Atmospheric Photooxidation Reactions. Rates, Reactivity, and Mechanism for Reaction of Organic Compounds with Hydroxyl Radicals Richard A. Cox* and Richard G. Derwent Environmental Sciences Division, AERE, Harwell, Oxfordshire, OX1 1 ORA, United Kingdom
Michael R. Williams Department of Chemistry, University of Birmingham, Birmingham. B15 2TT United Kingdom
w The photooxidation of a number of organic species, representing several classes of atmospheric pollutants, has been studied in the presence of HONO, NO, and NO2 a t parts per million concentrations. The experiments give information on the rate and mechanism of the reaction with organic molecules of HO radicals produced by photolysis of HONO under these conditions. The data allow evaluation of relative rate constants for the reaction of HO with these organic species and the stoichiometry for NO to NO2 conversion during the subsequent oxidation steps. The implications of the results for the assessment of the atmospheric reactivity of organic pollutants with respect to photooxidant formation are briefly discussed. Hydrocarbons and other organic compounds are removed from the atmosphere by gas-phase photooxidation reactions, and in many cases photochemically generated HO is established as the major attacking species in these reactions. The overall process is a low-temperature oxidation comprising short reaction chains involving nitrogen oxides as chain carriers. Ozone is produced during the atmospheric photooxidation of hydrocarbons in the presence of NO,, and, in heavily polluted air, photochemical smog results. The potential of a given hydrocarbon for ozone or oxidant formation, Le., its reactivity, has been recognized for some time as an important factor in assessing the role played by these compounds in photochemical air pollution. A number of criteria have been used to define reactivity and to classify hydrocarbons according to a reactivity scale. These have been based on chemical reaction rate data from smog chamber experiments ( I ) , on intensity of eye irritation or other measurable effects of smog ( 2 ) ,and on the elementary rate constants for attack of HO on hydrocarbons and oxygenates ( 3 , 4 ) . In order to understand fully the role played by individual types of hydrocarbon and other organic species both in smog formation and in atmospheric chemistry generally, a detailed knowledge of the chemistry of the photochemical oxidation process is required. Some good data are now available for very simple species, e.g., CH4 ( 5 ) ,CH3CHO ( 6 ) ,CzH4, and C3H6 ( 7 ) ,but it is not very clear how useful this understanding is for extrapolation to the other more complex, but commonly encountered, organic atmospheric pollutants. In the present work, laboratory studies of the photooxidation of a number of individual organic species, representing several classes of commGn pollutants, have been carried out under conditions simulating those in the real atmosphere. Particular emphasis in the investigation has been placed on the factors that affect reactivity, i.e., rate of oxidation of the hydrocarbons, rate of the NO to NO2 oxidation process that leads to ozone generation, and the nature of the degradation products. Although this study is of a preliminary nature and only semiquantitative, it has demonstrated the usefulness of the technique for providing the required information. Experimental
Chemical Systems. The photolysis of gaseous nitrous acid, HONO, provides a convenient source of HO radicals for 0013-936X/80/0914-57$01 OO/O
@
1980 American Chemical Society
studying photooxidation reactions under simulated atmospheric conditions, Le., in air at 760 Torr pressure and ambient temperature (8,9).The reactions that occur when HONO is photolyzed in the near-ultraviolet region (300-400 nm) are as follows: HONO+hu+HO+NO HO
+ HONO
+
H20
+ NO2
kl
When the products NO and NO2 accumulate, the following become significant also: HO + NO(+M) HO
+ N02(+M)
+
+
HONO(+M)
k2
HON02(+M)
k3
When a hydrocarbon (RH) is present, it may be attacked by HO to yield free-radical products, e.g.: HO
+ RH
-
H20
+ R.
k4
The effect of adding RH depends on the subsequent chemistry of the radical Re. The general pattern that has emerged is that Re reacts with 0 2 to form peroxy radicals, which can oxidize NO to NO2 and eventually form a stable oxidized organic product together with an HO2 radical (5, 7). The latter is readily converted to HO by reaction with NO, so completing a chain reaction that is identical with that which may occur in the atmosphere. Re + 0
+ NO RO. + 0 2
ROy
2
+
+
H 0 1 + NO
+
RO2
RO.
+ NO2
+ HO2 HO + NO2
R’O
+
In this general scheme, two molecules of NO are oxidized to NO2 following each HO hydrocarbon reaction. Depending on the detailed chemistry, more or less NO to NO2 oxidation may occur and measurement of the NO to NO2 oxidation stoichiometry together with the yield of stable reaction products gives information concerning the reaction mechanism. The rate of removal of the hydrocarbon is equal to the quantity k4[OH][RH]. If two hydrocarbons are present in the system, their relative removal rates will be proportional to the ratio of the rate constants for HO attack, provided conditions are maintained so that HO is the only significant attacking species. These conditions are readily maintained when HONO photolysis is used as a source of HO, because the accompanying production of NO maintains low concentrations of 0 3 and HOz in the system. If k 4 is known for one hydrocarbon, the rate constant for reaction of HO with the second species may then be evaluated. Experimental Method. The HONO photosensitized oxidations were carried out in a 250-L bag made of Tedlar (Dupont), which was irradiated by two banks of 10 X 20 fluorescent lamps (Philips L20/05, wavelength range 300450 nm). The temperature during irradiation was -300 K. Mixtures containing 3 to 20 ppm of HONO with 4 . 3 - 3 ppm each of NO and NO2 in synthetic air were prepared from aqueous NaNO;?
+
Volume 14, Number 1, January 1980 57
Table 1. Relative Rate Constants for the Reaction of OH with Organic Species 101'k, cm3 r n o l e c u l e s - ' ~ - ~
this cornpd
I,
2b
6
I
I
I
I
12
18
2L
30
t iminl
Figure 1. Decay of hydrocarbons in photolysis with nitrous acid in
air
as described before (8). The required amount of organic compound was added either in liquid or vapor form by syringe injection, and the bag contents were allowed to mix until the measured concentrations of NO, and organic species were constant and reproducible. The mixture was then photolyzed for 30 min or longer, during which time the concentration changes of the various components were followed by analysis of successive samples withdrawn from the bag. Analysis of the NO, species was by chemiluminescence from the NO 0 3 reaction using a calibrated commercial instrument (Thermo-Electron Model 12A). The techniques for distinguishing NO, NOz, HONO, and "03 have been described in detail before (9). Organic species were measured by gas chromatography using both flame ionization (FID) detection (for hydrocarbons, ketones, aldehydes, etc.) and electron-capture detection (ECD) (for alkyl nitrates, peroxyacetyl nitrates, and biacetyl). The column packings used were either PEG 400 on Chromosorb (in various lengths of 4-mm glass columns) and Porapak Q (0.5 m X 4 mm). The temperature and other conditions were adjusted to suit the particular analysis. The FID was calibrated using aliquots of dilute mixtures of known composition of the pure compounds in air. The ECD response was not calibrated, since pure samples of the compounds of interest were not available. I t was only used for qualitative work. Products were identified as far as possible by comparison of retention times with pure compounds. Total aldehydes were also measured using the MBTH colorimetric method (10)and formaldehyde using the chromotropic acid technique (11).
+
Results Relative HO Rate Constants. Relative rate constants for HO attack were obtained by photolyzing mixtures of HONO with two or more organic substrate molecules. Figure 1 shows typical data plotted in the form In [RH] vs. t , where [RH] is the concentration of the hydrocarbon species. These logarithmic plots were slightly curved. This behavior is expected since the steady-state concentration of HO and hence the effective first-order rate constant k4[HO] should decline with time as HONO is photolyzed. To determine the relative values of k4, the decay constants for the initial stages of the reaction were compared. Table I shows a summary of the relative rate data; absolute values are based on a value of 8 X cm3 58
Environmental Science & Technology
J
olefins ethylene propene isoprene cyclohexene aromatics benzene toluene o-xylene m-xylene ketones acetone methyl ethyl ketone methyl isobutyl ketone methyl vinyl ketone paraffins propane n-pentane isopentane 2-methylpentane 2,3-dimethylbutane
work
lit. ( 72)
8 24 74 62
8.0 5 78 68
a
1.3
d
6.1 13.5
e
0.8 7.2
13.3 18.6
21.9
0.5
notes
b C
f f
9
2.6 12.4 14.0
1.9 5.0 3.5 5.0
3.8
3.4 14.0
h h
1.8 5.0 3.5 5.0 5.6
i
a Assumed value, which is mean of studies at high pressure including both relative and absolute measurements. Relative rate. Based on data of Grimsrud, Westberg, and Rasmussen (22) analyzed by Winer et al. ( 13). Mean of two relative rate studies. Large uncertainty due to slow decay of C6H6. a Mean of ail available values: absolute determinations give values approximately a factor of 1.5 higher than relative measurement. 'Both relative and absolute measurements available. Decay of acetone in previous relative rate study ( 13)too slow to be measured. Present value from two experiments with C2H4and with toluene as reference hydrocarbon. Rate constant is upper limit, since some photolysis of acetone may occur. Relative rate studies only: upper limit rate constant, since some photolysis of ketone may occur. Uncertainty rather large due to slow decay of propane.
'
molecule-1 s-l for the reaction of HO with ethylene. This is a mean value of three different measurements of this rate constant a t pressures near 1 atm ( 1 2 ) . The resultant rate constants are compared with available literature values taken mainly from a recent review article on HO organic reactions ( 1 2 ) .The experimental error in the determination of k4[0H] for the faster reacting species was &lo%,but for species which decayed more slowly, the error was correspondingly larger: values of k 4 5 10-12 can only be considered accurate to f a factor of 2. A further possible source of systematic error arises from the photolysis of the aldehydes and ketones. However, since [HO] is relatively high in our system, this problem is not so serious. Based on the observations of Winer e t al. ( 1 3 )we estimate that the direct photolysis of methyl ethyl ketone in our system occurred at a rate of -4 X min-l, Le., a t least a factor lo2 slower than, HO attack. The steady-state concentration of HO during HONO photolysis with no added organic species is given by:
+
[HO] = 4[HONO]/(kl[HONO]+ kz[NO] + ~ ~ [ N O Z ] ) Using k z = h3 = 1.7kl = 1.1X lo-" cm3 molecule-1 s-l (5), and typical initial HONO, NO, and NO2 concentrations, we obtain [HO] = 6.0 X lo7 molecules ~ m - In ~ .all the experiments in which C2H4 was present with the alkenes and lower molecular weight alkanes, the initial first-order decay constant for CzH4 was essentially constant a t 4.0 f 1.2 X s-l. The corresponding [HO], calculated using the adopted value of k (HO CzH4),was 5.0 f 1.5 X lo7molecule ~ m - Individual ~.
+
Table II. Stoichiometry for (NO -,NO2) Oxidation in Photooxidation of Organic Compounds in the Presence of HONO c0mpou nd
olefins ethylene propen e isoprene cyclohexene aromatics benzene toluene o-xylene rn-xylene aldehydes and ketones acetaldehyde propionaldehyde acetone methyl ethyl ketone methyl isobutyl ketone methyl vinyl ketone paraffins n-pentane isopentane propane 2-methylpentane 2,3-dimethylbutane
5.2
5.1 5.9 4.8
2.1 1.a 1.1
1.1
4.4
0.8
2.1
1.2 0.9 0.8
4.7
3.4 3.7 2.4 2.8 2.8 1.7 2.7
1.7 1.1 1.7 2.4 2.3 1.1
4.5
2.1 2.3 1.9 1.a
3.4
2.3
4.8
4.9 5.6
1.3 1.o 1.6 1.o
1.8 1.0 1.o 1.o
1.3 1.9 1.a
0.8 2.6 2.0 2.2 2.7
1.2 1.7 1.1
values for the [HO] in the presence of each compound are shown in Table 11. Clearly [HO] is only slightly reduced by the addition of lower alkanes and alkenes, as expected if the chain process occurring after HO attack on the organic molecule regenerates HO radicals. In the presence of ketones and aromatics, [HO] was significantly reduced, which suggests that the intermediate radicals formed from these molecules can react by pathways that do not regenerate HO. The extent of reduction of HO in this case is a function of kq and the amount of hydrocarbon present, so the actual values in Table I1 cannot be interpreted in a simple way. Oxidation of NO t o NO2 d u r i n g Photooxidation. The rate of oxidation of NO to NO2 resulting from the presence of each organic compound in the HONO photolysis experiments was determined from the difference between observed initial rates of change in NO concentration in the presence of organic, R,, and the rate of formation of NO from photolysis of HONO, RHONO, in its absence. Since some NO is removed by reaction 2, an appropriate small correction (I 10%) was made to the quantity d(HONO), to calculate RH&, for each initial HONO, NO, and NO2 mixture. The rate of NO oxidation is given by:
This quantity was then compared directly with the observed initial rate of decay of the organic compound to obtain the stoichiometry factor, SNO, for NO t o NO2 oxidation: SNO= R ~ o / k 4 [ 0 H[RH]. ] In practice, the concentration of NO could be observed to increase or decrease with time in the initial stages, depending on the amount and reactivity of the hydrocarbon present. It was usually found that the total amount NO2 formed during photolysis was constant and of NO approximately equal to that formed in HONO-air mixtures with no organic additive, showing that the reduction in the NO formation rate was due mainly to oxidation of NO to NOZ.
+
Table I1 shows the values of SNO for a variety of hydrocarbon species together with the initial ratios of [N02]/[NO]in was measured. This is given since, in the mixtures where SNO some cases, notably the aldehydes, the stoichiometric factor decreased markedly as the [NOP]/’[NO]ratio increased. For most of the experiments [NOz]/[NO] was initially between 1 and 3, which is typical for an atmospheric situation. SNO was also markedly reduced in experiments with high total NO NO2. For example, SNO for isopentane, although showing little dependence on [NOz]/[NO],decreased from 2.3 to 0.8 when the total NO NO2 increased from 0.9 to 10.2 ppm. The results in Table I1 show that for the conditions [NO] [NO?] N 1 to 2 ppm and [NOz]/[NO] 1 to 3, the stoichiometry factors SNOwere mostly in the range 1 to 2.5. These results are in rather good agreement with those reported recently by Washida et al. ( 1 4 ) ,who measured values of the stoichiometry factor for NO to NO2 conversion in the photooxidation of hydrocarbon-NO-NO2 mixtures in air. There are some differences; notably, we find that for the aromatic hydrocarbons, SNO was consistently lower than the other classes of organics, even with relatively low N02/NO ratios. On the other hand, Washida et al. (14) obtained stoichiometry factors of between 2.1 and 3.4 for the isomeric xylenes. P r o d u c t Formation d u r i n g Photooxidation. Products were sought for all the organic species investigated, since this can provide useful information for elucidation of the oxidation pathways. Products were designated primary or secondary depending on the shape of the observed product vs. time behavior-secondary products exhibited an induction period before their maximum rate of formation was reached and presumably resulted from subsequent reaction of the initial products of photooxidation. A summary of the products found is shown in Table 111. It should be noted that because of the very high sensitivity of the ECD to some compounds, e.g., PAN and MeON02, species detected by ECD may be very minor products. In some cases no products a t all were observed and since low molecular weight aldehydes, ketones, and nitrates could be readily detected on the analytical system employed, the absence of this class of product is indicated in these cases. The following major points emerge from the results shown in the summary. Simple aldehydes and/or ketones are normally formed in the oxidation of alkenes and branched chain alkanes. The presence of the (CH3)CH group normally gives rise to high yields of acetone. Also, the straight-chain C j and C6 alkanes do not apparently give good yields of low molecular weight carbonyl compounds. This is in agreement with the proposal of Carter et al. (15) and also Baldwin et al. ( 1 6 ) that straight-chain (2C4) alkoxy radicals, which are intermediate in the oxidation mechanism of these compounds, undergo isomerization leading ultimately to hydroxy-substituted carbonyl compounds, which we would not expect to detect on our system. Many compounds give rise to PAN (peroxyacetyl nitrate) formation, and this is invariably associated with methyl nitrate, which is known to result from decomposition of PAN (6, 17). The aromatics particularly give substantial yields of PAN, showing that ring fissure occurs following HO attack. The C5 and c6 alkanes give rise to the formation of substantial amounts of Cg and c6 alkyl nitrate products. Similar observations have been made by Darnall et al. ( I 8 ) ,who have suggested that these nitrates are formed by isomerization and stabilization of the peroxy nitrite species initially formed in the reaction of peroxyalkyl radicals with NO. Isoprene is oxidized to give mainly gaseous products including formaldehyde and two other carbonyl compounds, one of which may be methyl vinyl ketone and/or methyl glyoxal. This is contrary to the previous implication that oxidation of
+
+
+
Volume 14, Number 1, January 1980
59
Table 111. Summary of Products from Photooxidation of Organic Compounds in the Presence of HONO compound
products found
notes
ethylene
propylene isoprene
cyclohexene toluene &xylene
m-xylene acetaldehyde methyl ethyl ketone methyl isobutyl ketone methyl vinyl ketone
no products detected but major product is HCHO ( 7 ) HCHO is also a product ( 7 ) : no ECD analysis for PAN methyl vinyl ketone tentatively identified but another possible product, methyl glyoxal, had t h e same retention time: a second major product with a shorter retention time was not identified; HCHO, > 5 0 % yield; PAN and CH30N02 are secondary products no products detected: FID analysis only aromatic nitro compound was tentatively identified as a nitrophenol or nitrocresol by visible-UV absorption spectrum of product trapped in 0.1 N NaOH biacetyl measured both on FID and ECD no Droducts observed on FID
CH3 CHOa HCHO, CH3 COC* CH2, PAN, CH3ONO2
Ar/N02(?), PAN CH30N02 (CH3C0)2, PAN, CH30N02 PAN, CH30N02 HCHO, PAN, CH30N02
no products observed on FID acetone yield = 68%
PAN, CH3ONO2 CH3COCH3, PAN, CH30N02
PAN a
primary product indicating CH3C0 radical released directly during breakdown methyl vinyl ketone PAN was a secondary product acetone yield = 56 % ; no ECD analysis acetaldehyde and propionaldehyde absent; PAN and methyl nitrate were secondary products no propionaldehyde present; higher alkyl nitrates only tentatively identified on the basis of retention times
PAN, CH30N02
of
propionaldehyde propane n-pentane isopentane 2-methylpentane
acetone and propionaldehyde yields rather small; 2 isomers of hexyl nitrate appeared to be formed; PPN and PAN secondary products 1.5 mol of acetone formed per mol of reactant: PAN a secondary product
2,3-dimethylbutane n-hexane a
2 isomers of hexyl nitrate formed: no simple aldehydes and ketones detected on FID: PAN a secondary product
Major products in italics. Peroxyacetyl nitrate. Peroxypropionyl nitrate.
isoprene and other terpenes yielded principally aerosol products ( 1 9 ) . PAN appears to be a secondary product in isoprene photooxidation, and this was further indicated by observation of PAN formation in the photooxidation of methyl vinyl ketone. These observations on isoprene photooxidation are of particular interest in relation t o the recent suggestion by Zimmerman e t al. (20) that natural emissions of isoprene could provide a significant source of atmospheric carbon monoxide. It is hypothesized that HCHO and methyl vinyl ketone should be major oxidation products of isoprene, and that these are further oxidized to produce mainly CO. Our work now provides strong experimental support for this hypothesis. Since many of the products were not unambiguously identified and quantitatively determined, a detailed discussion of the mechanisms involved in the photooxidation reactions will not be conducted here. As a n example of the utility of the information, we illustrate in Figure 2 a plausible oxidation pathway for isopentane, which satisfies the observations. This scheme explains the formation of acetone and acetaldehyde as primary products as a result of HO attack on the weaker tertiary H atom of isopentane. Analogous reactions will occur following the (slower) attack on the secondary and primary H atoms. PAN is formed as a secondary product, in our system resulting primarily from the attack of HO on acetaldehyde. In the atmosphere where [HO] is lower, photolysis of acetone and acetaldehyde will be relatively more important. Methyl nitrate, which is always found when PAN is present, results from the reaction of CH30 with NOn. Its formation is favored by the relatively high concentrations of NO, present in these laboratory systems. The maximum NO NO2 conversion stoichiometry is 3, but the lower observed value, -2.3,
-
60
Environmental Science & Technology
is expected since the side reactions forming the alkyl nitrates lead to net removal of radical chain carrying species and this decreases the amount of NO to NO2 oxidation. Conclusions in Relation to Atmospheric Reactivity and Photochemical Smog Modelling. The results summarized above show clearly that different classes of organic compounds not only have markedly different rate constants for reaction with OH radicals but also exhibit different efficiencies for NO to NO2 conversion and can form a variety of products, some of which can undergo rapid secondary photochemical oxidation. All these factors need to be taken into account when assessing the potential of a given organic species for ozone formation in the sunlit polluted atmosphere. A photochemical reactivity scale based on OH-radical reaction rates has been suggested ( 3 ) ,which has several advantages over previous criteria for determining reactivity, particularly in the assessment of the less reactive species that can contribute significantly to ozone formation in areas more remote from the precursor source region. However, since O3 formation is a result of NO to NO2 conversion, hydrocarbons with similar OH reaction rates may differ in reactivity if the stoichiometries for NO oxidation differ. On the other hand, a hydrocarbon that forms photochemically active products such as aldehydes and a-dicarbonyl compounds may have a higher overall reactivity than a hydrocarbon forming the less reactive ketones, even though the HO reaction rates may be comparable. In the present system using HONO photolysis as a source of HO, the role of secondary photochemical reactions of carbonyl compounds is deemphasized, since the concentration of HO was a factor of 10 higher than in the sunlit atmosphere and in typical smog chamber experiments, such as those reported by Washida e t al. (14). This may ac-
h/
* O H
(Primary1
-
+
H2O
I Secondary 1
mechanistic information necessary to make a satisfactory formulation of the lumped parameters is now becoming available as a result of studies of the type reported here. Acknowledgment We thank Dr. R. Atkinson and co-workers for providing us with a preprint of their review on HO reactions. Literature Cited (1) Dimitriades, B., in “Proceedings on Solvent Reactivity Confer-
I N 0 2 1 or 0’
i
( 2 Methvl
cZng 1;hHotoo:ysls
OH -
o2
CH3CHO
LA c e t a Ideb y
CH,COO;
O2 HO2
NO
OHI*N021
M
CH3C002N02
1 NO +
~ 2 ~ 5 0 0 ’
C2H50’ I + NO2 I
CHI C O ’
1
02
NO/
/
CH;
-
-
NO2 2 Butvlnitratel
(PAN1
CO2
1 + NO2 I
02.NO
I
CHJO’I.
NO2
1
02 ---
HCHO
+
HOz
If o r m a 1de b y b )
Figure 2. Photooxidation of isopentane in the presence of HONO
count for some of the apparent differences between the observed efficiency of NO to NO2 conversion in the two laboratory systems. Although the classical smog chamber may yield an overall stoichiometry for NO to NO2 oxidation more comparable to that in a typical atmospheric photooxidation situation, the results from the HONO photolysis system provide more unambiguous information regarding the reaction mechanism for the oxidation following attack by HO. The large number and type of organic species present in polluted air preclude the modeling of each individual species in most practical systems and, to overcome this difficulty, “lumped parameter” models have been developed for modeling the formation of ozone, NOz, PAN, and other photochemical products in urban air (21). The validity of the chemical aspects oft hese models depends on judicious choice of the parameters giving average rate and stoichiometry for the various classes of organic compound. The kinetic and
ence,” U.S. EPA, Research Triangle Park, N.C., EPA 650/3-74010, Nov 1974. (2) Huess, J. M., Glasson, W. A,, Enuiron. Sci. Technol., 2, 1109 (1968). (3) Darnall, K. R., Lloyd, A. C., Winer, A. M., Pittg, J. N., Jr., Enuiron. Sci. Technol. 10,692 (1976). (4) Bufalini, J. J., Walter, T. A., Bufalini, M. H., Enuiron. Sci. TQChnCJ/.10,908 (1976). (5) Cox, R. A,, Derwent, R. G., Holt, P. M., K e n , J. A,, J . Chem. Soc., Faraday Trans. I , 72,2044 (1976). (6) Cox, R. A,, Derwent, R. G., Holt, P. M., Kerr, J. A,, J . Chem. Soc., Faraday Trans. I, 72,2061 (1976). ( 7 ) Niki, H., Maker, P. D., Savage, C. M., Breitenbach,L. P.,J . Phys. Chem., 82,135 (1978). (8) Cox, R. A., J . Photochem., 3,175 (1974). (9) Cox, R. A., Int. J . Chem. Kinet. Symp., 1, 379 (1975). (10) Intersociety Committee, “Methods of Air Sampling and Analysis”, American Public Health Association, 1972, p 199. (11) Bricker, C. E., Johnson, H. R., Ind. Eng. Chem. (Anal.Ed.), 17, 400 (1945). (12) Atkinson, R., Darnall, K. R., Lloyd, A. C., Winer, A. M., Pitts, J. N., Jr., Adu. Photochem., 11,375 (1979). (13) Winer, A. M., Lloyd, A. C., Darnall, K. R., Pitts, J. N., J . Phys. Chem., 80,1635 (1976). (14) Washida, N., Inoue, G., Akimoto, H., Okuda, M., Bull. Chem. Soc. Jpn., 51, 2215 (1978). (15) Carter. W. P. L.. Darnall. K. R.. Llovd. A. C.. Winer. A. M.. Pitts. J. N., Jr.,’Chem. h y s . Lett., 42,’22 (i976). (16) Baldwin, A. C., Barker, J. R., Golden, D. M., Hendry, D. G., J . Phys. Chem., 81, 2483 (1977). (17) Cox, R. A., Roffey, M., Enuiron. Sci. Technol., 11,900 (1977). (18)Darnall, K. R., Carter, W. P. L., Winer, A.M., Lloyd, A. C., Pitts, J . N., Jr., J . Phys. Chem., 80,1948 (1976). (19) Gay, B. W., Arnts, R. R., U.S. Environmental Protection Agency Report No. EPA-600/3-77-001b, EPA, Research Triangle Park, N.C., 1977, p 745. (20) Zimmerman, P. R., Chatfield, R. B., Fishman, J., Crutzen, P. J., Hanst, P. L., Geophys. Res. Lett., 5,679 (1978). (21) Hecht, T. A., Seinfeld, J. H., Dodge, M. C., Enuiron. Sci. Techno/., 8 , 327 (1974). (22) Grimsrud, E. P., Westberg, H. H., Rasmussen, R. A,, Int. J . Chem. Kinet., Symp., No. 1,183 (1975).
Receiued f o r review April 10, 1979. Accepted September 10, 1979. This ulork was supported by the United Kingdom Department of the Environment.
Volume 14, Number 1, January 1980
61
