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
report the  results  of scientific and  engineering  studies of man's environment:
The  community, whether  urban, suburban,  or rural,  where he lives, works,
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                  AP - Air Pollution
                  AH - Arctic Health
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                  FP - Food Protection
                  OH - Occupational Health
                  RH - Radiological Health
                  WP - Water Supply and Pollution Control
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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

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(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

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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

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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

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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

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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

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  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

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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

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 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

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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 82O—SSB-3

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 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

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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

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 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

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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

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                           REFERENCES


 1.  E. A. Schuck and G. J. Doyle. Photooxidation of hydrocarbons in mixtures
     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.

 5.  J.  E.  Sigsby,  Jr.,  T.  A. Bellar,  and L. J. Leng. Dynamic irradiation
     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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