitrogen
oxides under high humidity conditions. In the AATCC test procedure for ni-
trogen oxide fading, nitrogen oxides are generated by combustion of natural
or propane gas under conditions of low humidity. Dyed cotton fabrics that
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8-18
have received service complaints of fading duplicate the fading change under
the German test method whereas the AATCC method does not show fading change.
This deficiency of the AATCC test method in predicting changes on cotton and
also on nylon has led the AATCC to develop a test method for nitrogen oxide
fading at high humidity.
Options for Protection Against Damage by Nitrogen Oxides^
When nitrogen oxides were established as the cause of cellulose acetate
fading and the high absorption rate of nitrogen dioxide by cellulose acetate
was determined, the textile industry initiated an extensive research effort
to prevent this color loss.
459
Inhibitors are effective controls which are used in two ways. Most
commonly, fabrics are co-dyed with an inhibitor of diphenylethylenediamine
structure. This inhibitor reacts rapidly with nitrogen dioxide thereby producing
nitroso compounds. Inhibitor reaction with nitrogen oxides blocks the
competitive reaction with the dye. This method has two disadvantages.
The light yellow coloring of the nitroso compound is often as objectionable
as the reddening of the unprotected dyeing. Careful control is therefore
necessary to avoid excessive inhibitor action. The inhibitor must be
purified to prevent excessive discoloration. This method does not give
permanent results since the dye becomes more vulnerable to absorbed gases
as the inhibitor is used up. The use of nonyellowing inhibitors of the
diphenylacetamidine structure has not proved effective due to their lack
of affinity.
The second method for inhibition is the maintenance of an alkaline
condition in the fabric since nitrogen oxides are not effective when
neutralized with alkali. Diphenylacetamidine is a weak organic base whose
effectiveness is its alkalinity. Fabric is padded in the finishing step
with solutions of water-soluble alkaline salts such as sodium formate
-------
8-19
(HCOONa), sodium acetate (CH COONa), or organic amines such as triethanolamine
3
[(HOCH CH ) N]. This protects dyed fabrics against change, and is espe-
cially useful in fabrics requiring dry cleaning, since the padded water-
soluble inhibitor is removed on washing.
Although inhibitors protect textile products in warehouses and on
retail shelves, the protection is not effective enough for such end uses
as upholstery, draperies, linings, etc. Blue dyes of high resistance to
gas fading have been known almost from the first complaints of atmospheric
fading. Since these dyes of azo structure (-N=N-) have very poor light -
fastness, they are used primarily on suit linings, which are not exposed
to light. OH n OH
Anthraquinone blue dyes
N02 0 NH
which combine both excellent lightfastness and resistance to nitrogen oxides,
474
were synthesized by Salvin, Whereas the conventional blue dyes contain
alkyl amino groups (e.g., Disperse Blue 3) which react readily with nitrogen
oxides, the resistant dyes do not contain the reactive alkyl amino group.
The structure contains an aryl amino group which is much less reactive be-
cause of the lower basicity of the anilino or substituted anilino group.
This work has led to the use of Disperse Blue 27 and Disperse Blue 70 as
dyes resistant to nitrogen oxides. . 2
OH 0 NHC H
6 5
A reduction in reactivity to nitrogen oxides may also be effected by
introduction of slightly negatively-charged (polar) groups which are adjacent
on the benzene ring (ortho) to the amino groups. These abstract electrons
from the amino group reduce basicity and reactivity to nitrogen oxides.
This dye is marketed as Disperse Blue 60.
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8-20
These anthraquinone blue dyes require costly Intermediates and several
manufacturing steps. They have slower dyeing rates, poorer leveling prop-
erties, and their dyeing procedures are lengthy, complex, and require great
care in application.
Dyes with high resistance to nitrogen oxides are recommended by the
producers of acetate and triacetate. Their quality standards require that
fabric resistance to gas fading be approved before either the trademark
Arnel for triacetate or Kodel for polyester is granted.
The desire for fastness to atmospheric fading and high light exposure
resulted in the development of drapery fabrics in which pigment dyes are
cospun in the filament.
The fading of disperse dyes, especially blues and violets on cellulose
acetate, is well known. However, a survey of reported fading of acetate
fabrics revealed the number of complaints to be quite low from all areas—
fiber producers, dye suppliers, fabric mills, retail organizations, chain
or major department stores, and consumer organizations. This improvement
stems from increased awareness of the problem by both the fabric producer
and the retailer. Resistant dyes, sufficient quantities of inhibitors,
pigment-dyed fabrics, and fibers other than acetate in vulnerable end uses
combine to minimize material damage. The fading of dyes on acetate fabrics
is an example of a problem whose causes are known and for which effective
controls have been implemented.
Fading on Cellulosics - Cotton and Viscose
Although the effects of nitrogen oxides on acetate dyes is now well-
known and preventative measures have been taken, effects on cotton and other
fibers have only recently be publicized.
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8-21
A survey of the effects of air pollutants on fabrics as reported by
551a
Upham and Salvin has produced documentation of fading of direct dyes on
cellulose and viscose, of vat dyes on cotton, and of sulfur and reactive
dyes on cellulosic fiber. The effect of nitrogen oxides on fiber reactive
dyes has been reported by Imperial Chemical Industries in its shadecard of
Procion dyes and by J. Hertig in his critical study of the International
228
Standards Organization test for fastness to nitrogen oxides.
457
Salvin has reported the fading of various dye classes on a range
of fabrics including cotton when exposed to ambient air in the high pol-
lution areas of Los Angeles and Chicago for periods of 30 to 120 days. He
documented the fading of dyes on cotton due to nitrogen oxides, ozone, or
both, and also the fading of direct dyes, vat dyes, sulfur dyes, and fiber
reactive dyes on cellulose. The AATCC test for fading from oxides of ni-
trogen shows no change; the AATCC tests for ozone and sulfur dioxide fading
show only insignificant changes. However, the nitrogen oxides test pro-
cedure involving oxides of nitrogen generated under high humidity does
produce changes which correspond to those observed in service. Salvin
reported fading of direct dyes using the AATCC nitrogen oxides test under
conditions of high humidity.
8
Ajax et al. conducted tests as long as 24 months in light-sheltered
cabinets exposed to various urban and rural environments. A high suscep-
tibility to fading resulted for many fabrics exposed to urban environments
with monitored nitrogen oxides and ozone content.
Further effects of nitrogen oxides were documented by McLendon and
346
Richardson in their study of color changes in dyed cotton placed in gas
clothes dryers. In the heated air of the gas dryer nitrogen oxides were
-------
8-22
present in sufficient amounts to change dyes on cotton fabrics as well as
to yellow cationic finishes. In a study of the destructive action of home
16
gas-fired dryers on dyes as carried out by the AATCC Midwest section, the
dyes sensitive to generated nitrogen oxides were also those which were
affected by combustion products on moist dyed fabrics.
Fading on Nylon and Polyester
The amount of color fading for a given combination of pollutant depends
on the humidity, temperature, and migration of dyes to the more gas absorbent
surface or fabric finishes.
Such conditions are important in determining the causes of air pol-
lutant fading on polyester/cotton permanent press fabrics and on nylon
carpeting dyes. For example, high humidity is necessary to duplicate fading
of nylon carpets where ozone is the air pollutant. Results of the nitrogen
oxides test reveal that nylon carpets do not fade under low humidity con-
ditions, but do fade when subjected to high humidity.
Fading of dyed polyester/cotton and textured polyester knits has been
a problem. This fading does not occur with the disperse dyes on polyester
as first dyed. It occurs only after the curing process for polyester/cotton
permanent press fabrics and after the heat setting process for polyester
knit fabrics in which temperatures of 182-204°C are used. The dyestuff
within the polyester migrates to the finish. The finish contains a surfactant
or softener that acts as both a solvent for the dye and an absorbing medium
for both ozone and oxides of nitrogen. The criteria for fading are met—
dyes that are susceptible to oxides of nitrogen change and the air pol-
lutant is absorbed.
-------
8-23
Fading of Dyes by Ozone and Other Air Contaminants
The fading of dyes resulting from exposure to the atmosphere is not
due to oxides of nitrogen alone. Exposure to light causes a photochemical
destruction of the dye molecules. In fact, many cases of color fading have
been falsely attributed to action of light rather than air pollutants. In
addition to oxides of nitrogen, color change on certain dyes is known to
be caused by sulfur dioxide, ozone, and, to a minor extent, the acid base
change of acidic air pollutants.
Ozone concentrations of 20-50 ppb cause destructive action on disperse
dyes on cellulose acetate. This was discovered when cellulose acetate dyed
with blue dyes, experimentally determined to be fast to oxides of nitrogen,
460
faded in service trials. In 1955 Salvin and Walker reported that
drapery fabrics dyed with Disperse Blue 27, a dye fast to oxides of nitrogen,
were simultaneously exposed with drapery fabrics dyed with Disperse Blue
7 and Disperse Blue 3 (dyes which are vulnerable to oxides of nitrogen
fading). Inhibitors were used to protect the blues sensitive to oxides of
nitrogen. To indicate the quantity of oxides of nitrogen, the AATCC ribbon,
used as a control in the test procedures, was exposed with all draperies.
Service tests were made in homes located in Pittsburgh, Pennsylvania
during periods of high concentrations of air pollutant gas, and in Ames,
Iowa, a nonindustrial town with minimum air pollution. Tests were made
on swatches of fabric from inside hems where light fading was not a factor.
-------
8-24
The AATCC ribbon in Pittsburgh reddened, indicating an appreciable
concentration of nitrogen oxides; in Ames, very little reddening occurred.
The color changes noted after 6 months became more pronounced after
a 12-month exposure. The changes in nitrogen oxide-resistant blues
were most noticeable in Ames where nitrogen oxides were of lower concentra-
tions. In Pittsburgh the nitrogen oxide resistant blues did not fade sig-
nificantly. The unexpected fading of the Disperse Blue 27 was attributed
to ozone. In Pittsburgh, the sulfur dioxide interacted with the ozone to
reduce its concentration. Moreover, the inhibitor used against fading is
the anti-oxidant used to protect rubber against degradation by ozone. The
discovery that dye fading may be due to ozone as well as nitrogen oxides
is important since the photochemical effect achieved when hydrocarbons from
auto exhaust are combined with oxides of nitrogen produces increased con-
centrations of ozone and oxidants (Los Angeles smog).
Yellowing of Whites
The susceptibility of dyes to fading from chemical action of air pol-
lutants is well known. But fiber interaction and action of specific air
pollutants, such as sulfur dioxide, ozone, and oxides of nitrogen that
cause yellowing of white fabrics has not been well established. The survey
551a
of the effects of atmospheric pollutants on fabrics documented a con-
siderable number of color changes on white fabrics during storage of
garments in warehouses or retail shelves as well as in homes.
458
Salvin examined 18 white fabrics for yellowing. The air
pollutant responsible for yellowing was determined through tests for exposure
to sulfur dioxide, nitrogen dioxide, ozone, and hydrogen sulfide. Tests of
exposure to heat and humidity in the absence of air pollutants were also made.
-------
8-25
The results are summarized as follows:
1. Nitrogen dioxide is the air pollutant responsible for yellowing
of white fabrics in the complaint fabrics tested. Yellowing is
not caused by ozone, sulfur dioxide, or hydrogen sulfide.
2. The standard AATCC test procedure for effects of oxides of nitrogen
does not always show the yellowing effect observed on service
exposure in warehouses or retail shelves away from light.
3. The tests include the use of high humidity and temperatures to
determine the effects of oxides of nitrogen. The migration of
chemicals from the interior of the fabric to the surface or to
the finish on the surface under the conditions of high humidity
and temperature contributes to the yellowing observed.
Fibers yellow only in Spandex, a polyurethane segmented fiber which
reacts directly with nitric dioxide to form the yellow nitroso compound.
In rubberized fabrics, the anti-ozonant used to protect the rubber
migrates or sublimes to the cotton where it yellows on contact with nitrogen
dioxide forming the yellow nitroso derivative. Optical brighteners of
coumarin structure react with nitrogen dioxide to form yellow
compounds. This is shown by standard AATCC test procedure for cellulose
acetate. No color change is noted on nylon until a high humidity test
procedure is used. Coumarin brighteners are used on acetate and nylon but
other brighteners which do not yellow are available for use. They are dif-
ficult to apply and are more expensive. Several cationic softeners con-
taining amines will yellow on exposure to nitrogen dioxide when used on
natural and manmade fiber. Anti-stats used on nylon and polyester have
also shown yellowing.
-------
8-26
Use of Dyed Fabrics as Area Monitors for Nitrogen Oxides
The EPA has produced an "exposure package" which includes , among other
things , a strip of acetate ribbon dyed to a medium shade with Disperse Blue 3.
This is the AATCC control ribbon mentioned above which is used in the oxides
of nitrogen test method. It changes to violet-red as it reacts with nitro-
gen oxide from combustion gas, cylinder gas, or generated nitrogen oxides.
Its color change is easily recognized. The degree of reddening indicates
the severity of the nitrogen oxide pollution. Because this change is
cumulative and sensitive to low nitrogen dioxide concentrations, the ribbon
should be exposed over a long period. The ribbon will not produce results
in the short periods used in instrumental methods. Monitoring factors
can be used to characterize a specific area for integrated effects over
a period of 30 to 90 days.
The use of the AATCC ribbon containing Disperse Blue 3 is not free
from interference. Sulfur dioxide and sulfuric acid derived from sulfur
trioxide (SO ) do not cause a color change, but ozone does. Ozone produces
a bleaching effect resulting in color lightening rather than the reddening
conferred by oxides of nitrogen. Thus, the color change recognition is
open to question.
It is possible to formulate a blue dyeing on cellulose acetate which
resists ozone but reacts with nitrogen oxides. This is accomplished by co-
application with the Disperse Blue 3 of an anti-ozonant which is an inhib-
itor against ozone fading. Such a fabric would be useful for producers
of dyed or white fabrics who, in their concern about effects of nitrogen
oxides, wish to learn the degree of exposure to which their fabrics or
garments will be subjected.
-------
S-27
It is also possible to formulate a ribbon that can be used to monitor
ozone similar to the AATCC ribbon used as control for the ozone fading
test.
Time - Concentration Exposures - Field Studies - AATCC Field Trial
The AATCC conducted service exposure trials to determine the extent
457
of fading of representative types of dyes on a range of fiber types.
These trials consisted of 90-day exposures during October, November, and
December of 1961 in Los Angeles, Phoenix, Sarasota, and Chicago, where the
concentrations of atmospheric pollutants were reasonably well-known. The
contaminants studied in these trials were nitrogen oxides, ozone, sulfur
dioxide, and the products of photochemical action on hydrocarbons. The
physical effects of humidity in both the swelling of the fiber and in in-
creasing absorption were considered.
A range of fibers were dyed—wool, cotton, rayon, nylon, Orion, acetate,
and polyester. Although not all-inclusive, the dyes chosen were those
commonly used for each fiber. For example, the dyes used for cotton in-
cluded directs, fiber-reactives, vats, and sulfurs. The colors chosen were
medium depths of yellow, red, and blue with no after finish.
Color Changes in Service Tests
The quantity of atmospheric contaminants, e.g., sulfur dioxide, oxides
of nitrogen, and ozone, present in the area of exposure was taken from air-
pollution data recorded for that period in Chicago and in Los Angeles.
These data approximated the values in Table 8-1.
Table 8-2 shows fading on a range of fibers (cotton, rayon, nylon,
wool, and acetate) of dyes of various structures (vats, directs, fiber re-
actives, acid, and disperse).
-------
8-28
TABLE 8-1
£1
Typical Concentration of Atmospheric Contaminants, ppm—
Oxides of Nitrogen
Sulfur Dioxide
Carbon Monoxide
Ozone
Aldehydes
Rural
0.01
0.03
—
0.06-0.11
—
Los Angeles
0.26
0.05
23.00
0.21
0.3
Chicago
0.22
0.25
16.00
0.005
— —
a 457
Trom V.S. Salvin. '
-------
8-29
TABLE 8-2
Color Changes on Dyed Fabric—Exposed
Without Sunlight in Pollution and Rural Areas—
International Grey Scale;— 5 = no change.
Y = yellow; W = weaker; G = greener; R =
redder; and B = bluer
Code Index No.
ACETATE
Disperse Red 35
Disperse Blue 27
Oxides of nitrogen
fading control
Disperse Blue 3
Ozone control—grey
dyed with:
Disperse Blue 27
Disperse Red 35
Disperse Yellow 37
POLYESTER
Disperse Yellow 37
Disperse Blue 27
Disperse Red 60
WOOL
Acid Black 26A
Acid Red 89
Acid Violet 1
Acid Blue 92
Acid Red 18
COTTON
Direct Dyes
Direct Red 1
Congo Red B
Direct Red 10
Direct Blue 76
Direct Blue 71
Direct Blue 86
Vats
Vat Yellow 2
Vat Blue 29
Vat Blue 6
Vat Red 10
Fiber Reactives
Reactive Yellow 4
Reactive Red 11
Reactive Blue 9
Reactive Yellow 16
Reactive Yellow 13
Phoenix
4.5Y
3.0W
3.5
3.-0
4.5
4.5
5.0
5.
5.
,0
.0
4.5
4.5
5.0
4.0
4.0
4.5
4.0
4.0
4.0
5.0
3G
4.0
5.0
5-0
5.0
4.5
5.0
5.0
Los Angeles
4.0Y
2.0W
1.5R
1.5
5.
4.
5.
4.5
3.5
4.0
4.0
4.0
1.5
2.5
3.5
2 grey
2.5R
1G
4.0
3G
3.5R
4.5
5.0
5.0
4R
5.0
5.0
Chicago
4.5Y
2.5W
2.OR
3.5
4.0
3.5
4.0
3.5
3.5
3.0
2.5
3.5
1.5
2.5
3B
1R
2R
1G
3G
2.5G
3.0
3.5
Sarasota
4.5Y
2.0W
3.5
2.5
4.0
4.5
4.5
4.5
2.5Y
4.0
4.0
4.5
3.0
2Y
4.0
3.0
3R
2.5G
5.0
1.5G
4R
5.0
4.5
4.5
3. OR
4.0
4.0
4.5
5.0
4.5
5.0
5.0
-------
TABLE 8-2 - continued
8-30
Code Index No.
Reactive Red 23
Reactive Red 21
Reactive Blue 19
Reactive Blue 21
Reactive Yellow 12
Reactive Red 19
Reactive Red 20
Reactive Blue 17
COTTON
Sulfur Dyes
Sulfur Yellow 2
Sulfur Brown 37
Sulfur Green 2
Sulfur Blue 8
Sulfur Black 1
Phoenix
5.0
5.0
4.5
4G
5.0
5.0
5.0
4.5
Los Angeles
5.
5.
3.
1.
,0
.0
.0
,5G
4.5
5.0
4.0
3.0
3.5R
5.0
3B
4.0
4.5
3R
4.5
2. OB
3.5G
4.5
Chicago
4.5
4.0
1R
1.5G
3.5
3.5B
4.0
1 grey
Sarasota
5.0
5.0
4.0
3G
4.5
4.5
4.5
4.0
2. OR
4.5
2. OB
3G
4.5
2.5R
4.0
2B
4.0
4.5
NYLON
Acid Red 85 5.0
Acid Orange 49 5.0
Disperse Blue 3 5.0
Disperse Red 55 5.0
Disperse Red 1 5.0
Alizarine Light Blue C 5.0
ORLON
Basic Yellow 11 5.0
Basic Red 14 5.0
Basic Blue 21 4.5
Disperse Yellow 3 4.0
Disperse Red 59 5.0
Disperse Blue 3 5.0
4.5
4.5
4.0
4.5
4.5
4.5
4.0
5.0
4.5
,0
,0
5.0
3.0
3.0
3.5
4.0
3.0
3.5
4.0
4.5
4.0
4.5
4.5
4.0
5.0
4.5
3.0W
3.5
4.5
4.5
4.5
4.5
5.0
4.5
4.5
4.5
-From V. S. Salvin.457
—The International Grey Scale is a numerical method of showing the degree of
shade change. It is geometric rather than arithmetic. Essentially, a shade
change of 4 shows a change which is slight and is not too easily recognized.
A shade change of 3 is appreciable and is easily recognized. A shade change
of 2 is severe. A shade change of 1 is disastrous. These numbers are in-
dicative of the shade change with 4 being passable and 3.5 a matter of judg-
ment.
-------
3-31
The fading observed in air pollution areas can be attributed
to the effect of oxides of nitrogen and ozone. However, such changes were
also found in Phoenix and Sarasota where oxides of nitrogen are absent
but ozone is present.
In Chicago's sulfur dioxide-rich atmosphere color destruction occurs
which is not observed in Los Angeles. The direct cotton dyes suffer more
pronounced hue change in Chicago.
In Sarasota, exposures created greater changes than in Phoenix. Both
areas are free of oxides of nitrogen and contain ozone in the same concen-
trations. The major difference is the greater humidity found in coastal
Florida. This discovery led to accelerated testing of the entire series
at low and high humidities.
EPA Field Trial
The Environmental Protection Agency (EPA) initiated a field study of
the fading of the dye-fiber combinations exposed by the AATCC Committee on
40
Colorfastness to Atmospheric Contaminants. The fading characteristics
of the 67 dye-fabric combinations were obtained by exposure in lightfast
cabinets for consecutive 3-month periods in 11 nationwide urban and rural
sites. The objectives of the study were:
• to determine how the colorfastness of dye-fabric combinations are
affected by existing levels of gaseous atmospheric pollutants,
• to find a possible relationship between dye-fading and exposure
location, and
• to identify pollutants or environmental factors that influence dye
fading.
-------
0- 0.5
0.6- 1.5
1.6- 3.0
Appreciable
Much
Very much
3.1- 6.0
6.1-12.0
>12.1
The National Bureau of Standards (NBS) has examined quantitatively the
degree of color change in which the NBS unit represents a value based on
spectrophotometric readings. The International Grey Scale shows color dif-
ferences by visual comparison to color standards that are directly compared
to the original shade. The NBS units are used for comparison of color
depths as well as fading changes.
40
In the Beloin article, shade changes instrumentally shown to be less
than 3 NBS units are not easily recognizable by visual comparison. They
would correspond to the International Grey Scale reading of 4 or in certain
cases 3.5.
Fading Category NBS Units Fading Category NBS Units
Trace
Slight
Just noticeable
Exposure Sites
The 11 exposure sites (Table 8-3) for this study were Los Angeles, Cali-
fornia; Chicago, Illinois; Tacoma, Washington; Washington, B.C.: Cincinnati,
Ohio; and Phoenix, Arizona; plus corresponding rural sites for each city. These
cities were selected because they represent the various types of pollution and
climates throughout the country. The rural sites provided controls with the
same climatic conditions, but with low levels of pollution.
Phoenix, Arizona and Sarasota, Florida were chosen because AATCC has used
these sites in the past, and because they have extremes in relative humidity at
high temperatures with low levels of pollution. Cincinnati was chosen for its
proximity to EPA laboratories which facilitated sampling.
The Los Angeles, Chicago, Cincinnati, and Washington sites were located a
few blocks from the Continuous Air Monitoring Program (CAMP) stations of the
National Air Sampling Network (NASN). The Los Angeles site was near a locally-
operated continuous air monitoring station. The data from these stations give
a continuous record of selected pollutant concentrations.
-------
8-33
Santa Paula, CA
Chicago, IL
Argonne, IL
Phoenix, AZ
Sarasota, FL
Cincinnati, OH
TABLE 8-3
Exposure SitesJL
City
Washington, DC
Poolesville, MD
Tacoma , WA
Purdy, WA
Los Angeles, CA
Location
Municipal Building
Poolesville High School
Franklin Gault School
PHS Shellfish Laboratory
LA County Air Pollution Control
Type
Urban
Rural
Urban
Rural
Urban
Average
Fade,
NBS
Units
5.0
4.3
4.3
2.9
5.7
District Building
Federal Post Office
Central Office Building
Argonne National Laboratory
Desert Sunshine Exposure Tests
Sun Test Unlimited
Taft High School
Rural 4.0
Urban 7.2
Rural 4.0
Suburban 2.7
Rural 3.1
Urban 4.8
a 40
-From N. J. Beloin.
-------
8-34
This 2-year study consisted of eight consecutive 3-month
seasonal exposures beginning in spring (March-April-May, 1966).
Conclusions of the EPA Study
1. Appreciable fading occurs in the absence of light. Sixty-four
percent of the fabrics tested faded appreciably.
2. The amount of fading varies among urban areas and seasons. A
significant difference in fading for the four urban areas of Los
Angeles, Chicago, Washington, and Tacoma occurred for 63% of the
fabrics having more than "trace" fading, and 86% of them showed
a seasonal variation.
3. Urban sites produce significantly higher fading than corresponding
rural sites. Eighty percent of the fabrics showing an appreciable
color change had significant urban-rural fading.
4. By themselves, temperature and humidity appear to have little
effect on dye fading, but they may increase fading when pollution
is present.
5. Sulfur dioxide, ozone, and nitrogen dioxide appear to be the pol-
lutants that cause dye fading.
EPA Laboratory Exposures
The EPA study confirmed that high percentage of dye-fabric combinations
changed appreciably as first noted in the AATCC study. However, the EPA
was particularly interested in the minimum level of pollution at which
fading will occur. A controlled environment laboratory study was conducted
in which dyed fabrics were exposed for 12 weeks to two levels of sulfur
dioxide, nitric oxide, nitrogen dioxide, and ozone under four combinations
-------
a-35
41
of temperature and humidity. The combinations that showed greatest
tendency to fade in the field study reported above were selected. With
the exception of Fabric Number 20, the AATCC Ozone Ribbon, all combinations
listed in Table 8-4 are used commercially.
Experimental Variables
Exposing 20 fabrics to atmospheric pollutants (sulfur dioxide, nitrogen
dioxide, nitric oxide, and ozone) at two temperatures and two relative
humidity levels, resulted in a very large number of exposures. Since
humidity and temperature are variables in the environment, the use of more
than one temperature and humidity would determine whether these variables
are of importance in establishing conditions for change. To obtain realistic
results, the high pollutant concentration that corresponded to the maximum
hourly average in a polluted urban atmosphere and the low concentration that
corresponded to the yearly average of a polluted urban atmosphere were selected.
3
The concentrations of sulfur dioxide, 260 yg/m + 15% (0.1 ppm) and
o
2,620 yg/m ± 15% (1.0 ppm), were continuously monitored during the 12-week
exposure period with a Beckman K-76 Acralyzer by using a hydrogen peroxide
459
(HO) method.
3 3
The ozone concentrations, 100 yg/m ± 20% (0.05 ppm) and 980 yg/m +
20% (0.5 ppm), were generated by passing dry air over an ultraviolet lamp;
25
concentration was monitored with a Mast Model 924-2 ozone meter.
o
The nitric oxide concentrations used were 120 ug/m ± 15% (0.1 ppm)
3
and 1,230 yg/m ± 15% (1.0 ppm); the nitrogen dioxide concentrations were
Q 3
90 yg/m ± 15% (0.05 ppm) and 940 yg/m ± 15% (0.5 ppm). Both gases were
monitored continuously with a Beckman K-76 Acralyzer by a modified Saltzman
471
method.
-------
-Jb
TABLE 8-4
Numbering Code for Dyed Fabric Study
Fabric
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Color Index No.
Red 1
Red 1
Red 151
Red 2
Red 14
Red
Orange 45
Yellow 65
Yellow 11
Green 2
Violet 1
Blue 86
Blue 3
Blue 3
Blue 27
Blue 27
Blue 1
Blue 2
Blue 14
AATCC Ozone Ribbon
Dye
Direct
Direct
Acid
Reactive
Basic
Azoic-
Acid
Acid
Basic
Sulfur
Acid
Direct
Disperse
Disperse
Disperse
Disperse
Reactive
Reactive
Vat
£.
Material
Cotton
Rayon
Wool
Cotton
Acrylic
Cotton
Nylon
Wool
Acrylic
Cotton
Wool
Cotton
Cellulose Acetate
Nylon
Cellulose Acetate
Polyester
Cotton
Cotton
Cotton
Acetate
Amount
of Dye
1.0%
0.5%
—
2.0%
0.5%
474 g/1
0.5%
—
0.05%
40 g/1
—
1.0%
1.0%
1.0%
1.0%
20 g/1
2.0%
2.0%
45 g/1
~~
-From N. J. Beloin.41
—Coupling Component 2, Azoic Diazo Component 32.
-C.I. Disperse Blue 27, C.I. Disperse Red 35, C.I. Disperse Yellow 37.
-------
8-37
The chamber average temperature was kept at either 32.22°C
or 12.78°C, and the relative humidity was maintained at either 50 + 5% or
90 + 5%. Both variables were automatically controlled and continuously
monitored. Pollutant concentrations were maintained within the stated
range at least 90% of the time.
Results from these experiments are presented in Hunter Color Units
which are based on spectrophometric readings of color on original fabrics
and on exposed fabrics with values less than three units designated as
trace. (See Tables 8-5, 8-6, and 8-7.)
To summarize, the study showed that nitrogen dioxide, ozone, and, to
a lesser extent, sulfur dioxide can cause appreciable dye fading; that
nitric oxide has little or no effect; that higher temperatures and relative
humidities increase dye fading; and that the rate of fading as a function
of exposure time appears to be nonlinear.
EFFECT OF AIR CONTAMINANTS ON FIBERS
The degradation of fibers during atmospheric exposure is affected not
only by natural agents such as air, sunlight, and atmospheric moisture, but
also by air contaminants such as ozone, nitrogen oxides, sulfur dioxide,
airborne dirt, etc. Cotton and nylon are vulnerable to chemical action of
acids which reduce the strength of the polymer.
Fiber degradation assumes additional economic importance in uses other
than garments and home furnishings. From 18 to 20% of all fiber uses are
industrial. In some of these products, such as tarpaulins and cords, pre-
mature losses in strength are costly and create safety hazards. Brysson
et al. ' show that cotton fabrics deteriorate in less than two months.
This explains manufacturers search for fibers best suited to resist airborne
acids and protective fabric coatings which reduce acid absorption.
Degradation of Cotton by Airborne Acids
430
Race studied the degradation of cotton in industrial regions in
England. In these regions the degradation of cotton is greater during the
winter than during the summer. Further, he observed that the smallest loss
in strength occurs in spring and autumn. Race determined that the longer,
-------
8-38
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8-41
more direct sunlight in summer and the increased acidity of atmospheric
moisture in winter were causes of the increased damage in those seasons.
The specific action of nitrogen oxides as an important cause of deg-
radation of fibers is not evidenced in the studies published to date. The
concentration of sulfur acids resulting from high sulfur fuel combustion
is higher than that of oxides of nitrogen in industrial areas. The ni-
trogen oxides present increase the total concentration of airborne acids.
These factors preclude the isolation of the effects of nitrogen oxides
from that of sulfur oxides in field exposures.
Airborne acids such as sulfuric acid and sulfur dioxide on cotton
appear mainly in industrial and thickly populated areas. The effects of
244
these acids on cotton have been studied by Howard and McCord. These
acid constituents behave as aerosols and are deposited on exposed fabric
in several ways. Precipitation (rain, snow, fog, or dew) forms around such
particles and transports them to the fabric. Gravity, electrostatic attrac-
tion, and direct interception also play a part on the deposition of particles
on the fabric.
424
Pomroy and Stevens conducted studies in two different locations:
Tallahassee, Florida (intense sunlight) and Charleston, West Virginia
(industrial air pollution). Six drapery lining fabrics (cotton and acetate)
were subjected to outdoor and indoor seasonal exposures.
The results, reported as breaking strength retained, showed statistically
significant differences between fabrics weathered in the two geographic
regions. The cotton sateens exposed to the contaminated air of Charleston
lost about 50% more strength than those exposed in Tallahassee. Charleston's
climate and topography, together with its industries, provide conditions
-------
8-42
for outdoor exposure similar to those reported by Race. Accordingly, the
cellulose degradation and deterioration is attributed to the action of
atmospheric moisture, airborne acids, and sunlight. The authors observed
that most deterioration occurs during the summer and least during the
winter in both Charleston and Tallahassee. This is in contrast to Race's
observations that deterioration in winter was due to the sulfur content of
domestic fuel (coal). Sunlight was an important factor in Charleston;
it became the determining one in fiber deterioration. But when the sunlight
was minimal, air pollution appeared as the major factor.
60,61
Brysson et al. established a definite relationship between air
pollution and accelerated degradation of cotton fabrics. These authors
conducted studies over 12-month periods at sites in the St. Louis, Missouri
and Chicago, Illinois areas. The strength retention and degree of soiling
of untreated heavy, and untreated and treated lightweight, cotton fabrics
were related to the air pollution data determined by periodic measurement
of dustfall, suspended particulate matter, sulfation, and sulfur dioxide.
Sulfation and sulfur dioxide measurements correlate best with fabric
degradation and soiling. Measurements of other air pollutants, including
nitrogen oxides, dust, and suspended particulate matter, also correlate
but to a lesser degree.
Degradation of Nylon
The destruction of nylon stockings by acid droplets, generated in pol-
luted atmosphere under certain weather conditions, has been reported in
many newspaper articles over several years.
/ * 541
Travnicek investigated the effect of air pollution on nylon. He
studied the conditions leading to fiber damage by analyzing the different
-------
8-43
contaminated atmospheres and types of damage they caused. He also devel-
oped a smog simulator which produced exhaust gases in an aerosol chamber.
Corrosive gases cause most damage to high polymers when absorbed on
soot, smut, and other carrier substances. Using the aerosol chamber,
/ w
Travnicek simulated the effect of various corrosive gases on textiles when
absorbed on solid carrier aerosols. So far, there is no protection against
sulfur dioxide-laden soot attaching itself to fibers, if outside conditions
favor a rapid oxidation of sulfur dioxide to sulfur trioxide and its trans-
fer to the fiber surface (high humidity, higher temperature). The formed
sulfuric acid will degrade or dissolve the polymer. There is some simi-
larity, or common action, between the degradation by light and that by
exhaust gases. Some light stablizers and dyestuffs protect fibers from
degradation by exhaust gases, expecially if these gases are transformed
into oxidizing agents by nitrogen oxides and light. Polyamide fibers that
are not light stabilized are easy prey for atmospheric pollutants originating
from exhaust gases. Polyamides are susceptible to hydrolysis by mineral
acid and also suffer degradation by oxidants.
322
Little and Parsons recorded data on cotton, nylon, and polyester
at eight exposure sites in England during 1961 and 1962. The loss in
strength that occurred when textiles were subjected to unprotected outdoor
exposure varied considerably with the location. In rural areas cotton
fabric was found to be more resistant than polyester or nylon. In urban
or industrial areas, cotton is inferior to polyester and, to a lesser
extent, to nylon. The effect of contaminated air on nylon as compared with
clean air is demonstrated by figures resulting when strength is plotted
against relative viscosity. After a loss of about 40% in strength, the
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8-44
samples fall into two groups. The curve for samples taken from urban
industrial exposure is different than the curves from samples exposed to
cleaner air. The figures for the exposed nylon samples plotted against
amino end-groups show strength decreasing rapidly with reduction in end
groups as the retained strength falls below 70%. The recorded values for
the intrinsic viscosity of polyester confirm its insensitivity to photo-
chemical attack. The pH of the aqueous extract is also an insensitive
factor, possibly from the leaching effect of rain. The figures obtained
indicate, however, that aqueous extracts of samples exposed to urban areas
are more acidic than those in rural areas.
Laboratory Studies of Nitrogen Oxides on Fibers
The direct effects of nitrogen oxides on cellulose fibers have not
370
been studied specifically. Morris conducted a service exposure in
Berkeley, California in which observed damage was attributed to ambient
levels of nitrogen oxides. Cotton yarn samples were exposed in two cabinets
open to sunlight. In one cabinet, air was filtered through carbon to re-
move nitrogen dioxide. In the other, ambient air was circulated. The
samples were exposed for three separate 28-day periods. During this
exposure, air pollution and weather measurements were made. Essentially,
the experiment determined the effect of sunlight with and without air pol-
lutants—oxidants and NO —on fiber deterioration. Because the Berkeley
x
area is low in sulfur dioxide, this pollutant was not measured in this
study. Resulting data show that unfiltered air deteriorated the cotton
yarn to a greater extent than filtered air. It is known that sunlight
will cause fiber degradation. The results of this study
-------
8-45
show that sunlight also accelerates the reaction between the air pollutants
and fibers. Carbon filter is not a good absorber of nitric oxide which
could have been present and which could convert to nitrogen dioxide. If
this is the case, air free of nitric oxide would be expected to show less
degradation.
620
In laboratory trials, Zeronian exposed manmade fibers to simultaneous
action of light energy (Xenon Arc) and air containing 0.2 pph nitrogen di-
oxide at 48°C and 39% RH. The fabric samples used were modacrylic (Dynel),
acrylic (orlon), nylon 66, and polyester (dacron). The fibers were exposed
to light in a Weatherometer and were intermittently sprayed with water.
Identical fabrics were subjected to the same tests under the same weathering
conditions but without introduction of nitrogen dioxide.
After exposure for 168 hrs, modacrylic fabrics showed no difference
with and without nitrogen dioxide. Orion and polyester showed only slight
differences. The results for nylon were not clear cut, although temperatures
did cause significantly greater degradation in the presence of NO .
During examinations of the reaction of linear polymers, including nylon
262
and polypropylene, Jellinik found that nylon 66 suffers chain sission in
the presence of dry nitrogen dioxide, whereas polypropylene tends to cross
link.
110
Dimaio and Manganelli investigated the effects on Lycra, nylon,
and polyester of nitrogen dioxide at 20 pphm, 25°C, in the absence of
light. The degradation of the fibers was measured by increases in viscosity
and by changes in chemical structure of the fibers as shown by infrared
spectra.
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a-46
EFFECTS OF NITROGEN OXIDES ON METALS AND ALLOYS
Oxidation - General
The rate of corrosion is significant to the life of a metallic
component. Almost all structural alloys in use are thermodynamically
unstable with a tendency to revert to their more stable state, generally
the oxide. Fortunately, the product of the reaction between the substrate
and the environment is usually an adherent scale which separates the reac-
ting species. Anything that changes the characteristics of this scale
could increase or decrease the reaction rate and, hence, alter the life of
the substrate.
The rate of scaling or consumption of substrates that do not form
protective films is usually linear in relation to time of exposure. In
general, such materials are not of great concern to designers since they
can account for the material loss in their design. The unexpected failure
of a system,which should form a protective scale but does not, is of more
concern. Reasons for potential failures are outlined below.
In order for the films formed to be protective, they must completely
cover the surface of the substrate from which they are formed and they must
be able to accommodate the stress within the film caused by differences in
specific volumes between the scale and the substrate, which is called the
420
Pilling-Bedworth ratio. The nature of the scale could depend upon the
environment, and in areas of high humidity the scale formed could be a
hydrated oxide or hydroxide instead of a simple oxide. If the scale does
not adhere, the rate of corrosion is increased.
The growth of protective layers is controlled by diffusion of ionic
defects through the scale. The rate at which the metallic substrate recedes
-------
8-47
can be increased or decreased by altering the concentration of ionic defects
in the scale. The incorporation of lower valent materials in "p" type
scales generally decreases the oxidation rate, whereas for "n" type scales,
the reverse is true.
The rates of reactions generally increase with increasing temperature
although the driving force, the change in free energy for the reaction, is
decreasing. The rate of reaction (K) can be described by an Arrhenius-type
equation,
K - Ae (1)
where E = the activation energy, R = gas constant, T = absolute tern-
3.
perature, and A is the preexponential factor, a constant for the system.
The rate of corrosion of structural steels in air, Y, has been described
215
by the empirical equation
log Y = A + A, [SO 1 - A0 [Oxidant] + b ln(t)
o 1 i *• °
- b-L [Oxidant]ln(t) + b2 [NO ]ln(t),
where the A's and b's are constants, [ ] is concentration, and t is exposure
time. It was reported that sulfur dioxide increases corrosion and oxidants
decrease corrosion by oxidizing ferrous oxide (FeO) and f errosoferric oxide
(Fe.,0,) to the more protective ferric oxide (Fe90 ) . This explanation is
incorrect since sulfur dioxide itself is a strong oxidant. Furthermore, as
long as the metallic substrate (in this case, iron) exists and the metal
exhibits more than one valency state, the oxide in contact with the metal
will be that representative of the "reduced" or lowest valency and the oxide
in contact with the oxidant will be the highest valency state. Therefore,
the equilibrium product next to the steel will always tend to be ferrous oxide.
-------
8-48
Increasing or decreasing the rate of reaction requires changing the defect
structure of the oxide(s) formed, since the relative thickness of the scale
of a multicomponents system depends on the relative diffusion rates. If no
protective scale forms, then oxidation ceases only when the chemical activ-
ity of the oxidant is reduced to a level below that necessary for scale
formation.
Wet Corrosion
149
Fontana and Greene define eight forms of corrosion: uniform and
general attack, galvanic, crevice, pitting, intergranular, selective leaching,
erosion, and stress corrosion. Although this list is arbitrary, it covers
a large variety of problems.
A liquid film or the presence of a hydrated salt plays an important
role in most of these types of corrosion. Each mechanism of corrosion must
be understood to attenuate the problem and to predict the effect of a variable
change.
Galvanic, pitting, crevice, and selective leaching require the presence
of an electrolyte or solvent. Hydrated salt crystals can serve as the
electrolyte in galvanic corrosion. The more conductive condensate found
near the sea is more corrosive than the condensates found inland, even
under equal humidity and temperature conditions. Moreover, the position of
149
a metal or alloy in the galvanic table varies with geographic location.
Pits or pitting corrosion, whose incubation periods can range from
days to years, usually grow in the direction of gravity. This corrosion is
generally associated with the presence of the chloride ion (Cl~), which may
be related to the formation of the strong, highly ionized hydrochloric
acid (HC1). Hence, the salts of fluorine and iodine are not regarded as
-------
8-49
strong pit formers, but nitrates may be strong pit formers. Since solutions
containing ferric chloride (FeCl ) and cupric chloride (CuCl ) or nitrate
[Cu(NO,,) • 3H«0] are aggressive because of both the presence of the chloride
ion and the reducibility of the cation, oxygen is not necessary.
In both crevice corrosion and pitting corrosion, the rate at which
metal dissolves when in contact with the solution is accelerated by the
presence of the chloride ion. For example, in the vicinity of the pit or
crevice, the oxidation reaction
M + M+ + e (2)
is increased while the reduction reaction
0 + 2H2 OH~ (3)
can continue on all surfaces. The greater mobility of the halide ion is
postulated to be significant since the chloride content of the liquid
within a crevice can be more concentrated than that in the bulk solution.
Stress corrosion and corrosion fatigue require the presence of a
corrosive media and a mechanical force. The corrosive media initiates the
stress riser from which the crack progagates. The applied stress cracks
any protective scale that separates the corrodent from the substrate. The
application of stress initiates corrosion. The pit thus formed then in-
creases the stress.
Material failure caused by the selective leaching of a component of
the alloy is exemplified by dezincification (loss of zinc), or dealumi-
nification (loss of aluminum), etc. Dezincification is a well-established
form of corrosion common to brasses. A material which has undergone
dezincification is weak, brittle, and possesses little strength.
-------
8-50
Past Experiences
The composition of a corrodent, gaseous or liquid, influences the
rate of corrosion of substrates. The materials engineer must understand
the various processes that occur in order to negate the harmful effects
prior to their occurrence.
Nickel/brass wire springs in telephone equipment failed prematurely,
226
primarily in the Los Angeles area. Although most failures occurred on
the West Coast, springs have also failed sporadically in other parts of the
country. Investigators found that the springs had a fogged appearance and
were covered with a hygroscopic dust rich in nitrates. Failure was attri-
buted to stress corrosion. The solution to the problem was a change in
materials. A zinc free alloy did not fail in service. To minimize failure
of equipment which still contained the nickel/brass springs, air conditioning
and air filtration units were installed.
Although the failure symptoms of the nickel/brass springs were attri-
buted to stress corrosion, these failures could be due to selective leaching,
especially since the cure to the problem was the removal of zinc from the
alloy. Zinc-containing alloys such as brass are most susceptible to selective
leaching corrosion. To minimize this corrosion, the composition of the
alloy is altered, usually by changing the zinc content.
Stress corrosion is alleviated by reducing the stress on the component,
eliminating the critical environmental species, changing the alloy, or
cathodic protection.
The telephone company changed the alloy and altered the environment
by installing air filters and air conditioning as mentioned above. They
reduced the quantity of "dust" and controlled the humidity. These steps
were effective for both stress corrosion and selective leaching.
-------
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Air filtration to control total accumulation is, under many conditions,
questionable due to traffic patterns of people entering and leaving the
503
area. Humidity control is important to all wet corrosion problems since
hydroscopic salts pull the moisture out of the air. Although nitrogen-
containing compounds have not been shown to cause accelerated corrosion,
evidence indicates that they may contribute to the overall common corrosion
problems. Specifically, the nitrate salts are more hydroscopic than the
226
chloride and sulfate salts. As such, they may lower the "threshold"
humidity requirements for the formation of an aqueous media which can serve
as the electrolyte or solvent for wet corrosion. The "creeping green cor-
rosion" associated with nickel-palladium-capped contacts probably falls
into this category. Nitrogen can change the defect structure of many oxides
310
thereby increasing or decreasing the rate of oxidation of alloys. If
the magnitude of the problem associated with the presence of NO , etc., is
X
to be evaluated, specific studies should be performed and statements such
as: " . did not show noticeable relation to dialing trouble, so it
was thought that nitrogen oxides were responsible" be disregarded until a
cause and effect is shown.
In summary, evidence exists that (a) nitrogen can affect the defect
structure of oxides thereby increasing or decreasing the rates of oxidation
of metals and alloys, and (b) nitrates adsorption of water moisture aids
the formation of a solvent or electrolyte for all the aforementioned forms
of wet corrosion; however, a direct relationship has not been shown to exist
between a given NO content and change in corrosion behavior of structural
X
materials. Since it is not conclusive that no such relationships exist or
that synergistic relationships are not possible, more careful examinations
must be made.
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3-52
POTENTIAL EFFECTS OF NITROGEN OXIDES ON RUBBER COMPOUNDS
Ozone is the only major pollutant whose ability to shorten the life of
rubber products is well-documented. However, there is some experimental
evidence suggesting that limited damage to foamed rubber products is caused
376
by nitrogen dioxide. This observed damage takes the form of discoloration
and deterioration of the strength of the foam. Because information on ni-
trogen dioxide damage to rubber products is limited, additional data must
be accumulated to definitively establish the effect of nitrogen dioxide on
rubber compounds.
SUMMARY
Field studies and laboratory research have demonstrated that atmospheric
pollutants can cause certain textile dyes to fade and certain white fabrics
to yellow. Because consumer awareness of pollutant fading is minimal, color
fading has often been erroneously attributed to sunlight. Nitrogen dioxide,
ozone, and sulphur dioxide can cause appreciable dye fading, while nitric
oxide has little or no effect. The chemical mechanisms for nitrogen oxide
fading of dyes are fairly well-established. Higher temperatures and relative
humidities can increase the degree of dye fading. Various methods for color
protection are available, including inhibitors, nitrogen oxide resistant
dyes, cospun pigment dyes, and substitution of alternate fabrics. The cost
to the consumer of color fading of dyes has been estimated to be more than
$100 million annually. The effect of pollutants on yellowing of white
fabrics has not been well-established. Recent studies show that nitrogen
dioxide is the principal pollutant responsible for this problem.
Evidence supporting specific action of nitrogen oxides as an important
cause of degradation of textile fibers has not yet been published. However,
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3-53
the presence of nitrogen oxides could increase the concentration of airborne
acids which degrade cotton fabrics and nylon.
Experimental evidence suggests that nitrogen oxides can affect the
defect structure of metal oxides thereby increasing or decreasing the
oxidation rates of metals and alloys. Furthermore, airborne nitrates can
adsorb water and aid in the formation of a solvent or electrolyte for wet
corrosion. However, a direct relationship has not been established between
a given nitrogen oxide concentration and a change in the corrosion behavior
of structural materials. Additional data are required if the role of ni-
trogen oxides in damage to metals and alloys is to be defined.
Information on direct nitrogen oxide damage to other materials, such
as foam rubber, is limited, and further study is required to establish the
effect of nitrogen dioxide on rubber compounds.
CONCLUSIONS
Field studies and laboratory research have demonstrated that nitrogen
dioxide, as well as sulfur dioxide and ozone, can cause certain textile
dyes to fade and certain white fabrics to yellow. The chemical mechanisms
for nitrogen oxide fading of dyes are fairly well-established, and various
methods for color protection are available. The cost to the consumer of
color fading of dyes by nitrogen oxides has been estimated to be more than
$100 million annually. The effect of pollutants on yellowing of white
fabrics has not been well-established; however, recent studies suggest that
nitrogen dioxide is the principal pollutant responsible for this problem.
No estimates of the cost to consumer of yellowing of white fabrics are
available.
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8-54
Available data do not indicate a direct role for nitrogen oxides in
the degradation of textile fibers. However, the presence of nitrogen
oxides are likely to increase the concentration of airborne acids which
degrade cotton fabrics and nylon.
Experimental evidence exists which suggests that nitrogen oxides can
affect the defect structure of metal oxides thereby increasing or decreasing
the rates of oxidation of metals and alloys. Furthermore, airborne ni-
trates can adsorb water, thereby aiding in the formation of a solvent or
electrolyte for wet corrosion. However, a direct relationship has not
been established between a given nitrogen oxide concentration and a change
in the corrosion behavior of structural materials.
RECOMMENDATIONS FOR FUTURE RESEARCH
1. Additional data are required to define the role of nitrogen oxides
in the degradation of textile fibers and rubber compounds.
2. Further studies should be made of the effects of nitrogen oxides
and airborne nitrates on metals and alloys.
3. Estimates should be made of the cost of yellowing of white fabrics,
degradation of textile fibers, and damage to metals and alloys by
nitrogen oxides and airborne nitrates.
-------
CHAPTER 9
EFFECTS OF NITROGEN OXIDES ON VEGETATION
Nitrogen oxides are less phytotoxic than other constituents of the
photochemical oxidant complex. Most of the vegetation injury they cause
is indirect through their participation in photochemical reactions that
produce atmospheric oxidants, including both ozone (0 ) and peroxyacyl-
nitrates (PANs) (see Chapter 6).
The direct effects of atmospheric oxides of nitrogen on vegetation
are usually associated with and confined to areas near specific industrial
sources. For example, injury to vegetation from exposures to nitrogen di-
oxide (NO-) has been observed near nitric acid (HNO_) plants, but there
are no published reports of vegetation injury in the field due to nitric
oxide (NO) or other oxides of nitrogen.
The direct effects of NO on vegetation are reviewed in this chapter.
X
Greatest emphasis is placed on reports relating NO effects to known con-
X
centrations and durations of exposure. Since the majority of available
data pertain to nitrogen dioxide, this pollutant receives the most attention
in the following pages. The effects of nitric oxide on vegetation are dis-
cussed less because existing data is sparse.
NATURE OF EFFECTS
General
To evaluate the effects of NO on vegetation, the ultimate use of a
plant must first be considered. These uses include three broad categories:
commercial, aesthetic, and ecologic. Commercial plants are those grown for
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9-2
food, forage, fiber, or fuel. Plants grown in private gardens and public
parks have aesthetic value. Examples of complex ecologic functions are the
participation of plants in soil and water conservation, the oxygen-carbon
dioxide (0 -C09) balance in nature, the accumulation and recycling of
elements, and the provision of food and habitats for wildlife.
Although a plant or plant community can serve any or all of these
functions simultaneously, the effect of NO on vegetation becomes signifi-
X
cant when it is related to the function of the plant. In this regard
196
Guderian et al. have differentiated between air pollution injury and
damage;
The term "injury" should be considered to include all plant
responses resulting from the action of air pollutants.
Temporary reductions of assimilation rate or alterations in
content of plant constituents thus are included, along with
leaf necrosis, leaf abscission, or retarded growth. Of the
many functional disturbances and dramatic visible effects
that can result from air pollutants, only those of sig-
nificance to the desired use of the plant should be con-
sidered damage.196
For example, in root crops (e.g., beet, carrot, radish, and turnip) or in
tree fruits, foliar injury is not necessarily "damage" because it does not
affect the usable crop. Until injury is sufficient to affect yield, it
cannot be considered as damage. Conversely, injury to leaves of ornamental
plants or leafy vegetables (e.g., spinach and lettuce), however slight, is
classified as damage since the appearance, of these leaves determines the
market value of the plant. The NO effects on vegetation described in this
X.
chapter have been observed in experiments designed to answer specific ques-
tions. Interpretation is therefore required in order to determine the
effects on the functional role of plants.
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9-3
Acute vs. Chronic
Exposures to most pollutants, including NO , are usually classified
X
arbitrarily as acute or chronic. In experimental fumigations, acute expo-
sures are those of short duration at high pollutant concentrations; chronic
exposures are for longer periods (occasionally intermittent, but usually
continuous) at low pollutant concentrations. The ranges of concentrations
and durations of exposure (doses) for acute and chronic exposures have not
been defined. Most botanical investigators would designate nitrogen dioxide
o
exposures of 1.6 to 2.66 ppm (3 to 5 mg/m ) or greater for periods up to 48
hr as acute, and those for longer periods at lower concentrations as chronic.
However, these definitions do not apply in the field near sources of NO
X
emissions. There, an acute exposure would be any single exposure causing
plant injury; the term "chronic" would be applied to a series of exposures
that result in injury, where no single exposure has an effect by itself.
These two types of exposures often elicit completely different responses.
For example, leaf injury is usually characterized by necrosis in acute expo-
sures, but by chlorosis in chronic exposures.
Metabolic Responses
Leaf injury is the most obvious effect of NO on plants, but it is only
X
the end result of a series of events that have occurred at lower levels of
biological organization. The responses of plants at these various levels
581
have been described by Vogl et al. for sulfur dioxide (SO ) and by
593 2
Weinstein and McCune for fluoride. These authors have related effects
at the cellular level to effects on leaves, the entire plant, and even com-
munities of plants. A similar scheme may be applied to NO effects.
x
-------
9-4
Atmospheric NO is received by plants primarily through the stomata,
X
after which the pollutant changes from a gaseous to an aqueous form. In
its aqueous form, the pollutant may alter the pH of the aqueous solution in
or around cells, or react with one or more constituents of the plant in
such a manner that both reactants are changed to forms affecting plant
metabolism.
In Vitro Responses. NO effects on plants at the cellular level are
x
not well understood. The reaction of nitrogen dioxide with water, which
occurs in plant tissues, results in a mixture of nitrous acid (HNO ) and
nitric acid. Enzymic experiments in vitro have shown that 1.0 M nitrous
acid inactivated 3-amylase isolated from barley and malted barley, possibly
592
through the oxidation of essential sulfhydryl groups. Later studies with
108
a-amylase extracts from Bacillus subtilis showed that nitrous acid inter-
acted with amino and tyrosine groups as well as sulfhydryl groups. The
oxidizing action of nitrous acid was reversible with sulfhydryl groups, but
irreversible with amino and tyrosine groups. Nitrous acid causes oxidative
deamination of amino groups of viral nucleic acids which reduces infectivity
469 381,489,548
of some viruses, or mutagenic effects in tobacco mosaic virus,
50
and polio virus. However, the concentrations of nitrous acid used in all
of these in vitro experiments were much greater than those that would result
from gaseous NO exposures.
X
595
In Vivo Responses. Wellburn et al. studied the effects of nitrogen
dioxide exposures on the ultrastructure of chloroplasts i.n vivo. Broad bean
(Vicia faba) plants were exposed for 1 hr to 1.0, 2.0, or 3.0 ppm (1.9,
3
3.8, or 5.6 mg/m ). The leaves were harvested immediately after exposure and
prepared for electron microscopy. Examination of the chloroplasts showed
-------
9-5
that nitrogen dioxide caused a swelling of the thylakoids associated with
the stroma. These swellings appeared to be reversible because thylakoid
swelling was not observed in chloroplasts of leaves exposed to unpolluted
air immediately following nitrogen dioxide fumigation.
234
In vivo experiments performed by Hill and Bennett have shown that
both nitric oxide and nitrogen dioxide inhibit apparent photosynthesis of
oat (Avena sativa) and alfalfa (Medicago sativa) plants at concentrations
below those that cause foliar lesions. The threshold dose for this inhi-
bition was about the same for both pollutants [0.6 ppm (1.1 mg/m ) for
3
nitric oxide and 0.4 ppm (0.8 mg/m ) for nitrogen dioxide in 90 min fumi-
gations], but the inhibition occurs faster for nitric oxide than nitrogen
dioxide. The NO -induced inhibition of photosynthesis is not a permanent
x
effect. Plants exposed to nitric oxide for 2 hr recover and assimilate
carbon dioxide at prefumigation rates within 1 hr after exposure, whereas
plants exposed to nitrogen dioxide require more than 4 hr to recover. In
fumigations introducing nitric oxide and nitrogen dioxide (1:1) simultaneously,
the degree of inhibition of apparent photosynthesis was the same as the sum
of that induced by each pollutant when introduced simultaneously. Unlike
235
ozone-induced inhibition of photosynthesis, which was determined to be a
234
consequence of stomatal closure, Hill and Bennett suggested that nitric
oxide and nitrogen dioxide affect photosynthesis directly. They found no
evidence that NO affects stomata.
x
To summarize, pollutant-altered pH of the cellular milieu and pol-
lutant reactions with cellular constituents lead to altered metabolism,
ultrastructural changes, reduced photosynthesis, and probably many other
effects that have not been observed or measured. These, in turn, lead to
effects at progressively higher levels of biological organization. Pre-
-------
9-6
mature senescence, chlorosis, necrosis, or abscission of leaves affect the
entire plant causing reduced growth or reproduction, and even death of
individual plants or entire plant communities.
Visible Symptoms
Although foliar symptoms are the most obvious effects of NO exposure,
X
less obvious effects have also been reported. Continuous exposure of Pinto
bean (Phaseolus vulgaris) seedlings for 10 and 19 days to 0.33 ppm (0.62
o
mg/m ) of nitrogen dioxide resulted in a gradual change in the appearance
529
of leaves without causing necrosis. The leaves developed a downward
curvature and a darker green color. The chlorophyll content (per unit weight)
of unifoliate leaves was greater for nitrogen dioxide-fumigated plants than
for control plants, but there was no difference in the amount of chlorophyll
per leaf. Deeper green color and downward curvature of leaves were also
noted for tomato (Lycopersicon esculentum) seedlings exposed to slightly
529
higher nitrogen dioxide concentrations. But the loss of chlorophyll is
a more common response to NO exposure. Extraction and chromatographic
X
separation of pigments from nitrogen dioxide-injured leaves by Kandler
274
and Ullrich revealed a loss of carotene, a reduction in chlorophyll, and
the appearance of a decomposition product of chlorophyll that was specific
for nitrogen dioxide. They proposed that these pigment changes could be
used as a diagnostic tool to differentiate between acute sulfur dioxide
and nitrogen dioxide injury.
In continuous long-term exposures of tomato plants to 0.25 ppm (0.47
3 50°
mg/m ) of nitrogen dioxide, Spierings noted premature senescence of older
leaves after 37 days which, with time, gradually increased in intensity,
affected younger leaves, and resulted in premature abscission of leaves.
536
Thompson et al. observed chlorosis and severe defoliation of navel orange
-------
9-7
3
trees (Citrus sinensis) after exposure for 35 days to 0.50 ppm (0.9 mg/m )
of nitrogen dioxide. Acute nitrogen dioxide exposures of Citrus sp. [200
3 3
ppm (376 mg/m" ) for 4 to 8 hr or 250 ppm (470 mg/m ) for 1 hr] were observed
330
by MacLean, et al. to cause rapid wilting and abscission of nearly all
spring leaves. Abscission frequently occurred before necrosis developed,
and some succulent young shoots were killed. Abscission of potato (Solanum
95
tuberosum) flowers and lodging of oats were observed by Czech and Nothdurft
3
after 1 hr exposures to 100 ppm (188 mg/m ) nitrogen dioxide.
There is no "typical" leaf symptom for diagnosing nitrogen dioxide
injury to plants. The extent and type of leaf injury are determined by
many factors. As mentioned above, acute and chronic exposures produce dif-
ferent symptoms. In addition, susceptibility of plants to nitrogen dioxide
varies among species and even among varieties and cultivars within a species.
Climatic and other external factors also influence the form of leaf injury.
Descriptions of nitrogen dioxide injury to leaves of various plant
species and color plates showing nitrogen dioxide symptoms have been pub-
577 530
lished by van Haut and Stratmann and by Taylor and MacLean. Their
studies indicate that the primary leaf symptom of chronic nitrogen dioxide
exposure is chlorosis, whereas acute injury to leaves of dicotyledonous
plants is usually characterized by the rapid appearance of irregularly-
shaped intercostal necrotic lesions. On most conifers and monocotyledonous
plants, nitrogen dioxide-induced foliar necrosis usually occurs at or near
the tips of leaves. The length or area of affected tissues varies with the
severity of the nitrogen dioxide exposure.
The best available descriptions of nitrogen dioxide-induced symptoms
577
on various plant species were published by van Haut and Stratmann:
-------
9-8
Acute leaf injuries (necrosis)
Diffuse gray-green or lightly brownish discolored spots appear
on the leaves as early symptoms of acute nitrogen dioxide injury.
They dry rapidly and bleach, through various intermediate colors,
becoming ivory to white; or, as with many deciduous trees, they
take on a reddish-brown, rarely black-brown final color. The
time course of acute injury depends on light and temperature.
On warm, sunny days, usually only 24 hr pass between the less
well recognizable early symptoms and the more distinct later
stages. In deciduous trees, the final coloration begins about
one week after the injury.
With some types of plants, such as clover, lucerne, or rape,
the characteristic ivory-bleached spots can even appear on the
day of exposure.... Necrosis is often surrounded by cork-like
dark brown tissue....
Dicotyledonous plants: Intercostal necrosis prevails in broad-
leafed dicotyledonous plants. After severe exposure, the damage
pattern extends over onto the leaf veins, which then also die
and discolor. Necrotic portions of tissue can tear apart and
break out of the leaf width, leaving a colored edge behind. This
"window formation" is observed mainly in broad leaves with tender
and succulent tissue, as well as in the leaves of various kinds
of deciduous trees. In deciduous trees, intercostal spots often
combine to form intercostal stripes. Point-like necrosis distri-
buted irregularly over the lamina can give the leaf a sprinkled
or rusted appearance. In other dicotyledonous plants the edges
of the leaves are the preferred points of attack. Leaf edge
necrosis is particularly characteristic for representatives of
the family of the papilionaceous flowers.... Injuries [in clover]
begin at the edges of the pinnate leaves and from there extend
inwards. Not uncommonly, necrosis first develops at a small dis-
tance from the edge of the leaflets and only after that does it
affect the outer marginal zone, as in peas.... Occasionally small
fleck-like necrosis also appears near the margin. The younger
pinnate leaflets of lucerne and lupine often show point necrosis,
while marginal necrosis at the leaf base is most common in older
leaflets.... Marginal necrosis can appear in Robinia.... Oak
and maple leaves develop light red-brown spots and marginal
necrosis, while those of apple are dark red-brown, and those of
pear are black-brown. With more severe exposure, necrosis
penetrates in tongue shapes from the edges into the intercostal
areas. [These necrotic areas] often coalesce so that: a green
zone remains only along the central vein. Furthermore, compound
leaves that are strongly segmented (carrot and parsley plants)
show injuries predominantly at the edges and tips. Simple leaves
with deep incisions (gooseberry bush) also react to the action of
nitrogen dioxide more with edge and tip damage than on the leaf
width. Finally, discolorations proceeding from the leaf tip are
also observed in species with lanceolate leaves (flax, sweet
William) as well as on small bracts and sepals.
-------
9-9
Monocotyledonous plants; Acute injuries are expressed in most
monocotyledonous plants in the form of yellow-white necrosis
which spreads mainly from the tips of blades or just below them.
In addition to this injury syndrome, necrosis also occurs at the
leaf margins, and as necrotic stripes on the leaf.
...tip necrosis, which can extend along the leaf edges, is found
in cereals, corn, and meadow grasses. After severe exposures,
as much as half of each of the leaves dies, and they may even die
back to the base. Gray-green necrosis is observed just below the
leaf tips in garden hyacinths and narcissus. Here, though, the
injury soon spreads to the tips. Marginal necrosis is typical
for oval-leafed monocotyledonous plants. Not uncommonly, the
leaves are damaged at opposite sides of the lamina, as in the
tulip, for example. In young leaves these damage zones often
form just below the tips, while in old leaves they appear more
toward the base. Marginal necrosis spreads over the leaf more or
less widely, depending on the intensity of the action.... Longi-
tudinal necrotic strips should be mentioned as another damage
pattern. They occur oftener and more distinctly in cereals than
in corn and gladiolus. Later, adjacent strips can merge so that
large longitudinally extended damage zones cover the lamina. Tip
necrosis occasionally spreads as streaks also.
The sensitivity of the beards on the inflorescences of rye and
barley is noticeable. They usually bleach down from the.tips.
After severe exposure the discoloration spreads over the hulls.
With oats, the necrotic stripes can join the yellow-white dis-
colored tips of the hulls.
In monocotyledonous plants, too, diffusely green colored groups
of cells, which soon assume a greenish-brown tone, occur as early
symptoms of leaf damage. In cereals, the gray-white final color
usually appears one day after the damage. The bleaching spreads
for several days. One week after the exposure the necrosis may
be surrounded by a thin red-brown border.
Apparently, there is a relationship between the location of
necrosis and the age of the leaf. While the injury proceeds
preferentially from the tips in young leaves—particularly narrow-
leaved types (Graminea)—the damage zones in older leaves are more
often in the middle or basal part of the leaf. This can be explained
as due to basal leaf growth. Those groups of cells that were last
to complete their development are always the most sensitive....
Conifers; Acute damage in conifers is expressed as a red-brown
or fuchsia discoloration of the needles, usually beginning at
the tips, which can extend to the base. There are also limited
local damage zones at the tips, in the centers, or at the bases
of the needles. Distinct boundaries between the necrotic and
uninjured parts of the needles can be observed in pines and firs.
Here the dead red-brown part of the needle is delineated from the
green tissue by a sharp boundary line. Later, some one to two
months after the exposure, a ring-shaped, dark red-brown zone
appears at this transition. At this time, however, the initial
-------
9-10
red-brown discoloration of the needles has bleached somewhat. [The
retention time for necrotic needles varies with the species. Spruce
needles fall] soon after the red coloration [develops]. In con-
trast, injured larch needles can remain on the twigs for several
months, and pine needles for more than a year. The fir needles
also remain for months, or even until the following year if the
injury is limited to the tips. But if the damage extends over half
of the needles, most of them fall within the first month after ex-
posure.
In the initial stage of injury the needle Lips or the upper sides
of the needles discolor to a pale or gray-green, and the damaged
part of the needle loses its gloss. With intense sunlight a light
brown intermediate color appears after only a few hours. In sunny
weather the red-brown final color appears the first week after ex-
posure. However, this may take 14 days in cloudy weather. This
explains why the needles exposed to sunshine often discol&r sooner
and more intensely than those in the shade on the same tree. Later,
the needles can bleach and take on light red-brown tones. The
spring twigs show less severe necrosis than the needles from the
previous year. The color tone of the necrosis is noticeably light
on very young needles, particularly in pine. Shortly after leaving
the sheathing leaves, the damaged needles are differentiated by
their yellow-brown color from the red-brown of the older needles.
In the pines, it is possible to detect relations between the
appearance of necrosis in certain portions of the needles and the
ages of the needles. In young needles which have just left the
sheathing leaves the damage progresses from the tips, while older
needles often show damage zones shortly below the tips. This
banding is usually only transient because the discoloration soon
spreads to the tips. With advancing age, necrosis occurs more
frequently in the center or basal part of the needle also....
Leaf Chlorosis
Along with acute damage, other phenomena are observed, for which the
term "chronic damage" has become accepted. We can mention leaf
chlorosis as a striking characteristic of this type of injury. This
is expressed as greenish yellow spots or as yellowing of the entire
leaf surface. Chlorosis occurs due to the long-continued effects
of low nitrogen dioxide concentrations, and less often from peak
concentrations which can also cause necrosis at the same time.
Spotty chlorosis is characterized by small yellow zones. They are
distributed across the lamina in large numbers, giving the leaf
a marbleized appearance. Later, adjacent spots can merge. In
other cases, chlorosis is limited to the leaf margins, or it may
spread from there across the surface. In cereals, chlorotic dis-
colorations often begin at the tips and progress towards the base
if the exposure continues. At the beginning of damage, conifers
often show chlorotic zones on the upper side of the needles or at
the tips. Later the light green or yellow-green discoloration
spreads over the entire needle. Finally, the yellow-green tone
in the upper part of the needle can change to a pale yellow and
the tip itself may die....
-------
y-il
A general chlorosis of the older leaves is often the only symptom
of injury after long-continuous exposures to low concentrations.
It resembles early senescence and can be linked with premature
falling of leaves. With young leaves, on the other hand, regen-
eration of the chloroplast pigments frequently begins after the
exposure stops, so that the leaves are a normal green again after
some weeks. The green-brown discolorations that occasionally
appear can also be reversible. It is true for all chlorotic
injury patterns that they are nonspecific and indicate gas expo-
sures only [when they occur] in combination with symptoms of acute
injury (necrosis). Both types of injury can occur on the same
plants, as well as adjacent to each other on one leaf. In general,
the older leaves tend to become chlorotic while the younger leaves
show severe necrosis. In tobacco, for instance, leaves in the
lower segment of the plant become yellow over their entire surface;
in the center section chlorotic discolorations appear along with
necrosis; while the younger leaves in the upper part of the plant
show only necrosis. In deciduous trees, necrosis and yellowing
appear preferentially at the same time in older leaves.... In
the narrow-leafed monocotyledonous plants diffuse chlorotic zones
often form a transition from the necrotic leaf tip to the green
basal part of the leaf. Cereals also occasionally show fine
chlorotic longitudinal strips which sometimes transform into
longitudinally extended necrosis. Corn often exhibits distinct
longitudinal chlorotic stripes before the appearance of necrosis.
In conifers there can be a chlorotic zone adjacent to the ne-
crotic needle tip, which advances toward the base or, as in the
narrow-leafed monocotyledonous plants, forms a transition zone
between the dead leaf tip and the green basal part.
Growth and Yield
Data concerning the effects of NO on plant growth and yield are
limited. Nevertheless, it is reasonable to assume that NO -induced reduc-
x
tions in the assimilatory capacity of plants through altered metabolism,
leaf injury, or abscission also affect the growth of plants. One of the
most comprehensive reports on the effects of nitrogen dioxide on growth
500
and yield is that of Spierings. Continuous exposures of tomato plants
2
(Lycopersicon esculentum) to 0.25 ppm (0.47 mg/m ) during the entire
growth period (128 days), caused growth reduction of leaves, petioles,
and stems. The crop matured slightly earlier with substantial decreases
in fresh weight yield (22%), average fruit weight (12%), and the number
3
of fruits (11%). After exposures to 0.5 ppm (0.94 mg/m ) for 10 days or
-------
9-12
3
0.25 ppm (0.47 mg/m ) for 49 days, tomato plants were taller than control
plants, but stems were smaller in diameter, leaves were not as large, and
536
the fresh weights of entire plants were less. Thompson et al. exposed
navel orange trees to nitrogen dioxide continuously for 290 days. Com-
pared to trees exposed to filtered air, those exposed to nitrogen dioxide
3
concentrations between 0.06 and 0.25 ppm (0.12 and 0.47 mg/m ) showed an
increase in fruit drop throughout the exposure period and a significant
reduction in both the number and weight of fruit at harvest. Czech and
95 3
Nothdurft, using 1-hr exposures to 1,000 ppm (1,880 mg/m ) nitrogen di-
oxide, reported that the fresh weight of sugar beet (Beta sp.) roots was
one-third less than that of control plants.
Both stimulation and reduction of plant growth have been reported for
500
nitrogen dioxide. As noted above, Spierings found growth reduction in
529
tomato plants, and Taylor and Eaton reported growth suppression based
on leaf weights. The fresh and dry weights of unifoliate leaves of Pinto
o
bean plants exposed to nitrogen dioxide at 0.33 ppm (0.62 mg/m ) for 10
and 19 days, or tomato leaves from plants exposed for 10 to 22 days to 0.11
3
to 0.62 ppm (0.21 to 1.17 mg/m ) were usually significantly less than
529
corresponding leaves of nonfumigated plants.
139
Conversely, Faller found that nitrogen dioxide stimulated growth.
Fumigations providing nitrogen dioxide at 0.8, 1.6, 2.39, and 3.19 ppm
3
(1.5, 3.0, 4.5, and 6.0 mg/m ) during daylight hours for three weeks re-
sulted in increases in plant height and in the dry weight of leaves, stems,
and roots of sunflower (Helianthus annuus) plants.
FACTORS AFFECTING PLANT RESPONSES
The extent, severity, and type of NO effects on plants can be altered
x
by both external and internal factors. Environmental conditions, the
-------
presence of additional pollutants in the atmosphere, and the condition or
status of the plant itself can all influence the response of the plant to NO
Biological Factors
530,577
Grouping plants according to their susceptibility to NO reveals
X
the influence of genetic factors. As with most other pollutants, suscep-
tibility to NO varies greatly among plant species and even among varieties,
X
cultivars, or clones of the same species.
Another important biological factor is the stage of development or
577
age of the plant. Van Haut and Stratmann found that the stage of devel-
opment at which nitrogen dioxide exposures occur affects the degree of yield
reduction. For example, fumigation of oats during flowering has the
greatest effect on the yield of grain. Exposures during the earlier,
vegetative state of development, or later, when the grain is yellow-ripe,
have no effect on yield. The critical stage for radish (Raphanus sativus)
is the period of root development.
The age of leaves can affect their susceptibility to NO . In tobacco
x
(Nicotiana sp.) the oldest leaves become chlorotic, middle-aged leaves
become chlorotic with necrotic lesions, and injury to the younger leaves
577 42
is limited to necrosis. Benedict and Breen reported that age-
dependent foliar susceptibility is not the same for all species. Middle-
aged leaves are most susceptible in dandelion (Taraxacum officinale),
cheeseweed (Malva parviflora), lamb's-quarter (Chenopodium album), pigweed
(Amaranthus retroflexus), and Kentucky bluegrass (Poa pratensis), whereas
the susceptibility of middle-aged and old leaves is about the same in
sunflower, annual bluegrass (Poa annua), and nettle-leaved goosefoot
(Chenopodium sp.). Emerging or elongating needles of conifers are more
577
susceptible than mature needles.
x
-------
9-14
Environmental Factors
Theoretically, any external factor than can Influence plant growth or
development may also affect the response of plants to a pollutant. For
sulfur dioxide, fluoride, and ozone, some of the influences of climatic
(temperature, relative humidity, light intensity, light quality, and photo-
period) and edaphic (soil temperature, soil moisture, and mineral nutrition)
factors on plant responses have been reported. But little is known of these
influences on the susceptibility of plants to NO .
529 x
Taylor and Eaton suggested that the rate of nitrogen dioxide absorp-
618
tion by plants is greater in darkness than in light, and Zahn reported
that exposures of alfalfa at night were more injurious than daytime expo-
95
sures. These findings are supported by Czech and Nothdurft who dis-
covered the toxic dose for 1-hr exposures of sugar beet plants to be 10
(18
577
o
times greater in daytime fumigations [100 ppm (188 mg/m )] than at night
3
[10 ppm (18.8 mg/m )]. Van Haut and Stratmann found leaf injury in oats
to be greater at night than during the day. But they also reported that
once injury was initiated, the development of necrotic lesions was most
rapid on warm sunny days.
Susceptibility varies at different times of the day. In a series of
2-hr exposures, beginning at 0800-1000 hr and ending at 2000-2200 hr,
577
injury to rye (Secale cereale) plants was greatest at midday. Soil
moisture stress reduces the susceptibility of various weeds to nitrogen
dioxide. In 4-hr exposures to 20 or 50 ppm (37.6 or 94 mg/m ), plants
grown in moist soil developed injury covering up to 50% of the surface area
of their leaves; but little foliar injury occurred under conditions of
42 618
moisture stress. With respect to mineral nutrition, Zahn reported
that an increase in the amount of available nitrogen resulted in a reduc-
tion in nitrogen dioxide-induced foliar injury.
-------
9-15
Other Pollutants
The presence of other pollutants in the atmosphere with nitrogen
dioxide is another important consideration. When sulfur dioxide and ni-
trogen dioxide occur together in the atmosphere, each at a concentration
that is not harmful to plants, their combined effect can result in plant
122
injury. For example, Dunning et^ a^. evaluated six different crop species
and found the threshold for nitrogen dioxide and sulfur dioxide injury to
3 3
be 2.0 ppm (3.8 mg/m ) and 0.5 ppm (1.3 mg/m ), respectively, in 4-hr
exposures. When the gases were provided together [at concentrations of
0.05 to 0.25 ppm (0.09 to 0.47 mg/m ) of nitrogen dioxide and 0.05 to 0.25
o
ppm (0.13 to 0.66 mg/m ) of sulfur dioxide] leaf injury was induced.
538
Tingey
-------
9-16
SUSCEPTIBILITY OF PLANTS TO NITROGEN DIOXIDE
The relative susceptibilities of some plant species to nitrogen dioxide
are listed in Table 9-1. The three classes—susceptible, intermediate, and
resistant—are approximate because they are based on subjective criteria
42,95,330,331,353,530,577
from several sources. Most of the classifications
are derived from experimental fumigations at various locations at different
times of the year using different nitrogen dioxide exposures. Methods for
assessing such injury as percentage of leaves injured, amount of leaf
surface affected, defoliation, etc. also varied. Because of these variables,
a plant species considered resistant by one investigator may be considered
susceptible by another.
PLANT RESPONSES IN RELATION TO AIR QUALITY
The concentration of the pollutant and the duration of exposure are
collectively referred to as dose. The lowest dose that produces an effect
is termed a threshold dose. Because of the interrelationship between con-
centration and time, there is no single threshold dose for an effect to
occur. For example, leaf injury on a given plant or set of plants exposed
o
to 20 ppm (38 mg/m ) of nitrogen dioxide might occur after only 1 hr,
o
but an exposure of 1 ppm (1.9 mg/m ) might require up to 100 hr to produce
leaf injury. Threshold doses are therefore often described as functions
344
of the pollutant concentration and exposure time.
329
MacLean recently reviewed the responses of plants to NO . His
X
threshold curves for complete defoliation of citrus (Citrus sinensis),
azalea (Rhododendron canescens), and hibiscus (Hibiscus rosa-sinensis)
illustrate the relation between nitrogen dioxide concentration and duration
329
of exposure (Figure 9-1). The discussion presented by MacLean stated that:
Although these threshold curves [Figure 9-1] are based on limited
data, they suggest that a function relating exposure time and
-------
9-17
TABLE 9-1
Q
Susceptibilities of Selected Plants to Nitrogen Dioxide-
Plant Susceptible Intermediate Resistant
Conifers
Abies alba Mill. - White fir +
Abies homolepis Sieb. & Zucc. - Nikko +
fir
Larix decidua Mill. - European larch +
Larix leptolepis Gord. - Japanese +
larch
Picea glauca (Moench) Voss - White +
spruce
Picea pungens cv. glauca, Regel - +
Colorado blue spruce
Pinus nigra Arnold - Austrian pine +
Taxus baccata L. - English yew
Deciduous trees and shrubs
Acer palmatum Thunb. - Japanese +
maple
Acer platanoides L. - Norway maple +
Betula pendula Roth. - European white +
birch
Carpinus betulus L. - European +
hornbeam
Fagus sylvatica L. - Beech +
Gingko biloba L. - Gingko +
Quercus robur L. - English oak +
Robinia pseudoacacia L. - Black locust +
Sambucus nigra L. - Elder +
Tilia cordata Mill. - Little-leaf +
linden
Ulmus glabra Huds. - Scotch elm +
Field crops and grasses
Avena sativa L. - Oats +
Hordeum distichon L. - Barley +
Medicago sativa L. - Alfalfa (lucerne) +
Nicotiana glutinosa L. - Tobacco +
Poa annua L. - Annual blue grass +
Poa pratensis L. - Kentucky blue grass +
Secale cereale L. - Rye +
Solanum tuberosum L. - Potato +
Trifolium incarnatum L. - Spring clover +
Trifolium pratense L. - Red clover +
Triticum sativum Lam. - Wheat +
Vicia sativa L. - Spring vetch +
Zea mays L. - Sweet corn +
-------
9-18
TABLE 9-1 - continued
Plant Susceptible Intermediate Resistant
Fruit Frees
Citrus sp. - Orange, grapefruit, tangelo
Malus sp. - Apple (wild) +
- Pear (wild) +
Garden crops
Allium cepa L. - Onion +
Allium porrum L. - Leek +
Apium graveolens L. - Celery— + +
Asparagus officinalis L. - Asparagus +
Brassica oleracea cv. capitata L. - +
Cabbage
Brassica caulorapa Pasq. - Kohlrabi +
Daucus carota L. - CarrotS. + +
Lactuca sativa L. - Lettuce +
Lycopersicon esculentum Mill. - +
Tomato
Petroselinum hortense Nym. - Parsley +
Phaseolus vulgaris cv. Pinto L. - +
Pinto bean
Phaseolus vulgaris cv. humilis Alef. +
L. - Bush bean
Pisum sativum L. - Pea +
Rheum rhaponticum L. - Rhubarb +
Ornamental shrubs and flowers
Antirrhinum majus L. - Snapdragon +
Begonia sp. - Tuberous begonia +
Bougainvillea Spectabilis Willd. - +
Bougainvillea
Carissa carandas L. - Carissa +
Chrysanthemum leucanthemum L. - Daisy +
Codiaeum variegatum Blume - Croton +
Convallaria majalis L. - Lilly-of-the- +
valley
Dahlia variabilis Willd. - Dahlia +
Erica carnea L. - Spring heath +
Fuchsia hybrida Voss - Fuchsia +
Gardenia jasminoides Ellis - Cape jasmine +
Gardenia radicans Thunb. - Gardenia +
Gladiolus sp. - Gladiolus +
Hibiscus rosa-sinensis L. - Chinese +
hibiscus
Hosta sp. - Plantain lily +
-------
9-19
TABLE 9-1 - continued
Plant Susceptible Intermediate Resistant
Ornamental shrubs and flowers cont'd.
Ixora coccinea L. - Ixora +
Juniperus conferta Parl. - Shore juniper +
Lathyrus odoratus L. - Sweet pea +
Ligustrum licidum Ait. - Ligustrum +
Melaleuca leucadendra L. - Bottle +
brush
Nerium oleander L. - Oleander +
Petunia multiflora - Petunia +
Pittosporum tobira Ait. - Japanese +
pittosporum
Pyracantha coccinea Roem. - Pyracantha +
Rhododendron canescens Sweet - Azalea +
Rhododendron catawbiense Michx. - +
Catawba rhododendron
Rosa sp. - Rosek + +
Weeds
Amaranthus sp. - Pigweed +
Brassica sp. - Mustard +
Chenopodium album L. - Lamb1s-quarters +
Chenopodium sp. - Nettle-leaved +
goosefoot
Helianthus annuus L. - Sunflower +
Malva parviflora L. - Cheeseweed +
Stellaria media Cyrill. - Chickweed +
Taraxacum officinale Weber - Dandelion +
Compiled from references: 42,95,330,331,353,530, and 577
—Different investigators reported different susceptibilities.
-------
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-------
9-21
concentration can be computed for every effect of N02 if enough
information is known about that effect over a relatively wide
range of concentrations and exposure periods.
Three threshold curves are shown in Figure [9-2], These are
approximate estimates based on the various responses of many
plant species determined in all of the acute and chronic N02
doses reported in references [42,43,95,122,234,330,331,336,353,
500,529,536,538,599]. The threshold curve for N02 doses that
result in the death of plants is short because it is based on
limited information.I'" N02 doses approaching this threshold
result in complete defoliation of some plant species, but are
not lethal.1330] The threshold curve for leaf injury is based
on observations at many N02 doses. The shift in leaf injury
from necrosis to chlorosis for N02 doses along this curve
generally occurs between 10 and 100 hr. Because no measurable
effects have been reported for N02 doses below the lower curve,
it can be considered as the threshold for metabolic and growth
effects. N02 doses in the area between this curve and the
threshold curve for leaf injury are those that do not injure
leaves, but often result in growth suppression or effects on
photosynthesis or other plant processes.
These thresholds [Figure 9-2], assuming that they are reasonable
estimates for vegetation in general, can serve as points of
reference to evaluate air quality standards for N02, and they
can be viewed with respect to the N02 concentrations that occur
in the atmosphere.
The U.S. air quality standard for nitrogen dioxide [0.05 ppm (0.10
Q
mg/m ) for an annual average] is below the threshold curve for metabolic
and growth effects in vegetation, and it should afford reasonable protection
from nitrogen dioxide-induced plant damage (economic losses).
329
MacLean also pointed out that:
Localized episodes resulting from accidental releases or spills
of NOX account for most N02-induced plant injury and damage.
But for agricultural activities near urban areas where the emis-
sions of NOX to the atmosphere are primarily from stationary
sources that burn coal, oil, natural gas, and other fuels, and
from the combustion of motor vehicle fuels, the probability of
direct effects of N02 on vegetation is very low. Average N02
concentrations in most major cities of the United States are
well below the threshold curve for metabolic and growth effects.
In fact, the maximum N02 concentrations recorded in Los Angeles,
California[563] for an entire year (1966) are only slightly
higher than the threshold curve for metabolic and growth effects
for averaging times of one day or less and below this threshold
curve for longer averaging periods [Figure 9-2],
-------
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Indirect effects of NC>2 on vegetation, however, probably do
occur. The participation of N0£ in atmospheric reactions
leading to the production of ozone and peroxyacyl nitrates,
and the synergistic effects on plant injury of relatively
low concentrations of mixtures of N0£ and S02 in the atmo-
sphere pose a real threat to vegetation growing in or near
metropolitan areas.
FUTURE RESEARCH
Since plants are relatively insensitive to nitric oxide as compared
with nitrogen dioxide, the emphasis in NO research should be directed
2C
toward the latter. The gross effects of nitrogen dioxide have been fairly
well documented, and the utility of additional research would be limited
primarily to the evaluation of injury due to localized accidental nitro-
gen dioxide releases. Future research, therefore, should emphasize the
effects of chronic and intermittent nitrogen dioxide exposures, at
realistic concentrations and exposure times, on symptomatology, and on
plant growth and yield. Furthermore, little is known about the primary
effects of nitrogen dioxide at the cellular or metabolic levels of organ-
ization. Information in this area is needed to provide an understanding
of nitrogen dioxide effects on vegetation.
The combined effects of NO and other substances in the atmosphere
x
on plants is another important area in which information is limited. In
a few plant species, atmospheres containing both nitrogen dioxide and
sulfur dioxide cause greater injury than when either pollutant occurs
122,336,538
alone, whereas the combination of nitrogen dioxide and ozone
336
results in reduced leaf injury. Experimental exposures of nitrogen
dioxide in combination with other atmospheric constituents should be carried
out on a wide variety of plant species to reveal the interacting effects
of nitrogen dioxide and other pollutants on phytotoxicity.
-------
9-24
SUMMARY AND CONCLUSIONS
Nitric oxide is less injurious to vegetation than nitrogen dioxide,
and both are less phytotoxic than sulfur dioxide, gaseous fluoride, or
photochemical oxidants. Although injuries to plants by both nitric oxide
and nitrogen dioxide have been experimentally induced, there have been
no confirmed reports for nitric oxide injury to vegetation in the field.
Nitrogen dioxide-induced injury in the field has been associated with
accidental acute exposures from certain industrial processes.
Nitrogen dioxide doses in or near metropolitan areas rarely exceed
the threshold dose for effects on plant metabolism or growth; those that
occur in suburban or rural areas are even lower. The existing U.S. air
-j
quality standard for nitrogen dioxide [0.05 ppm (0.10 mg/m~) for an annual
average] is below the threshold for detectable effects on vegetation.
Therefore, secondary standards to protect vegetation from the direct
effects of nitrogen dioxide are not necessary.
Indirect effects of nitrogen dioxide on vegetation, however, are more
likely. Nitrogen dioxide participation in atmospheric reactions leading
to ozone and peroxyacylnitrate production, and the synergistic effects on
plant injury of low concentrations of nitrogen dioxide and sulfur dioxide
mixtures in the atmosphere may pose a real threat to vegetation growing in
or near metropolitan areas. As knowledge of these phenomena increases,
air quality standards for nitrogen dioxide to protect vegetation may have
to be reevaluated.
-------
CHAPTER 10
HEALTH EFFECTS OF OXIDES OF NITROGEN
Considerable evidence has recently accumulated indicating that oxides
91, 177, 199,486,487, 518
of nitrogen are deleterious components of air pollution.
Most of this evidence is based on the relationship of human or animal expo-
sure to oxides of nitrogen and resultant measurements of pulmonary dysfunc-
tion. It is implicit in these experiments that the measured abnormality either
results in chronic pulmonary disease (emphysema or bronchitis) or increases
the deficit from an already present chronic respiratory disease. Inasmuch as
176
these diseases affect approximately 4% of the American population, this
postulated sequence of events is of paramount importance. More recently,
human exposure to oxides of nitrogen has been associated with an increased
411,486,487
susceptibility to acute respiratory infections. The morbidity from
these frequently occurring illnesses can have considerable economic and
social significance. Exposure to nitrogen dioxide (NO ) also causes visual
z 51,224
and olfactory disturbances of uncertain medical significance.
Nitrous oxide (N~ O), nitric oxide (NO), nitrogen dioxide, dinitrogen
trioxide (N« O~ ), dinitrogen pentoxide (N« O,-), and nitrate ions (NO., -) are
the oxides of nitrogen that are present in the atmosphere. Of these oxides,
nitrogen dioxide is by far the most significant biologically.
Interest in the toxicity of nitrogen dioxide has led to many investigations
with laboratory animals in which pollutant exposure has been related to mor-
tality, respiratory pathology and physiology, pulmonary resistance to micro-
bial infection, and, in a few instances, to extrapulmonary dysfunction.
Before evaluating these studies, some aspects of the experiments are
worth emphasizing. Under the true environmental conditions the deleterious
-------
10-2
effects of air pollution caused by exposures to a mixture of interacting
pollutants, one of -which is nitrogen dioxide. Realistic assessment of the
significance of health effects of nitrogen dioxide must be considered within
this context. However, before toxic effects of mixtures of air pollutants
can be determined, the effects of individual pollutants must be known.
Studies conducted in laboratory animals provide information on the maximum
tolerated dose in various animal species, define the target organs, and pro-
vide clues as to the mechanisms of damage. Considerable information is
now available on the toxic effects of nitrogen dioxide in animals and man
and engineering technology has advanced to a point where exposure to mix-
tures of airpollutants can be carefully monitored. Consequently, effects of
nitrogen dioxide on humans can now be studied under conditions more
realistically resembling true environmental exposures.
A second important factor concerns the relation of the animal model to
the human. Animal models are often similar but not completely analogous
to humans. As an example, the respiratory anatomy of rats (the animal
most frequently used in studies of nitrogen dioxide-induced emphysema)
differs from that of humans in that they do not have interlobular septa, they
have fewer generations of airways, and their respiration occurs through
551
distal bronchioles rather than alveoli and pulmonary vasculature. The
epithelium of the tracheobronchial tree of rats also differs from that of
man. It has more large mucus-secreting glands lining the trachea and
445
fewer along the bronchi. Important anatomic differences between humans
551
and rabbits, mice, guinea pigs, monkeys, and dogs explain why identical
concentrations of nitrogen dioxide can cause diverse pathologic disturbances
•which make it difficult to apply such results to disease in man.
-------
10-3
The few studies of inhaled nitrogen dioxide indicate that this pollutant
is probably distributed throughout the lung and that a high percentage of it is
248, 584
absorbed. Von Nieding and his coworkers exposed volunteers to ni-
3
trogen dioxide concentrations of 0. 94-9. 4 mg/m (0. 5-5. 0 ppm) and analyzed
584
the gas concentrations in exhaled air to determine the extent of absorption.
These investigators reported that 81-87% of inhaled nitrogen dioxide is ab-
sorbed during normal ventilation and that more than 90% is absorbed during
periods of maximal ventilation. Ichioka designed a model airway to simulate
the dynamic behavior of nitrogen dioxide within the respiratory system. When
3
nitrogen dioxide at 9.4 mg/m (5.0 ppm) flowed through the model, most of
248
the pollutant penetrated the more distal regions. Although more exacting
studies of intrapulmonary distribution and absorption, subh as those avail-
153
able for sulfur dioxide, have not been performed, much of the inhaled pol-
lutant seems to react throughout the lung.
TQXICOLOGIC EFFECTS OF NITROGEN DIOXIDE IN LABORATORY
ANIMALS
Mortality
Figure 10-1 compares mortality for rats, mice, guinea pigs, rabbits,
and dogs after a 1-hr exposure to increasing concentrations of nitrogen di-
237
oxide. These data, which are abstracted from a study by Hine e_t aU ,
3
indicate a mortality threshold concentration of 75-94 mg/m (40-50 ppm)
after 1-hr exposures of these laboratory animals. Rats, mice, and guinea
pigs appear to be more susceptible to the pollutant than rabbits or dogs. Ex-
periments in which nonhuman primates were exposed to nitrogen dioxide for
3
8 hr at 122 mg/m (65 ppm) have shown that monkeys are slightly more sus-
504
ceptible to nitrogen dioxide than similarly exposed rodents. Once the
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the threshold concentration is exceeded, the death rate for each species in-
creases as the exposure period is lengthened. When compared, brief expo-
sures to high concentrations of nitrogen dioxide are much more toxic than
exposures to low concentrations for long periods.
All animal species studied survive continuous exposures of a year or
27, 129, 144, 158, 159,
more to peak ambient concentrations of nitrogen dioxide.
160, 161, 586
(See Table 10-1. ) Because these studies did not include higher
concentrations of nitrogen dioxide, it is not known whether any of these species
would tolerate even more severe exposures.
Pulmonary Effects
Pulmonary Function. The effect of short-term exposure to nitrogen di-
382 221
oxide on pulmonary function has been studied in the guinea pig, monkey,
583
and rabbit. Guinea pigs manifest an increase in respiratory rate and a
decrease in tidal volume after a 2-hr exposure to nitrogen dioxide at 122
3 382
mg/m (65 ppm). Similar results occur after monkeys are exposed for
3 221
2 hr to 28 mg/m (15 ppm). These abnormalities are reversible. Lesser
exposures have not caused dysfunction. Pulmonary diffusing capacity is re-
3 583
duced in rabbits after a 15-min exposure to 56 mg/m (30 ppm).
Table 10-2 summarizes the respiratory effects that follow chronic expo-
sure to nitrogen dioxide. Data in that table indicate that prolonged exposure
to peak ambient concentrations of nitrogen dioxide causes abnormalities in res-
piratory function, which appear to revert to normal after the exposure ends.
Pathologic Changes. Pathology in the lungs occurs primarily in animals
exposed for less than 24 hr to concentrations of nitrogen dioxide exceeding
3 397
13. 16 mg/m (7 ppm). Various degrees of vascular congestion, edema,
bronchiolitis, consolidation, exudative plugging of the bronchi, tissue destruction,
-------
10-6
TABLE 10-1
Survival of Animals Exposed Chronically to
High Concentrations of Nitrogen Dioxide
Species
Mice
Rats
Guinea pigs
Squirrel Monkeys
(Saimiri sciureus)
Stump-tailed macaque
(Macaca speciosa)
Dogs
Rabbits
Concentration
mg/m^ ppm
0.94 0.5
1.52 0.8
3.76 2.0
23.50 12.5
7.52 4.0
28.20 15.0
1.88 1.0
3.76 2.0
9.40 5.0
2.44 1.3
Duration of
Exposure
12 months
Lifetime
Lifetime
213 days
4 hr/day,
5 days /week
for 6 months
6 months
16 months
2 years
15 months
17 weeks
Fatalities
Attributed
to Exposure
None reported
pneumonitis
None
None
11% fatality
None
None
None
None
None
None
Ref
129
159
161
160
27
27
144
158
586
358
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abscess formation, and pneumonitis occurred in all species that have been
78, 118,221,237,286,287,309,504
studied.
The extent of damage corresponds to exposure dose. Acute exposures
3
to nitrogen dioxide concentrations of 3. 760-5. 640 mg/m (2-3 ppm), -which
are close to ambient levels, have not resulted in canine or murine morphologic
118,513 513
abnormalities, nor have those exposures affected cellular structure.
At higher concentrations, ultrastructural and scanning electron micoscopic
studies have revealed loss of cilia, swelling, and disruption of type I alveolar
cells, fibrin deposition along basement membranes, and an influx of macro-
408,513
phages. These pathologic lesions appear to be transient. When the
insult is discontinued, the damaged tissue returns to normal.
Rodents have most commonly been used to study the pathology resulting
48, 65,85, 156-161,
from prolonged exposure to nitrogen dioxide (Table 10-3).
194,213, 214,286, 360,383, 504, 512, 617
In rats exposed to nitrogen dioxide at
3
18. 8, 23. 5, or 47 mg/m (10, 12. 5, or 25 ppm) for 3 or more months, the
thoracic cavities become larger, dorsal kyphosis develops, and the animals
acquire an inflated appearance. There is distention of alveolar ducts, dilatation
of alveoli, and hyperplasia of bronchiolar epithelium. Alveolar septa are
157,214
occasionally missing, but destruction of parenchyma is unusual.
These pathologic features are similar but not identical to those of human
emphysema. A critical difference is the absence of alveolar necrosis.
19
Destructive bullous lesions are the sine qua non of emphysema, but
these bullae do not develop in rodent models, even in rats exposed for a life-
3
time to nitrogen dioxide concentrations of 1. 5 and 3. 8 mg/m (0. 8 and 2. 0 ppm).
The lungs from these animals are grossly normal; microscopic examination
shows only minor ciliary loss, epithelial hypertrophy, and "cytoplasmic
159,214
blebbing. " These animals have a normal lifespan and die of diseases
156
unrelated to nitrogen dioxide exposures.
-------
10-9
TABLE 10-3
Pathology in Animals Exposed Chronically to
Rats
Guinea pigs
Rabbits
Hamsters
Dogs
Squirrel Monkeys 9.4
(Saimiri
sciureus)
High Concentrations of Nitrogen Dioxide
0
0
0
75
1
3
31
18
9
18
28
41
15
28
84
9
48
9
Concentration
mg/m ppm
.9 0.5
.9 0.5
.6- 0.9 0.3- 0.5
.2 40.0
.5- 3.8 0.8- 2.0
.8 2.0
.9 17.0
.8- 47.0 10.0-25.0
.4 5.0
.8 10.0
.2- 37.6 15.0-20.0
.4 22.0
.0- 22.6 8.0-12.0
.2- 47.0 15.0-25.0
.6-103.4 45.0-55.0
.4 5.0
.9 26.0
.4 5.0
Duration of
Exposure
3 months
6, 18, and
24 hr/day for
3-4 months
6 months
6-8 weeks
Lifetime
2 or more
years
20 months
3 or more
months
7.5 hr/day,
5 days /week,
5 . 5 months
6 weeks
2 hr/day,
5 days/week,
21 months
2 hr/day,
3 weeks
3-4 months
2 hr/day up
to 2 years
21-23 hr/day
15-18 months
6 months
169 days
Pathology Attributed
to Exposure
Ciliary loss, alveolar cell
disruption
Expanded alveoli, reduction
of distal airway size, pro-
gressive parenchymal damage
Destruction bronchial epithe-
lium, lymphocytic infiltration
Epithelial abnormalities of
terminal bronchioles
Ciliary loss, epithelial
hyperplasia
Thickened basement membrane
Massive increase in collagen
fibrils
Enlarged thoracic cavities,
dorsal kyphosis, distended
alveoli and alveolar ducts
Perivascular and tracheal in-
flammation, desquamative
pneumonit is
Hyperplasia of type 2 pneumat-
ocytes
Inflammation bronchiolar
epithelium
Multifocal emphysema tous
changes
Necrosis of alveolar walls
with enlargement of air spaces
No emphysematous lesions
Dilated alveolar spaces,
inflammatory cells, epithelial
hyperplasia, increased lung
volume
No abnormalities
Bullous emphysema
Focal alveolar edema
Ref
156
48
85
65
159,
214
512
512
157,
214
27
617
287
194
213
286
286
586
320
143
-------
10-10
Electron microscopic studies of the lungs of rats exposed to nitrogen
3
dioxide at 3. 8 mg/m (2. 0 pprn) revealed hypertrophy and focal hyper-
plasia in the epithelium of the terminal bronchiole and a loss of cilia.
These minor abnormalities, which appear after the third day of exposure,
apparently heal in the presence of nitrogen dioxide and disappear at 21
513
days. After 2 or more years exposure to the same concentration, the
only abnormality in animals was thickening of the basement membrane
under the epithelium of the terminal bronchiole due to enlarged collagen
512
fibrils.
3
Exposure to 32 mg/m (17 ppm) causes much more severe ciliary
loss, injury to the epithelium lining the alveoli adjacent to the terminal
bronchioles, sloughing of type I alveolar epithelial cells, thickening of
the air-blood barrier, and fibrin deposition along the basement membrane.
But even in animals exposed to this high concentration, the destructive
513
phase was followed by a period of adaptation and repair. The princi-
3
pal abnormality in 2-year-old rats after exposure to 32 mg/m (17 ppm)
512
for 610 days was a massive increase in the size of the collagen fibrils.
Mice are more susceptible to the toxic effects of nitrogen dioxide than
3
rats. Continuous exposure to nitrogen dioxide at 0. 94 mg/m (0. 5 ppm)
for 3 months causes loss of cilia, alveolar cell disruption, and obstruc-
156
tion of respiratory bronchioles. Longer exposures cause more severe
bronchiolar inflammation, pneumonitis, and increases in alveolar area,
48
secondary to alveolar expansion rather than septal breakage.
Epithelial cell proliferation of the peripheral bronchus occurred in
3
mice that were exposed to 0. 94-1. 5 mg/m (0. 5-0. 8 ppm) for 30-45
383
days. Electron microscopic studies of these lungs revealed mild
-------
10-11
ciliary abnormalities, degeneration of the mitochondria of Clara and
383
alveolar cells, and edematous changes -within the cytoplasm. These
85
abnormalities have been confirmed histopathologically, and in later
360
scanning and electron microscopic studies. Some investigators classi-
fied these changes as representative of chronic bronchitis and chronic
85 3
tracheitis. Germ-free mice that -were exposed continuously to 75 mg/m
(40 ppm) for 6-8 weeks developed epithelial abnormalities throughout the
bronchial tree. These were most pronounced at the terminal bronchioles
65
and in the alveoli immediately surrounding the terminal bronchioles.
Inflammation, characterized by edema and exudation of white blood cells,
65
was not observed in the nitrogen dioxide-provoked lesions.
Haydon et a_L exposed rabbits continuously to atmospheres containing
3
nitrogen dioxide at 15-23 mg/m (8-12 ppm) for 3-4 months and reported
destructive changes in alveolar walls and abnormal enlargement of the
213
distal air spaces. These findings approximate the emphysematous
lesions observed in humans. Unfortunately, there are no reports of data
obtained from rabbits exposed to lower concentrations of nitrogen dioxide.
Other investigators have failed to find emphysematous changes in rabbits
3
exposed for 2 hr/day to nitrogen dioxide at 28-47 mg/m (15-25 ppm) for
286
periods up to 2 years.
A multifocal type of emphysema has been induced in guinea pigs after 3
3 194
weeks of exposure for 2 hr/day to nitrogen dioxide at 41 mg/m (22 ppm).
An ultrastructural study of the lungs of 12 guinea pigs exposed continuously to
3
18. 8 mg/m (10 ppm) for 6 weeks revealed greatly increased proportions of type
617
II pneumatocytes, presumably resulting from hyperplasia, and minor cellu-
lar changes consisting of increased numbers of tightly packed lamellae and of
small and giant lipid bodies.
-------
10-12
Kleinerman and Cowdry did not find emphysematous changes in ham-
3
sters after exposing them to 85-103 mg/m (45-55 ppm) for 21-23 hr/day
for 10 -weeks. Dogs are also very resistant to the toxic effects of nitrogen
3
dioxide. Wagner ^t a_L determined that lungs of dogs exposed to 9.4 mg/m
(5. 0 ppm) for 15-18 months did not exhibit differences when compared with
586
lungs of control animals. These negative results -were confirmed by
504,579
later investigators. Bullous emphysema has been induced in dogs
but only after prolonged exposure (6 months) to high concentrations, 48. 9
3 320
mg/m (26 ppm).
Vascular Permeability and Edema Formation. A potentially important
pathophysiologic consequence of exposure to nitrogen dioxide is injury to
vascular membranes with a resulting increase in capillary permeability,
191, 237,482, 518
transudation of protein into alveoli, and edema formation.
Edema formation has been a significant abnormality in experiments in
which animals were exposed acutely to high concentrations of nitrogen
dioxide.
The difficulties encountered with present measuring techniques should
be thoroughly understood before attempts are made to evaluate chronic expo-
sures that do not generally produce edema. The most common method
determines a wet:dry weight ratio that is a very crude estimate of edema
11,132
fluid. This ratio remains unaltered in rats that are exposed chroni-
157
cally to high concentrations of nitrogen dioxide. Pulmonary weights,
both dry and wet, increase proportionately in these animals, indicating
that pulmonary mass (cell and fibrous tissues) increases as a consequence
156,157, 160
of pollutant exposure. Although histologic and electron micro-
scopic examination of pulmonary tissue provides a more precise determi-
195
nation of the presence of edema, small quantities of edema fluid may
11, 195
still remain undetected.
-------
10-13
Focal alveolar edema was reported in squirrel monkeys that were
3
exposed continuously to nitrogen dioxide at 9.4 mg/m (5. 0 ppm) for
143 157, 504, 586 48, 98,
169 days. Edema was either absent or not mentioned
213, 617
in most other pathologic studies. Edema was not noted in three
electron microscopic investigations of the effect on the lung of prolonged
408,512
exposure to nitrogen dioxide. The results of one ultrastructural
study indicated early edematous changes within alveolar epithelial cells
and alveolar interstitium in mice that were exposed continuously to nitro-
3 383
gen dioxide at 0. 94-1. 5 mg/m (0. 5-0. 8 ppm) for 30-45 days.
A new and potentially precise method for detecting small alterations
in pulmonary capillary permeability has been reported by Sherwin and
482
Richters. This method, in which tritium-labeled serum is used to
assess intrabronchial protein leakage, showed that exposure of mice to
3
nitrogen dioxide at 9. 4 mg/m (5. 0 ppm) for 14-72 hr was associated with
transient increases in intrabronchial radioisotope. Because of the pre-
liminary nature of these studies and the small number of animals that have
been studied, these data cannot be used to form conclusions about edema
formation. More recently, Sherwin and Carlson have used a disk-gel
electrophoresis method to measure intrabronchial protein in guinea pigs
3 480
that were exposed for a week to nitrogen dioxide at 0. 75 mg/m (0. 4 ppm).
Their preliminary results suggest that transudation of protein results from
such exposures.
Biochemical Abnormalities. The biochemical mechanisms by which
nitrogen dioxide causes cellular dysfunction are in the initial stages of
investigation. Although a few potentially important biochemical abnormali-
ties have been described, their significance in relation to the observed
pathophysiologic disturbances is speculative.
-------
10-14
531
Nitrogen dioxide causes lipoperoxidation of murine lung lipids and
decreases in pulmonary lipid content and total saturated phospholipid fatty
24,278
acids. These changes, similar to those noted with ozone, have been
interpreted as evidence that free radical formation is the mechanism by-which
475, 518, 578
injuries from nitrogen dioxide exposure occur. Of potential prac-
531
tical significance is the additional finding by Thomas and his coworkers
that pretreatment with antioxidant (10 mg of vitamin E per day) was partially
effective in preventing the lipid peroxidative changes induced by nitrogen
dioxide. Such sulfur compounds as a-naphthylthiourea (C-, n^i-j NHCSNH ~ )
and phenylthiourea (Cfi He NHCSNFL, ) also protect rodents from nitrogen
dioxide-induced damage, presumably by acting as antioxidants.
Exposure to nitrogen dioxide adversely alters pulmonary surfactant,
24, 118
which may explain the observed impairments in lung compliance.
In studies of the interaction of nitrogen dioxide and cellular enzyme
27,64,354,578
systems, it was discovered that some enzyme systems,
578 27
such as catalase and lactic dehydrogenase, are susceptible to damage
by the pollutant, whereas other enzymes such as cathepsin D, seem to be
354
impervious.
A possible relation between exposure to nitrogen dioxide and pulmonary
169,407
neoplasia has been the subject of preliminary studies. Gardner
obtained some evidence that nitrogen dioxide exposures may contribute
169
to lung tumor development in mice. Because benzopyrene hydroxylase
presumably inactivates the carcinogenic potential of polyaromatic hydro-
carbons, the effect of an acute exposure to nitrogen dioxide on this enzyme
system was evaluated in studies of rabbit tracheobronchial mucosa. No
407
effect was found.
-------
10-15
Extrapuknonary Effects
Nitrogen dioxide has been associated with some systemic effects. A
diminished rate of weight gain occurs in rats exposed chronically to
3 156
nitrogen dioxide at a high concentration [23 mg/m (12 ppm)] and in
3
rabbits exposed to lower concentrations [2. 4 and 5. 6 mg/m (1. 3 and 3. 0
358
ppm)]. Growth abnormalities do not occur uniformly. Dogs have been
3 586
exposed to 1. 8 and 9. 4 mg/m (1. 0 and 5. 0 ppm) for 6 hr/day for 18 months
3
and mice to 0. 94 mg/m (0. 5 ppm) for 5 days each week for up to 12 months
129
without alterations in weight gain.
Minor changes in hematologic indexes have been reported in some species
after prolonged exposure to nitrogen dioxide. Rats develop a transient poly-
cythemia characterized by increases in hematocrit and hemoglobin and
158
decreases in mean corpuscular volume and hemoglobin. These changes
appear 2-3 weeks after exposure and spontaneously regress, despite con-
tinued exposure, after approximately 3 months. Leukocytosis has been
3
reported in rabbits that were exposed to 2. 4 and 5. 6 mg/m (1. 3 and 3. 0 ppm)
358
for 15-17 weeks. Other investigators have not reported hematologic
504 504 504,586 504
abnormalities in rats, rabbits, dogs, guinea pigs, and
143
squirrel monkeys resulting from continuous exposure to nitrogen dioxide.
The effect of nitrogen dioxide on organs other than the lung has received
48,481, 504
little attention. Pathologic studies of the heart, thymus, spleen,
liver, and kidney did not disclose alterations in mice exposed continuously
3 48
to 0. 94 mg/m (0. 5 ppm) for 12 months. Studies using other animal species
504
have also failed to reveal extrapulmonary abnormalities.
Proteinuria has been reported in guinea pigs after continuous exposure
3 481
to nitrogen dioxide at 0. 94 mg/m (0. 5 ppm) for 7-14 days. This urinary
abnormality was not associated with renal pathology.
-------
10-16
EFFECTS OF COMBINED EXPOSURES TO NITROGEN DIOXIDE AND
OTHER AGENTS IN LABORATORY ANIMALS
The importance of nitrogen dioxide as a causative or aggravating agent
91, 126-130, 144, 177,178, 221, 257,
in respiratory infections is well documented.
428,486,487, 519
The pollutant appears to enhance host susceptibility to
bacterial infection by injuring the defense mechanisms that ordinarily main-
tain pulmonary sterility. Aspirated microorganisms in the lungs are either
removed via the mucociliary transport mechanism or destroyed in situ by
187
alveolar macrophages. The mode of microbial clearance depends on the
site of deposition which in turn is affected by the particle size of the inhaled
microorganism. In one method of clearance microorganisms are trapped in
the mucous covering of the bronchial epithelium and transported from the
lungs by ciliary activity. Expectoration is another mode. Microorganisms
that reach the distal regions of the lung are phagocytised by alveolar macro-
127,129,130,178
phages. There is considerable evidence that nitrogen dioxide,
108, 245,428,450, 588
like other air pollutants, damages these bronchopulmonary
defenses, thereby allowing bacterial proliferation and invasion.
Mortality and Survival
Short-Term Exposures. Ehrlich and his coworkers demonstrated in a
series of studies that acute exposure to low concentrations of nitrogen dioxide
126,127,129,428 127
reduces the resistance of mice, hamsters, and squirrel
143,144,221, 222
monkeys to bacterial pneumonia. This increased suscepti-
bility is evidenced by increased mortality rate, reduced life span, and
reduced ability to clear viable bacteria from the lungs. In their experiments
Ehrlich and his colleagues exposed animals to atmospheres containing
measured quantities of nitrogen dioxide before or after respiratory challenge
with aerosols of virulent Klebsiella pneumoniae.
-------
10-17
In studies of acute exposures, mice were subjected for 2 hr to nitrogen
3
dioxide concentrations of 2. 8 to 47. 0 mg/m (1. 5 to 25 ppm) and then
challenged with an aerosol of K_. pneumoniae •within 1, 6, or 27 hr after the
1277428
nitrogen dioxide exposure. The minimal nitrogen dioxide concentra-
tion required to produce a statistically significant (p_<0. 05) rise in mortality
3
after a 2-hr exposure was 6. 6 mg/m (3. 5 ppm) when the infectious challenge
occurred within 1 hr after that exposure. When the infectious challenge was
delayed, a statistically significant effect was noted after 6 hr, but not after
3
27 hr after exposures to nitrogen dioxide concentrations of 9.4 mg/m
3
(5 ppm) and above. Exposure to 47. 0 mg/m (25 ppm) 6 or 14 days before
126
the challenge with K. pneumoniae did not increase mortality. When mice
3
were infected with K. pneumoniae then later exposed to 47. 0 mg/m (25 ppm)
for 2 hr within 1, 6, 27, 48, or 72 hr after the infectious challenge, the
mortality increase was statistically significant. This effect was not observed
3 428
at 4. 7 mg/m (2. 5 ppm).
A similar increase in mortality was observed in hamsters exposed to
3
nitrogen dioxide at 65. 8 mg/m (35 ppm) or more for 2 hr and then challenged
127
within less than 1 hr with K. pneumoniae.
221
Henry _e_t _al. exposed three squirrel monkeys to nitrogen dioxide at
3
18. 8-94. 0 mg/m (10-50 ppm) for 2 hr. He then challenged them with
K_. pneumoniae introduced intratracheally. Neither the infectious challenge
3
nor the 2-hr exposure to 94. 0 mg/m (50 ppm) alone •was fatal; whereas
exposure to the same nitrogen dioxide concentration followed by challenge
with 1C. pneumoniae was fatal for all three monkeys. Increased mortality
rates were not observed in squirrel monkeys exposed to the lower concen-
3
trations between 9. 4 and 65. 8 mg/m (5-35 ppm). In all three species of
-------
10-18
animals (mice, hamsters, and squirrel monkeys), the increased mortality
rates were consistently paralleled by significant decreases in mean sur-
vival times.
In Figure 10-2 the acute exposure data for the three species of labora-
tory animals are summarized. The data show the actual mortalities of
animals in each species that were not exposed to nitrogen dioxide but were
challenged with K. pneumoniae, of those that were exposed to low nitrogen
dioxide concentrations and K. pneumoniae but did not incur much increase in
mortality rates, and of those that were exposed to high nitrogen dioxide
concentrations and PC. pneumoniae that reacted synergistically to cause
high mortality rates.
Mortality rates among the control animals reflect the natural resistance
of each species to the infectious agent (41% for mice, 11% for hamsters, and
no mortality for monkeys). Moreover, the estimated dose of inhaled K.
pneumoniae for monkeys and hamsters was approximately 10 microorganisms,
3
compared with 10 microorganisms for mice. Thus, the susceptibility of
a species to respiratory infection was partially responsible for the increased
mortality rate after exposure to nitrogen dioxide.
The rate at which inhaled bacteria were cleared from the lungs of
mice and hamsters decreased after exposure to nitrogen dioxide. In con-
trol animals that were not exposed to nitrogen dioxide, the bacterial popu-
lation was markedly reduced during the 6-hr period after the challenge.
Thereafter, the population increased, reaching its initial concentration
3
after approximately 8 hr. In mice and hamsters exposed to 9. 4 mg/m
(5 ppm), the period of initial clearance was reduced to 4. 5 and 5 hr,
respectively, and the original concentration was reestablished in less
-------
10-19
KB
90
0\°
Pi
O
70
60
50
30
20
10
0
lllttR OF ANIMALS
MICE
0 2.82- 6.58-
4.7 47
(1.5- (3.5-
2.5) 25)
0 9.4- 65.8-
47 122.2
(5- (35-
25) 65)
*L5
0 9.4- 94
65.8
(5- (50)
35)
N02 CONCENTRATIONS, mg/md (ppm)
FIGURE 10-2.
Mortality in mice, hamsters, and monkeys after challenge with K. pneumoniae
(103 microorganism for mice, and 105 each for hamsters and monkeys) alone
C3 and when administered after 2-hr exposures to low H and high K^8
concentrations of nitrogen dioxide. Synergy is evident from increased
fatalities after high concentration exposures. From Ehrlich, Henry, and
Fenters, 1970.13°
-------
10-20
127 3
than 7 hr. Squirrel monkeys exposed to 18. 8 mg/m (10 ppm) for 2 hr
and then challenged with K. pneumoniae had bacteria present in their lungs
221
19-51 days after challenge.
178
These findings -were corroborated by Goldstein et al. who studied
3
the effect of nitrogen dioxide at concentrations from 3. 6 to 28. 0 mg/m
(1. 9 to 14. 8 ppm) on the rate of bacterial removal and bactericidal activity
of mouse lungs. Mice were infected -with an aerosol of Staphylococcus
aureus labeled with radioactive phosphorus and then exposed to nitrogen
dioxide for 4 hr. The removal of the bacteria from the lungs was unaffected
by exposure to nitrogen dioxide. However, pulmonary bactericidal activity
decreased progressively on exposure to nitrogen dioxide concentrations
3
above 13. 2 mg/m (7. 0 ppm). A similar effect was observed in mice
3
exposed to 4. 3 or 12. 4 mg/m (2. 3 or 6. 6 ppm) for 17 hr before challenge
with the infectious agent, but the effect was slight at the lowest concentra-
tion. These observations suggest that prolonged exposures to nitrogen
dioxide cause bactericidal dysfunction even at low concentrations.
Autopsies of mice that died after the bacterial challenge revealed a
high incidence of purulent exudate in the pleural cavities. Lungs of mice
3
exposed to nitrogen dioxide concentrations of 6. 6 mg/m (3. 5 ppm) or
higher incurred various degrees of congestion and dilation of veins and
capillaries. Pathologic changes were not observed in lungs of mice
127
exposed to lower concentrations of nitrogen dioxide.
The lungs of squirrel monkeys that were exposed to nitrogen dioxide
and then challenged with I£. pneumoniae developed massive infection.
The infectious microorganisms were recovered from kidney, heart,
liver, adrenals, and spleen, as well as lung. Exposure to increasing
-------
10-21
concentrations of nitrogen dioxide caused progressively severe histopath-
ology of the alveoli. The primary defects -were various degrees of alveolar
expansion and a high incidence of septal breaks in the alveoli. The combi-
nation of exposures to nitrogen dioxide and infectious challenge appeared to
result in superimposed effects, rather than an enhancement of the individual
effect of either exposure.
Chronic Exposures
Bacterial pneumonia: In several studies mice or squirrel monkeys •were
exposed to nitrogen dioxide for extended periods and, at various intervals
127
during the exposure, challenged with 1C. pneumoniae aerosols. Ehrlich
3
exposed Swiss albino mice for 30 days to nitrogen dioxide at 0. 94 mg/m
(0. 5 ppm) continuously, 24 hr/day for 7 days/week, and intermittently,
6 hr/day for 5 days/week. Both exposure conditions resulted in increased
mortality rates, compared with controls that were not exposed to nitrogen
dioxide. Intermittent exposure appeared to cause greater effects than
continuous exposure.
129
Ehrlich and Henry exposed groups of mice to nitrogen dioxide at
3
0. 94 mg/m (0. 5 ppm) for 6, 18, or 24 hr/day for 7 days/week for up to 12
months. Challenge with an aerosol of K_. pneumoniae occurred after 1, 3,
6, 9, or 12 months of exposure. Statistically significant increases in
3
mortality were observed after continuous exposure to 0. 94 mg/m (0. 5
ppm) for 3 months (Figure 10-3), and after 6- and 18-hr/day exposure for
6 months. After 12 months of exposure, increases in mortality were
evident only when the nitrogen dioxide exposure was continuous.
The rate at which viable bacteria were cleared from lungs of mice
was also affected by the long-term exposure to nitrogen dioxide at
-------
CO
CO
LU
O
CD
I—«
CO
10-22
CS!
CD
CL
LA
CD
&
8
CO
CO
£P t
o
o
8.
,
- CO
cxi o:
8:
CTLLJ
COU_
LU
OL
CO LU
LU
O
LU
CJ
CO
« Q
CO "Z.
LU LU
CO Q_
CO
CD.
SSJK3
-------
10-23
3
0. 94 mg/m (0. 5 pprn). When mice were exposed to nitrogen dioxide for
6 or 18 hr/day, during 9 and 12 months, capacity to clear bacteria from
lungs decreased; mice that were exposed for 24 hr/day incurred reduced
capacity after 6 months of exposure.
No decrease in pulmonary clearance rate in germfree and conventional
mice was observed by Buckley and Loosli after exposing the mice 6 weeks
3
to nitrogen dioxide at 71 mg/m (38 ppm), then to a respiratory challenge
65
with S^. aureus aerosols. Although the rates of bacterial clearance
differed between the germfree and conventional mice, neither group was
affected by exposure to nitrogen dioxide. The investigators interpreted
their data as showing that nitrogen dioxide did not influence bacterial
clearance rates. These studies may be criticized on two counts. First,
only three animals were studied for each exposure period--too few for
statistical analysis. Second, S. aureus is not pathogenic for mice, and
99% of the microorganisms are removed from the lungs within 24 hr.
Hence, significant differences in clearance that might have occurred
during the first 24 hr with other microorganisms would not have been
caused by S^. aureus.
222
Henry ^t ad. exposed male squirrel monkeys continuously to
nitrogen dioxide and then challenged them with aerosols of K_. pneumoniae.
3
One of the four monkeys that were exposed to 18. 8 mg/m (10 ppm) for
1 month died, and in two others the infectious agent was present in the
3
lungs at autopsy. Two of seven monkeys that were exposed to 9. 4 mg/m
(5 ppm) for 2 months died, and in five the infectious agent was present
in the lungs at autopsy.
-------
10-24
Influenza: Squirrel monkeys were infected with mouse-adapted influenza
3
A/PR/8 virus 24 hr before exposure to nitrogen dioxide at 18. 8 or 9.4 mg/m
3
(10 and 5 ppm). The six monkeys that were exposed continuously to 18. 8 mg/m
(10 ppm) died within 3 days, whereas only one of three that were exposed to 9. 4
3
mg/m (5 ppm) succumbed to the disease. There were no deaths in a control
group of monkeys that were supplied with filtered air and then challenged with
222
influenza virus.
Mouse-adapted influenza A/PR/8 virus was instilled intratracheally into
monkeys that were exposed for approximately 5 months to nitrogen dioxide
3 128, 143
at 9.4 mg/m (5 ppm). The virus was injected 24 hr before the con-
tinuous exposure began, and again, 37 and 77 days after. The production of
serum neutralizing (SN) antibodies appeared to be influenced by the nitrogen
dioxide exposure. After a 3-month exposure, two of the three monkeys that
were exposed to filtered air had SN titers of 1:256 or higher, whereas the titer
in three of the four monkeys that were exposed to the nitrogen dioxide did not
exceed 1:21. However, after 5 months of exposure, when the study ended, the
SN antibody contents of the exposed monkeys had returned to the levels of
the controls.
In another group of experiments, monkeys were exposed to nitrogen
3
dioxide at 1. 9 mg/m (1 ppm) for 16 months. They -were challenged by
intratracheal instillations of a monkey-adapted influenza A,/PR/8 virus
128, 144
24 hr before the exposure began, and again 41, 83, and 146 days after.
An additional challenge was introduced on the 266th day of a 20-min exposure
to influenzal virus aerosol. Monkeys exposed to nitrogen dioxide produced
SN antibodies within 21 days after the first virus infection, whereas only
one of the three control monkeys, who were given the same challenge but
-------
10-25
exposed to filtered air, showed a, comparable response. At 41 days after
the first challenge, three of five monkeys exposed to nitrogen dioxide had
SN titers of 1:128 or greater, reflecting a sixfold increase over the titer
in the one control monkey that showed any response. After 12 months,
monkeys exposed to nitrogen dioxide showed consistently higher SN antibody
titers than did those that were exposed to air (Figure 10-4). The pathologic
changes in the lungs of monkeys exposed to nitrogen dioxide and challenged
with the virus consisted of slight emphysema and thickened bronchial and
bronchiolar epithelium. These changes were not observed in monkeys
exposed to air and challenged with the virus.
The effects of low concentrations of nitrogen dioxide on the immunologic
response of squirrel monkeys appeared to be partially related to the ability
of the virus to multiply in the lung tissue of the host and to the native
resistance of the host to the infectious agent. With the mouse-adapted strain
of influenza virus, the ability to form SN antibodies apparently decreased
3
in monkeys exposed to nitrogen dioxide at 9. 4 mg/m (5 ppm). Conversely,
after challenge with a monkey-adapted strain of the same virus, SN anti-
bodies appeared sooner and in higher amounts in monkeys that had been
3
exposed to nitrogen dioxide at 1. 9 mg/m (1 ppm). In this instance, expo-
sure to nitrogen dioxide appeared to enhance the establishment and multi-
plication of the monkey-adapted influenza virus.
257
Ito £t al. exposed mice to nitrogen dioxide concentrations of 18. 8
3
mg/m (10 ppm), for 2 hr/day over 1, 3, or 5 days and then challenged
them with mouse-adapted influenza A/PR/8 virus. Other mice were
challenged with the same virus after a continuous exposure to nitrogen
3 3
dioxide concentrations of 0. 94 mg/m (0. 5 ppm) or 1. 9 mg/m (1. 0 ppm)
-------
10-26
LL)
C_3
CO
3
-------
10-27
3
for 39 days. Exposure to 18. 8 mg/m (10 ppm) for 2 hr/day for 5 days
increased the susceptibility of mice to influenza virus resulting in in-
creased mortality. Interstitial pneumonia -was more extensive in mice
challenged -with the virus after both acute and chronic exposures than in
the controls exposed to air. Moreover, adenomatous proliferations of
bronchial and bronchiolar epithelium were marked in the mice challenged
with influenza virus after the continuous exposure to the low nitrogen
dioxide concentrations.
65
Buckley and Loosli exposed germfree and conventional mice to air
3
containing nitrogen dioxide concentrations of 71 mg/m (38 ppm) for 6
weeks. After this exposure, mice were challenged with aerosols of mouse-
adapted influenza A/PR/8 virus. Mortality rates and average day of death
after the challenge with the virus indicated that exposure to nitrogen dioxide
resulted in a marked increase in resistance to influenza infection. The
mice that inhaled nitrogen dioxide had higher survival rates after inocula-
tion with an L-LVg virus and increased survival days after an LD ..„„ virus
inoculation. These observations differ from those reported by other
investigators.
Immunologic Effects
334
Matsumura recently reported results of studies on guinea pigs that
were sensitized to egg albumen. A 30-min exposure to nitrogen dioxide
3
at 132 mg/m (70 ppm) increased the susceptibility of those guinea pigs
to systemic anaphylaxis after inhalation of egg albumen aerosols. Expo-
3 335
sure to nitrogen dioxide at 75 mg/m (40 ppm) caused increased dyspneia.
Circulating antibodies reactive with pulmonary tissue have also been found
3
in guinea pigs exposed to 9. 4-28. 0 mg/m (5-15 ppm) for up to 12 months.
-------
10-28
The effects of extended exposures to low concentrations of nitrogen
dioxide on the immunologic response were studied by vaccinating mice with
490
279 CCA units of chick embryo A /Taiwan/1/64 influenza virus vaccine.
Before vaccination, one group of mice was exposed continuously for 3 months
to nitrogen dioxide at 3. 8 mg/m (2 ppm); another group was exposed to 0. 94
3 3
mg/m (0. 5 ppm) with daily 1-hr-long peaks of 3. 8 mg/m (2 ppm) for 5 days
per week. After the 3-month exposure, the mice were vaccinated with the
influenza vaccine and held in either of the exposure conditions for up to 7
months. At various intervals, the hemagglutination inhibition (HI) and SN
antibody titers and the concentrations of immunoglobulins were measured.
Two weeks after vaccination, the SN antibody decreased and seroconversion
3
rates were markedly lower especially among mice exposed to 0. 94 mg/m
3
(0. 5 ppm) with the daily 1-hr peaks of 3. 8 mg/m (2 ppm). After 4 weeks of
exposure, the SN titers and seroconversion rates did not differ significantly
from those in control mice exposed to filtered air.
Serum imrnunoglobulin concentrations in nonvaccinated mice -were
altered during the 3 months of exposure to nitrogen dioxide. In general,
IgA decreased while IgM, IgG^ and IgG2 increased. During the 28 weeks
after vaccination, exposure to nitrogen dioxide did not further influence
IgA concentration. However, serum IgM, IgG-^ , rand IgG? were higher in
mice exposed to nitrogen dioxide than in those maintained in filtered air.
More specifically, mice consistently showing higher imrnunoglobulin
3
concentrations were those exposed continuously to 0. 94 rng/m (0. 5 ppm)
3
with daily 1-hr peaks of 3. 8 mg/m (2 ppm) before and after vaccination.
Moreover, mice exposed to filtered air before vaccination and to nitrogen
3
dioxide at 3. 8 mg/m (2 ppm) after vaccination and those exposed to
-------
10-29
3
nitrogen dioxide at 3. 8 mg/m (2 ppm) before vaccination and to filtered
490
air after vaccination showed similar increases in serum immunoglobulins.
These results suggest that fluctuation in atmospheric nitrogen dioxide
has more influence on the immune response than constant, although higher,
3
concentrations of nitrogen dioxide. Continuous exposure of mice to 3. 8 mg/m
(2 ppm) for approximately 10 months did not appear to influence the formation
of antibodies or the immunoglobulin concentrations. Conversely, exposure to
3 3
0. 94 mg/m (0. 5 ppm), -with daily 1-hr peaks of 3. 8 mg/m (2 ppm), appeared
to decrease the ability to form SN antibody and significantly altered the con-
centrations of serum IgM, IgGp and IgG 2 '
Mucociliary Clearance Mechanisms
When a highly irritant gas reaches the mucous membranes of the trachea
or bronchi, it can increase mucus secretions, paralyze the cilia, and eventu-
ally result in mucosal thickening, thereby reducing the effectiveness of the
physical air-way-clearing mechanisms. In severe cases, it may cause
desquamation of the epithelium, exposing the deeper and more sensitive cell
layers to infectious agents.
The majority of the few existing reports describing the effect of nitrogen
dioxide in the mucociliary activity are based on observations of in vitro
94
rabbit tracheal preparations. Cralley reported that a nitrogen oxide gas
mixture (nitric oxide, nitrogen dioxide, nitric acid (HNO ), etc. ) at approxi-
3 3
mately 188 mg/m (100 ppm) caused irreversible cessation of ciliary beating
96
after a 5-min exposure. Dalhamn and Sjoholm reported that nitrogen
3
dioxide between concentrations of 188 and 280 mg/m (100 and 150 ppm)
caused cessation of ciliary movement. These investigators did not state
279
whether the effect was permanent or temporary. Kensler and Battista
-------
10-30
3
reported that an 18-sec exposure to nitrogen dioxide at 790 mg/m (420
ppm) produced a 50% inhibition in the rate particles •were transported by
the mucociliary apparatus.
175
More recently, Giordano and Morrow studied the mucociliary clear-
ance in anesthesized rats after a 6-week exposure to nitrogen dioxide at
3
11. 3 mg/m (6 ppm). The two transport characteristics measured -were
"first edge time" and a "20% transport time. " Exposure to nitrogen dioxide
caused a significant increase in both, indicating a significant inhibition of
the mucociliary clearance mechanism. The decrease in ciliary activity was
not accompanied by any observable abnormality of the airways. Pathologic
changes of edema and vascular congestion did occur, but these were con-
fined to the alveolar regions. The authors suggest that one point of inter-
action between the nitrogen dioxide and the mucociliary apparatus might be
64
the energy source of the cilia. Buckley and Balchum have reported that
adenosine triphosphate, ATP (C -^ H-, ^N,- O-^H^ P oOg), is the source of
energy for ciliary motion and that nitrogen dioxide can affect lung homogenate
enzymes linked to ATP formation. Other possible reactions affect the mucus,
i. e. , through modification of the cross-linking of polymer chains -which affects
172
the viscosity of the mucus.
The in vivo clearance of inhaled bacteria from the lungs can be hindered by
188
the action of a number of different agents presumably by inhibition of ciliary
activity. However, removal of the inhaled bacteria by the ciliary mechanisms
and mucus flow is probably not a major part of lung antibacterial defense.
Other mechanisms must be involved, inasmuch as the loss of viability of in-
179, 276
haled bacteria in the lungs occurs much faster than their removal.
-------
10-31
Phagocytic Activity of Alveolar Macrophages
When inhaled bacteria first enter the lungs, they are free in the air
spaces; later, they are almost entirely contained within phagocytic alveolar
179,188, 276
cells. The removal of inhaled bacteria by these cells is con-
sequently of utmost importance as a defense mechanism against respira-
tory infections.
The action of nitrogen dioxide on the alveolar macrophage system was
168
reported by several investigators. Gardner ^t al. stated that exposure
to nitrogen dioxide reduced the number of alveolar macrophages obtained
by pulmonary lavage. Moreover, the phagocytic activity of the surviving
fraction of the macrophages was also reduced. Exposure of rabbits to
3
nitrogen dioxide at 15 mg/m (8 ppm) resulted in a significant increase in
the percentage of intra-alveolar heterophiles obtainable by pulmonary
3
lavage. A 3-hr exposure to 18. 8 mg/m (10 ppm) resulted in a marked
decrease in phagocytic activity, as reflected by an approximately 50%
reduction in phagocytized Streptococcus pyogenes cells. In addition,
although 83% of the macrophages from control rabbits contained at least
one bacterium, only 66% of the macrophages from exposed rabbits showed
phagocytic activity.
575
Valand _et al. reported that alveolar macrophages harvested from
rabbits that received intratracheal injections of para-influenza-3 virus
were resistant to later in vitro challenge with rabbit pox virus. However,
3
when the rabbits were exposed to nitrogen dioxide at 47 mg/m (25 ppm)
for 3 hr immediately after challenge with the para-influenza-3 virus,
during the challenge, or 3, 5, 12, or 24 hr before, this resistance did
not develop. This refractory state lasted at least 96 hr; the alveolar
-------
10-32
macrophages isolated from rabbits exposed to nitrogen dioxide -were
unable to produce interferon when inoculated in vitro with the para-
influenza-3 virus. Exposure to nitrogen dioxide also appeared to increase
the adsorption rate of the virus in the lungs of rabbits, but did not enhance
the infectivity of the virus.
The inhibition of alveolar macrophage function was further demon-
strated by Acton and Myrvik. They exposed rabbits to nitrogen dioxide
3 4
concentrations of 28. 2, 47, or 94 mg/m (15, 25, or 50 ppm) for 3 hr
and inoculated them with killed BCG vaccine. Macrophages lavaged from
the lungs of these rabbits had a markedly impaired ability to phagocytize
the vaccine, compared with normal controls. This effect was not observed
3
in cells from animals exposed to nitrogen dioxide at 9.4 mg/m (5 ppm).
578
Vassallo
-------
10-33
exposure to either of the nitrogen dioxide concentrations showed distinct
alteration in surface structure when examined with a scanning electron
microscope.
Combined Exposures
89
Coffin and Blommer studied effects on resistance to infection caused
by exposure to nitrogen dioxide in combination with other pollutants. They
exposed mice to light-irradiated automobile exhaust for 4 hr and then
infected them with airborne streptococci. An increased mortality rate was
observed in mice exposed to exhaust containing concentrations of carbon mon-
3 3
oxide (CO) at 47 mg/m (25 ppm) and oxidants at 0. 28 mg/m (0. 15 ppm).
3
The nitrogen dioxide concentration in the exhaust gas was 0. 6 mg/m (0. 3
ppm), approximately 10% reportedly required to increase susceptibility to
127
respiratory infection. Because oxidant concentrations were within the
3
effective range for ozone [threshold values of 1. 88 mg/m (0. 1 ppm) have
88
been reported ], the reduction in resistance to bacterial infection was
attributed to the presence of oxidants in the automobile exhaust.
Hamsters were exposed to combinations of nitrogen dioxide, tobacco
223
smoke, and 1C. pneumoniae. A 2-hr exposure to nitrogen dioxide at
3
28. 2 mg/m (15 ppm) followed by a 1-hr exposure to 3% (v/v) tobacco
smoke decreased their resistance to bacterial pneumonia, as evidenced
by increased mortality and reduced survival time. Furthermore, the com-
bined exposures reduced the rate at -which viable bacteria were cleared
from their lungs to a greater extent than individual pollutant exposures.
Scanning electron microscopic examination of lung tissues indicated
that 1C. pneumoniae had no demonstrable effect and exposure to nitrogen
dioxide had only a slight effect on the surface structure of the respiratory
-------
10-34
220
airways. Exposure to tobacco smoke and bacteria produced reversible
alterations in the surface structure of the bronchi. The changes were present
2, but not 7, days after the exposure terminated. Exposure to nitrogen di-
oxide and tobacco smoke followed by inhalation of K. pneumoniae produced
marked loss of cilia and alteration of surface morphology of nonciliated cells
2 days after exposure. These changes were progressive, rather than reversi-
ble, and the surface structure of the affected bronchi showed an even greater
disruption 7 days after the exposures.
A similar loss of cilia from the terminal bronchiolar surface and leveling
of the terminal bronchiolar epithelium were observed in rats after a 24-hr
3 408
exposure to nitrogen dioxide at 28. 2 mg/m (15 ppm). When examined
with a scanning electron microscope, the terminal bronchiole presented an
even surface and most areas were free of cilia. Moreover, tiny microvilli
covered most of the nonciliated areas, and the population of brush cells
increased markedly. These changes in terminal bronchioles were more pro-
nounced after 2 and 7 days of exposure to this nitrogen dioxide concentration.
181
Goldstein jst a_l. studied the pulmonary defense mechanisms as mani-
fested by bactericidal dysfunction in mice exposed to various combinations
of concentrations of nitrogen dioxide and ozone, and infected with aerosols
of S. aureus. Their results suggested that the pollutants did not act syner-
gistically, since the reduction in bactericidal function was equivalent to
that expected if each pollutant acted independently. These experiments also
indicated that measurement of oxidant-induced disturbance in air-way
resistance may be a more sensitive indicator of pollutant damage than
measurement of bactericidal function.
-------
10-35
TOXICOLOGIC EFFECTS OF OTHER OXIDES OF NITROGEN IN
LABORATORY ANIMALS
Nitrous Oxide
Nitrous oxide is an inert gas with anesthetic characteristics. Its balanced
system of biologic production and biologic and stratospheric degradation is
441
independent of the cycles of other oxides of nitrogen. The normal ambient
3 91, 199
concentration of nitrous oxide is 0. 9 mg/m (0. 50 ppm), -which is con-
siderably below the threshold concentration for a biologic effect. Consequently,
the potential toxicity of this gas as a pollutant is nil.
Nitric Oxide
In addition to its biologic formation, large quantities of nitric oxide form
as a result of industrial processes, especially those which burn petroleum.
3 69
Its ambient concentrations are almost always less than 0. 6 mg/m (0. 5 ppm).
Although this pollutant is a principal component of photochemical smog and
accounts for a major portion of the oxides of nitrogen, its biologic toxicity is
much less than that of nitrogen dioxide. After rats have been exposed for
2. 5 to 24 months to automotive exhaust gases containing carbon monoxide
3 3
at 67. 4 mg/m (57. 8 ppm); nitric oxide at 27. 8 mg/m (22. 6 ppm); and
3
nitrogen dioxide at 1. 7 mg/m (0. 92 ppm), they incur decreases in body
weight, diminution of the sound-avoiding reflexes, and an increase in the
522
number of spontaneous tumors. Similarly, although nitric oxide has
undoubtedly been present in cases of human poisoning due to oxides of nitro-
518
gen, poisoning from nitric oxide alone has never been reported.
The rapid conversion of nitric oxide to nitrogen dioxide, a much more
toxic gas, makes nitric oxide very difficult to investigate. The few reported
studies suggest that exposure to high concentrations of nitric oxide will
184,189, 540
result in tissue injury caused by methemoglobin formation.
-------
10-36
Experiments in dogs have demonstrated that these animals are capable of
3
breathing air containing nitric oxide at 1, 230 mg/m (1,000 ppm) for 136
189
min before cyanosis appears. The arterial oxygen pressure in these
animals was 309 mm of mercury and the methemoglobin concentration was
5%.
3
Dogs that were exposed to nitric oxide at 6, 200 mg/m (5, 000 ppm) for
24 min or more succumbed to pulmonary edema after exposure. Shorter
exposures resulted in respiratory distress but not deaths.
Physiologic abnormalities in addition to the decrease in oxygenation were
respiratory acidosis, diminished lung compliance, increased airway resistance,
189
diminished cardiac output, and a marked reduction in heart rate. The
principal biochemical abnormality was the time-related formation of methemo-
189, 540
globin. The major findings from autopsies of both the dogs that survived
and the ones that died were pulmonary edema, hyperinflation, hemorrhage,
483
desquamation of mucosa, and bronchopneumonia. With the exception of
congestion, the liver, spleen, and kidneys appeared normal. The brain mani-
fested changes that were consistent with hypoxia. The similarity of these
pathologic findings to those attributed to nitrogen dioxide, and the known con-
tamination of nitric oxide with nitrogen dioxide support the probability that
some, if not all, pathologic abnormalities were caused by nitrogen dioxide,
rather than nitric oxide.
Experiments with mice have yielded similar responses. For example,
3
lethality was not observed after 8-hr exposures to nitric oxide at 380 mg/m
416,417
(310 ppm).
-------
10-37
Dinitrogen Trioxide and Dinitrogen Pentoxide
The chemical instability of these oxides of nitrogen reduces the likeli-
hood of their causing significant hazard in other than acute circumstances.
Dinitrogen trioxide is extremely unstable and decomposes almost
184
immediately to nitric oxide and nitrogen dioxide.
Dinitrogen pentoxide is produced from the interaction of ozone and
109,441, 518
nitrogen dioxide. This gas is an anhydride of nitric acid, and
in moist air it rapidly converts to nitric acid. Although dinitrogen pent-
oxide is slightly toxic, the following arguments suggest that this toxicity is
not of practical importance. At ozone and nitrogen dioxide concentrations
3 3
of 0. 2 mg/m (0. 1 ppm) and a nitric oxide concentration of 0. 01 mg/m
(0. 01 ppm), the upper-limit equilibrium concentration of dinitrogen pent-
3 518
oxide is 0. 001 mg/m (0. 0002 ppm). The formation of dinitrogen pent-
109, 518
oxide requires an equivalent reduction in the concentration of ozone,
517
which is probably the more toxic gas. Although results of different tests
109, 517
vary, it is certain that dinitrogen pentoxide can be only slightly more
109, 181
toxic than ozone. Consequently, of all the oxides of nitrogen, only
nitrogen dioxide can be classified as a significant hazard to health.
SUMMARY OF EXPERIMENTAL ANIMAL STUDIES
Mice exposed for less than 24 hr to nitrogen dioxide concentrations of
3
3. 8 mg/m (2 ppm) or greater develop defects in pulmonary microbial
defense mechanisms. Inhibition in intrapulmonary killing of inhaled
3
Staphylococcus aureus occurs in mice exposed to 3. 8 mg/m (2. 0 ppm)
of nitrogen dioxide for 17 hr and pneumonia and death occurs in mice in-
3
fected with Klebsiella pneumoniae following 2-hr exposures to 6. 6 mg/m
(3. 5 ppm). In other rodents, physiologic and pathologic abnormalities
-------
10-38
3
appear at concentrations above 9.4 mg/m (5 ppm). Mortality occurs at
3
nitrogen dioxide concentrations of 75 mg/m (40 ppm). Even the lowest
concentrations at which abnormalities are occasionally observed are higher
than concentrations found in the ambient atmosphere.
Prolonged exposure to nitrogen dioxide has affected normal animals only
3
at concentrations of 0. 94 mg/m (0. 5 ppm) and above. No deaths resulted
3
in any animals exposed for a year or more to 0. 94 mg/m (0. 5 ppm) of nitro-
gen dioxide. Such animals as dogs and guinea pigs survive exposures of a
year or more to much higher concentrations of nitrogen dioxide.
Pathologic abnormalities (ciliary loss, alveolar cell disruption, and
obstruction of respiratory bronchioles) occur in mice and rats after continu-
3
ous exposure to nitrogen dioxide concentrations of 0. 94 mg/m (0. 5 ppm).
Exposure to higher nitrogen dioxide concentrations causes more severe
cellular and structural damage, which in the rat and rabbit resembles
emphysema.
Physiologic alterations (tachypnea, increases in airway resistance,
decreases in tidal volume and in static compliance) occur in rodents and
in nonhuman primates after exposures of 2 months or longer to nitrogen
3
dioxide at concentrations of 9. 4 mg/m (5. 0 ppm) or higher. More serious
abnormalities in pulmonary function (decreases in blood oxygenation) occur
3
in rabbits after continuous exposure to 15 mg/m (8. 0 ppm). These data
are not comprehensive because neither the measurements of small airway
function nor the effect of exercise have been reported.
Continuous exposures for 3 months and intermittent daily exposures
3
for 6 months or longer to nitrogen dioxide at concentrations of 0. 94 mg/m
(0. 5 ppm) and higher diminishes murine resistance to pulmonary bacterial
-------
10-39
3
infection. Exposures of 9. 4 mg/m (5. 0 ppm) have produced similar
results in nonhuman primates. These decreases in resistance to
infection have caused pneumonia and death. Immunologic deficits, not
associated \vith overt infection, occur during exposures of 16 months to
3
1. 8 mg/m (1. 0 ppm). These investigations of the combined effect of
nitrogen dioxide and infection have provided the most sensitive indices of
pollutant-induced damage.
In a few reported studies animals were exposed to mixtures of pollu-
tants including nitrogen dioxide. Combinations of nitrogen dioxide with
carbon monoxide, ozone, or sulfur dioxide (SO ) have usually resulted in
additive or indifferent effects. Synergy has, with one exception of uncertain
significance, not been reported, and to our knowledge antagonism has never
been reported.
EFFECTS IN HUMANS FROM SHORT-TERM NITROGEN
DIOXIDE EXPOSURES
Sensory Effects
Sensory perception, in the form of either odor or dark adaptation, is
the most sensitive indicator in humans of the presence of nitrogen dioxide.
224 476
Henscheler _et al_. and Shalamberidze have studied odor perception
in volunteers. Henscheler and his colleagues exposed several groups of
two to four volunteers to concentrations of nitrogen dioxide fixed at various
3
levels between 0. 23-56. 8 mg/m (0. 12-30. 2 ppm), for either 30 or 120 min.
The nitrogen dioxide was prepared by adding sulfuric acid to sodium nitrite
to form nitric oxide and then reacting the nitric oxide with oxygen in a 1-m
reaction tube. The pollutant was introduced into exposure chambers at
high flow rates, and its concentration was measured by the Saltzman method.
-------
10-40
Olfactory responses were recorded for groups of two to four volunteers
immediately after their entrance into the nitrogen dioxide-containing
chambers. After 30 min of exposure, the volunteers left the chamber.
They were then reexposed to several inhalations of test concentrations of
nitrogen dioxide at 30-sec intervals, and their responses were again recorded.
The odor of nitrogen dioxide was perceived by 3 of 9 volunteers at concentra-
3
tions of 0. 23 mg/m (0. 12 ppm) and by 8 of 13 subjects at concentrations of
3
0. 41 mg/m (0.22 ppm). (See Table 10-4.) The volunteers perceived the odor
3 .
for 1-10 min after entry into the chamber at concentrations of 7. 52 mg/m
(4. 0 ppm) or less. The duration of odor perception was unrelated to the
concentration of nitrogen dioxide within the chamber. The obliterated
olfactory response returned within 1-1. 5 min after the subject left the
chamber. Some subjects reported a metallic taste and pharyngeal dryness,
3
roughness, and constriction from exposure to 0. 23 mg/m (0. 12 ppm) or
more. These symptoms lessened with repeated exposures and eventually
3
disappeared even at concentrations as high as 37. 0 mg/m (19. 7 ppm).
3
Exposure to 56.4 mg/m (30 ppm) resulted in overt discomfort-burning
sensation in the nose and chest, cough, dyspnea, and sputum production.
These symptioms persisted for several hours after termination of the
exposure. When the volunteers entered the exposure chamber before the
addition of nitrogen dioxide, and the pollutant was added in gradually
3
increasing concentrations up to 47 mg/m (25 ppm), olfactory perception
did not occur.
The role of humidity was investigated by exposing the volunteers to
3
2. 26 mg/m (1. 2 ppm) at 55% humidity which was then increased to 78%.
-------
10-41
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This resulted in a sharp increase in odor perception and in irritation of
the mucous membranes of the respiratory tract.
In summary, these studies demonstrate that sensitive subjects can
3
detect the odor of nitrogen dioxide at a concentration of 0. 23 mg/m
(0. 12 ppm), but that this perception can be prevented by gradual exposure
to the gas. In addition, tests have shown that increases in humidity increase
odor perception and respiratory irritation resulting from nitrogen dioxide
exposure. Data establishing the threshold concentration of odor perception
3 476
at 0. 23 mg/m (0. 12 ppm) have been reported.
Exposure to low concentrations of nitrogen dioxide impairs dark adapta-
51,476
tion. According to Shalamberidze, the threshold for this sensory
3
defect may be as low as 0. 14 mg/m (0. 075 ppm), which is about 40% lower
476
than that for odor perception. This result, however, was not reproduced
51
by Bondareva in his studies of the effect of nitrogen oxides on eye sensitivity
to darkness. Since Bondareva did not indicate whether he used nitrogen dioxide
alone or in combination with nitric oxide, the higher threshold values he found
may have resulted from differences in test atmospheres.
Bondareva determined the normal adaptation curve in five volunteers
and then exposed the subjects to atmospheres containing nitrogen oxides at
3
0.15-0. 50 mg/m (0. 08-0. 26 ppm). The nitrogen oxides were measured
colorimetrically by the Griess-Ilosvay reaction. These data indicated that
3
exposure to a nitrogen oxide concentration of 0. 3 mg/m (0. 16 ppm) did
not cause changes in dark adaptation. A sharp rise in the adaptation curve
3
did occur at 0. 5 mg/m (0. 26 ppm). Repeated exposures over 2. 5 to 3
months resulted in a slight decrease in the adaptation curve, suggesting
some physiologic adjustment to the pollutant.
-------
10-43
Pulmonary Function
The effect of exposure to nitrogen dioxide on pulmonary function has been
1, 384, 523, 532, 58Z-585, 613-616 1
studied by several investigators. Abe exposed
five healthy men (four nonsmokers and one light smoker), between 21 to 40 years
3
of age, to nitrogen dioxide concentrations of 7. 5-9. 4 mg/m (4. 0-5. 0 ppm) for
10 min. Measurements of pulmonary compliance, inspiratory flow resistance,
maximal midexpiratory and peak flow rates, and vital capacity were made
immediately before and 10, 20, and 30 rnin after exposure. Compliance curves
were obtained with intraesophageal balloons and flow transducers. Nitrogen
dioxide concentrations were determined by the Saltzman method. The study
3
•was repeated but with 10-min exposures to sulfur dioxide at 11.4-14. 3 mg/m
3
(4. 0-5. 0 ppm) and to combinations of nitrogen dioxide at 4. 7 mg/m (2. 5 ppm)
3
and sulfur dioxide at 7. 5 mg/m (2. 5 ppm).
The effects of the different pollutant exposures on expiratory flow resist-
ance are shown in Table 10-5. The maximal response for nitrogen dioxide
occurred 30 min after the exposure. In contrast, exposure to sulfur dioxide
caused an immediate increase in expiratory flow resistance, which rapidly
returned to normal within 10 min after the exposure. The effects of sulfur
dioxide and nitrogen dioxide combined were neither cumulative nor antago-
nistic; each gas affected expiratory flow resistance independently, creating
a bimodal curve characterized by the early effect of sulfur dioxide and the
delayed effect of nitrogen dioxide. Early in the course of exposure to combi-
3
nations of sulfur dioxide at 7. 5 mg/m (2. 5 ppm) and nitrogen dioxide at
3
4. 7 mg/m (2. 5 ppm), an 18% increase in expiratory flow resistance occurred
and was attributed to the effect of sulfur dioxide. Flow resistance returned to-
wards normal in the first 10 min after exposure and then increased again by 45%
-------
10-44
TABLE 10-5
Effect of Nitrogen Dioxide and Sulfur Dioxide on Expiratory Flow Resistance
tontrol period
Ixposure period (10 min)
Expiratory Flow
N02
7.5-9.4 mg/m
(4-5 ppm)
100
96
Resistance , %
SO
11.4-14.3 mg/m3
(4-5 ppm)
100
141
N02 + S02
4.7+7.5 mg/m3
(2.5+2.5 ppm)
100
118
Recovery period
10 min 126 90 106
20 min 145 95 132
30 min 177 90 145
-------
10-45
at 30 mm after exposure, an effect similar to the response to isolated nitro-
gen dioxide exposure demonstrated earlier. Similar findings were reported
for inspiratory flow resistance and pulmonary compliance. Measurements of
the 1-sec forced expiratory volume, maximal midexpiratory and peak flow
rates, and vital capacity during and after exposure to nitrogen dioxide alone
did not reveal changes, suggesting that these tests were insensitive to the
experimental conditions.
1 384,523
Abe cites two Japanese studies with similar results. Suzuki and
523
Ishikawa measured pulmonary compliance and inspiratory and expiratory
resistance in 10 healthy individuals after a 10-min exposure to nitrogen dioxide
3
concentrations of 1. 32-3. 76 mg/m (0. 7-2. 0 ppm). The test values did not
change immediately; but, 10 min after exposure, increases of 50% and 15%
in inspiratory and expiratory flow resistance and decreases of 10% in compli-
ance were recorded. Prompt increases in airway resistance have been
produced by other investigators by exposing healthy individuals for 5 min to
3
nitrogen dioxide concentrations of 11. 8 mg/m (6 ppm) to as high as 75. 2
3 384
mg/m (40 ppm).
Although increased airway resistance resulting from nitrogen dioxide
exposure has been found consistently, its threshold concentration is uncertain.
Using a plethysmograph on volunteers who had been exposed to nitrogen
613-616
dioxide for 10-120 min, Yokoyama measured increases in airway
3
resistance at 13. 2 mg/m (7 ppm) and higher. He also reported wide
variations in individual sensitivity. Some volunteers tolerated concentra-
3
tions up to 30 mg/m (16 ppm). Because atropine effectively blocked the
bronchoconstrictive effect of sulfur dioxide, but not nitrogen dioxide, these
investigators suggested that the mechanism for the increase in airway
resistance was unrelated to vagal stimulation.
-------
10-46
58Z, 583, 585
Von Nieding and his associates exposed 63 patients with
chronic bronchitis to 30 breaths of nitrogen dioxide from concentrations of
3 3
less than 1.8 mg/m (1 ppm) to 9.4 mg/m (5 ppm). The concentrations
were determined by the Saltzman method and airway resistance was measured
by body plethysmography before and immediately after each exposure. Signifi-
cant increases in airway resistance occurred upon exposure to nitrogen dioxide
3
concentrations of 3, 0 to 3. 8 mg/m (1. 6 to 2. 0 ppm). (See Fig. 10-5. ) After
3
exposure to 9. 4 mg/m (5 ppm), normal subjects also exhibited diminished
583
single-breath diffusing capacity and significant increases in the alveolar-
582
arterial ;p_O 2 gradient. These findings suggest that nitrogen dioxide
affects respiratory gas exchange.
582
Von Nieding also observed that the effect of nitrogen dioxide on airway
resistance and arterial oxygen pressure was prevented by administration of an
antihistamine (Meclostine) but not by atropine or orciprenaline (a sympathomi-
metic drug). This suggests that nitrogen dioxide causes pulmonary tissue
to release histamine, rather than inducing a parasympathetic reflex mechanism
similar to that resulting from sulfur dioxide exposure.
532
Thomas and his associates failed to observe nitrogen dioxide effects at
3
0. 94-6. 58 mg/m (0. 5-3. 0 ppm) on sputum histamine concentrations or on
total sputum -weight either in five healthy subjects or in four patients with
chronic respiratory disease.
Ten individuals who inhaled nitrogen dioxide while exercising were studied
447
by Rokaw _et_ a.l_. Six of their subjects were healthy; four had chronic
pulmonary disease. Temperature and humidity were constant at 21. 11 C, 50%
RH. Nitrogen dioxide was generated from cylinders of liquid nitrogen peroxide-
nitrogen dioxide and analyzed by the Saltzman method. Airway resistance was
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10-48
measured during both rest and bicycling in chamber conditions at nitrogen
3
dioxide concentrations of 0, 0.94, 1.88, 2.82, 3.76, 4.7, and 5. 64mg/m
(0, 0. 5, 1. 0, 1. 5, 2. 0, 2. 5, and 3. 0 ppm), and 45 mm after filtering all
the nitrogen dioxide from the chamber. Airway resistance increased in
most normal subjects during rest when exposed to a concentration of 5. 64
3
mg/m (3.0 ppm). Resistance increased during exercise, beginning in
3
some subjects at 2. 82 mg/m (1. 5 ppm). Subjects with chronic respiratory
disease experienced increases in airway resistance during rest beginning
3 3
at 3. 76 mg/m (2. 0 ppm), and during exercise at 2. 82 mg/m (1. 5 ppm).
After staying in the clean, filtered chamber for 30 min at the start of
each test run, most subjects showed a decline in airway resistance, compared
with measurements taken immediately after entering the chamber from the
ambient environment.
EFFECTS IN MAN FROM EXPOSURES TO NITROGEN DIOXIDE IN THE
PRESENCE OF OTHER POLLUTANTS
Experimental Studies
• Sensory Effects. The effects of nitrogen dioxide alone and in combin-
ation with other gases on thresholds for odor and dark adaptation have been
476
reported by several Russian scientists. Shalamberidze compared the
effects of nitrogen dioxide, sulfur dioxide, and combinations of the two
gases on 15 human subjects. Nitrogen dioxide was prepared by heating lead
nitrate with quartz sand (Maser's method). Concentrations were determined
by the Griess-Ilosvay reagent. Sulfur dioxide concentrations were measured
with a Nephelometer. When inhaled together, sulfur dioxide and nitrogen
dioxide acted additively. That is, an odor was detected when the sum of
the fractional concentrations of each gas, expressed as a ratio of the gas
-------
10-49
concentration in the mixture to the odor threshold concentration for each gas by
itself, equaled 1.0 or more.
Using the same gas mixtures, Shalamberidze studied the effects of nitrogen
dioxide, sulfur dioxide, and combinations of these gases on the threshold for
dark adaptation. Observations were made during three series of tests: nasal
inhalation of gases separately and combined for 5 mm, oral inhalation as above,
and nasal inhalation of gases separately for 25 min. These tests revealed that
the threshold for dark adaptation occurred at nitrogen dioxide concentrations of
3 3
0. 14 mg/m (0. 075 ppm) and sulfur dioxide at 0. 60 mg/m (0. 21 ppm). When
tested as components of a mixture, the two gases acted additively, as they had
for odor perception. The threshold for dark adaptation with each gas was lower
3
than that for odor. The odor threshold with nitrogen dioxide was 0. 23 mg/m
3
(0. 12 ppm); but for dark adaptation was about 60% lower, or 0. 14 mg/m
3
(0. 075 ppm). With sulfur dioxide, the threshold for dark adaptation [0. 6 mg/m
3
(0. 32 ppm)], about 38% of the odor threshold [l. 6 mg/m (0. 32 ppm)].
292
Korniyenko similarly evaluated odor perception as affected by a mixture
of nitrogen oxides (not further specified), sulfur dioxide, sulfuric acid aerosol,
and ammonia. After conducting a series of 439 tests, the author determined
that the odor threshold of the gas mixture occurred at a concentration coefficient
of 1. 0, expressed as a sum of the fractional odor threshold concentrations of
the component gases. He reported the odor threshold of the combination to
3 3
be 0. 02 mg/m (0. Oil ppm) for nitrogen oxides, 0. 17 mg/m (0. 090 ppm) for
3 3
sulfur dioxide, 0. 11 mg/m (0. 059 ppm) for sulfuric acid aerosol, and 0. 3 mg/m
(0. 160 ppm) for ammonia.
Korniyenko also investigated the effect of the gas mixture on the electrical
activity of the brain. He studied the amplitude of the alpha rhythm during a
-------
10-50
series of 187 tests in which four subjects were exposed to combinations of
the four gases at fractional concentrations of the threshold level at -which
changes in the amplitude of the alpha rhythms occurred. The earliest
effect of combined exposure to these compounds was detected when the sum
of the fractional concentrations, as determined above, was equal to one.
3
Fractional concentrations for the mixture were 0. 008 mg/m (0. 425 ppm)
3 3
for nitrogen oxides, 0. 08 mg/m (0. 043 ppm) for sulfur dioxide, 0. 08 mg/m
3
(0. 043 ppm) for sulfuric acid aerosol, and 0. 2 mg/m (0. 106 ppm) for
ammonia. The author states that these tests verified the principle that the
effect of gas combinations is a simple summation of the isolated effects of
individual components. This report does not describe the methods for gen-
erating and measuring gas concentrations.
V. Melekhina of the USSR studied the effect of combinations of three
mineral acids [nitric (HONO2 ), hydrochloric (HC1), and sulfuric (H2SC>4 )]
352
on thresholds for odor and for eye adaptation. Tests were performed on
16 volunteers, from 17 to 36 years of age. Acid concentrations were deter-
mined spectrophotometrically. The average odor threshold concentrations
3
for individual acid aerosol exposures were 0. 85 mg/m (0. 452 ppm) for
3 3
nitric acid, 0. 40 mg/m (0. 213 ppm) for hydrochloric acid, and 0. 74 mg/m
(0. 394 ppm) for sulfuric acid. When acid aerosol concentrations were ex-
pressed as mg of hydrogen ions per m^ , the odor thresholds for the different
3
gases were nearly identical [0. 0108-0. 0148 mg/m (0. 006-0. 008 ppm)].
The response to an acid aerosol mixture was equivalent to an additive
response to the individual acids, if each acid concentration was expressed as
a fraction of the molecular concentration necessary to produce the threshold
response. When the aerosol concentration of the combined gas -was expressed
-------
10-51
3
as mg of hydrogen ions per m , the odor threshold was 0. 130--es sentially
the same concentration as for individual acid aerosols. Thus, the effect
resulting from the combination of acids in an aerosol appeared to be deter-
mined by the hydrogen ion concentration of the mixture and was nearly constant
for individual or acid aerosol mixtures. Studies of dark adaptation thresholds
produced similar results indicating that effects are determined by hydrogen
ion concentrations of individual and of combined acid aerosols. The hydrogen
ion concentration altering dark adaptation was the same as that causing an
effect on odor threshold.
• Pulmonary Function. The response of the pulmonary airways to
aerosol combinations of nitrogen dioxide and sodium chloride (NaCl) was
384
investigated by Nakamura. Airway resistance was measured both before
and after exposure of three groups of healthy subjects from 18 to 27 years
of age (Table 10-6). Nitrogen dioxide was measured by the Saltzman method.
Each group was exposed to the sodium chloride aerosol alone for 5 min at a
3
concentration of 1. 4 mg/m (0. 75 ppm), rested for 10-15 min, exposed to
3
to nitrogen dioxide alone at concentrations of 5. 6 and 11.3 mg/m (3 and 6
ppm) for 5 min, rested for 10-15 min, and then exposed to the combination
for 5 min. All three groups exhibited increased air-way resistance on exposure
to the gas alone, but not on exposure to the isolated aerosol. Nitrogen di-
3
oxide concentrations of 5. 6 and 11. 3 mg/m (3 and 6 ppm) produced 16 and
34% increases, respectively, in airway resistance. When the gas was com-
bined with an aerosol, "with an average particle size of 0. 95 ym, airway resis-
tance was double that caused by gas exposure alone. Aerosols with smaller
particles, 0. 22 P m, failed to increase the effect of nitrogen dioxide on
airway resistance.
-------
10-52
TABLE 10-6
Exposure of Humans to Sulfur Dioxide or Nitrogen Dioxide Alone
and in Combination with Sodium Chloride Aerosol~~
Exposures
Pollutant
Gas Concentration
mg/m3 (ppm)
Aerosol
Particle Size,
Median Diameter, urn (Baseline = 100%)
Average
Effect on Airway
Resistance, %
so
2
n - 10
N°2
n - 8
N02
n = 7
25-172
25-172
5.6-43.2
5.6-43.2
11.3-75.2
11.3-75.2
(9-60)
(9-60)
(3-23)
(3-23)
(6-40)
(6-40)
None
0.95
None
0.22
None
0.95
127
160
119
118
124
141
-------
10-53
Schhpkoter and Brockhaus evaluated the effects of nitrogen dioxide,
carbon monoxide, and sulfur dioxide on pulmonary deposition of inhaled
463
dust in three persons. Homogenized soot with particle sizes from 0. 07
to 1. 0 Mm was suspended in Tween-80 solution. This suspension -was sprayed
through a heating tube, combined separately with maximal allowable concen-
3
trations of nitrogen dioxide [9 mg/m (4. 79 ppm)], carbon monoxide
3 3 603a
(55 mg/m ), and sulfur dioxide (13 mg/m ), and then adminis-
tered to the three subjects by inhalation.
Differences between the concentrations of inhaled and exhaled dust were
measured to indicate total pulmonary retention. Fifty percent of the inhaled
dust was retained under control conditions after the sulfur dioxide and carbon
monoxide exposures. Maximal allowable concentrations of nitrogen dioxide
caused 76% pulmonary dust retention; the greatest difference from control
conditions occurred when dust particle sizes ranged from 0. 3 Mm to 0. 8 M m.
Epidemiological Studies
In all epidemiological studies of health effects of air pollution, multiple
pollutants have been present in the ambient atmosphere. Respiratory
illness or impaired pulmonary function may well have been a result of
exposure to combinations of pollutants that included gaseous nitrogen di-
oxide as the pollutant of major interest but have not been limited to this
agent. It is not possible, on the basis of epidemiological studies avail-
able, to ascribe a health hazard to nitrogen dioxide alone at a given con-
centration in the ambient atmosphere. However, since these studies are
critical to the understanding of human health hazards from air pollution,
the data reviewed below are important to the assessment of health effects
of atmospheres containing nitrogen dioxide.
-------
-0-54
361 610
• Pulmonary Function. Mogi e_t aj_. and Yamazaki e_t al_. examined
the possible effects on pulmonary function from prolonged exposure to nitrogen
dioxide in diesel exhaust from railroad engines in Japanese railroad tunnels
and in inspection and repair sheds. The average nitrogen dioxide concen-
trations in four inspection and repair sheds, as determined by the Saltzman
3
method, were 0. 3-1. 13 mg/m (0. 16-0. 60 ppm). Maximal nitrogen dioxide
3
concentrations varied from 0. 34 to 3. 0 mg/m (0. 18 to 1. (> ppm). The
spirometric measurements of lung function of 475 railroad -workers included
vital capacity, 1-sec forced expiratory volume, maximal m idexpiratory flow,
and peak flow rates. The effects on lung function of age, height, smoking,
family allergy history, pollutant concentrations at work location, and job
description were factors considered in the analysis.
Spirometric test results were highest in workers from "no-pollution" job
areas, as opposed to light-, medium-, and high-pollution work locations.
However, values of lung function tests were not associated with a pollution
gradient across the light-, medium-, and high-pollution work categories.
The spirometric data, presented in the English translation,, are not clearly
adjusted for age and height. There is also some question whether multivariate
analysis was used to adjust for possible age and height differences among work
locations.
486
In the Chattanooga study of schoolchildren, lung function in 7- and 8-year
olds was assessed in relation to the nitrogen dioxide concentrations in several
communities. Determinations of 0. 75-sec forced expiratory volumes (FEV )
were made weekly during 2 months of the school year among 306 children living
in a neighborhood with relatively high concentrations of nitrogen dioxide among
264 children from an intermediate exposure area, and among 225 children from
-------
10-55
a low exposure area. FEV _ ?r- values of children in high concentration
areas were significantly lower (p_<0.05) than those measured for children
in the other areas. These differences, however, were of borderline signifi-
cance and were not consistent during the 2 months of testing. The lung
functions of children in the intermediate and low concentration areas were
about the same. The association of diminished lung function with nitrogen
dioxide concentrations as suggested in the Chattanooga study is weakly
supported because results lacked consistency throughout the test period
and a clear dose-response relation was not established.
498,499
In their studies of chronic respiratory disease, Speizer and Ferris,
68
and Burgess _e_t jil_. found no differences in results from pulmonary
function tests administered to 128 Boston traffic police and 140
suburban patrol officers after exposure to nitrogen dioxide concentrations
3 3
of 0. 100 mg/m (0. 053 ppm) in the city and 0. 080 mg/m (0. 040 ppm) in
3
the suburbs; and sulfur dioxide at 0. 092 mg/m (0. 049 ppm) in the city
3
and 0. 026 mg/m in the suburbs.
90
Analysis by Cohen _et al_. of several ventilatory tests, including spiro-
metry and flow-volume curves, did not reveal any differences between non-
smoking Seventh Day Adventists living in the San Gabriel Valley of the Los
Angeles Basin and nonsmokers of the same religion living in San Diego.
Average and 90th percentile nitrogen dioxide concentrations in the San Gabriel
3
Valley were respectively 0. 096 and 0. 188 mg/m (0. 051 and 0. 100 ppm), and
3
in San Diego 0. 043 and 0. 113 mg/m (0. 023 and 0. 060 ppm).
The four epidemiologic surveys of lung function in relation to nitrogen
dioxide exposure, cited above and summarized in Table 10-7, reveal that
at the concentrations given, nitrogen dioxide in a mixed urban atmosphere
has little effect on lung function.
-------
0-56
• Acute Respiratory Disease. Petr and Schmidt found a twofold
excess in acute respiratory disease among 7- to 12-year old children
living near a large chemical complex in Czechoslovakia compared with
children of the same age living in a low-exposure community of similar
414,415
socioeconomic characteristics. Exposures, as determined by
colorimetric methods, are shown in Table 10-8. The composition of
the nitrogen oxides was not specified in the report, although the con-
centrations appear to represent nitrogen dioxide of moderately low
3
concentration [< 0. 066 mg/m (0. 035 ppm)] in the "high-exposure"
town s.
A greater number of hypertrophied tonsils and cervical lymph nodes
were found in children from both towns "with high pollution. Also, more
children with retarded physical development were recorded as living in
these areas. Several hematologic indexes, not clinically used in the United
States, were evaluated. These included the lymphocytogram (ratio of lym-
phocytes with narrow cytoplasm to lymphocytes with broad cytoplasm), index
of proliferation of monocytes (ratio of promonocytes to polymorphic mono-
cytes), and an index of monocytic differentiation (ratio of rnonocytes to
polymorphic monocytes). Children from the low-exposure area (the town
of Bohdanec) had much lower lymphocyte gram values and higher indexes
of proliferation and differentiation of monocytes. Children from Rosice,
the town •with higher nitrogen oxides but lower sulfur dioxide, deviated
most from the control area (Bohdanec). According to the authors, these
data imply that air pollutants affect the maturation and proliferation of
lymphocytes and monocytes. The clinical significance, if any, of this
effect is not known. Methemoglobin content in 50% of the tested children
from Rosice was above the normal physiologic range of 0. 86 4_ 0. 3%, but
-------
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10-58
TABLE 10-8
Concentrations of Nitrogen Oxides and Sulfur Dioxide
in Several Czechoslovakia!! Communities
Where Effects on Children were Studied'^ '
Concentrations, mg/m (ppm)
Distance from Nitrogen Oxides
Community Source, km (undefined) _2_
High-exposure
Rosice 1.5-2.0 0.02-0.07 0.01-0.12
(0.011-0.037) (0.005-0.064)
Ohrazenice 1.5-2.0 0.005-0.05 0.03-0.32
(0.003-0.027) (0.016-0.070)
Low-exposure
Bohdanec 6 None detected Traces
-------
10-59
below the upper limit of normal in control children. These results suggest
relatively high exposures of Rosice children to atmospheric nitric oxide
or to nitrites in food or water. Analysis of water samples from Rosice
showed that most of the wells contained high nitrite concentrations, ranging
from 100 to Z90 mg/1. On reexamination of the children from Rosice after
an undisclosed interval, the authors report that methemoglobin values had
returned to normal, after the concentration of atmospheric nitrous gases
in the area had been reduced. The authors contend that these reversions
support their belief that the methemoglobinemia was caused by atmospheric
nitrogen oxides.
Petr and Schmidt's results are difficult to evaluate because their
studies lack information on air pollutant concentrations over time. In
Rosice, measured exposures to sulfur dioxide and nitrogen oxides were
below US and USSR standards, and observed health effects could well have
been produced by higher past exposures or by other environmental factors.
Evidence of methemoglobinemia suggests much higher atmospheric nitric
oxide concentrations or nitrites in food or water than are indicated in the
report. The authors' conclusion that combined exposure to elevated con-
centrations of nitrogen oxides and sulfur oxides have a greater adverse
effect on children than exposure to high levels of sulfur oxides in the
presence of low nitrogen oxides, requires confirmation.
The health of preschool- and schoolchildren living 0. 5 to 1. 7 km south
and 0. 5 and 3. 0 km west of a superphosphate fertilizer manufacturing
321
plant in the USSR was compared by Lindberg with children from a con-
trol area 10 km away with similar socioeconomic status. Medical exami-
nations were conducted in the winter of 1953-1954. Pollutant concentrations
near the plant were reported only in reference to the maximal permissible
-------
0-60
3
concentrations for the USSR, and ranged from 0. 32 to 3. 4 rng/m (0. 17-1. 8
3
ppm) for nitrogen dioxide, 0. 25 to 1. 42 mg/m (0. 09-0. 50 ppm) for sulfur
3
dioxide, and 0.03 to 0.16 mg/m for sulfuric acid aerosol. Children living
near the plant experienced a 16- to 17-fold excess in acute upper respiratory
disease, a 6- to 12-fold excess in nontuberculous chest film abnormalities,
and a 1. 6- to 2. 5-fold excess in enlarged lymph glands when compared
with children in the control area. An association between length of resi-
dence in the polluted town and respiratory disease frequency was also
reported. Methods of air monitoring and health examinations are not
described in the report. At the high concentrations recorded for the
polluted town, the excess in respiratory disease may be attributed to any
one or to combinations of the measured pollutants.
Adolescents 16 to 19 years old undergoing vocational training in a
factory manufacturing fertilizers in Kemerovo, USSR, and in a chemical
173
works at Novokemerovskii, USSR, were compared by T. L. Giguz
with 85 controls of the same age and from the same schools but without
occupational exposure to chemical pollutants. A total of 140 adolescents
in training were exposed to nitrogen oxides and ammonia at concentrations
that did not exceed the maximum permissible concentrations (MPC) [MPC
3 337
for average daily nitrogen dioxide, 0.10 mg/m (0.053 pprn) in 1964],
Exposures lasted 3 hr/day for 150 days in the first year of training, and
6 hr/day for 200 days in the second year. Studies of trainees, during
their 2 years of training, indicated that exposed adolescents had an
increased incidence of acute respiratory disease and increased serum
levels of beta-lipoproteins, cholesterol, and albumin.
-------
The investigator states that the main influence on the health of
adolescents -working in the fertilizer and chemical plants was exposure
to low concentrations of nitrogen oxides and ammonia. Although his
report lacks information on the methods used, sampling frequencies, and
results of atmospheric monitoring, his data suggest an association between
acute respiratory disease and nitrogen oxide exposures at ambient con-
centrations. Polyak reported that residents living within 1 km of the
Shehelkovo, USSR, chemical works made 44% more visits to the health
clinic for respiratory, visual, nervous system, and skin disorders
423
than residents who live farther than 3 km from the chemical complex.
Atmospheric nitrogen dioxide exceeded the MPC by a factor of 5. 8-12
3
[MPC, 0. 10 mg/m (0. 053 ppm), 24-hr average]. Sulfur dioxide and
sulfuric acid exceeded the MPC by factors of 1. 5 and 4, respectively
3
[MPC, 0.15 and 0.10 mg/m , respectively]. None of the persons studied
in the two areas was employed at the chemical works; housing and living
conditions -were the same in the two communities. Residents of the
high-exposure area also complained of odors, damage to vegetation,
rusting of iron, and destruction of bee colonies as a result of emissions
from the chemical works. The reported effects -were probably attributable
to the combination of pollutant exposures, namely, nitrogen dioxide at
3 3
0.58-1. 2 mg/m (0. 31-0. 64 ppm), sulfur dioxide at 0. 225 mg/m (0.08
3
ppm), and sulfuric acid at 0.40 mg/m .
39
Belanger studied the possibility of an association between daily
admissions to the Philadelphia General Hospital in 1966 and daily concen-
trations of nitrogen dioxide, sulfur dioxide, total suspended particles (TSP),
coefficient of haze (COLO, and oxidants. Although during a few months
levels of COH, nitrogen dioxide, and sulfur dioxide were significantly and
-------
10-b2
positively associated with hospital admissions for respiratory disease, no
consistent pattern of association between individual pollutants and hospital
admissions was observed. Hospital admissions are not sensitive indicators
of air pollution effects because of strong "extraneous" factors, such as
availability of beds, daily and seasonal variations in hospital admission
rates, and differences between inpatient and outpatient treatment for the
same disorder. Hospital admission records in any US city are probably
inadequate reflections of a disease burden in a community.
411,486-488
In the Chattanooga schoolchildren studies, the effect of
community exposure to nitrogen dioxide was evaluated in relation to the
incidence of acute respiratory disease among schoolchildren, their
siblings, and parents. Different distances of three study neighborhoods
from a trinitrotoluene manufacturing plant resulted in an area, gradient
for exposure to nitrogen dioxide. The acute respiratory disease experience
of 871 families -with 4,043 individuals -was assessed at biweekly intervals in
these study areas between November 1968 and April 1969. As shown in
Table 10-9, respiratory illness rates per 100 persons -were significantly
higher for each family segment in the high nitrogen dioxide exposure
neighborhood. The relative excess in respiratory illness in the high nitro-
gen dioxide areas •was 18. 8% averaged over all family members compared
with average rates in intermediate and low nitrogen dioxide areas. In the
486,487
original reports nitrogen dioxide concentrations were determined by
the Jacobs-Hochheiser method which has subsequently been shown to be
562
unreliable. Alternate nitrogen dioxide exposure data were available
from measurements obtained at an 11-station monitoring network operated
by the US Public Health Service and the US Army in Chattanooga from
488
September 1967 through November 1968. The continuous Saltzman
-------
1 0 - L. 3
method was used for these latter measurements. Results are given in
Table 10-9.
As a follow-up to the first Chattanooga studies, the same neighborhoods
were surveyed in 1970 for lower respiratory disease frequency among first-
and second-grade elementary schoolchildren and among infants born between
411
1966 and 1968. Bronchitis rates per 100 children corresponded to the
area gradient in nitrogen dioxide among the population of schoolchildren
who had lived in the same neighborhood for 3 or more years; rates in
children from the high nitrogen dioxide area were significantly greater
(32. 2 per 100) than in children from the low area (23. 2 per 100), and rates
in the intermediate exposure area (31. 2 per 100) were nearly as high as
those of children from the high exposure area. For schoolchildren who
had lived in the neighborhood for less than 2 years, bronchitis rates did
not correspond to an area exposure gradient. Bronchitis rates among
infants born between 1966 and 1968 who had lived in the same neighborhood
for 1, 2, and 3 years did not always correspond to the area gradient for
nitrogen dioxide, although the general pattern of highest rates in the high
or intermediate exposure areas was observed.
Excess respiratory disease in the high exposure neighborhoods of
Chattanooga was attributed to nitrogen dioxide concentrations exceeding
3
annual averages of 0. 113 mg/m (0. 06 ppm). As in all population studies,
other pollutants were present in the ambient atmosphere, and these
included elevated concentrations of suspended sulfates (0. 010 to 0. 013
3 3
mg/m ) and suspended nitrates (0. 0038 to 0. 0072 mg/m ). Reports
from the US Environmental Protection Agency's Community Health and
574a
Environmental Surveillance System suggest that respiratory tract
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10-65
irritation may occur at these sulfate and nitrate concentrations. There-
fore, excess respiratory illness observed in the Chattanooga studies may
well have been a result of exposure to combinations of pollutants that
included gaseous nitrogen dioxide and particulate sulfates and nitrates,
as well as intermediary compounds such as sulfuric and nitrous acids.
Until more is known about the toxicities of these pollutants, both
separately and in combination, it is not possible to ascribe a causal
relationship to any one pollutant at a given concentration in the ambient
atmosphere.
A summary of the several epidemiological studies associating nitrogen
dioxide exposure, in the presence of other pollutants^with acute respiratory
disease is given in Table 10-10.
• Chronic Respiratory Disease. Fujita and his associates conducted
surveys of chronic bronchitis prevalence in 1962 and again in 1967, in
165
the Tokyo, Tsurumi, and Kawasaki areas of Japan. In each survey,
7, 800 post office employees from the same offices in each of the three
cities were evaluated and categorized by work location into downtown and
industrial districts, intermediate sectors, and suburban areas. Chronic
bronchitis rates were consistently higher in the 1967 survey. Increased
rates were reported for all age groups, in all smoking categories, and
for both indoor and outdoor employees. Average chronic bronchitis pre-
valence doubled in 1967. (See Table 10-11.) The authors attributed this
to increases in the concentrations of sulfur dioxide, nitric oxide, and
nitrogen dioxide in the atmosphere between 1962 and 1967, not to increases
in cigarette smoking.
-------
Although scant information on ambient air concentrations is given, the
authors state that in all districts there has been an increase of such harm-
ful gases as sulfur dioxide, nitric oxide, and nitrogen dioxide, in contrast
to a pronounced decrease in the amount of suspended dust particles.
Pollutant concentrations reported by Fujita were based on measurements
taken •with automatic instruments by the Environmental Pollution Section of
the Tokyo metropolitan government during 1962-1966 in Tokyo, Tsurumi, and
Kawasaki (See Table 10-12).
The Air Quality Bureau of the Japanese Environmental Agency published
air quality data from one urban and two suburban sectors of Tokyo obtained
7
from 1964 through 1970 during annual surveys conducted by the National
Institute of Hygienic Sciences. (See Table 10-13. )
The air quality data in Tables 10-12 and 10-13 are insufficient documenta-
tion of increases in sulfur dioxide or nitrogen dioxide between 1962 and 1967.
Although the authors carefully point out the age- and smoking-specific changes
in bronchitis rates between 1962 and 1967, they do not substantiate the attribu-
tion of such changes to increasing sulfur oxide or nitrogen oxide pollution.
The report of the Expert Committee on Air Quality Criteria for Oxides
82
of Nitrogen and Photochemical Oxidants (Japan), includes a survey of
82
bronchitis prevalence in housewives living in six Japanese communities.
Four hundred women, nearly all of whom •were nonsmokers, 30-69 years old,
were selected for study. The prevalence of chronic respiratory disease was
determined through administration of the British Medical Research Council's
standardized questionnaire. Table 10-14 contains the data for pollutant con-
centrations and symptom prevalence in each community during the winter of
1970-1971. Although simple correlation coefficients between persistent cough,
-------
10-67
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10-68
TABLE 10-11
Bronchitis Prevalence Rates per 100 Employees
Work District
Year
1962
1967
Downtown and
Industrial
5.0
8.4
Intermediate
3.7
8.0
Suburban
3.7
8.1
-------
10-69
Table 10-12
Pollutant Concentrations during 1962 and 1966
by Work District,
Downtown and
Industrial Intermediate Suburban
Pollutant 1962 1966 1962 1966 1962 1966
Nitrogen dioxide — 0.036
Nitric oxide — 0.100
Sulfur Dioxide 0.147 0.214 0.161 0.188 0.054 0.054
-------
-0-70
phlegm., and air pollutants were highest for nitrogen dioxide, nitric oxide,
and total nitrogen oxides, the concentrations of these pollutants were usually
3
below the US nitrogen dioxide standard of 0. 10 mg/m (0. 05 ppm) in five of
six communities. However, less significant correlation coefficients were
obtained between respiratory symptoms and suspended particulates, even
though the concentrations of particulates greatly exceeded the US standard
3
of 0. 075 mg/m annual average. In the two communities with highest
bronchitis rates, Ohmuta and Higashi-Osaka, pollutant concentrations
were 4 to 5 times higher than the US air quality standard for suspended
3
particulates and 42-79% above the standard of 0. 080 mg/m (0. 03 ppm)
for sulfur dioxide; in only one of the communities (Higashi-Osaka) was
3
the US standard for nitrogen dioxide [0. 100 mg/m (0. 05 ppm)] exceeded.
From these results, excess bronchitis seems to be attributable largely
to high concentrations of sulfur oxides and suspended particulates although
nitrogen oxides may have contributed to the observed excess bronchitis
prevalence. No single pollutant can explain the observed community
gradient in respiratory symptom prevalence.
In addition to the pulmonary function tests described earlier in this
498,499 68
chapter, Speizer and Ferris and Burgess _e_t a_l. compared the
prevalence of chronic respiratory disease among 1Z8 traffic officers
working in central Boston with 140 suburban patrol car officers. The
exposure of each group to nitrogen dioxide (Saltzman method) and sulfur
dioxide (West-Gaeke method) was determined at several work locations
for the central city officers and in the patrol cars of suburban officers. A
slight but not statistically significant excess in chronic respiratory disease
was found in smokers as compared with nonsmokers and exsmokers from
the central city group (Table 10-15).
-------
10-71
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10-73
Chronic respiratory disease among parents of high school students
83
was studied in three exposure areas in Chattanooga. Sample sizes, ni-
trogen dioxide exposures, and chronic bronchitis rates are given in Table
10-16. Higher nitrogen dioxide concentrations occurred in the high nitro-
gen dioxide area from 1966 to 1969 than during the course of the study
(1970), because of decreased trinitrotoluene production at the point source.
As shown in Table 10-16, chronic bronchitis prevalence rates were not
associated with the area gradient in nitrogen dioxide exposure. Past ex-
posures in the high nitrogen dioxide area had exceeded the national primary
standard, and exposures at the time of testing -were below the standard.
In their study of nonsmoking, 45- to 64-year-old Seventh Day Adventists
90
(see earlier section on Pulmonary Function), Cohen and his associates
compared chronic respiratory disease prevalence and lung function among
136 residents of the San Gabriel Valley in the Los Angeles Basin with that
among Z07 residents of San Diego. Exposures to photochemical oxidants
and nitrogen dioxide were determined by the potassium iodide and Saltzman
methods, respectively. (See Table 10-17. ) Area differences in average
oxidant concentrations were small, whereas nitrogen dioxide averages
differed by as much as a factor of 2. Area differences in peak oxidant
and nitrogen dioxide exposures were similar for the two pollutants. The
3
national primary standard for nitrogen dioxide, 0. 1 mg/m (0. 053 ppm),
was not exceeded, whereas the national primary standard for oxidants,
3
0. 16 mg/m (0. 08 ppm), a maximal 1-hr concentration not to be exceeded
more than once per year, was exceeded during 10% of the testing in San
Diego, and more frequently in Los Angeles. The two study groups showed
no difference in the prevalence of chronic respiratory disease; the rates
of this disease were less than 4% in each age-sex-exposure combination.
-------
10-74
TABLE 10-15
Exposures and Respiratory Disease in Traffic Officers
a
Concentration Prevalence of Chronic
mg/m (ppm) Respiratory Disease, %
N°2 SQ2 NS ^ HS
Central city 0.103 0.092 17 50 60 28
traffice officers (0.055) (0.035)
(n=128)
Suburban patrol 0.075 0.026 15 43 55 31
car officers (0.040) (0.010)
(n=140)
National primary 0.103 0.080
air quality (0.055) (0.030)
standard
a.
Average of 2 sampling days per season of the year for 1 year at each
of 16 work stations.
NS, nonsmoker; LS, light smoker (10-24 cigarettes/day) ; HS, heavy smoker
(25 + cigarettes/day) ; ExS, Ex-smoker.
-------
10-75
TABLE 10-16
Chronic Respiratory Disease in Chattanooga
Parents of High School Students
NC>2 Exposure
Group
N00 Concentration
3
mg/mj (ppm)
June-Dec 1970 Dec 1967-Nov 1968
Prevalence of Chronic
Bronchitis, %a
Early
Disease
Advanced
Disease
Low
(n=234)
Intermediate
(n=755)
High
(n=652)
0.058 (0.031) 0.053 (0.028)
0.071 (0.038) 0.117 (0.062)
0.092 (0.049) 0.150-0.282
(0.080-0.150)
30
33
25
11
20
13
Rates adjusted for smoking, sex, race, and age.
-------
10-76
The study suggested that the twofold difference in nitrogen dioxide
exposures that remained below the national primary standard, in the
presence of oxidant exposures that exceeded the national standard in
both groups, was unassociated with effects on chronic respiratory disease
or lung function (previously cited) in a nonsmoking population.
• Excess Mortality. Mortality rates for various cancer categories,
cardiovascular disease, and respiratory disease in 38 US Standard
Metropolitan Statistical Areas (SMSA) during 1959-1961 and 1961-1964
were analyzed by Hickey _e_t al_. in relation to air pollutant measurements
230,231
taken at the National Air Surveillance Network station in eeich SMSA.
Nitrogen dioxide, sulfur dioxide, suspended sulfates, total particles,
calcium, chromium, copper, iron, lead, manganese, nickel, tin, titanium,
vanadium, zinc, and water hardness were included in the analysis.
Mortality rates were analyzed with and -without regard to age, sex,
and race differences. The author applied a modification of multiple
regression analysis in which combinations of variables were selected
from the complete set of independent variables with the objective of
2
maximizing the square of the multiple correlation coefficient (R ) for the
selected subset. Only variables significantly predictive of each mortality
rate were selected.
As shown in Table 10-18, nitrogen dioxide and sulfur dioxide were thus
repeatedly positively associated with age-, race-, and sex-adjusted or
unadjusted mortality rates for various cancers and for arteriosclerotic
heart disease. Other pollutants were variably and often negatively asso-
ciated-with these mortality categories. Hickey et_ aJ. speculate that, if
cancer and heart disease are cumulative somatic genetic diseases, then
-------
10-77
TABLE 10-17
Concentrations of Photochemical Oxidants and Nitrogen
Dioxide, 1963-1967, Used as Exposure Data in a
Study of Nonsmoking Seventh Day Adventists.3.
San Gabriel Valley San Diego
Pollutants „
o 3
(hourly average) mg/m (ppm) mg/m (ppm)
Oxidants
Arithmetic mean 0.092 (0.046) 0.076 (0.038)
90th percentile 0.260 (0.130) 0.160 (0.080)
Nitrogen dioxide
Arithmetic mean 0.096 (0.051) 0.043 (0.023)
90th percentile 0.188 (0.100) 0.113 (0.060)
a 151
From Cohen et al., 1972.
-------
10-78
the genetic effects of exposure to nitrogen dioxide and sulfur dioxide
should be demonstrable in exposed populations as excess ca.ncer and
heart disease. They present several arguments to support the possibility
that atmospheric nitrogen oxides and sulfur oxides could be mutagens.
The weakness of their analysis is not in the methodology or biologic
plausibility of the associations, though the latter is tenuous, but in the
quality of the exposure data. Neighborhood differences in pollutant
exposures within the same SMSA are often larger than pollutant differences
between SMSA's. Consequently, the one to three stations of the National
Air Surveillance Network within an SMSA are poor estimators of the
pollutant burden to the population or of differences in this burden between
SMSA's. Furthermore, population mobility in the United States is so great
that the place of death is often different from the longest residence location.
If cximulative exposures were responsible for a postulated "cumulative soma-
tic genetic disease," these exposures should be estimated from data from
the air monitoring station -where the patient lived the longest.
The consistency of Hickey1 s findings concerning nitrogen dioxide and
sulfur dioxide prevents complete dismissal of his conclusions, which are
stated more in the form of hypotheses for later verification. The biologic
evidence marshaled in support of his statistical findings is not unreason-
able, and the statistical analysis appears to be sound. In 27 of the 38 SMSA's
3
studied, nitrogen dioxide concentrations ranged from 0. 080 and 0. 116 mg/m
(0. 043 and 0. 062 ppm). These concentrations bracket the existing US
3
standard of 0.100 mg/m (0.053 ppm) and suggest the need for analysis of
mortality and pollutant concentrations in later years. The hypothesis of
Hickey £t al. that nitrogen oxides and sulfur oxides may be exerting genetic
effects in the population should be explored in experimental studies.
-------
10-79
TABLE 10-18
Multiple Repression Analysis of Pollutant
Cause of Death
Breast Cancer
Breast Cancer
Breast Cancer
Lung Cancer
Lung Cancer
Lung Cancer
Total Cancer
Exposures and
Cancer Mortality
Year of Age-Sex-Race Variables Associated
Death Adjustment with Death Rate
1959-1961
1959-1961
1961-1964
1959-1961
1959-1961
1961-1964
1961-1964
No
Yes
No
No
Yes
No
No
N02,
N02,
N02,
N02,
N02,
N02,
N02,
S02, Cd, Cu
Ni, Ti
S02, Cd
SO^, Cu, Ti, As
Mn, V, Ti, As
S02, Cu
S02, Cd
0
0
0
0
0
0
0
R2
.58
.56
.55
.73
.61
.51
.55
Arterio-
sclerotic
heart disease
Arterio-
sclerotic
heart disease
1959-1961
1959-1961
No
Yes
water
N02, S02, Cd, hardness 0.47
S0,
, Cu, Zn
0.56
-------
10-80
Lebowitz studied variations in daily mortality in relation to daily
air pollution and weather variables in New York, Philadelphia, and Los
311
Angeles during 1962-1965. Nitrogen oxide measurements -were avail-
able only for the New York and Los Angeles analyses. Mortality data
•were statistically transformed to control for extraneous variables including
day of •week and seasonal effects. A relation between air pollution and
weather variables and daily mortality was found in each of the cities.
In New York, multiple regression analysis revealed a significant
but negative influence of daily nitrogen oxide concentration on mortality
in winter. Particulate matter (measured as coefficient of haze), low
temperatures, and wind speed were also significant determinants of
2
daily mortality. The R for the combined variable analysis was 0. 31.
Nitrogen oxide concentrations were not statistically associated with mor-
tality in summer.
hi Los Angeles, winter mortality of persons 45-64 years old, 65
and older, and all ages combined was significantly and positively related
to daily nitrogen oxide concentration during 1962-1965. Sulfur dioxide,
temperature, and wind speed were also significant determinants of
2
daily winter mortality variations. The R for this combination of vari-
ables was 0. 11. In summer, nitrogen oxides "were not a significant vari-
able, whereas sulfur dioxide, carbon monoxide, relative humidity, and
wind speed were statistical determinants of daily mortality. These
results fail to provide convincing evidence of a relation between nitrogen
oxides and daily mortality. Findings in New York -were opposite to
those in Los Angeles, and the associations were not consistent for all
seasons.
-------
10-81
• Acute Toxicity. Acute exposure to high concentrations of nitrogen
5, 207, 341,402
dioxide is an uncommon occupational hazard of -welders, silo
101,185,326,431 37,379 338,357 547
fillers, miners, chemists, firemen,
580
and workers employed in the manufacture of nitric acid. These expo-
sures are far in excess of ambient concentrations and are noteworthy
only in proving that high concentrations of nitrogen dioxide are extremely
toxic. Five distinct clinical responses to several such high concentrations,
based on observations of occupational exposures, are summarized in
Table 10-19. Acute respiratory and nasal irritation are first noted at ni-
3
trogen dioxide concentrations of 28-47 mg/m (15-25 ppm). Reversible
3
pneumonia and bronchiolitis result from exposures to 47-141 mg/m
3
(25-75 ppm), whereas concentrations of 282-564 mg/m (150-300 ppm)
have caused fatal bronchiolitis and bronchopneumonia. In contrast, workers
in Italy employed in nitric acid manufacturing and exposed to an average
3
of 56. 4-65. 8 mg/m (30-35 ppm) for an unspecified number of years
580
exhibited no signs or symptoms of injury.
Accidental exposure of a large group of hospital workers occurred in
147
1929 as a result of a fire in the X-ray room of the Cleveland Clinic.
Combustion of nitrocellulose films released high concentrations of nitric
oxide, carbon monoxide, and hydrocyanic acid. Ninety-seven persons
died within 2 hr after exposure to carbon monoxide and hydrocyanic
acid.
To investigate possible delayed effects of acute nitrogen dioxide expo-
sure, 87% of the individuals present at the fire were studied 36 years
192
later by Gregory and colleagues. Observed survival rates among those
exposed were no different than those of controls who were present at the
-------
10-82
TABLE 10-19
Effects of Acute Exposure to High
Nitrogen Dioxide Concentrations
NC>2 Concentration, Time between Exposure
mg/m-' (ppm) Clinical Effect and Termination of Effect
940 Acute pulmonary edema— Within 48 hr
(500) fatal
564 Bronchopneumonia—fatal 2-10 days
(300)
282 Bronchiolitis fibrosa
(150) obliterans—fatal 3-5 weeks
94 Bronchiolitis, focal
(50) pneumonitis—recovery 6-8 weeks
47 Bronchitis, broncho-
(25) pneumonia—recovery 6-8 weeks
-------
10-83
fire but not exposed to its fumes. Acute exposure to high concentrations
of nitrogen dioxide therefore appear to have no effect on long-term sur-
vival.
SUMMARY OF HUMAN STUDIES
Acute Effects of Nitrogen Dioxide
Studies of human volunteers have provided the most precise documenta-
tion of the acute effect of nitrogen dioxide exposure (Table 10-20). The
earliest response occurs in the sense organs. The odor of nitrogen dioxide
3
is perceived at concentrations of 0. 23 mg/m (0. 12 ppm), while changes
3
in dark adaptation occur at 0. 14 to 0. 50 mg/m (0. 075 to 0. 26 ppm).
These sensory perceptors are categorized as physiologic responses; there
is no evidence for sequelae in terms of human pathology, and the responses
in all cases were immediately reversible.
The studies of airway resistance by Von Nieding, Rokaw, and Suzaki
indicated acute effects after 15 to 45 min exposures to nitrogen dioxide
3
concentrations of 2. 8 to 3. 8 mg/m (1. 5 to 2. 0 ppm). Von Nieding and
Rokaw were unable to detect any effect on airway resistance from exposures
3
less than 2. 8 mg/m (1. 5 ppm). Although changes in airway resistance -were
reversible, these manifestations of nitrogen dioxide exposure are potentially
adverse, particularly for hypersensitive asthmatics or subjects with advanced
chronic obstructive pulmonary disease. Measurements of potentially more
sensitive indicators of pulmonary dysfunction, such as small airway resist-
ance and perfusion indices, have not been reported. In one study, sodium
chloride aerosol combined with nitrogen dioxide gas augmented the effect
on airway resistance when compared with exposure to nitrogen dioxide
alone.
-------
10-84
TABLE 10-20
Summary of Human Responses to Short-Term
Effect
Odor threshold
Threshold for
dark adaptation
Increased airway
resistance
Nitrogen Dioxide Exposures Alone
NO Concentration
3
mg/m
0.23
0.23
0.14
L 0.50
1.3-3.8
3.0-3.8
2.8
3.8
5.6
7.5-9.4
9.4
11.3-75.2
13.2-31.8
(ppm)
(0.12)
(0.12)
(0.075)
(0.26)
(0.7-2.0)
(1.6-2.0)
(1.5)
(2.0)
(3.0)
(4.0-5.0)
(5.0)
(6.0-40.0)
(7.0-17.0)
Time to Effect
Immediate
Immediate
Not reported
Not reported
20 mina
15 min.
45 min
45 min ,
45 min
c>
40 min
15 min
5 min,.
10 min-'
Decreased pulmonary
diffusing capacity
Increased alveolar-
arterial pp.,
difference
No change in sputum
histamine concen-
tration
7.5-9.4 (4.0-5.0) 15 min
9.4
(5.0)
25 min'
9
Reference
224
476
476
51
523
583-585
447
447
447
1
520
384
613-616
583
0.9-6.6 (0.5-3.0) 45 min
584
532
a
Exposure lasted 10 min. Effect on flow resistance was observed 10 min
after termination of exposure.
Effect was produced at this concentration when normal subjects and those
with chronic respiratory disease exercised during exposure.
j
'Effect occurred at rest in subjects with chronic respiratory disease.
I
Effect occurred at rest in normal subjects.
eExposure lasted 10 min. Maximal effect on flow resistance was observed
30 min later.
•'Also failed to find increased flow resistance over the range of nitrogen
dioxide exposures from 5.1 to 30.1 mg/m^ (2.7-16.0 ppm) .
^Effect occurred 10 min after termination of 15-min exposure.
b
d
-------
10-85
An accidental exposure of persons to very high nitrogen dioxide concen-
3
trations established the fatality level from acute exposures at 282 n;g/m
(150 ppm) and above. These high concentrations caused pulmonary edema or
bronchiolitis fibrosa obliterans which resulted in death. Concentrations
3
bet-ween 47 and 140 mg/m (25 to 75 ppm) produced reversible pneumonia
and bronchiolitis. Permanent sequelae, resulting in shortened lifespan,
were not found in the only reported follow-up study of survivors of acute
high level nitrogen dioxide exposure.
The literature contains little data on the health hazards of repeated
nitrogen dioxide exposures of 2- to 4-hr duration, a common occurrence
in the ambient environment. The few studies on exposures of hurt, in
volunteers indicate an increase in airway resistance at nitrogen dioxide
3
concentrations of 2. 8 to 3. 8 mg/m (1. 5 to 2. 0 ppm) for 15 to 45 min.
In order to make the results of these studies applicable to air quality
standards, these studies of effects should be conducted in atmospheres
that contain combinations of pollutants common to urban environments.
Nitrogen dioxide is often present with sulfur dioxide, small amounts of
ozone, and respirable sulfate and nitrate aerosols, at both low and high
humidities. The interaction or synergy between ozone and sulfur dioxide
on human airway resistance has been well demonstrated. Similar inter-
actions between nitrogen dioxide and other atmospheric pollutants must
also be investigated. Until this is done, we cannot be confident that
3
short-term ambient exposures of less than 2. 8 mg/m (1. 5 ppm)
nitrogen dioxide have no effect on airway resistance.
Clearly, other potential manifestations of short-term exposures of
humans to peak ambient nitrogen dioxide concentrations must be investi-
gated. As a first step, we need more extensive studies of impaired
-------
pulmonary defense mechanisms in animals caused by intermittent short-
term exposures to nitrogen dioxide alone and in combination with other
atmospheric pollutants. The importance of this effect of nitrogen dioxide
justifies the extensive research called for in these considerations.
Table 10-20 summarizes the sensory and pulmonary effects of human
exposure to nitrogen dioxide.
Effect of Prolonged Exposure on Lung Function
Two epidemiological studies suggest that the combination of nitrogen
3
dioxide at concentrations of 0. 15 to 0. 3 mg/m (0. 08 to 0. 16 ppm) with
other pollutants causes changes in ventilatory function. Two other
studies in -which lower levels of nitrogen dioxide were studied did not
reveal these effects. Because of the disparity in populations and in
pollutant conditions, conclusions cannot be reached regarding the effect, if
any, of chronic exposure to nitrogen dioxide on ventilatory function.
Effects on Acute Respiratory Disease in Populations
Some epidemiological data support the idea that excess acute respira-
tory disease may occur in healthy populations following exposure to atmos-
pheres containing nitrogen dioxide. Four studies have been reviewed in
the search for an association between exposure to ambient concentrations
3
of nitrogen dioxide from 0. 10 to 0. 58 mg/m (0. 053 to 0. 309 ppm) and
small excesses in respiratory illnesses. However, the variable pol-
lutant exposures and conditions of study make it difficult to quantify the
relationship of nitrogen dioxide by itself to the reported increases in
respiratory disease. In each study air contaminants likely to enhance
susceptibility to respiratory infection (sulfur dioxide, sulfuric acid, sulfates,
nitrates, etc. ) were also present.
-------
LO-87
Effects on Chronic Respiratory Disease in Populations
Evidence that nitrogen dioxide induces excess chronic respiratory
disease is not convincing. Reports of excess chronic respiratory disease
associated •with low concentrations of ambient nitrogen dioxide [less than
3
0. 10 mg/m (0. 053 ppm)] do not provide convincing evidence that other
pollutants -which were measured at relatively high concentrations -were not
the probable cause of the excess disease. In the presence of low concen-
trations of sulfur dioxide and particulates, three investigators failed to
detect excess chronic respiratory disease in areas where nitrogen
3
dioxide exposures were at or below 0.10 mg/m (0.053 ppm).
Acute Toxicity to Humans
High level nitrogen dioxide exposures of small occupational groups
3
establishes the fatal level of nitrogen dioxide as 282 mg/m (150 ppm) and
above. These deaths resulted from pulmonary edema or bronchiolitis
3
fibrosa obliterans. Concentrations between 47 and 140 mg/m (25 and 75
ppm) cause reversible pneumonia and bronchiolitis. Permanent sequelae
manifested as shortening of survival time were not found in the only long-
term follow-up of survivors of acute high-level exposure.
-------
CHAPTER 11
SUMMARY, CONCLUSIONS, AND
RECOMMENDATIONS FOR FUTURE RESEARCH
PROPERTIES OF THE NITROGEN OXIDES AND THEIR PHYSICIAL EFFECTS ON ATMOSPHERIC
LIGHT TRANSMISSION
Among the various oxides of nitrogen present in polluted atmospheres,
nitric oxide (NO) and nitrogen dioxide (NO ), designated by the composite
formula NO , are the most important in relation to chemical and photochemical
X
changes. A major source of these oxides is fuel combustion. Nitric oxide
is the dominant nitrogen oxide formed in combustion. A fraction of the
nitric oxide is converted to nitrogen dioxide by reaction with oxygen
during the exhaust dilution process; however, the major pathway leading
to formation of nitrogen dioxide from nitric oxide is the photochemical
interaction between NO , hydrocarbons, and various other compounds and
X
intermediate free radicals that are generated in the sunlight-irradiated
polluted atmosphere.
The degree to which nitrogen dioxide causes reduction of visibility
and coloration of the horizon sky is directly dependent on the concentra-
tion of the pollutant, the viewing distance, and the accompanying aerosol
concentration. The presence of photochemical aerosol or other particulate
matter diminishes the coloration effect of nitrogen dioxide and further
decreases visibility.
SOURCES AND CONTROL OF ATMOSPHERIC NITROGEN OXIDES
Annual global emissions of nitrogen oxides from manmade sources are
substantially less than from natural sources; however, manmade sources
-------
11-2
play a very significant role in atmospheric pollution on a local level.
The principal manmade source of nitrogen oxides is combustion. In this
category, motor vehicle and fossil-fueled electric power generating
stations are the most significant. Although industrial process losses
contribute only a small amount to the total manmade nitrogen oxide emissions,
they can be important sources in a local, industrialized area.
Existing methods for obtaining emission inventories have limited use
in evaluating the importance of specific categories of local sources of
nitrogen oxides since these methods generally involve inventories of large
geographic areas over extended periods. Furthermore, differences between
actual nitrogen oxide emissions from specific sources and tabulated emission
factors can result in significant inaccuracies in emission inventories.
Source inventories should discriminate between nitric oxide and nitrogen
dioxide emissions since each of these oxides interacts with the atmospheric
photolytic cycle in a different way.
The two principal sources of nitrogen oxides produced in combustion
are the oxidation of atmospheric (molecular) nitrogen and oxidation of
nitrogen compounds in the fuel (fuel nitrogen). Nitric oxide is produced
from atmospheric nitrogen in the high temperature regions in the combustion
chamber, whereas fuel nitrogen can be converted to nitric oxide at lower temper-
atures and can be a major source of nitrogen oxide emissions in some com-
bustion devices.
There are two basic approaches to control NO emissions from combustion
X
sources: modification of the combustion process and treatment of exhaust
gases. Present understanding of the principles of nitric oxide formation
in combustion is sufficient to permit development of techniques for reducing
NO emissions from combustion sources. However, implementation of these
-------
11-3
techniques may be limited by excessive cost, losses in combustion efficiency,
and by a number of operational problems in the combustion chamber.
Recommendations;
1. Improved inventories of nitrogen oxide emissions are needed to
estimate the relative importance of various sources in specific
geographic locations and to assist in the evaluation of control
strategies.
2. Potential techniques for reducing nitrogen oxide emissions from
combustion sources should be evaluated. The implementation costs
and the extent to which atmospheric levels of nitrogen oxides are
reduced should be estimated. Techniques requiring evaluation
include: two-stage combustion, catalytic combustion, combustion
of emulsified fuels and fuel blends, combustion of prevaporized
liquid fuels, combustion of gasified coal, and combustion of
synthesized fuels.
3. Fuel processing methods that can reduce the nitrogen content of
the fuel before combustion should be considered as a method to
control nitric oxide emissions from sources burning nitrogen-
containing fuels.
ANALYTICAL METHODOLOGY FOR THE DETERMINATION OF NITROGEN OXIDES IN AIR
Older chemical methods used to measure nitrogen oxides were based on
the transfer of the nitrogen from the nitrogen dioxide molecule directly
into a measurable compound. This method and its variations have generated
most of the data over the last 40 years and are still in use. The manual
Griess-Saltzman Method (not to be confused with the Griess-Saltzman reagent
-------
11-4
or principle) for calibrating continuous analyzers is based on the direct
reaction of nitrogen dioxide with a single reagent mixture to form a colored
azo dye. The Griess-Saltzman reagent and several modifications thereof are
the basis for many continuous analyzers. Nitric oxide cannot be measured
directly with this technique; it must be oxidized to nitrogen dioxide with
an appropriate catalyst.
Chemiluminescence techniques are based on optical measurements
of light emitted by the reaction of two gases. In one method, nitric oxide
is measured through its reaction with ozone within the chemiluninescent
instrument. To measure nitrogen dioxide, a prior reduction step to nitric
oxide over a heated catalyst is required. Another method involves the
photofragmentation of nitrogen dioxide by ultraviolet light and chemilu-
minescent measurement of the ozone produced. This last approach avoids
contact with a catalyst but requires nitric oxide as a reagent. For
calibration of the chemiluminescent methods, precisely known standards
containing appropriate concentrations of nitric oxide or nitrogen dioxide
are used. These concentrations can be obtained by dynamic dilution of
more concentrated gas mixtures or with permeation tubes having precisely
known leaking rates. By contrast, most wet chemical methods use an alkaline
nitrite or a nitrate as primary standards.
Alkaline methodology for sampling and measuring nitrogen dioxide has
been criticized. The departure from original sampling conditions and
absorbing reagent formulations, and the assumption of constant performance
generated equivocal results. A variety of new methods (collectively called
Jacobs-Hochheiser methods) were developed but not evaluated.
In these methods, nitrite in absorbing solutions is analyzed by applying
-------
11-5
the Griess principle of azo dye formation. More recent methods without
these shortcomings have been evaluated, including the 24-hr arsenite method.
Most methods in which nitrogen oxides are transformed into nitrate
are suited for higher than atmospheric levels of nitrogen oxides. These
higher levels can also be analyzed with the same methods used in atmo-
spheric analysis after proper aliquoting or dilution.
Recommendations:
1. The manual Griess-Saltzman method is recommended as the primary
reference method. All details of this method must be strictly
followed. The recommended primary standard for calibration is
sodium nitrite.
2. The continuous Saltzman method and the differential chemiluminescence
method are suggested as secondary reference methods. Chemilumin-
escence requires careful determination of the efficiency of the
nitrogen dioxide converter. Calibration of these methods with
permeation tubes instead of secondary standards changes their
category to primary reference methods.
3. Nitric oxide used as a secondary standard should be calibrated
using one of the recommended primary reference methods rather
than the gas-phase titration procedure.
4. Devices for continuous monitoring of particulate nitrates and
gas-phase nitric acids should be developed.
ATMOSPHERIC LEVELS OF NITROGEN OXIDES
While there has been an appreciable loss of data due to questionable
accuracy of certain methods used to measure nitric oxide and NO , sufficient
x
accredited data exist to give strong support to the following conclusions:
-------
11-6
1. The current national primary air quality standard for nitrogen
dioxide is 0.1 mg/m (0.05 ppm) annual average. This concentra-
tion is consistently exceeded in the Los Angeles area and, if
present trends continue, will soon be exceeded in the Chicago
and Philadelphia areas. Maximum 1-hr concentrations of several
milligrams per cubic meter may be recorded without exceeding the
existing primary air quality standard. In fact, 1-hr maximum
3
concentrations of 5 and 9 mg/m have been recorded in Chicago
and Philadelphia, respectively.
2. Highest average concentrations of oxides of nitrogen and nitrates
are found in heavily populated, industrialized urban areas.
These concentrations show a generally increasing trend for the
10-year period between 1962 and 1971.
3. Physicochemical models of pollutant distribution, based on ana-
lytical data, are of value in predicting maximum concentration
values of NO that will be reached under a given set of conditions
X
in different locations. Data from these models can be used to
give warning of impending "alert" conditions.
4. Relationships between nitrogen dioxide and nitrate concentrations,
and the types of nitrate formed in different areas, are under
study because of the potential health significance of atmospheric
nitrates.
5. Because of current concern with pollution of the stratosphere,
nitric oxide and nitrogen dioxide concentrations have recently
been determined in that region of the atmosphere. Nitric oxide
in the stratosphere is thought to result from oxidation of nitrous
-------
11-7
oxide (N 0). The concentration of nitrogen dioxide in the strato-
2
sphere has been estimated at 0.20 ppm. Changes in these levels
are anticipated as high altitude air traffic becomes more common.
Recommendations;
1. Augment existing data on the atmospheric concentration of nitric
oxide, nitrogen dioxide, and nitrates (NO ~). Studies should be
made on the chemical interaction of these species when they are
contained on discrete colloidal atmospheric particles (mineral,
carbon, organic liquid, and aqueous) incorporating the effects
of particle size and composition.
2. Physicochemical models used in predicting maximum concentration
values of NO should be expanded to include information obtained
x
from continuous monitoring in regions of high NO concentrations.
X
These data should include criteria for "alert" conditions in the
warning system.
3. Stratospheric data on nitric oxide, nitrogen dioxide, and nitrous
oxide concentrations are incomplete. Chemical interactions
occurring in this region are not precisely known and additional
study is required. Effects of stratospheric pollutants at the
surface of the earth require further investigation.
4. The relationship between nitrogen dioxide and nitrate concentra-
tions under various meteorologic and geographic conditions requires
further study. The relative health hazard of nitrogen dioxide and
nitrates, and the synergistic effects between these pollutants and
sulfates in the atmosphere, need further evaluation.
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11-8
CHEMICAL INTERACTIONS OF NITROGEN OXIDES IN THE ATMOSPHERE
Solar radiation induces a number of reactions in the atmosphere
between gaseous organic molecules and nitric oxide thereby producing a
variety of so-called secondary pollutants. These secondary pollutants
are present in extremely small concentrations and many are very transient.
Because of these two factors, it is very difficult to identify or measure
these pollutants by conventional analytical techniques.
Starting with simple hydrocarbons and nitric oxide, attempts have
been made to identify intermediate and end products of the photochemical
reactions by relating laboratory studies involving chemical and kinetic
modeling to atmospheric observation. Because certain key information is
still lacking, it is not yet possible to make quantitative predictions.
Enough information is available, however, to indicate that the main
features of the model are correct, placing considerable reliability on
the qualitative predictions.
Based on calculated concentrations and theoretical first half-lives,
it is possible to predict photochemical intermediates which might persist
into the respiratory system. The health significance of these must still
be determined.
It does not appear possible to set numerical standards on transient
secondary pollutants; but, based on available information, some relation-
ships can be used to develop air quality control planning.
The extent to which nitric and nitrous acids and nitrate and nitrite
(N0~~) salts result from gaseous nitrogen dioxide and nitric oxide absorption
into aerosol droplets is not known. It is known in general that the rate
of absorption of acidic gases is a function of the partial pressure of the
-------
11-9
gases, the rate gases diffuse into solution, the pH of the solution, and
the interfacial area. However, these factors must all be evaluated re-
garding nitrate salt formation in smog and related potential health problems.
Recommendations:
1. The health significance of such photochemical intermediates as the
hydroxyl radical (OH), the hydroperoxyl radical (HO ), and sym-
metrical nitrogen trioxide (NO ) , which can exist in typical
polluted atmospheres long enough to be transported to the respi-
ratory system, should be determined. These intermediates cannot
be studied independently. Rather, they should be produced in a
reaction vessel as an array of intermediates. When conclusive
information on these health aspects becomes available, numerical
standards on these intermediates should be set.
2. Kinetic studies of possible photochemical reactions that can be
induced by sunlight within the tropospheric region should be
continued. The inhibiting effect of very high nitric oxide con-
centrations on product rates and eye irritation should also be
studied further.
3. More detailed information concerning turbulent diffusion, local emis-
sions, and solar zenith angle changes as well as chemical reactions
must be included in the development of useful simulations of actual
urban atmospheric reactions.
4. The methods of nitrate salt formation in the atmosphere in both
homogeneous and heterogeneous reactions, primarily in gas—gaseous
liquid phase reactions, should be further investigated. Meteorologic
and geographic aspects should be considered.
-------
11-10
EFFECTS OF NITROGEN OXIDES ON NATURAL ECOSYSTEMS
No specific information is available on the effects of nitrogen oxides
on animals in ecosystems. Research on crop plants indicates that NO should
X
have effects similar to other air pollutants on some plant communities.
Consequently, we anticipate: differential species sensitivity to NO ;
X
complications due to synergistic or antagonistic interactions between NO ,
X
other air pollutants, and natural environmental stresses; and secondary
ecosystem responses due to changing symbiotic and competitive interactions.
Nitric oxide, nitrogen dioxide, and nitrous oxide affect the growth
or survival of individual microorganisms when tested at high concentrations
in defined media, but the effect on microorganisms or microbial processes
of nitrogen oxides at the levels naturally found in the atmosphere are
unknown. The effects of ambient NO concentrations on populations or
activities in both natural habitats and in vitro have not been studied.
Although little suppression of heterotrophs is caused from the presence
of these gases, a meaningful conclusion is not possible in the absence of
direct experimentation.
The algae that are extremely important to primary production in fresh
and marine waters, the algae and lichens that are significant in the
weathering of rocks and in certain soil processes, and the activity of
microorganisms colonizing leaves and causing plant disease are affected
by low concentrations of such air pollutants as sulfur dioxide (SO ),
hydrogen fluoride (HF), ozone (0 ), and cement kiln dust, but their
sensitivity to NO has yet to be tested.
Both nitric oxide and nitrogen dioxide react readily with soils, and
are generally converted to nitrate. Sorption of large amounts of NO
-------
11-11
decreases soil pH which presumably can be corrected by the addition of
lime. The scavenging role of soil in removing typical atmospheric con-
centrations of NO is uncertain.
x
Recommendations:
1. It should be determined whether ambient atmospheric concentrations
of nitrogen oxides affect plant and animal communities, and micro-
organisms and microbial processes in soils and water.
2. In addition, the efficacy of soil as a sink for atmospheric nitro-
gen dioxide should be ascertained.
EFFECTS OF NITROGEN OXIDES ON MATERIALS
Field studies and laboratory research have demonstrated that ambient
concentrations of nitrogen dioxide, as well as sulfur dioxide and ozone,
can cause fading of certain textile dyes and yellowing of some white
fabrics. The chemical mechanisms for nitrogen dioxide fading of dyes are
fairly well-known, and various methods for color protection are available.
The cost to the consumer of color fading of dyes by nitrogen dioxide has
been estimated to exceed $100 million annually. The effect of pollutants
on yellowing of white fabrics has not been well-established; however,
recent studies suggest that nitrogen dioxide is the pollutant principally
responsible for this problem. No estimates of the cost to the consumer of
yellowing of white fabrics are available.
Available data do not indicate a direct role for nitrogen oxides in
the degradation of textile fibers. If nitrogen oxides increase the con-
centration of airborne acids, this might lead to degradation of cotton
fabrics and nylon.
-------
11-12
Evidence suggests that nitrogen oxides can affect the defect
structure of metal oxides thereby increasing or decreasing the rates of
oxidation of metals and alloys. Furthermore, airborne nitrates can
adsorb water. This aids the formation of a solvent or electrolyte for
wet corrosion. However, a direct relationship has not been established
between a given nitrogen oxide concentration and a change in the corrosion
behavior of structural materials.
Recommendations;
Further studies are needed to determine the mode of action of
NO and nitrates on the degradation of textile fibers and rubber
x
compounds, and on the corrosion of metals and alloys.
EFFECTS OF NITROGEN OXIDES ON VEGETATION
Experimental fumigations have shown that nitric oxide is less injurious
to vegetation than nitrogen dioxide, and both are less phytotoxic than
sulfur dioxide, gaseous fluoride, or photochemical oxidants. Nitrogen
dioxide-induced injury in the field has usually been associated with
accidental acute exposures near industries that manufacture or use nitric
acid. There have been no confirmed reports for nitric oxide injury to
vegetation in the field.
The existing U.S. air quality standard for nitrogen dioxide (0.10
3
mg/m for an annual average) is below the threshold for detectable effects
on vegetation. Therefore, secondary standards to protect vegetation from
the direct effects of nitrogen dioxide are not necessary. Indirect effects
of nitrogon dioxide on vegetation, however, are more important. The par-
ticipation of nitrogen dioxide in atmospheric reactions leading to the
-------
11-13
production of ozone and peroxyacylnitrates (PANs), and the synergistic
effects on plant injury of low concentrations of nitrogen dioxide and
sulfur dioxide mixtures in the atmosphere may pose a real threat to
vegetation growing in or near metropolitan areas. As knowledge of these
phenomena increases, air quality standards for nitrogen dioxide to pro-
tect vegetation may have to be reevaluated.
Recommendations;
1. Since plants are relatively insensitive to nitric oxide, future
research on NO should concentrate on nitrogen dioxide. The
x
gross effects of acute nitrogen dioxide exposures have been
fairly well documented. Future research, therefore, should
emphasize the effects of chronic and intermittent nitrogen
dioxide exposures, at realistic concentrations and exposure
times, on plant metabolism, growth, and yield.
2. The combined effects of NO and other substances in the atmo-
X
sphere on plants is another important area in which information
is limited. Experimental exposures to nitrogen dioxide in com-
bination with other atmospheric constituents should be conducted
on a wide variety of plant species to reveal the interacting
effects of nitrogen dioxide and other pollutants on phytotoxicity.
HEALTH EFFECTS OF NITROGEN OXIDES
Effects of Short-Term Exposures (Less Than 24-hr)
Reduced resistance to respiratory infection is the most sensitive
response ->f animals to nitrogen dioxide exposure. Significant differences
in mortality from experimentally induced bacterial pneumonia were observed
-------
11-14
3
in mice after a 2-hr exposure to 6.6 mg/m (3.5 ppm) nitrogen dioxide.
This impairment was also observed in other animal species including hamsters
and nonhuman primates. The action of nitrogen dioxide appears to be me-
diated through alterations of specific defense mechanisms including the
alveolar macrophage system, anatomical structure of lungs, and humoral
immunity.
Other effects of short-term animal exposure include various physiologic
3
and pathologic abnormalities caused by exposures to 9.4 mg/m (5 ppm) ni-
trogen dioxide and above, and mortality which resulted when concentrations
3
of nitrogen dioxide exceeded 75-95 mg/m (40-50 ppm).
Human volunteer studies have provided precise documentation of the
acute effect of nitrogen dioxide. The earliest response to nitrogen di-
oxide occurs in the sense organs. The odor of nitrogen dioxide can be
3
perceived upon exposure to 0.23 mg/m (0.12 ppm) nitrogen dioxide, while
changes in dark adaptation were reported following exposures ranging from
0.14 to 0.50 mg/m (0.075 to 0.26 ppm). These effects are immediately
reversible, and there is no evidence for sequelae in terms of pathology.
Three studies have shown increases in airway resistance after 15 to
3
45 min exposures to 2.8 to 3.8 mg/m (1.5 to 2.0 ppm) nitrogen dioxide.
Although the increases in airway resistance were reversible, they may be
adverse for asthmatics or subjects with advanced chronic obstructive
pulmonary disease. Measurements of more sensitive parameters of pulmonary
dysfunction, such as small airway resistance and perfusion indices, have
not been reported.
The literature contains a paucity of data on the health effects of
repeated short-term (2 to 4 hr) exposures to nitrogen dioxide, or exposures
to nitrogen dioxide in mixtures of pollutants that commonly occur in the
-------
11-15
environment. In one study, sodium chloride aerosol combined with nitrogen
dioxide augmented the effect on airway resistance when compared with ex-
posure to nitrogen dioxide alone. The synergy between ozone and sulfur
dioxide on human airway resistance has also been reported. Therefore,
potential interactions between nitrogen dioxide and other atmospheric
pollutants must be investigated before it can be stated with confidence
3
that short-term exposures to less than 2.8 mg/m (1.5 ppm) nitrogen di-
oxide present in a mixture of other air pollutants have no effect on airway
resistance.
Accidental exposures establish the acutely fatal concentration of nitro-
o
gen dioxide for man at 282 mg/m (150 ppm) and above. Deaths were due to
pulmonary edema or bronchiolitis fibrosa obliterans. Concentrations between
3
47 and 140 mg/m (25 to 75 ppm) caused reversible pneumonia and bronchiolitis.
Permanent sequelae manifested as shortening of life were not found in the
only reported follow-up study of survivors of acute exposures to high nitro-
gen dioxide concentrations.
Chronic Effects of Long-Term Nitrogen Dioxide Exposure
As in short-term exposure studies, reduced resistance to respiratory
infection appears to be the most sensitive indicator of damage produced
by long-term nitrogen dioxide exposure. Continuous exposures for 3 months
and intermittent daily exposures for 6 months or longer to nitrogen dioxide
3
at concentrations of 0.94 mg/m (0.5 ppm) and higher diminishes murine
3
ability to resistant pulmonary bacterial infection. At 9.4 mg/m (5.0
ppm) nitrogen dioxide, similar results have been reported for nonhuman
primates after a 2 months exposure. The reduced resistance to infection
has been associated with pneumonia and death. Immunologic deficits
-------
11-16
unassociated with overt infection have been observed after prolonged
3
exposures to 1.8 mg/m (1.0 ppm). Such pathological abnormalities as
ciliary loss, alveolar cell disruption, and obstruction of respiratory
bronchioles, occur in lungs of mice and rats after continuous exposure
3
to 0.94 mg/m (0.5 ppm) of nitrogen dioxide.
Long-term exposures to nitrogen dioxide at concentrations of 9.4
3
mg/m (5.0 ppra) or higher also cause rapid breathing and increases in
airway resistance in rodents and in nonhuman primates. A more serious
abnormality in pulmonary function, namely decreases in blood oxygenation,
occurs in rabbits after continuous exposure to 15 mg/m (8.0 ppm).
Limited studies were reported in which animals have been exposed to
mixtures of pollutants including nitrogen dioxide. Combinations of ni-
trogen dioxide with carbon monoxide, ozone, or sulfur dioxide have usually
resulted in additive or indifferent effects. Synergy has, with one excep-
tion of uncertain significance, not been reported, nor, to our knowledge,
has antagonism.
Epidemiological studies also indicate that excess acute respiratory
disease was observed in healthy populations following exposure to nitrogen
dioxide. Considered broadly, four studies are consistent in suggesting
an association between exposures of 0.10 to 0.58 mg/m (0.053 to 0.31 ppm)
nitrogen dioxide and excesses in respiratory illnesses. However, the
variable pollutant exposures and conditions of study make it difficult to
quantify the direct relationship between nitrogen dioxide and the increases
in respiratory disease. In each of these studies other air pollutants
likely to enhance susceptibility to respiratory infection (sulfur dioxide,
sulfuric acid, sulfates, nitrates, etc.) were also present.
-------
11-17
Results of two epidemiological studies in which ventilatory function
o
was measured suggest that 0.15 to 0.3 mg/m (0.08 to 0.16 ppm) nitrogen
dioxide in combination with other pollutants causes changes in ventilatory
function. These effects were not reported in two other studies in which
2
nitrogen dioxide concentrations were less than 0.15 mg/m (0.08 ppm) and
in which a mixed pollutant atmosphere existed. The effective concentra-
tion at which ventilatory function is impaired more than likely is not a
function of chronic exposure to nitrogen dioxide alone, but the combin-
ation of pollutants and other atmospheric conditions most of which were
not monitored.
Reports of excess chronic respiratory disease associated with ambient
concentrations of nitrogen dioxide do not provide convincing evidence that
other pollutants, measured at high concentrations, did not cause the excess
disease prevalence. At low concentrations of sulfur dioxide and partic-
ulates, three investigators failed to detect excess chronic respiratory
disease in populations where nitrogen dioxide exposures were at or below
0.10 mg/m (0.053 ppm).
While urban air pollution may be related to chronic human diseases,
including cancer, a correlation with oxides of nitrogen in the atmosphere
has not been established. Tumors were not observed in rats after life-
3
time exposure to 1.5 to 3.8 mg/m (0.8 to 2.0 ppm) nitrogen di-
oxide although extensive lung disease occurred and epithelial hyperplasia
was observed in the trachea and bronchi of these animals. Although
evidence from man and animals is limited, possible participation of ni-
trogen oxides or their reaction products (organic and inorganic nitrates,
nitrites, and possible nitroso compounds) in carcinogenesis merits attention.
-------
11-18
Recommendations;
1. Lung function response should be studied in human volunteers to
determine the potentially important health effect of repeated 2
to 3 hr nitrogen dioxide exposures. Such studies should be
designed to establish the highest no-effect concentration of ni-
trogen dioxide. Resulting data would be directly relevant to the
establishment of a short-term nitrogen dioxide standard. Several
experimental conditions should be taken into account when designing
these studies:
• Lung function should be evaluated during exercise since the
importance of intermittent light exercise during exposure to
pollutants has been well demonstrated.
• Lung function tests should include measurements sensitive to
small airway changes, as well as the more traditional measures
of total airway resistance and flow or volume. The effect of
nitrogen dioxide on distal bronchioles should be studied be-
cause of the morphological evidence of damage in the terminal
bronchioles and alveoli observed in laboratory animals.
• To simulate ambient environmental conditions, the effect of
repeated short-term exposures to nitrogen dioxide alone, and
to nitrogen dioxide in combination with sulfur dioxide, ozone,
and particulate aerosols at low and high humidity should be
determined.
• Consideration should be given to studies of human volunteers
with potential high sensitivity to short-term peak nitrogen
dioxide exposures. These subjects might include asthmathics
and individuals with advanced chronic obstructive lung disease.
-------
11-19
• Studies using human volunteers should be designed to establish
the highest no-effect concentration of nitrogen dioxide.
2. Studies should further define the causal relationship between
nitrogen dioxide and acute respiratory infections in laboratory
animals. To simulate more closely true environmental air pol-
lution situations, such studies must include mixtures of gaseous
and particulate pollutants at various temperature and humidity
conditions. Moreover, long-term exposures should be conducted
at low concentrations of nitrogen dioxide with superimposed
short-duration peaks of higher nitrogen dioxide concentrations.
The sensitivity to nitrogen dioxide of defense mechanisms
against respiratory infections, e.g., phagocytosis, cellular,
and humoral immunity, should be investigated in several animal
species, including nonhuman primates. Whenever possible, the bio-
logical response endpoints used in such studies should be relevant
and of potential use to human epidemiological surveys.
3. The relationship between long-term nitrogen dioxide exposure and
chronic respiratory disease should be carefully evaluated. Future
epidemiologic surveys of chronic respiratory disease should take
into account history of past exposures, indoor as well as ambient
sources of nitrogen dioxide, and presence of other pollutants
commonly encountered in the urban atmosphere. It may be necessary
to take advantage of special occupational situations in which
relatively isolated nitrogen dioxide exposures may be encountered.
In addition, animal models of chronic obstructive respiratory
disease should be used to determine the effects of repeated inter-
mittent peak nitrogen dioxide exposures.
-------
11-20
4. Since respirable solid and liquid aerosol particles carrying
nitrogen oxides in condensed, dissolved, or adsorbed form are
deposited in the lungs, the composition of the aerosols and the
inhaled dose, deposition patterns, and pathological effects re-
sulting therefrom require further investigation.
5. To quantify individual human exposures similar to those measured
in animal dose-response studies, characteristic responses of
population cohorts to air pollutant mixtures require: evaluation.
Pilot studies should compare actual total exposures for workers
in industrial high exposure settings with groups exposed to lower
concentrations. This would provide preliminary data concerning
the range of individual exposures in different segments of the
population. Such studies will complement the available environ-
mental data that document large diurnal and seasonal variations
in ambient nitrogen dioxide concentrations. Exposure doses cal-
culated only from ambient air concentrations do not provide real-
istic estimates of actual human exposure times, particularly
because of the extensive movement into and out of polluted areas
by any individual on any given day.
6. The carcinogenic potential of airborne oxides of nitrogen or their
reaction products should be considered. In particular, studies
should be conducted on carcinogenic effects of organic nitrogen com-
pounds in air and of nitrogen dioxide in the presence of other pollutants,
7. Wherever possible, biological studies of nitrogen dioxide effects
cihould be made on a number of exposure levels sufficient to permit
a preliminary estimate of thresholds and the slope of the bio-
logical response.
-------
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO
EPA-600/1-77-013
3. RECIPIENT'S ACCESS! ON-NO.
A TITLE AND SUBT'TLE
NITROGEN OXIDES
5 REPORT DATE
February 1977
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Subcommittee on Nitrogen Oxides
. PERFORMING ORGANIZATION NAME AND ADDRESS
Committee on Medical and Biologic Effects of
Environmental Pollutants
National Academy of Sciences
Washington, D.C.
10. PROGRAM ELEMENT NO.
1AA601
11. CONTRACT/GRANT NO.
68-02-1226
12 SPONSORING AGENCY NAME AND ADDRESS
Health Effects Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, N.C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report is a review of current knowledge of the environmental health
basis for control of manmade sources of nitrogen oxide emissions. The literature
rex ic-w covered the period through 1974. The principal subject areas considered
in the report include: sources and control of atmospheric nitrogen oxides;
analytical methodology; concentrations and chemical reactions in the atmosphere;
and the effects of nitrogen oxides on human health, materials, vegetation, light
transmission, and natural ecosystems. Emphasis is primarily on nitroc oxide (NO)
and nitrogen dioxide (N02), designated by the composite formula NOX for nitrogen
oxides. The major manmade source is the combustion of fossil fuel. Highest
atmospheric concentrations are found in heavily populated, industrialized urban
areas. Both acute and chronic health effects resulting from short-term and
long-term exposures, are discussed in the report. Effects range from slight
incvaases in airway resistance to death depending upon exposure concentrations.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Nitrogen Oxides
Air Pollution
Toxicity
Health
Ecology
Chemical Analysis
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
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9. PERFORMING ORGANIZATION NAME AND ADDRESS
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10. PROGRAM ELEMENT NUMBER
Use the program element number under which the report was prepared. Subordinate numbers may be included in parentheses.
11. CONTRACT/GRANT NUMBER
Insert contract or grant number under which report was prepared.
12. SPONSORING AGENCY NAME AND ADDRESS
Include ZIP code.
13. TYPE OF REPORT AND PERIOD COVERED
Indicate interim final, etc., and if applicable, dates covered.
14. SPONSORING AGENCY CODE
Leave blank.
15. SUPPLEMENTARY NpTES
Enter information not included elsewhere but useful, such as: Prepared in cooperation with, Translation of, Presented at conference nf,
To be published in, Supersedes, Supplements, etc.
16. ABSTRACT
Include a brief (200 words or less) factual summary of the most significant information contained in the report. If the report contains a
significant bibliography or literature survey, mention it here.
17. KEY WORDS AND DOCUMENT ANALYSIS
(a) DESCRIPTORS - Select from the Thesaurus of Engineering and Scientific Terms the proper authorized terms that identify the major
concept of the research and are sufficiently specific and precise to be used as index entries for cataloging
(b) IDENTIFIERS AND OPEN-ENDED TERMS - Use identifiers for project names, code names, equipment designators, etc. Use open-
ended terms written in descriptor form for those subjects for which no descriptor exists
(c) COSATI FIELD GROUP - Field and group assignments are to be taken from the 1 965 COSAT1 Subject Category List Since the ma-
jority of documents are multidisciplinary in nature, the Primary Field/Group assignment(s) will be specific discipline, area of human
endeavor, or type of physical object. The application(s) will be cross-referenced with secondary Field/Group assignments that will follow
the primary posting(s)
18. DISTRIBUTION STATEMENT
Denote releasability to the public or limitation for reasons other than security for example "Release Unlimited." Cite any availability to
the public, with address and price.
19. &20. SECURITY CLASSIFICATION
DO NOT submit classified reports to the National Technical Information service.
21. NUMBER OF PAGES
Insert the total number of pages, including this one and unnumbered pages, but exclude distribution list, if any.
22. PRICE
Insert the price set by the National Technical Information Service or the Government Printing Office, if known.
PA Form 2220-1 (9-73) (Reverse)
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