ENVIRONMENTAL HEALTH
SERIES
Air Pollution
Reactivity
in Atmospheric
Photooxidation
U. S. DEPARTMENT OF HEALTH,
EDUCATION, AND WELFARE
Public Health Service
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REACTIVITY OF ORGANIC SUBSTANCES
IN ATMOSPHERIC PHOTOOXIDATION REACTIONS
A. P. Altshuller
Laboratory of Engineering
and Physical Sciences
Robert A. Taft Sanitary Engineering Center
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Division of Air Pollution
Cincinnati, Ohio
July 1965
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The ENVIRONMENTAL HEALTH SERIES of reports was established to
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Public Health Service Publication No. 999-AP-14
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ABSTRACT
The organic vapors emitted to urban atmospheres by motor vehicles
and other sources of emissions consist not only of paraffinic, acetylenic,
aromatic, and olefinic hydrocarbons, but also of aldehydes, ketones,
alcohols, phenols, and chlorinated hydrocarbons. To estimate the
contribution of each of these classes of compounds to photochemical
smog, one must know both their atmospheric concentrations and their
relative reactivities in atmospheric reactions.
A review of the available literature on concentration levels of organic
vapors in urban atmospheres indicates that much more analytical
work is needed. The existing data are adequate, however, for the
formulation of useful estimates.
Reactivities of organic substances in photooxidation reactions can
be considered from many standpoints. Rates of disappearance of the
organic substances, rates of disappearance of nitric oxide or of for-
mation and disappearance of nitrogen dioxide, and rates or maximum
yields of various products such as oxidant or organic nitrates all can
be used as chemical measurements of reactivity. Eye irritation, various
types of plant damage, and aerosol formation are indicators of
reactivity that can be related only to a limited extent to chemical
measurements of reactivity. The problems of developing a single index
of reactivity are considered.
The application of reactivity measurements to automobile exhaust
composition, to control devices, and to improvements in atmospheric
purity is discussed.
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REACTIVITY OF ORGANIC SUBSTANCES
IN ATMOSPHERIC PHOTOOXIDATION REACTIONS
"Photochemical reactivity," or "reactivity," may be defined as the
tendency of an atmospheric system containing organic substances and
nitrogen oxides to undergo, under the influence of ultraviolet radiation
and appropriate meteorological conditions, a series of chemical re-
actions that result in the various manifestations associated with the
photochemical type of air pollution. These manifestations include eye
irritation, plant damage, and visibility reduction. Although this type
of air pollution has been associated with the Los Angeles basin, it is
also experienced elsewhere in California and in many other urban
areas in the United States. The constant increase in the use of pe-
troleum products in motor vehicles and in other combustion sources
could result in photochemical air pollution becoming a problem
throughout the world in urban centers that are characterized by un-
favorable meteorological conditions.
The chemical reactions are initiated by the photochemical dissociation
of nitrogen dioxide by solar radiation below 4000 A. Subsequent
chemical reactions result in the rapid conversion of nitric oxide to
nitrogen dioxide, usually followed by a nitrogen dioxide maximum and
the further reaction of nitrogen dioxide. After nitrogen dioxide reaches
a maximum concentration, oxidant, including particularly ozone and
peroxy acyl nitrates, begins to form. Aerosol also is produced. Aldehydes
and organic nitrates are the predominant organic substances known
to be produced by these reactions. Included in the products are the
substances responsible for eye irritation and for several types of plant
damage. The aerosol formed causes visibility reduction.
On the basis of the available literature, it is not clear whether one
or several "measures" of reactivity are necessary to define the atmos-
pheric system. Measures include the rate of reaction of the various
participating organic reactants, the rate of oxidant formation, and the
degree of eye irritation. The characterization of these measures of
reactivity certainly is not an academic exercise. The most economical
and effective methods of control of photochemical "smog" depend on
decisions as to the most desirable modifications of the atmosphere.
It is essential to keep in mind that a control device is not an end in
itself, but merely a means of modifying an urban atmosphere. To
understand what may be accomplished by an air pollution control
device, its potential effect on the reactivity of the atmosphere also must
be understood. The reactivities obtained for various classes of com-
pounds can be combined with detailed analytical data on emissions
to determine the effectiveness of various engine exhaust systems or fuel
modifications.
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The reactivity of photochemical smog can be evaluated by using
experimental data on the following:
1. Rates of disappearance of organic substances from irradiated
single organic substance nitrogen oxide mixtures, from multi-
component organic substance nitrogen oxide mixtures, and
from auto exhaust mixtures.
2. Ability of organic substances to participateinthephotochemically
induced conversion of nitric oxide to nitrogen dioxide.
3. Rate of formation or yield of ozone or oxidant.
4. Rates of formation or yields of aldehydes and peroxyacyl
nitrates.
5. Aerosol formation.
6. Plant damage.
7. Eye irritation.
The rates of disappearance of individual hydrocarbons and some of
the other organic substances can be obtained irrespective of the com-
plexity of the mixture. In general, however, direct association of ,the
experimental results obtained in (2) through (7) with in dividual organic
reactants in the mixture is not possible. Exceptions would be limited
to comparisons or results involving two- or at most three-component
organic substance nitrogen oxide mixtures with results involving
the corresponding single-component systems. Even in such comparisons
as these, assumptions about additivity of effects have rarely been veri-
fied experimentally. Complex interactions may well occur in all of
those effects that are strongly dependent on the ratio of hydrocarbon
to nitrogen oxide as well as on hydrocarbon type and concentration.
The available experimental data cannot be used except after critical
evaluation. For example, as reported by different investigators, the
data on aldehydes and ketones produced from irradiation of single
hydrocarbon nitrogen oxide systems sometimes include large differ-
ences in yields. Differences of 50 to 100 percent in the yields reported
are not uncommon for certain aldehydes or ketones.
It must be decided whether comparisons are to be made at those
ratios of reactants giving maximum effects irrespective of the possible
differences in these reactant ratios for various systems. Effects certainly
cannot be assumed a priori to maximize at the same ratio for different
single- or multi-component organic substance nitrogen oxide systems.
Irrespective of the limitations cited and others that could be discussed,
it is important to attempt to make judgements on the parameters
that affect the reactivity of photochemical atmospheres. At the same
time, the assumptions made and the criteria for selection of data should
be explicitly stated so that the results may be modified conveniently.
Reactivity Of Organic Substances
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CHEMICAL MEASURES OF REACTIVITY
Chemical measures of reactivity include the rates of disappearance
of reactants and the rates of formation or the yields of various pro-
ducts. Great care must be exercised in equating such chemical measures
with the effects of more direct concern, namely, eye irritation, plant
damage, and visibility reduction. In this study these chemical measures
of reactivity are used to define differences in the reactivity of classes
of organic substances. The chemical measures of reactivity are not
used to assign precise contributions to photochemical air pollution from
each of a large number of individual organic substances. This latter
approach requires more accurate and detailed chemical and biological
experimental results than presently exist or are likely to exist in the
near future.
It appears doubtful that a single summed measure of biological
reactivity can be defined for individual substances. Eye irritation, the
various types of plant damage, and aerosol formation are not neces-
sarily caused by the same substances nor are these effects necessarily
associated with the same reaction mechanisms. It is not meaningful
to combine yields of products such as ozone, aldehydes, and peroxyacyl
nitrates and define the result as an overall measure of reactivity. Such
an exercise ignores multiple effects of some of the products and inter-
action effects. The association of either total reactivity or a single
measure such as eye irritation with a given set of chemical species
ignores the distinct possibility that unidentified species exist which cause
biological effects. This approach also is dependent on the sensitivity
of yields of some products to concentration and ratio of reactants.
These comments certainly are not meant as an argument against
chemical reactivity studies or the even more desirable combinations
of chemical and biological investigations. It is essential, however, that
the difficulties in the reactivity approach be recognized and that great
caution be taken in interpreting results and arriving at general con-
clusions.
Reactions of Organic Substances
Rates of reaction or times for partial consumption of various hydro-
carbons and aldehydes have been reported in a number of studies
(Table 1,2, and 3). These investigations include both laboratory
measurements on irradiations of individual organic substances with
nitrogen oxides in air and irradiations of diluted auto exhaust (1-7).
The measurements have been made both in static reactors-and in
stirred dynamic flow reactors. Irradiations involve the use of com-
binations of fluorescent lamps and sometimes mercury arcs to simulate
solar radiation in the ultraviolet region. The measurements of the
disappearance of the organic substances in static irradiations have
been reported as follows: (1) times to half-conversion to products,
In Atmospheric Photooxidation Reactions
-------
(2) percentage of substances converted per hour, and (3) normalized
"rate constants" for the most rapidly changing portion of the experi-
mental concentration versus time curves. In dynamic irradiations,
measurements are given as the percentage of the substance consumed
near dynamic equilibrium during a given average irradiation
time (5-7).
TABLE 1. RANKING OF REACTIVITIES OF HYDROCARBONS WHEN
PHOTOOXIDIZED IN PRESENCE OF NITROGEN OXIDES
UNDER STATIC CONDITIONS
Ranking of hydrocarbons
Hydrocarbon
Tetramethylethylene
Trans-2-butene
Cis-3-hexene
Isobutene
1,3- Butadiene
Propylene
m-Xylene
p-Xylene
Ethylene
Hexanes, Octanes
Pentanes
Schuck
and Doyle a
10
6
1.5
1
1
0.5
0.1
< 0.1
< 0.01
Stephens
and Scott b
10
6
2
2
1
0.5
0.3
Tuesday c
10
8
2
1
1 Reference 1. Reference 2. c Reference 3.
TABLE 2. RANKING OF REACTIVITIES OF AROMATIC HYDRO-
CARBONS WHEN PHOTOOXIDIZED IN PRESENCE OF
NITROGEN OXIDES UNDER STATIC CONDITIONS
Ranking of
Aromatic hydrocarbon reactivities ^
1,2,3,5-Tetramethylbenzene 10
1,2,5-Trimethylbenzene 8
1,2,4-Trimethylbenzene 6
1,2,3-Trimethylbenzene 6
m-Xylene, 3-methylethylbenzene 3
o-Xylene 1.5
p-Xylene, p-diethylbenzene 1.5
Toluene, ethylbenzene, isopropylbenzene 1
Benzene 0.5
aBased on percent consumed in first hour of reaction.
3 Reference 4.
Reactivity Of Organic Substances
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TABLE 3. RANKING OF REACTIVITIES OF ORGANIC SUBSTANCES
WHEN PHOTOOXIDIZED IN PRESENCE OF NITROGEN
OXIDES UNDER DYNAMIC CONDITIONS.
Auto exhaust studies "Pure component" studies
Organic substances Sigsby et al.a Leach et al.b Klosterman et al.c
2-Alkenes 10 d
Trans-2-butene 9
1,3,5-Trimethylbenzene 6 8
1,3-Butadiene 7 8
1-Butenes, 1-pentenes 7 d 7
Propylene 5 6
1,2,4-Trimethylbenzene 6
Xylenes 34 5
Formaldehyde 5
Propionaldehyde 4
Ethylene 133
Toluene 122
Pentanes 0.5
Butanes 0.5
Cyclohexane 0
Isooctane 0
Benzene 0
Acetylene 0
Ethane 0
Based on percent hydrocarbon consumed near dynamic equilibrium:
a Reference 5. c Reference 7.
b Reference 6. d 85% of all butenes and pentenes reacted.
The most reactive organic substance measured in each study involv-
ing static irradiations has been arbitrarily rated as 10, and the other
substances rated relative to 10 on the basis of the experimental data
available (Table 1 and 2). Data from the dynamic irradiation work
have been treated differently. The experimental data from these studies
have been rated on the following basis: 0 percent conversion, 0; 0 to
5 percent conversion, 0.5; 5 to 15 percent conversion, 1; 85 to 95
percent conversion, 9: and above 95 percent conversion, 10 (Table 3).
Since the experimental measures of rate are not the same in these
investigations, the rating scales are not completely compatible with
each other. The numerical values for a given substance cannot be
expected to be identical in the various investigations. Furthermore,
different concentrations of reactants, light intensities, and temperatures
were used, and rates of reaction do not necessarily change pro-
portionately with these differences in experimental conditions.
Despite all of these limitations, the rankings of sub-classes of organic
substances and of many of the individual compounds are very con-
In Atmospheric Photooxidation Reactions
-------
sistent. Branched or straight-chain olefins with internal double bonds,
e.g. tetramethylethylene or trans-2-butene, form the most reactive sub-
class. The trialkylbenzenes, the tetraalkylbenzenes, and olefins with
terminal double bonds (except ethylene) rank next, followed by the
dialkylbenzenes, aldehydes, and ethylene. Toluene is less reactive than
ethylene, and the paraffinic hydrocarbons, acetylene, and benzene are
less reactive than toluene.
The least reactive hydrocarbons are consumed so slowly in the photo-
oxidation reactions that accurate experimental measurements are
difficult to obtain. Variations of flow in dynamic flow irradiations
and adsorption losses and leaks in static and dynamic experiments
could account for some of the small, but non-zero rates reported in
some studies. The whole body of experimental evidence indicates that
most paraffinic hydrocarbons, acetylene, and benzene react at rates
at least 2 orders of magnitude less than the most reactive substances.
In the laboratory the substances rated at 10 are half-consumed in the
order of 0.1 hour, whereas the least reactive substances should require
10 to 100 hours for half-conversion to products. Some substantiation
of the upper end of this time scale is given by the report that no loss
of acetylene or ethane could be detected after an 18-hour irradiation
of an atmospheric sample (2).
Irradiations of polluted air masses by sunlight occur for from 8 to
more than 14 hours a day, depending on season and latitude (8).
Peak intensities vary by about a factor of 3, and 50 percent of peak
intensity is exceeded from 5 to 10 hours a day, again depending on
season and latitude. Thus the irradiation period of practical interest
is about 10 hours. If the air mass should not be dispersed, the same
air mass may be irradiated for more than 1 day. On the basis of such
considerations, it matters little within a large metropolitan area whether
a component reacts appreciably in 0.1, 1, or even several hours.
Ultimately, the effects of its reaction products may be felt somewhere
in the area in terms of eye irritation, plant damage, or visibility
reduction. Thus any substance that can be largely consumed during the
daylight hours should be considered potentially undesirable. On the
other hand, substances such as most of the paraffinic hydrocarbons
present in significant concentrations, acetylene, and benzene, which
do not appear to react appreciably during this time period, cannot
contribute significant amounts of products.
The reaction of the organic substance is necessary but not sufficient
evidence that an adverse biological effect could be associated with the
photooxidation of that substance. Although a substance reacts, it
does not follow a priori that it produces significant amounts of eye
irritants, phytotoxicants, or oxidants or that it will participate in re-
ducing visibility. Product yields and biological effects also must be
Reactivity Of Organic Substances
-------
measured if any generally meaningful evaluation of reactivities is
to be obtained.
Photo oxidation of Nitric Oxide to Nitrogen Dioxide in Presence of
Hydrocarbons.
The ability of a wide variety of hydrocarbons to participate in the
conversion of nitric oxide to nitrogen dioxide has been determined in
one study (9). The ranking of the hydrocarbons used in these static
irradiations is shown in Table4. The ranking follows the same general
order as that obtained from rates of hydrocarbon disappearance.
Although this measure of reactivity is no better fundamentally than
that based on hydrocarbon reaction, it does have the advantage of
simplicity in analytical measurement. Measurement of the hydrocarbons
requires either gas chromatography with several column substrates
or an infrared instrument with a long-path cell. The nitrogen dioxide
formation rate can be determined by manual measurement of nitrogen
dioxide or with a monitoring instrument.
TABLE 4. REACTIVITIES OF HYDROCARBONS BASED ON ABILITY
TO PARTICIPATE IN PHOTOOXIDATION OF NITRIC OXIDE
TO NITROGEN DIOXIDE a
Hydrocarbon Ranking
Trans-2-butene 10
1,2,5-Trimethylbutene 6
Isobutene 6
m-Xylene 5
Propylene 5
1,2,3,5-Tetramethylbenzene 5
1,3-Methylethylbenzene 3
1,2,4-Trimethylbenzene 3
o-Xylene, p-Xylene 2
Ethylenc 2
o- and p-Diethylbenzene 1.5
Propylbenzenes 1.5
Toluene, ethylbenzene 1
Benzene 0.5
n-Nonane 0.5
3-Methylheptane 0.25
Acetylene 0.25
Methylpentanes 0
d Reference 9.
Reaction of Nitrogen Dioxide to Products
Data also are available on the rate of conversion of nitrogen dioxide
to products in the presence of various hydrocarbons (9). The per-
In Atmospheric Photooxidation Reactions 7
-------
centage of total nitrogen oxides reacted was related to hydrocarbon
and nitrogen oxide concentration and ratio in a study of irradiated
auto exhaust under dynamic conditions (10). The percentage-reacted
curves versus initial concentrations of hydrocarbons and nitrogen
oxides resembled the oxidant curves versus the reactant concentrations.
Furthermore, when the percentage of nitrogen oxide reacted was zero,
the oxidant level was also zero. Percentage of nitrogen dioxide reacted
also followed the same trend as did theplant damage effects, with little
or no nitrogen dioxide converted or plant damage observed at low
ratios of hydrocarbon to nitrogen oxide. Again, this index can be
readily obtained by a nitrogen dioxide analysis.
Oxidant Yields
The term 'oxidant' refers to that group of substances formed in the
photochemical type of air pollution that are capable of oxidizing
potassium iodide and other chemical reagents. These oxidants include
ozone, nitrogen dioxide, peroxyacyl nitrates, possibly other types of
organic peroxy compounds or radicals, and atomic oxygen. Because
the relative responses of the various oxidant methods or instruments
to these substances vary, the concentrations of oxidant reported by
different procedures should not necessarily agree. Ozone, the major
constituent of the oxidant, is a phytotoxicant; it also causes rubber
cracking. There is some evidence that respiratory distress is caused
by atmospheric levels of ozone. Eye irritation has been suggested as
being statistically, but not causatively, associated with moderate to
high atmospheric oxidant levels.
Several methods that are claimed to be relatively specific for ozone
have been proposed. Two of these methods, one based on rubber
cracking and the other on infrared absorption, have been used to obtain
measurements of the ozone formed during the irradiation of a large
number of organic substance nitrogen oxide mixtures in air (1,11)
(Table 5).
Rubber cracking measured as cracking depth in millimeters has been
used to determine ozone produced by irradiating a wide variety of
hydrocarbons, aldehydes, alcohols, ketones, acids, and organic nitrogen
compounds with nitrogen dioxide (11). Measurements of total ozone
exposure of the rubber strips were made during a 10-hour irradiation
of 3 ppm of the organic substance and 2 ppm of nitrogen oxide at
30° to 33°C and 30 percent realative humidity.
Cracking depths of more than 5 millimeters were observed when
olefins with an internal double bond, a diolefin, dialkylbenzenes, a
trialkylbenzene, higher-molecular-weight alcohols, butyric acid, bi-
acetyl, and alkyl nitrites were irradiated with nitrogen dioxide. Cracking
depths of 3 to 5 millimeters were observed when olefins with a terminal
double bond (except ethylene), aldehydes, methanol, ethanol, and
propionic acid were irradiated along with nitrogen dioxide. Ethylene
Reactivity Of Organic Substances
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3
o
cc
a
g*
11
c
o
o
01
TABLE 5. OZONE OR OXIDANT YIELDS FROM PHOTOOXIDATION OF ORGANIC SUBSTANCE NITROGEN
OXIDE MIXTURES.
Static irradiations
Organic Substance
1,3- Butadiene
2-Alkenes
1,3,5-Trimethylbenzene
Xylenes
1-Alkenes
Methanol, ethanol
Formaldehyde
Propionaldehyde
3-Methylheptane
n-Nonane
Ethylene
Hexanes, heptanes
Toluene
Acetylene
c [ - c 5 Paraffins
Haagen-Smit
Cracking depth, mma
12
8
7
6-7
5
5
4
4
3
3
2
1
0.6
0.5
< 0.2
Schuck and Doyle
ppm by volume b
0.65
0.55-0.73
0.18
0.58 - 1.00
1.1
-0.2
0.0- 0.2
Altshuller and Cohen
ppm by volume °
1.1
0.65-1.0
1.0
0.2
0.5
0.0
Dynamic irradiations
Klosterman et al.,
ppm by volume c
0.72
0.37
0.4
1.05
0.80
0.69
0.0
0.36
Reference 11.
b Reference 1.
: Reference 9.
Reference 7.
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and most six-carbon and higher paraffinic hydrocarbons caused 1 to 3
millimeters of cracking depth when irradiated with nitrogen dioxide.
Irradiation of acetylene nitrogen dioxide mixtures caused only 0.5
millimeter of cracking depth. Very little or no cracking of the rubber
strips was observed when methane, ethane, propane, butanes, pentanes,
formic acid, or acetic acid were irradiated with nitrogen dioxide in air.
The infrared measurements of ozone in .the work reported by Schuck
and Doyle were essentially instantaneous compared to the rubber-
cracking measurements (1). Data were obtained ontheozone produced
by irradiating a wide variety of olefins and paraffinic hydrocarbons
along with a few other types of compounds at concentrations of 3 or
6 ppm with 1 or 2 ppm of either nitric oxide or nitrogen dioxide. The
ozone concentrations reported were the equilibrium values.
From 0.38 to 1.1 ppm of ozone was formed during irradiation of
the various olefins with nitrogen oxides (1). The ozone yields from the
terminal double-bonded monoolefin nitrogen oxide irradiations over-
lapped those from the internally double-bonded monoolefin nitrogen
oxide irradiations. This result is not in agreement with that obtained
by rubber-cracking measurements, which indicated that more ozone
was produced by irradiating internally doublebonded monoolefins
with nitrogen oxides. Similarly, irradiation of ethylene with nitrogen
oxide produced the highest ozone concentration reported in this latter
study, whereas irradiated ethylene nitrogen dioxide mixtures caused
less rubber cracking than that produced from other olefin nitrogen
dioxide mixtures. The xylene used in the study reported by Schuck
and Doyle was presumably p-xylene; when irradiated with nitrogen
oxide, this xylene participated in the formation of only 0.18 ppm of
ozone. Although p-xylene was not used in the rubber-cracking measure-
ments, m-xylene and o-xylene were used; when irradiated with nitrogen
dioxide, these aromatic hydrocarbons participated in the formation
of as much ozone as was produced by irradiation of olefin nitrogen
dioxide mixtures.
Irradiation of several five- and eight-carbon paraffinic hydrocarbons,
with the nitrogen oxides resulted in the production of only 0.15 to
0.26 ppm of ozone. Whenmethane, methylethylketone, trichloroethylene,
and tetrachloroethylene were irradiated with nitrogen oxides, very little
or no ozone was detected (1). These results appear to be in general
agreement with the small or undetectable amounts of rubber cracking
observed in the irradiation of similar systems.
In a later study (9) oxidant measured colorimetrically by the potas-
sium iodide method was determined after irradiation of a number of
different aromatic hydrocarbon nitric oxide systems, aldehyde
nitric oxide systems, and several others (Table 5). Oxidant con-
10 Reactivity Of Organic Substances
-------
centrations from 0.65 to 1.1 ppm were produced when p-xylene,
o-xylene, m-xylene, and 1,3,5-trimethylbenzene were irradiated with
nitric oxide. Toluene nitric oxide mixtures produced 0.5 ppm of
oxidant when irradiated. The oxidant yields from the irradiation of
acetylene nitric oxide, n-nonane nitric oxide, andpropionaldehyde
nitric oxide mixtures were 0.0, 0.2, and 1.0 ppm, respectively.
A recent investigation (7) with both static and dynamic reaction
conditions confirms the fact that aromatic hydrocarbons (except
benzene) and several of the aliphatic aldehydes, when irradiated with
nitrogen oxides, produce oxidant concentrations within the same range
as those measured in irradiated olefin nitrogen oxide mixtures
(Table 5). Under dynamic flow conditions no oxidant was detected
during the irradiation of four paraffinic hydrocarbons containing six
or eight carbon atoms.
The results discussed above indicate that olefins, aromatic hydro-
carbons, and aldehydes all should contribute to atmospheric ozone
and oxidant levels. The ability of higher-molecular-weight paraffinic
hydrocarbons to participate in oxidant formation under static conditions
is on the average only about a third to a fifth that of the aromatic
hydrocarbons, olefins, or aldehydes. The overall reactivity of the six-
carbon and larger paraffins with respect to oxidant formation also
appears to be small because no oxidant was detected when these hydro-
carbons were irradiated with nitrogen oxides under dynamic conditions.
The one- through five-carbon paraffins have no tendency or only a
very slight tendency to participate in oxidant formation, even in static
experiments.
Aldehydes
Some investigators attribute much of the eye irritation effects observed
to the presence of formaldehyde and acrolein in polluted atmospheres
(1). Plant damage also has been caused by irradiation of certain
aldehydes (7, 12). Although some of the atmospheric aldehydes come
directly from combustion sources, photooxidation reactions probably
contribute a large portion of these aldehydes.
Formaldehyde is produced by the photooxidation of almost all olefins
and aromatic hydrocarbons in the presence of nitrogen oxides (1,7,9).
Photooxidation of some of the higher aldehydes also could contribute
some formaldehyde (7). Other aliphatic aldehydes, i.e., acetaldehyde,
propanal, butanal, acrolein, and others are largely formed from olefin
nitrogen oxide reactions (1). The photooxidation of higher-molecular-
weight aldehydes produces aldehydes with smaller numbers of carbon
atoms (7). Although aromatic hydrocarbons (except benzene) produce
formaldehyde when irradiated with nitrogen oxides, there is no evidence
that acetaldehyde, propanal, or other such aliphatic aldehydes exist
In Atmospheric Photooxidation Reactions 11
-------
as reaction products. (7,9). Analyses for total aldehydes do indicate the
presence of aldehydes other than formaldehyde (7,9). Analyses for
dicarbonyl compounds indicate the formation of molecules with two
carbonyl groups as products from the photooxidation of aromatic
hydrocarbons with nitrogen oxides (9). Thus, it appears that the
aldehyde yields from aromatic hydrocarbons consist of formaldehyde
along with aldehydes containing two aldehydic groups.
Molar yields of formaldehyde based on initial olefm concentrations
usually are in the 0.3 to 0.5 range (Table 6). Molar yields of for-
maldehyde based on initial dialkyl or trialkylbenzene concentrations
range from 0.15 to 0.2. On the basis of these results, it may be expected
for equal concentrations of the two classes of hydrocarbons that the
yield of formaldehyde from aromatics should be about half that from
olefins.
Molar yields of aliphatic aldehydes cover a wider range than do
those of formaldehyde (Table 6). Molaryieldsfrom aliphatic aldehydes
TABLE 6. ALDEHYDE YIELDS FROM PHOTOOXIDATION OF HYDRO-
CARBON NITROGEN OXIDE MIXTURES
Moles of aldehyde per mole of hydrocarbon a
Hydrocarbon
Ethylene
Propylene
1-Butene
Isobutene
Trans-2-butene
1,3-Butadiene
1-Pentane
2-Methyl-2-butene
Cis-2-hexene
Toluene
Xylenes
1,3,5-Trimethylbenzene
Formaldehyde Acrolein
0.3-0.4
0.4
0.4
0.5- (5.7
0.35
0.3-0.5 0.2-0.3
0.5
0.3- 0.5
.05
0.15- 0.2
0.15- 0.2
Aliphatic
Aldehydes
0.3- 0.4
0.6-0.8
0.65
0.5-0.7
1.25- 1.55
0.5-0.8
1.0
0.8- 1.2
0.9- 1.0
.1
0.2- 0.3
0.3-0.4
lBased on hydrocarbon initially present as reactant.
also are more uncertain because of the limitations in the measurements.
The yields reported from aromatic hydrocarbons may be too low
because the analytical method used does not respond well to dicarbonyl
compounds.
12 Reactivity Of Organic Substances
-------
Very little or no formaldehyde or other aldehydes are detected in
the irradiation of any paraffinic hydrocarbon nitrogen oxide system.
Similarly, no formaldehyde is detected as a product when nitrogen
oxides are irradiated in the presence of methylethyl ketone, trichloro-
ethylene, or tetrachlorethylene.
Ketones
Ketones such as acetone and methylethyl ketone are formed from the
photooxidation of olefins such as isobutene, 2-methyl-2-butene, and
other branched-chain olefins (1). Acetone and methylethyl ketone show
little photochemical reactivity (1,9,11), and at present there is no
evidence that they cause any direct biological effect.
Peroxyacyl Nitrate Yields
The members of the peroxyacyl nitrate series have been shown to
cause plant damage and eye irritation (12). Peroxyacyl nitrate yields
from irradiation of a number of olefins and aromatic hydrocarbons
with nitrogen oxide at 5 ppm of a reactant have been reported (2-4).
Peroxyacyl nitrate yields from 0.55 to 1.0 ppm were formed from the
irradiation of propylene, several butenes, and hexenes with nitric oxide
or nitrogen dioxide. No peroxyacyl nitrate was formed from ethylene.
The internally double-bonded olefins when irradiated with nitrogen
oxides produced more peroxyacyl nitrates than did the olefins with
terminal double bonds. The xylenes, 1,2,4,5-tetramethylbenzene, and
1,3,5-trimethylbenzene reacted in the presence of nitrogen oxides to
form 0.4 to 0.8 ppm of peroxyacyl nitrates.
The peroxyacyl nitrate yields from four irradiated olefin nitrogen
oxide systems were studied as a function of initial nitrogen oxide
concentration. The yields depended on whether nitric oxide or nitrogen
dioxide was used as the reactant. The maximum yield did not occur at
the same ratio of reactants from system to system. The maximum yields
from the tetramethylethylene and trans-2-butene nitrogen dioxide
systems were about twice those from the propylene or isobutene
nitrogen dioxide systems. The effect of variations of initial nitrogen
oxide concentration on peroxyacyl nitrate closely resembled the be-
havior observed for ozone or oxidant.
The peroxyacyl nitrates were observed as products from irradiation
of propionaldehyde nitrogen oxide mixtures (7). Irradiations of C i
to C s paraffins, n-hexane, 3-methylpentane, 2,3-dimethylbutane, cyclo-
hexane, and isooctane with nitrogen oxides did not produce peroxyacyl
nitrates in detectable yields (1,2,7). Very small yields (0.02 to 0.05) of
peroxyacyl nitrates were reported from the irradiation of 2-methyl-
heptane and 2-methylpentane with nitrogen oxide (1).
In Atmospheric Photooxidation Reactions 13
-------
Aerosol Formation as a Measure of Reactivity
Visibility reduction caused by atmospheric aerosols is one of the
characteristics of the photochemical type of air pollution. It is well
known that particulate matter in the proper size range can effectively
penetrate the respiratory tract. A synergistic effecthas been demonstrated
with respect to certain respiratory function tests when some gases are
adsorbed on particulates. There does not appear to be clear proof,
however, that adverse respiratory effects are caused by aerosols formed
in photochemical reactions Eye irritation effects are not increased when
photochemically produced aerosols are present (13).
Olefins of five or fewer carbon atoms when irradiated in the presence
of nitrogen dioxide form very little aerosol (1,14) (Table?). The corres-
ponding olefin nitrogen dioxide sulfur dioxide mixtures form large
TABLE 7. EFFECT OF SULFUR DIOXIDE ON AEROSOL FORMATION
IN NITROGEN DIOXIDE HYDROCARBON REACTIONS a
Photometer reading b
Hydrocarbon Olefin, NO 2 Olefin, NO 2 SO 2
Ethylene
1-Butene
2-Butene
1-Pentene
2-Pentene
1-Hexene
2-Hexene
3-Heptene
2,4,4- Trimethyl-1-pentene
n-Butane
2-Methylpentene
0.1
0.05
0.05
0.1
0.0
0.05
0.25
0.45
3.3
2.2
2.85
3.0
3.05
3.0
3.35
3.05
3.35
3.3
0.1
0.0
aReference 14.
b Readings before irradiation were subtracted from readings during irradiation.
Reactant concentration, ppm: olefin, 10; NO , 5; SO , 2.
amounts of aerosol upon irradiation (1,14). The irradiated ethylene
nitrogen dioxide sulfur dioxide mixture produces much less aerosol
than do mixtures containing the other olefins. Olefins with internal
double bonds do not appear to participate in the formation of any
more aerosol than is formed by irradiating olefins with terminal double
bonds with nitrogen dioxide and sulfur dioxide. The aerosol in all of
these systems at appreciable relative humitities is sulfuric acid aerosol
(15,16). Six-carbon and higher-molecular-weight olefins when ir-
radiated with nitrogen dioxide can form aerosol, but a large increase
14 Reactivity Of Organic Substances
-------
in aerosol occurs when sulfur dioxide is added to most of these
systems (17).
Aromatic hydrocarbon nitrogen dioxide mixtures when irradiated
in the absence of sulfur dioxide can produce aerosol (7,17). Aerosol
also is produced by the irradiation of aromatic hydrocarbon
nitrogen dioxide sulfur dioxide mixtures (17). The quantitative effects
of sulfur dioxide on aerosol formation in these systems have not been
studied in detail. Intensity appears to be a more critical experimental
parameter in the systems containing aromatic hydrocarbons (16).
No aerosol formation has been observed when aldehyde nitrogen
dioxide systems have been irradiated. No data appear to be available
on irradiated aldehyde nitrogen dioxide sulfur dioxide mixtures. No
significant level of aerosols has been reported from the irradiation of
either paraffinic hydrocarbon nitrogen dioxide or paraffinic hydro-
carbon nitrogen dioxide sulfur dioxide mixtures (1,14).
When irradiated with nitrogen dioxide, reactive organic molecules
of higher molecular weight appear capable of forming what must be
an organic aerosol. Lower-and higher-molecular-weight olefms and
aromatic hydrocarbons both form aerosol when irradiated in the pre-
sence of nitrogen dioxide and sulfur dioxide together.
Plant Damage as a Measure of Reactivity
A number of plant-damaging substances are produced by reactants
or products of photochemical reactant. Ethylene can cause damage to
ornamental plants, cotton, and tomatoes. Ozone and peroxyacyl
nitrates also are phytotoxicants for many plant species. The irradiation
of most olefin nitrogen oxide and aromatic hydrocarbon nitrogen
oxide mixtures produces phytotoxicants including ozone and peroxy-
acyl nitrates. Irradiation of ethylene nitrogen oxide mixtures has
failed to produce plant damage (except damage due to ethylene itself)
to tobacco wrapper, pinto bean, petunia, cotton, or endive (7,18). No
peroxyacyl nitrate-type compound is formed from this system (2,7).
The diolefin 1,3-butadiene also failed to produce damage to tobacco
wrapper, pinto bean, or petunia (7). Other types of olefins when ir-
radiated with nitrogen oxide do produce phytotoxicants.
Damage to tobacco wrapper, pinto bean, and petunia is produced
by irradiated 1,3,5-trimethylbenzene nitrogen oxide mixtures (7).
Other of the more reactive aromatichydrocarbons that produce appreci-
able yields of peroxyacyl nitrates also should cause plant damage.
Olefin ozone mixtures that contain olefins with five carbon atoms
or more and particularly those that can give four-carbon or larger
fragments produce a short-lived phytotoxicant that causes damage to
pinto bean and tobacco (12). This phytotoxicant apparently is not
In Atmospheric Photooxidation Reactions 15
-------
responsible for most types of plant damage termed typical oxidant
damage. This type of damage is more similar to that caused by
peroxyacyl nitrate alone or to that produced by irradiating olefin or
aromatic hydrocarbon mixtures.
Irradiation of propionaldehyde and higher saturated aldehydes alone
(7,12) and in the presence of nitrogen oxides (7) causes damage to
tobacco wrapper, pinto bean, and petunia similar to that caused by
irradiating olefin nitrogen oxide mixtures. Formaldehyde and
acetaldehyde do not cause significant plant damage when irradiated
in air (7,12). Formaldehyde nitrogen oxide mixtures also do not
produce a phytotoxicant (7).
In a study of the relative pi ant-dam aging potential of four hydro-
carbons irradiated with nitrogen oxide, the severity of damage to
pinto bean was in the following descending order: cis-2-butene, propy-
lene 1-hexene, isopropylbenzene (12). In another investigation mod-
erate plant damage by irradiated propylene nitrogen oxide mixtures
was investigated in detail (18). In the same investigation no plant
damage was observed from irradiated ethylene nitrogen oxide or
acetylene nitrogen oxide mixtures (18).
Recently, the plant-damage indices were determined for ethylene,
1-butene, 1,3-butadiene, toluene, 1,3,5-trimethylbenzene, formaldehyde,
propionaldehyde, n-hexane, 3-methylpentane, 2,4,4-trimemylpentane,
and cyclohexane (7). On the basis of data from irradiations of 2 to 3
ppm of organic substance with nitrogen oxide, theseverity of damage to
pinto bean and petunia was as follows: 1,3,5-trimethylbenzene, severe;
propionaldehyde, severe; 1-butene, moderate to severe; all other hydro-
carbons, none.
The results discussed above are summarized in Table 8. If the
damage to young pinto bean and petunia is largely due to peroxyacyl
nitrates, then substances that form high yields of peroxyacyl nitrates,
such as 2-butene, 1-butene, and mesitylene, should cause moderate to
severe damage and substances that form little or no peroxyacyl nitrates,
such as ethylene, 1,3-butadiene, formaldehyde, acetylene, toluene, and
paraffinic hydrocarbons, should not cause any appreciable damage.
The degree of damage to pinto bean and petunia has been shown to
be sensitive to reactant ratio in a study of irradiated auto exhaust
under dynamic flow conditions (Table 9). When the ratio of hydro-
carbon (carbon ppm) to nitrogen oxide was 3:1 and sometimes 6:1,
no plant damage was observed. Under these experimental conditions,
no oxidant was produced in the irradiated mixtures and the conversion
of nitrogen dioxide to products that would include the peroxyacyl
nitrates was minimal. Consequently, the measurements of product
formation indicate, as they should, that little or none of the phytotoxi-
16 Reactivity Of Organic Substances
-------
TABLE 8. PLANTDAMAGEFROM IRRADIATED ORGANIC SUBSTANCE
NITROGEN OXIDE MIXTURES
Substance
2-Butene
1,3,5- Trirnethylbenzene
Propionaldehyde
1-Butene
Propylene
1-Hexene
Isopropylbenzene
Ethylene
Toluene
Formaldehyde
Acetylene
Hexanes
Isooctane
a References 7
and 12.
Severity of plant damage
to pinto bean or Petunia a
severe
severe
severe
moderate to severe
moderate
moderate
light
none
none
none
none
none
none
TABLE 9. PLANT INJURY INDEX ^b
Young pinto bean: lower-surface
glazing-type injury
0
3
6
12
0
3
6
12
NOX,
1/4 1/2
0.4 0.1
2.0
3.0
ppm
1 2
0
0 0
1.0 0
3.5 0
Petunia: total injury
NOX,
1/4 1/2
1.1 1.2
2.6
2.0
(all phytotoxicants)
ppm
1 2
0
0 0
1.3 0
3.0 0
aNote: Index based on scale of 0 to 4; 4-maximum damage.
b Reference 10.
In Atmospheric Photooxidation Reactions
17
-------
cants that have been identified are present when there is no plant
damage.
Irradiation of mixtures containing either about 0.5 ppm of ethylene,
1-butene, and trans-2-butene or 0.5 ppm of the xylenes with 1 ppm of
nitrogen oxides does not produce significant plant damage (7). When
mixtures containing 0.5 ppm of both the xylenes and the olefins with
1 ppm of nitrogen oxides (twice the hydrocarbon to nitrogen oxide
ratio) are irradiated, moderate damage is experienced by pinto bean
and petunia. Thus, the presence of both aromatic hydrocarbons and
olefins in the irradiated mixtures is necessary to reproduce the plant
damage results in irradiated auto exhaust.
Eye Irritation as a Measure of Reactivity
Eye irritation as it occurs in areas exposed to the photochemical
type of air pollution is an aggravating effect. No definite clinical
condition has been associated with this effect. Irritation of the
nose and throat also can occur. Laboratory measurements in
recent years have been confined to obtaining the subjective response
of human panelists to irritation of their eyes by various mixtures.
Time to initial irritation or intensity of irritation is reported by each
panelist. The results are averaged and reported as a single time in
seconds or minutes or as a number on an intensity scale.
Eye irritation responses reported in two studies are shown in Tables
10 and 11 (1,10). Additional work has been done elsewhere (19).
The available results show that ethylene participates in forming smaller
amounts of eye irritating substances than do other olefins and that
1,3-butadiene and other diolefins form larger amounts of irritating
substances than do monoolefins. The more limited data on aromatic
hydrocarbons and aldehydes indicate that upon irradiation with
nitrogen oxides these classes of substances form as much irritant
material as do all but the most reactive olefins. Irradiation of hexanes
or octanes at 3 ppm (18 to 24 carbon ppm) with nitrogen oxide
produces few or no eye irritants (1,7). Another study of eye irritation
from irradiated hexane nitrogen dioxide mixtures produced no irri-
tation at 5 ppm (30 carbon ppm) of hexane and 2.5 ppm of nitrogen
dioxide (19). At 10 ppm and above (60 carbon ppm and above)
of hexane and 5 ppm and above of nitrogen dioxide, irritation was
experienced after several hours of irradiation of such mixtures. Alcohols
and acetylene when irradiated with nitrogen oxide cause very little
if any eye irritation (19). The data on chlorinated hydrocarbons and
ketones are somewhat conflicting. Although acetone and perchloro-
ethylene have been reported as inactive (1,19), higher-molecular-
weight ketones and trichloroethylene have been reported as causing
significant eye irritation in one study and no eye irritation in another
(1,19).
18 Reactivity Of Organic Substances
-------
TABLE 10. COMPARISON OF OBSERVED EYE IRRITATION AND EYE
IRRITATION CALCULATED FROM FORMALDEHYDE AND
ACROLEIN CONCENTRATIONS a
Concentration
Compound ppm
Ethylene
Ethylene
Propylene
1-Butene
Trans- 2-butene
2-Methyl-2-butene
1-Pentene
3-Heptene
Cyclohexene
1,3-Butadiene
2-Methyl-l, 3-butadiene
Isopentane
n-Pnetane
3-Methylpentane
Methane
2, 2,4-Trimethylpentane
Methylethyl ketone
Xylene
3
6
3
3
3
3
3
3
3
3
3
9
6
6
6
3
3
3
Concentration Calculated
ofNOx, eye
ppm irritationb
1
2
1
1
1
1
1
1
1
1
1
3
2
2
2
1
1
1
1.8
8.8
8.0
5.6
4.0
6.0
10.8
9.2
6.0
3.6
19.9
0.8
0.8
0.8
2.0
0.8
0.0
0.8
Observed
eye
irritationb
2.4
8.8
7.9
5.6
7.6
4.0
14.8
7.6
8.0
2.4
16.8
3.2
2.8
2.4
3.2
2.0
1.6
5.2
^Reference 1. b Arbitrary Units (24 maximum).
TABLE 11. EYE IRRITATION RESPONSE TO IRRADIATED ORGANIC
SUBSTANCE NITROGEN OXIDE MIXTURE UNDER
DYNAMIC CONDITIONS a
Concentration,
Organic compound ppm
n-Hexane
3-Methylpentane
Cyclohexane
Isooctane
Ethylene
1-Butene
1-Butene
1-Butene
1,3-Butadiene
Tolune
1, 3, 5- Trimethylbenzene
Formaldehyde
Propionaldehyde
2.93
2.92
3.45
2.84
3.15
1.11
3.25
4.87
3.30
1.93
0.41
6.1
3.5
Concentration
of NOX, ppm
1.1
1.1
1.15
1.1
1.0
0.7
0.95
1.0
.85
1.05
1.1
0.9
0.9
Observed eye
irritation index b
3
0
0
3
7
7
15
17
22
10
8
17
10
a Reference 7. b Based pm 0 to 30 intensity scale.
In Atmospheric Photooxidation Reactions
19
-------
Irradiation of mixtures containing either about 0.5 ppm of ethylene,
1-butene, and trans-2-butene or 0.5 ppm of the xylenes along with 1
ppm of nitrogen oxide produced only slight eye irritation (7). When
mixtures containing both 0.5 ppm of the olefins and 0.5 ppm of xylenes
along with 1 ppm of nitrogen oxides were irradiated, moderate eye
irritation was experienced. Both the olefin and aromatic hydrocarbon
contents of diluted auto exhaust must be present in order to simulate
comparable eye irritation effects.
In other measurements on irradiated auto exhaust mixtures, eye
irritation was shown to increase with increases in either olefin or
aromatic hydrocarbon concentration in the fuel (19). The exhaust
from a fuel containing 72 percent aromatics, 8 percent olefin, and
20 percent saturates caused about as much eye irritation when
irradiated as was caused by irradiating exhaustfroma fuel containing
38 percent aromatics, 23 percent olefins, and 39 percent saturates.
When irradiated, exhaust from a high paraffinic fuel caused much less
eye irritation.
DISCUSSION
All of the previous discussion has been based on whatever laboratory
data were available, irrespective of theconcentrationsofreactants used.
Usually, the concentration of each individual component has been well
in excess of the total atmospheric concentration of hydrocarbons on
smoggy days. This approach is often necessary in the laboratory to
obtain even reasonably reliable experimental results. In the atmosphere
the concentrations of these substances may cover at least a 100-fold
range from 0.001 to 1 ppm or more (for methane).
It is essential to consider the range of concentrations in polluted
atmospheres and the highest reported concentrations of various com-
ponents. Although a considerable amount of data has been obtained
on light hydrocarbons (20,21), only a few measurements are available
on higher-molecular-weight aliphatic hydrocarbons and on aromatic
hydrocarbons (20). Consequently, the ranges and highest reported
values are more uncertain for these substances. The values used are
not those obtained immediately adjacent to heavy vehicle traffic. The
available measurements of this sort indicate that these concentrations
can be several times the values obtained at locations more represen-
tative of general area pollution.
The estimated ranges and highest values have been rounded off and
are shown in Table 12. These values are only approximate, but are
reasonably consistent with total carbon loadings. Higher values than
the highest reported to the present are certainly possible. The lower
limits in the range are not the lowest values present at times of low
pollution and high rates of ventilation. Under these circumstances,
20 Reactivity Of Organic Substances
OPO 82OSSB-3
-------
the concentrations of most individual components would not be detect-
able analytically and the total concentration of hydrocarbons can be
as low as 1.3 to 1.5 ppm.
TABLE 12. ATMOSPHERIC CONCENTRATIONS OF VARIOUS ORGANIC
SUBSTANCES
Concentration of organic, ppm by volume
Component
Methane
Ca- Cs Paraffins
Ce + Paraffins
Acetylene
Ethylene
Propylene
1-Butene + isobutene
2-Butene (cis + trans)
1,3-Butadiene
1-Pentene
2-Pentenes (cis + trans)
2-Methyl-l-butene
2-Methyl-2-butene
C e + Olefins
Benzene
Toluene
Ethylbenzene
Xylenes
Formaldehyde
Acrolein
Probable normal range in
polluted atmosphere
2
0.1
0.03 -
0.03 -
0.02
0.005 -
0.002
0.001 -
0.001
0.001
0.002
0.001 -
0.002 -
0.01
0.01
0.01
0.002 -
0.01
0.02
0.002 -
6
0.5
0.15
0.25
0.2
0.04
0.01
0.005
0.005
0.005
0.01
0.005
0.01
0.05
0.05
0.06
0.01
0.05
0.1
0.01
Highest reported
value
7
0.7
-0.3- 0.4
0.3
0.3
0.05
0.01 -0.02
0.01 '
0.01
0.01
0.01 -0.02
0.01- 0.02
0.01 - 0.02
- 0.1-0.2
0.07
0.09
0.01 - 0.02
0.07
0.2
0.01 - 0.02
Because of a lack of analytical data on atmospheric concentrations,
estimates of the atmospheric levels of nine-carbon and higher aromatic
hydrocarbons, most types of oxygenated organic compounds, chlori-
nated hydrocarbons, and organic nitrates have not been included in
Table 12. Rough estimates can be made of the atmospheric concen-
trations of some of these substances. Auto exhaust and blowby data
(22,23) indicate that the nine-carbon aromatic hydrocarbons and
styrene may be present at concentrations almost equal to that of the
three xylenes, which could correspond to atmospheric concentrations
as high as 0.05 ppm. The trimethylbenzenes in auto exhaust and
blowby are about equal in concentration to m-xylene plus p-xylene,
which could correspond to atmospheric concentrations as high as
0.03 ppm. Higher aldehydes appear to about equal formaldehyde in
concentration (6,24). Saturated aldehydes of higher molecular weight
than acetaldehyde may made up half of the total aldehydes above
In Atmospheric Photooxidation Reactions
21
-------
formaldehyde. Peroxyacetyl nitrate has been analyzed in some atmos-
pheric samples recently, and concentrations appear to range from a
few parts per billion to several hundredths of a part per million in
polluted air (25).
The paraffinic hydrocarbons, acetylene, and benzene make up 90
percent or more of the hydrocarbon on the basis of parts per million
by volume and 60 to 80 percent on a carbon basis, according to
various analyses of urban atmospheres. The methane alone can con-
stitute 70 to 80 percent of the hydrocarbon and one-third to two-thirds
of the carbon, even in a highly polluted atmosphere. Thus, much of
the hydrocarbon present consists of substances that have been found
to be essentially unreactive in laboratory investigations.
The olefins, including ethylene, in the atmosphere probably occur
at almost twice the concentration of the aromatic hydrocarbons, ex-
cluding benzene. The aldehydes are unique in that they are products
of the hydrocarbon nitrogen oxide reactions and also are capable of
reacting further with nitrogen oxide in sunlight. Formaldehyde and
acrolein are eye irritants produced in major part by irradiation but
also by combustion processes. The photooxidation of acetaldehyde and
higher saturated aldehydes results in the formation of more formalde-
hyde, peroxacylnitrates, and oxidant and thus with prolonged ir-
radiation tends to maintain the biological reactivity of the atmospheric
mixture.
If the concentrations on the high end of the probable concentration
ranges estimated in Table 12 are used along with the estimates dis-
cussed above, the total concentration of these substances at an early
morning traffic peak in Los Angeles could be about 7.7 ppm by volume
or approximately 13 carbon ppm. These values do correspond to the
higher values actually measured. Excluding the paraffins, acetylene,
and benzene, this atmospheric mixture contains 0.7 ppm by volume or
4 carbon ppm of olefins, aromatics (except benzene), and aldehydes.
This estimate excludes ketones, organic nitrates, chlorinated hydro-
carbons, and other possible organic constituents. It is of interest to
compare this composition with that in auto exhaust having about the
same total carbon loading (6). The auto exhaust mixture, excluding
the paraffins, acetylene, and benzene, contained 1.1 ppm by volume
or 5.7 carbon ppm of olefins, aromatics (except benzene), and alde-
hydes. Consequently, for a given total carbon loading, if the same
distribution of reactive hydrocarbons is assumed, the irradiated auto-
mobile exhaust mixture would be expected to cause high levels of
biological effects.
The effects of auto exhaust irradiations on eye irritation and aerosol
formation do not appear to be as marked as might be predicted from
the relative concentrations of reactive substances in auto exhaust com-
22 Reactivity Of Organic Substances
-------
pared to those in the atmosphere. In the laboratory the concentration
of organic substances often must be several parts per million before
severe eye irritation and greatly reduced visibility occur. Such con-
centrations are three or four times higher than the maximum atmos-
pheric concentrations (1,7,12). The differences in laboratory and
atmospheric responses can be attributed to a variety of factors. These
factors include the presence of much lower levels of small particles and
condensation nuclei in laboratory mixtures than in the atmosphere, less
than adequate analytical data on polluted atmospheres, or artifacts
created by laboratory conditions. This latter category would include
wall and sampling line losses of particles and active species and the
arbitrary use in the laboratory of eye exposures alone rather than eye,
nose, and throat exposures, along with differences in the intensity and
distribution of light in the laboratory and in the atmosphere.
From the control standpoint it is desirable that olefins and aromatic
hydrocarbon levels from vehicle exhausts, evaporation losses, and sol-
vent usage be reduced as much as possible. The reduction of most
paraffinic hydrocarbons, acetylene, or benzene appear not to be justified
on the basis of reactivity considerations alone. Naturally, there is no
reason why these substances should not be reduced along with reactive
substances if no additional costs are involved. Since aldehydes also are
reactive, hydrocarbons should be converted by control devices to water
and carbon dioxide, not to intermediate oxygenated substances.
If either the atmosphere or emission sources are to be monitored
routinely for reactive substances, it is obvious that new instrumental
approaches for organic substances ar.e essential. Although gas
chromatography will provide detailed hydrocarbon analysis, an
analysis for one- to eight- or ten- carbon hydrocarbons is involved,
time-consuming, and expensive. For certain research projects such
detailed measurements are desirable; however, these techniques are
difficult to apply to large numbers of routine analyses for extended
periods of time.
One approach is to find a way to remove the unreactive substances
or conversely to remove the reactive substances and determine them
by difference. For atmospheric analyses a method of monitoring
methane is of considerable utility, since methane constitutes the greater
part of the unreactive material. A method being developed permits such
an analysis (26,27). Another procedure involves the removal of
unsaturates by use of various chemical adsorbents in a differential
determination by flame ionization analysis (28).
A compromise can be made between analysis with an elaborate gas
chromatograph and an analysis limited to a flame ionization or non-
dispersive infrared analyzer. The methane through pentane and acety-
lene composition can be determined with a single chromatographic
In Atmospheric Photooxidation Reactions 23
-------
unit. If total carbon is also determined with a flame ionization analyzer,
the sum of the lower-molecular-weight hydrocarbons expressed as
carbon ppm can be subtracted from the total carbon loading. The
difference can be attributed to reactive substances. In atmospheric
studies this procedure would exclude almost all of the unreactive sub-
stances. In auto exhaust this procedure should subtract much of the
unreactive material. Most of the remaining carbon in the essentially
unreactive or very slightly reactive substances would be in the hexanes
and higher-molecular-weight paraffinic hydrocarbons.
There are many uncertainties in defining the relationship between
the chemical, physical, and biological properties of complex atmospheric
systems. Much additional research is needed on both the chemical and
biological aspects of these systems. It isnotfeasible to assign a precise
measure to each organic substance, but this is not essential for most
purposes. If a reasonable background of research data is available on
the major classes and sub-classes of organic substances, it is possible
to define the problem and recommend the most desirable modifications
in atmospheric composition. Certainly, our present knowledge is suf-
ficient to suggest strongly that control measures can be modified from
the present gross emission standards to standards based on the
chemical, physical, and biological reactivity of the organic substances
produced by various sources of emissions. An important need now is
to develop techniques and instrumentation that allow monitoring for
compliance with such standards for emission sources and also for the
atmosphere in a practical and convenient manner.
24
-------
REFERENCES
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containing oxides of nitrogen and sulfur dioxide. Report No. 29, Air
Pollution Foundation, San Marino, Calif. Oct. 1959.
2. E. R. Stephens and W. E. Scott. Relative reactivity of various hydrocarbons
in polluted atmosphere. Proc. Am. Petrol. Inst. 42 (III):665-70. 1962.
3. C.S. Tuesday. The atmospheric photooxidation of olefins: the effect of
nitrogen oxides. Arch. Environ. Health. 7:188-201. 1963.
4. S. L. Kopczynski. Photooxidation of alkylbenzene nitrogen dioxide
mixtures in air. Intern J. Air Water Pollution. 8:107-20. 1964.
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chamber tests of automotive exhaust. Part II Chemical effects. JAPCA.
12:522-23. 1962.
6. P. W. Leach, L. J. Leng, T. A. Bellar, J. E. Sigsby, Jr., and A. P.
Altshuller. Effect of HC/NO ratios on irradiated auto exhaust. Part II.
JAPCA. 14:176-83. 1964.
7. A. P. Altshuller, D. L. Klosterman, P. W. Leach and J. E. Sigsby. The
irradiation of single and multi-component hydrocarbon and aldehyde
nitric oxide mixtures in air under dynamic and static flow conditions.
Presented at 147th natl. Meeting, Am. Chem. Soc., Philadelphia, Pa.,
April 5-10, 1964. To be published in Intern. J. Air Water Pollution.
8. P. A. Leighton. Photochemistry of air pollution. Academic Press, New York,
N. Y. 1961.
9. A. P. Altshuller and I. R. Cohen. Structural effects on the rate of nitrogen
dioxide formation in the photooxidation of organic compound nitric
oxide mixtures in air. Intern. J. Air Water Pollution. 7:787-97. 1963.
10. M. W. Korth, A. H. Rose, and R. C. Stahman. Effects of hydrocarbon to
oxides of nitrogen ratios in irradiated auto exhaust. Part I. JAPCA.
14:168-74. 1964.
11. A. J. Haagen-Smit and M. M. Fox. Ozone formation in photochemical
oxidation of organic substances. Ind. Eng. Chem. 48:1484-87. 1956.
12. E. R. Stephens, E. F. Darley, O. C. Taylor, and W. E. Scott. Photochemical
reaction products in air pollution. Intern. J. Air Water Pollution. 4:79-
100. 1961.
13. G. J. Doyle, N. Endow, and J. L. Jones. Sulfur dioxide role in eye
irritation. Arch. Environ. Health. 3:657. 1961.
25
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14. M. J. Prager, E. R. Stephens, and W. E. Scott. Aerosol formation from
gaseous air pollutants. Ind. Eng. Chem. 52:521. 1960.
15. N. Endow, G. J. Doyle, andJ. L.Jones. The nature of some model photo-
chemical aerosols. JAPCA. 13:141-47. 1963.
16. L. Reckner, W. E. Scott, and R. H. Linnell. Aerosol formation from SO
photochemical systems. Presented at Air Pollution Symposium, 145th
natl. Meeting, Am. Chem. Soc., New York, N. Y., Sept. 8-13, 1963.
17. H. J. R. Stevenson, D. E. Sanderson, and A. P. Altshuller. Some effects
of sulfur dioxide on formation of photochemical aerosols. Presented at
Industrial Hygiene Meeting, Philadelphia, Pa., Apr. 1964. To be published
in Intern. J. Air Water Pollution.
18. W. W. Heck. Plant injury induced by photochemical reaction products
of propylene nitrogen dioxide mixtures. JAPCA. 14:255. 1964.
19. L. G. Wayne. The chemistry of urban atmospheres. Technical Progress
Report III. Los Angeles Air Pollution Control District. Dec. 1962.
20. A. P. Altshuller. Gas chromatography in air pollution studies. J. Gas
Chromatog. 1:6-20, 1963.
21. R. E. Neligan. Personal communication on data obtained by Los Angeles
County Air Pollution Control District.
22. J. E. Sigsby, Jr. and M. Korth. Composition of blowby emissions.
Presented at 57th Annual Meeting, APCA, Houston, Texas, June 21-25,
1964.
23. S. Goldenberg. Unpublished data, Scott Research Laboratories, San
Bernardino, California, March 1964.
24. A P. Altshuller and S. P. McPherson. Spectrophotometric analysis of
aldehydes in the Los Angeles atmosphere. JAPCA. 13:109-11, 1963.
25. E. R. Stephens and E. F. Darley. Atmospheric analysis for PAN. Pre-
sented at 6th Conf. on Methods in Air Pollution Studies, Berkeley, Calif.
Jan. 1964.
26. G. C. Ortman. Continuous selective monitoring of hydrocarbons: appli-
cation to determination of methane. Air Pollution Symposium, Division
of Water, Air and Waste Chemistry, 148th meeting, Am. Chem. Soc.,
Chicago 111., Aug. 30-Sept. 4, 1964.
27. A. P. Altshuller, G. C. Ortman, and B. E. Saltzman. Positive hydro-
carbons and non-methane hydrocarbons. 7th conf. of methods in Air
Pollution Studies, Los Angeles, Calif., Jan. 24-26, 1965.
28. W. B. Innes, W. E. Bambrick, and A. J. Andreatch. Hydrocarbon gas
analysis using differential chemical absorption and flame ionization de-
tectors. Anal. Chem. 35: 1198-1203. 1963.
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BIBLIOGRAPHIC: AltshuUer, A.P. Reactivity of organic substances in
atmospheric photooxidation reactions. PHS Publ. No. 999-AP-14.
July 1965. 26 pp.
ABSTRACT: The organic vapors emitted to urban atmospheres by motor
vehicles and other sources of emissions consist not only of paraffinic,
acetylenic, aromatic, and olefmic hydrocarbons, but also of aldehydes,
ketones, alcohols, phenols, and chlorinated hydrocarbons. To esti-
mate the contribution of each of these classes of compounds to photo-
chemical smog, one must know both their atmospheric concentrations
and their relative reactivities in atmospheric reactions.
A review of the available literature on concentration levels of
organic vapors in urban atmospheres indicates that much more
analytical work is needed. The existing data are adequate, however,
for the formulation of useful estimates.
Reactivities of organic substances in photooxidation reactions can
be considered from many standpoints. Rates of disappearance of the
organic substances, rates of disappearance of nitric oxide or of for-
mation and disappearance of nitrogen dioxide, and rates or maximum
yields of various products such as oxidant or organic nitrates all can
be used as chemical measurements of reactivity. Eye irritation, various
types of plant damage, and aerosol formation are indicators of re-
activity mat can be related only to a limited extent to chemical
BIBLIOGRAPHIC: AltshuUer, A.P. Reactivity of organic substances in
atmospheric photooxidation reactions. PHS Publ. No. 999-AP-14.
July 1965. 26 pp.
ABSTRACT: The organic vapors emitted to urban atmospheres by motor
vehicles and other sources of emissions consist not only of paraffinic,
acetylenic, aromatic, and olefinic hydrocarbons, but also of aldehydes,
ketones, alcohols, phenols, and chlorinated hydrocarbons. To esti-
mate the contribution of each of these classes of compounds to photo-
chemical smog, one must know both their atmospheric concentrations
and their relative reactivities in atmospheric reactions.
A review of the available literature on concentration levels of
organic vapors in urban atmospheres indicates that much more
analytical work is needed. The existing data are adequate, however,
for the formulation of useful estimates.
Reactivities of organic substances in photooxidation reactions can
be considered from many standpoints. Rates of disappearance of the
organic substances, rates of disappearance of nitric oxide or of for-
mation and disappearance of nitrogen dioxide, and rates or maximum
yields of various products such as oxidant or organic nitrates all can
be used as chemical measurements of reactivity. Eye irritation, various
types of plant damage, and aerosol formation are indicators of re-
activity that can be related only to a limited extent to chemical
Accession No.
Key Words:
Accession No.
Key Words:
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measurements of reactivity. The problems of developing a single
index of reactivity are considered.
The application of reactivity measurements to automobile exhaust
composition, to control devices, and to improvements in atmospheric
purity are discussed.
measurements of reactivity. The problems of developing a single
index of reactivity are considered.
The application of reactivity measurements to automobile exhaust
composition, to control devices, and to improvements in atmospheric
purity are discussed.
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