Investigation of hydroxyl radical reactions with o-xylene and m-xylene

Products of the Gas-Phase Reactions of OH Radicals with p-Xylene and 1,2,3- and ... Formation of ring-retaining products from the OH radical-initated ...
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Investigation of Hydroxyl Radical Reactions with o-Xylene and m-Xylene in a Continuous Stirred Tank Reactor Michael W. Gery" Systems Applications, Inc., San Rafael, Californla 94903

Donald L. Fox and Richard M. Kamens Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, Chapel Hill, North Carolina 27514

Leonard Stockburger Environmental Sciences Research Laboratory, US. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1

rn The gas-phase reactions of hydroxyl radicals with o-

t

xylene and m-xylene were studied in a continuous stirred tank reactor. Gas and aerosol products accounted for 6545% of the reacted carbon. Approximately 19 and 13% of the original o-xylene and m-xylene oxidation were estimated to have occurred through methyl hydrogen abstraction by OH, primarily leading to methylbenzyl nitrates and tolualdehydes. The remaining mass reacted through the OH addition pathway forming dimethylphenols, nitrodimethylphenols, nitroxylenes, and stable products resulting from reaction of metastable 02-OH adducts. For o-xylene, the ratio of the rate constants for formation of nitroxylenes vs. dimethylphenols was estimated to be 5.9 X lo4,while the same value for m-xylene was only about 1.0 X lo4. The ratios of the dimethylphenol formation rates to the oxygen addition rates were found to be greater than or equal to 0.15 for o-xylene and 0.27 for m-xylene. Introduction Alkylbenzene species comprise a significant portion of the reactive hydrocarbon compounds in automobile exhaust (1, 2) and urban atmospheres (3, 4). Although toluene is usually the most prominent aromatic compound, the combined xylene isomers have contributed similar mass in most studies (1, 3, 4). Side-by-side outdoor smog chamber experiments, in which equal amounts of automobile exhaust or urban hydrocarbon mixtures are replaced by toluene on one side and m-xylene on the other, show a significantly higher rate of ozone production on the m-xylene side (5). These data indicate other important differences between m-xylene and toluene chemistry besides the OH reaction rate constants. Most notably, mxylene seems to be a more prolific radical source than toluene (6). Despite these facts, the chemistry of the xylene isomers is poorly understood beyond the initial reactions (7). The atmospheric impact of alkylbenzene emissions, however, cannot be accurately described without additional information concerning the photochemical kinetics and reaction mechanisms of xylenes.

Under conditions typical of photochemical smog, significant xylene oxidation occurs only through reactions with the hydroxyl radical (8, 9). Reactions with ozone, atomic oxygen [O(3P)]and NO3 have been shown to be comparatively slow (10-12). Some high molecular weight, gas-phase, o-xylene oxidation products have been identified as o-tolualdehyde, 2,3- and 3,4-dimethylphenol, 3- and 4-nitro-o-xylene, and o-methylbenzyl nitrate (13-15). Smaller amounts of dimethyl-p-benzoquinone and nitrodimethylphenols were also detected (13). Ring cleavage products have been monitored in the gas-phase oxidation of all xylene isomers. o-Xylene forms glyoxal, methylglyoxal, and biacetyl (13-la), and longer chain a- and y-dicarbonyl products have been identified by Shepson and co-workers (14). Also, yields of glyoxal and methylglyoxal have been determined (15, 18, 19) in m- and pxylene irradiations (biacetyl has not been found to form unless adjacent methyl groups occur on the original aromatic ring). The yields of dicarbonyl compounds in these studies were far lower than those predicted by current chemical reaction mechanisms (20-24). This circumstance is no doubt due to the low product yields in earlier experimental studies, which have forced modelers to present mechanisms based on best estimates. The result has been the routing of excess reacted carbon into these highly reactive products. In this work the unique temporal characteristics of a continuous stirred tank reactor (CSTR) were utilized to provide a more complete account of the product mass generated from gas-phase reactions of hydroxyl radicals with o-xylene and m-xylene. The reaction pathway yields in these systems were determined and, when appropriate, compared with earlier xylene studies and results from similar CSTR experiments with toluene and benzaldehyde (25). Experimental Section The experiments were performed in a 200-L Teflon CSTR operated at steady-state conditions. The operation and dynamics of the system have been discussed elsewhere

0013-936Xl8710921-0339~01.5010 0 1987 American Chemical Society

Environ. Sci. Technol., Vol. 21, No. 4, 1987

339

Table I. Summary of Analytical Techniques

substance or parameter

method

hydrocarbons (reactants) hydrocarbons (gas products) hydrocarbons (gas and aerosol products) hydrocarbons (gas and aerosol products) hydrocarbons (aerosol products) PAN, biacetyl

co

dicarbonyl compounds (glyoxal, methylglyoxal, biacetyl) formaldehyde NO, (including NO, NOz, HONO, HNOJ

uv

GC-FID/direct sample GC-FID/direct sample XAD-2 and filter extraction, GC-FID and TSD XAD-2 and filter extraction, GC/MS filter extraction, GC-FID GC-ECD GC-FID HPLC/DNPH colorimetric chemiluminescent with denuder system pyranometer thermometer cooled mirror chemiluminescent electrical aerosol analyzer laser scattering OPC

temperature dew point ozone aerosol size distribution aerosol size distribution

error tolerance, minimum detectable 0.001 ppm 0.010 ppm

qualitative only qualitative only 0.100 ppm 0.0001 ppm 0.010 ppm 0.010 ppm 0.001 ppm var. 0.001-0.02 ppm 0.2 mV ca1-l (cmz mi&' 0.1 OC 0.1 OF 0.001 ppm 0.013 pm 0.09 pm

%

15% *20 (50)" 120 ( 5 0 ) O f5 *IO f20 120 15 f5-30 110 f2 A10 *5

variable variable

"Higher associated error exists if no pure calibration standards are available (about 10% of identified species).

(25). Hydroxyl radicals were generated in situ through the photolysis of nitrous acid (at ca. 310-390 nm): HONO

+ hv

OH

-+

+ NO

(1)

The CSTR was irradiated with 34 UV fluorescent (GE, F20 BLB) lamps. Maximum intensity provided an NO2 (in air) photolysis rate of 0.30 min-l and a biacetyl photolysis rate of 1.8 X min-l (derived from the results of two biacetyl CSTR experiments). Usual carrier gas throughput rates corresponded to average residence times of about 20 min. This value was chosen to allow the OHxylene reaction mixture to advance to rapidly occurring secondary reactions but to decouple the system from long-term loss of less reactive products. The reactor was generally operated in the flow mode. However, a set of batch experiments was performed under HONO/hv conditions similar to those of the flowing system to find which secondary products derived from identified, primary products. Blank chamber, dark HONO, dark hydrocarbon/HONO, and hydrocarbonlhv experiments were performed, in both flow and batch regimes, to investigate background and interference responses and system stability at steady state. Eleven flow experiments and 14 batch experimentswere performed. These data and similar toluene results (25) indicate that product distributions are reproducible (in the range of f10-15%) when similar initial conditions can be established. Reactor operation occurred at 300-305 K and 1atm for approximately 500-700 min. Steady-state conditions existed for approximately 300-500-min periods in the later portions of an experiment; analysis interxals varied from 1to 40 min, depending on the instrument (summarized in Table I). Temporary variations due to flow fluctuations were found to be always less than 10%. Nitrous acid was generated by the liquid-phase reaction of sodium nitrite with sulfuric acid. HONO, NO, and NO2 were off-gassed as products of this reaction. The technique and apparatus used for generation was a composite of previously described reactors (25-29). The average operating range for the ratio of HONO to total generated NO, was 15-30%. It was found that production of greater amounts of HONO leads to undesirably high NO2 concentrations. A Bendix 8101-B chemiluminescent NO, analyzer was used as the oxides of nitrogen sensor. Although NO and total NO, are relatively easy to measure, intermediate 340

Environ. Scl. Technol., Vol. 21, No. 4, 1987

oxidation states (such as NO2 and HONO) must be selectively separated from the other species and calculated by comparing the differences in NO, signals. Methods of separation for nitrogen oxides and oxy acids were initially described by Cox (27) and later utilized and confirmed by Hoshino et al. (26). Ozone was monitored by a Bendix 8002 series chemiluminescent analyzer. Because of the high NO concentration in the reactor, only traces of 0, were detected. A Beckman 6800 gas chromatograph (GC) was used to monitor carbon monoxide. Formaldehyde was analyzed with a CEA Instruments, Inc., 555 colorimetric analyzer (30). The xylene reactants were monitored every 4 min by a GC with flame ionization detector (FID) and a 50.8 cm X 1.6 mm i.d. stainless steel column packed with 10% TCEP on 100/120 Chromasorb PAW. Aromatic products were concentrated at -77 "C in a 11-cm3trap packed with 40-pm glass beads and then electrically heated and injected to a 30 m X 0.25 mm DB1 capillary column and FID. XAD-2 resin and quartz fiber filters were used to collect gas- and aerosol-phase organic samples for later analysis. Both were extracted with methylene chloride and concentrated. Qualitative analysis was performed by a HP-5992 GCMS system and by a second GC equipped with both a flame ionization detector and a thermionic-specific detector. A GC with an electron capture detector was used to monitor PAN, biacetyl, and organic nitrites with a 1.2 m X 1.6 mm glass column packed with 10% Carbowax 600 on 60/80 Gas Chrom Z operated at 25 "C with argon/ methane carrier gas. Glyoxal, methylglyoxal, and biacetyl were collected with a bubbler containing an absorbing reagent of acidified 2,4-dinitrophenylhydrazine(31) at 0 "C. Collection efficiency was between 80 and 85% for these species. Sample vials were heated at 150 "C for 30 min, and the resulting derivatives (32) were analyzed by HPLC with a Varian MCH-10 reverse-phase, 30 cm X 9 mm column and a fixed 254-nm detector. Compressed gas cylinders of 0- and m-xylene in nitrogen were used for continuous hydrocarbon injection; these and the calibration cylinders were obtained from Scott Specialty Gases. Other reagents and calibration species were obtained from Aldrich Chemical Co. and Phaltz and Bauer, Inc. Either an AADCO clean air generator or cylinders of compressed nitrogen were used to deliver dry carrier gas. Initial HzO was estimated to be about 500-1000 ppm, and CO was less than 10 ppb.

Table 11. Product Distributions for m-Xylene CSTR Experiments

Table 111. Product Distributions for o-Xylene CSTR Experiments expt no. 2/1 1/27

expt no. 1/20' 12/16. 1/17 Conditions residence time, min dark exit concn, ppm of C lights-on exit concn, ppm of C A[m-xylene], ppm of C A[rn-xylene], % inlet [NO,], ppm [HCI/[N0,1, P P of~ C / P P ~ chamber temp, K

7.79 19.37 22.24 136.12 151.50 222.64 128.08 140.97 203.77 8.04" 10.53 18.87 5.91' 6.95 8.48 8.2 7.9 nae 17.8 nag 28.1 305.1 304.0 302.9

Conditions residence time, min dark' exit concn, ppm of C lights-on exit, concn ppm of C A[o-xylene], ppm of C A[o-xylene], % inlet [NO,], ppm [HCl/[N0,1, P P of~ C / P P ~ chamber temp, K

10.78 188.43 183.61 4.82 2.56 8.2 23.3 303.5

22.55 194.17 180.68 13.49 6.95 8.0 25" 300.2

Product Distribution (Percent of Reacted Carbon) carbon monoxide 6.1 6.0 4.5 formaldehyde 2.9 1.2 3.6 2.1 glyoxal 3.1 1.7 methylglyoxal 20.6 10.0 13.0 biacetyl peroxyacetylnitrate nae 0.6 1.0 1.1 0.8 0.4 rn-methylbenzyl nitrate 18.1 8.4 6.8 rn-tolualdehyde 2-nitro-rn-xylene 0.0 0.0 0.0 4-nitro-rn-xylene 1.5 3.2 1.5 0.6 1.4 0.6 5-nitro-rn-xylene 2,6-dimethyl-p-benzoquinone 13.8 8.4 5.7 0.2 0.3 0.2 2,4-dimethylphenol 6-nitro-2,4-dimethylphenol 11.7 9.8 7.7 0.2 0.7 0.5 2,6-dimethylphenol 4-nitro-2,6-dimethylphenol 7.6c 6.3c 5.OC 3,5-dimethylphenol 0.0 0.0 0.0 unidentified gas-phase species (approx)d 11.4 9.7 8.9 aerosol 1.7 5.6 1.6 percent of reacted carbon detected 100.5"~~73.9 63.0

Product Distribution (Percent of Reacted Carbon) carbon monoxide nae 1.5 2.2 formaldehyde 4.8 glyoxal 3.1 2.0 methylglyoxal 12.2 9.7 biacetyl 8.5 7.2 1.2 0.9 peroxyacetylnitrate 2.1 0.7 o-methylbenzyl nitrate o-tolualdehyde 19.2 11.7 3-nitro-0-xylene 1.2 0.8 4-nitro-0-xylene 6.6 5.1 2,3-dimethyl-p-benzoquinone 3.3 3.1 3.8 2.8 2,3-dimethylphenol 6-nitro-2,3-dimethylphenol (tentative) 2.7 1.5 4-nitro-2,3-dimethylphenol 2.7c 1.5e 3,4-dimethylphenol 0.9 1.1 2- or 5-nitro-3,4-dimethylphenol 0.4 0.3 6-nitro-3,4-dimethylphenol 1.5c LOc unidentified gas-phase species (approx)d 13.7 11.5 nae nag aerosol percent of reacted carbon detected 87.gb 64.5b

Yield values have a large associated error because the denominator, A[m-xylene], had low measurement precision. Total reacted carbon value low because some data were not available on this day. Calculated from XAD-2 response ratio with isomer that responded on real-time gas chromatograph. See Table 111. e na = not available.

"Estimated. bTotal reacted carbon value low because some data were not available on this day. Calculated from XAD-2 response ratio with isomer that responded on real-time gas chromatograph. Probably high molecular weight species; concentrations obtained from average GC response to carbon atoms. e na = data not available.

Results

Table IV. Products Formed in Batch Experiments

Product and kinetic data can be obtained from the two o-xylene and three m-xylene CSTR experiments that were performed with HONO and light. Results are provided in Table I1 and 111. Product yields decrease with larger residence times due to the increased reaction of primary products to form a more diverse set of secondary species (some of which were undetectable). This finding is consistent with that of the earlier study (25). To supplement the information 'from the CSTR xylene systems, the chemistry of some major reaction products was also studied. Eight batch-type experiments were performed to determine the relationships between apparent primary products and suspected secondary species. The reactor was operated in batch mode for HONO/hv systems of 2,3and 3,4-dimethylphenol, o-tolualdehyde, and o-methylbenzyl alcohol (o-xylene products) and 2,6-dimethyl-pbenzoquinone and 2,4-, 3,s-, and 2,6-dimethylphenol (mxylene products). The 2,6-dimethyl-p-benzoquinone photolysis system was also studied without HONO. Products found in these batch photolysis experiments are listed in Table IV. The product species were always of high enough yield for positive GC/MS identification. In all dimethylphenol experiments, only products with the original methyl-to-hydroxylorientations were formed. No trace products with other structures were found. Off-site GC and GC/MS analysis was performed for XAD-2 cartridge extractions from one o-xylene/HONO/hv and two rn-xylene/HONO/hv CSTR experiments and for

reactants

products

2,3-dimethylphenol 3,4-dimethylphenol o-methylbenzyl alcohol o-tolualdehyde

o-Xylene Systems 6-nitro-2,3-dimethylphenol (ID tentative) 4-nitro-2,3-dimethylphenol 2,3-dimethyl-p-benzoquinone 2- (or 5-) nitro-3,4-dimethylphenol 6-nitro-3,4-dimethylphenol no products found 3-nitro-2-hydrdoxytoluene 5-nitro-2-hydroxytoluene o-toluic acid phthalide o-cresol (trace) methyl-p-benzoquinone (trace)

2,4-dimethylphenol 2,6-dimethylphenol

rn-Xylene Systems 6-nitro-2,4-dimethylphenol 2,6-dimethyl-p-benzoquinone 4-nitro-2,6-dimethylphenol 2,6-dimethyl-p-benzoquinone

3,5-dimethylphenol nitro-3,5-dimethylphenol 2,6-dimethyl-p-benzo- no products found quinone (with and without HONO)

all batch experiments. Chemical speciation of aerosol products was performed for experiments of both xylene isomers, although filter mass was only determined for m-xylene experiments. In addition to these data, small Environ. Sci. Technol., Vol. 21, No. 4, 1987

341

amounts of alkyl nitrates and formic acid may have been detected but were not quantified; conjugated y-dicarbonyl species were probably not detectable with the HPLC. Also, although acetone, acrolein, and higher aliphatic aldehydes, including acetaldehyde, could have been detected with the HPLC, none were found. Absent in all real-time analyses were the xylene analogues of the OH-toluene reaction products nitrophenol and benzyl alcohol (nitrocresols and 0- and m-methylbenzyl alcohols). In addition, although 2,4- and 2,6-dimethylphenols were detected in the mxylene/HONO/hu CSTR experiments, no 3,5-dimethylphenol or nitro-3,5-dimethylphenol products were found. o-Phthalaldehyde was not found in the gas phase of either o-xylene experiment, and 2-nitro-m-xylenewas not identified in any m-xylene system. One filter extraction was performed for each xylene/ HONO/hv experiment. Because the results were not reproduced, the aerosol information is considered qualitative and possibly incomplete. Condensed-phaseproducts from the o-xylene system were o-phthalaldehyde,phthalide, and 3-nitro-0-xylene. Traces of 2,3- and 3,4-dimethylphenol were also found. Approximately equal amounts of 6nitro-2,4-dimethylphenol and 4-nitro-2,6-dimethylphenol were detected on the m-xylene/HONO/hv CSTR filter.

Discussion In gas-phase reactions with alkylbenzene compounds, hydroxyl radicals either abstract a side-chain hydrogen atom or add to a ring carbon (7, 33). The pathways for o-xylene are

(-

&cH3

CH3 + OH

t

H20

OH

OH

?

NO

(2)

0

CV3

Flgure 1. Hydroxyl radical-o-xylene abstraction reaction scheme. OH

+

isomers

(3)

Hydrogen Abstraction Pathway. MethyI hydrogen abstraction (reaction 2) yields a methylbenzyl radical that, in the atmosphere, should rapidly add oxygen to form the appropriate methylbenzylperoxy radical. Subsequent reactions probably result in the formation of the methylbenzyl nitrates and nitrites, along with tolualdehydes. By analogy with toluene chemistry (7,8,20), this is expected to occur for o-xylene through the set of reactions shown in Figure 1and for m-xylene through an analogous set of reactions. Methylbenzyl nitrites probably photolyze rapidly (34) and should be unimportant. CSTR yields for methylbenzyl nitrates and tolualdehydes are given in Tables I1 and 111. If the initial reactions in Figure 1accurately represent the xylene abstraction pathways, then the CSTR, by virtue of its short residence time and small secondary product formation, can be used to determine the total xylene abstraction pathway yields. These values can be calculated for each xylene isomer by summing the methylbenzyl nitrate and tolualdehyde outlet concentrations with the estimated yields for products formed from tolualdehyde reaction during the CSTR residence time. Unfortunately, the gas-phase reaction and photolysis rates for o- and m-tolualdehyde have not yet been determined. It is expected, however, that only small errors resulted by approximating tolualdehyde kinetics from known benzaldehyde values. This is possible because the short CSTR 342

a3

Environ. Sci. Technol., Vol. 21, No. 4, 1987

residence times minimized the corrections. The factors for correcting the tolualdehyde outlet concentrations to values that would have occurred without reaction loss ranged from 1.05 to 1.18 for residence times from 7 to 23 min. It has been shown that only photolysis and OH reactions are rapid enough to appreciably deplete benzaldehyde under CSTR experimental conditions (25). An average photolysis rate of (1.1f 0.4) X min-l was calculated for benzaldehyde. However, because of the high OHbenzaldehyde rate constant (35,36), this reaction predominated over CSTR photolysis loss. Because tolualdehydes have a ring methyl group, some reaction with OH probably occurs at that position. To account for this, an increment equal to the difference between OH-toluene and OH-benzene reaction rates was added to the OHbenzaldehyde reaction rate to estimate the reaction rate of OH and tolualdehyde. This approximation is limited, however, and cannot account for isomeric differences or any differences in the photolysis rates between benzaldehyde and the tolualdehydes. The corrected average tolualdehyde yields (outlet tolualdehyde and calculated product concentrations) for o- and m-xylene systems were 17.2 f 7.0% and 12.2 f 5.9% (associated error values are two standard deviations plus experimental error when applicable). The 0- and mmethylbenzyl nitrate yields were about 7 and 8% of the corresponding 0- and m-tolualdehyde yields. In toluene experiments benzyl nitrate yields were about 7 % of ben-

zaldehyde yields, and Hoshino et al. (26)found this value to be 12 f 4% of benzaldehyde yields. Such results indicate that the formation of alkylbenzyl nitrates is relatively unimportant, either because formation rates are small or photolysis of these compounds is rapid. From these results, the total abstraction pathway yields for 0-and m-xylene were calculated to be 18.6 f 8.3% and 12.8 f 6.4%. The overall ratio of abstraction to addition can be calculated by first accounting for the small amount of O(3P)oxidation in the remaining reacted xylene isomers (slightly less than 1% of the total xylene loss) and assuming the difference represents the amount of OH addition. The best value of ka~,/(ka,,,+ kadd) [k,/k, + kJ1 in these data is 0.19 f 0.09 for o-xylene and 0.13 f 0.07 for m-xylene. Both values are within the ranges of previous results from Perry et al. (33). They found values for kabs/(kab kadd) of 0.202:: for o-xylene and 0.042b0,"for m-xylene. Uncertainty concerning the mechanism and products of aromatic aldehyde photolysis has been discussed elsewhere (7,25). Although no products were detected to clarify this process for o-toluaIdehyde, the absence of toluene as a product indicates that o-tolualdehyde photolysis to CO and toluene is probably a minor process at these wavelengths. OH abstraction of the aldehydichydrogen in benzaldehyde has been shown to produce 0- and p-nitrophenol, pbenzoquinone, peroxybenzoyl nitrate (PBzN), and possible benzoic acid (25, 37, 38). With the exception of PBzN, products found in the batch photolysis experiment of otolualdehyde and HONO (Table IV) were methylated homologues of these species. The proposed reactions that might lead to formation of 0-and p-nitrophenol through the nitration of phenoxy radicals were originally presented by Niki and co-workers (38). The 3- and 5-nitro-2hydroxytoluene probably formed from the reactions of NOz with the o-methylphenoxy radical (Figure 1). 0-Toluic acid and traces of methyl-p-benzoquinone were detected in XAD-2 extractions of the o-tolualdehydeexperiment. The methyl-p-benzoquinone may have formed by oxygen bridging across the methylphenylperoxy radical to form a bicyclic radical, followed by scission of the oxygen bridge and creation of a second carbonyl bond (Figure 1). Yields indicate, however, that such a process was unimportant in the o-tolualdehyde batch experiment. Peroxymethylbenzoyl nitrate concentrations were not determined in any experiment. It has been shown (38,39), however, that NO effectively competes with NOz for reaction with the peroxyybenzoyl radical (39),and assuming that these rates also apply for peroxymethylbenzoyl radical, computer simulations of the CSTR chemistry indicate that peroxymethylbenzoyl nitrate yields would have accounted for less than 1% of reacted xylene carbon. Traces of o-cresol were also found in the o-tolualdehyde batch experiment. This may have resulted from the reaction of o-methylphenoxy radical with HOz, although such a process also appears to be unimportant in the CSTR since the levels of o-cresol were low. Finally, evidence for hydrogen abstraction from both methyl groups exists in the detection of o-phthalaldehyde in o-xylene aerosol. Also, phthalide was found in o-xylene aerosol and in the o-tolualdehyde batch experiment. This product may have been formed by the intramolecular rearrangement of a difunctional product after sampling but prior to GC/MS analysis. These results suggest that formation of condensed-phase difunctional products could constitute a significant carbon loss process over an extended period. Hydroxyl Radical Addition Pathway. OH addition to o-xylene can occur at three unique sites, while there are

+

four possible OH-m-xylene adduct isomers. By use of the measured distribution of O(3P)sites of addition (40) as a guide, OH can probably add to o-xylene at all ring carbons with approximately equal frequency. It appears, however, that only the 2- and 4-OH adducts are significant addition sites for m-xylenes (7,40). No products that would have resulted from methyl displacement by OH (8)were found from the reaction of either xylene isomer. This indicates that methyl substitution by OH is probably unimportant for xylenes, although OH addition to the methylated sites may occur but results in other products. Figures 2 and 3 are schematic representations of the suspected OH addition reactions for 0- and rn-xylene. Regular comparison between these figures will clarify the differences that occur between xylene isomers. It should be kept in mind, however, that secondary reactions at various ring carbons also create isomeric differences. For simplicity, only one isomer is shown in the figwes, although various isomers are discussed next. (a) Phenol Formation. After addition of OH (reaction 3), aromatic structure can be regenerated by displacement of hydrogen to form the appropriate dimethylphenol and HOz (Figures 2 and 3). Results from xylene CSTR and dimethylphenol batch experiments indicate that dimethylphenolsreact further in HONO/hv systems to form nitrodimethylphenols. From the batch experiments it was found that each dimethylphenol isomer reacted to form only nitrodimethylphenol products with methyl-tohydroxyl orientations identical with that of the parent dimethylphenol (Table IV). Hence, it was possible to attribute each nitrodimethylphenol product to reactions of specific dimethylphenol isomers that were produced in the initial OH-xylene reaction (Table I1 and 111). The mechanism by which nitration occurs, however, is not clear. It was shown previously for toluene (25) that the reaction of OH with cresols (41) and NO, with cresols (7) could not contribute enough mass to account for the gas-phase nitrocresol yields. Although GC sample concentration prior to injection lasted for only about 2 min at -77 "C, nitration of cresols (and probably, in this case, dimethylphenols) may have occurred during this period (26). For modeling purposes it was assumed that (1)all species identified as gas-phase nitrodimethylphenols were dimethylphenols (25, 26) and (2) nitrodimethylphenols measured on filter samples represented condensed-phase nitrodimethylphenols. These assumptions could lead to the overestimation of dimethylphenol yields. However, because each nitrodimethylphenol resulted only from reaction of specific dimethylphenols (those with identical methyl-to-hydroxyl orientations), the mass that passed through specific isomeric reactions could be determined, regardless of the final disposition of that mass. Given these assumptions, the average gas-phase dimethylphenol yield from the o-xylene/HONO/hv systems was 10.2 f 3.9% of reacted xylene carbon. A total of 74 f 11% of that yield was 2,3-dimethylphenol and nitro2,3-dimethylphenol products, with the remainder as 3,4dimethylphenol and nitro-3,4-dimethylphenols(Table 11). Similarly, for m-xylene/HONO/hv systems, the dimethylphenol carbon yield was 17.8 f 6.5%. The isomeric yield of 2,4-dimethylphenolproducts was 58 f 19%, with the remainder as 2,6-dimethylphenolproducts (Table 111). Results from toluene/HONO/hv CSTR experiments (25) showed cresol yields to be 24.3 f 5.7% of reacted carbon, with o-cresol and its products accounting for 81 f 9% of the yield. (b) Nitroxylene Formation. Besides formation of dimethylphenols,regeneration of the aromatic ring can also Environ. Sci. Technol., Vol. 21, No. 4, 1987

343

1l-j-j HO H

(44%

molar y i e l d )

CH3

y 3

Y3

I

@ 3:

OH H

,

(6-8%

\

FH3

OH

molar y t e l d )

(4-8%

a-dicarbonyls and other unidentified products Glyoxal + kthylglyoxal + Biacetyl = 60% molar y i e l d

molar y i e l d )

'0

'

H02

(appx. 3% molar y i e l d )

Figure 2. Hydroxyl radical-o-xylene addition reaction scheme. The OH adduct isomers shown were chosen to represent the xylene addition chemistry because they probably undergo all proposed reactions to some extent. The other isomers may yleld higher amounts of certain products but are structurally restricted in other cases. Recall that NO, concentrationswere high in these experiments. Lower values In the atmosphere limit the importance of the NO, reaction pathway. All yields are molar. Major isomers are used to represent all possible isomers. Percent yields are for all isomers, even if only one is shown. See text-for isomeric ratios.

occur through addition of NOz, probably forming a nitroxylene isomer by ejecting a water molecule. Examples of this reaction mechanism are shown in Figures 2 and 3. It can probably be assumed that NOz addition to a methylated ring site is a low-yield process. This was evidenced by the lack of any products in either xylene system (such as cresols or nitrocresols) that might result from such an addition. Thus, the o-xylene/HONO/hv experiments should have formed only the two nitro-0-xylenes detected, 3- and 4-nitro-0-xylene (Table 11). The total yield was 6.8 f 1.9% of reacted o-xylene carbon, with 4-nitro-0-xylene accounting for 86 f 7% of that yield. Takagi et al. (13), in o-xylene/NO/H20/air irradiations of about 5 h, found an average nitro-0-xylene yield of 8.0 f 4.7%, with 94 f 4% of the isomers as 4-nitro-0-xylene. The nitration reaction shown in Figure 2, along with a similar reaction for the 3-OH adduct, was probably the mechanism of 4nitro-0-xylene formation in the CSTR. For m-xylene/HONO/hv systems, three nitro-m-xylene isomers could have formed, though 2-nitro-m-xylene was not detected. The lack of this product may be due to either additional steric hindrance of NO2 approach be344

Environ. Sci. Technol., Vol. 21, No. 4, 1987

tween the meta methyl groups or lack of the proper OH adduct radical structure. The analogous toluene mechanism (20) suggests that a 1adduct would be necessary to produce significant 2-nitro-m-xylene yields, and this is probably a low-yield OH adduct (7,40).Nitro-rn-xylene yields averaged only 3.3 f 2.5% of reacted m-xylene carbon (Table 111). 4-Nitro-rn-xylene accounted for 71 f 49% of this, with the remainder as 5-nitro-m-xylene. These yields were lower than those from both the o-xylene and toluene CSTR experiments, suggesting that even at elevated NOz concentrations this was a relatively unimportant process for m-xylene. It is possible to estimate the relative importance of nitration vs. dimethylphenol formation ( k + N o 2 / k p H E N 0 L ) for each xylene isomer by using the yields of dimethylphenols and nitroxylenes, combined with the yields of their reaction products, in the following approximation: k+NOz --

PHENOL

-

C [nitroxylenes]

[04

C [dimethylphenols] [NO,]

(4)

For o-xylene, the average value was calculated to be ap-

adai tion

t

t

Other

isomers

OH H

m-Xylene

\

I

1

\to2

OH

( l e s s than 1 % molar yield)

CH3

\

/

$$IH3

H

HO H

\

J?fo*

HO H

CH3 (2-5%

molar yield) a-dicarbonyl s and other unidentified Droducts Glyoxal t Methylglyoxal molar yields = 451

(12-19% molar yield)

0 (5-14% ' Hoi molar yield)

Flgure 3. Hydroxyl radical-rn-xylene addition reaction scheme. The OH adduct isomers shown were chosen to represent the xylene addition chemistry because they probably undergo all proposed reactions to some extent. The other isomers may yield higher amounts of certain products but are structurally restricted in other cases. Recall that NO, concentrationswere hlgh in these experiments. Lower values in the atmosphere limit the Importance of the NO2 reaction pathway. All yields are molar. Major Isomers are used to represent all possible isomers. Percent yields are for all isomers, even if only one is shown. See text for isomeric ratios.

proximately 5.9 x lo4 (k7 X lo3). The same calculation for m-xylene gave only 1.0 X lo4 ( k 3 X lo3),showing the effect of lower nitroxylene and higher dimethylphenol yields as compared to o-xylene. A similar lumped value for toluene was determined to be 3.3 X lo4 (25). Since NO2 concentration was higher in the CSTR than in the atmosphere and because NO, is an effective radical terminator, processes such as nitroxylene formation were more significant in the CSTR. Hence, although larger amounts of measurable products allowed calculation of more precise isomeric product ratios, nitrated product yields in Tables I1 and I11 can appear misleadingly high. Equation 4 and the above values for k + N o 2 / k Y H E N O L can be used to approximate the relative yields of nitroxylenes over dimethylphenols at ambient NO2 concentrations. In all xylene CSTR experiments the NOz concentration was near 2.5 ppm. Therefore, the relative yield of nitro-oxylenes to o-dimethylphenols would range from 7 to 0.7% for a NO, range of 0.25-0.025 ppm. This ratio, for mxylene products over the same NO, range, would be only 1.2-0.12%. Regardless of the low nitro-m-xylene yields discussed above, the abundance of 4-nitro-m-xylene, as compared to 5-nitro-m-xylene,is interesting from a molecular structure viewpoint. The prominent OH adducts assumed from O(3P)rates (the 4-OH adduct) should have produced high yields of 5-nitro-m-xylene according to the mechanism in

Figure 3. Assuming this mechanism is correct, 4-nitro-mxylene would have formed from NO2addition to either the 1- or 5-OH adduct with subsequent ejection of a water molecule. It was expected that the 1 adduct was a lowyield adduct isomer because no product evidence was found in either xylene or toluene CSTR experiments. 2-Nitro-m-xylene, which could have formed from the 1-OH adduct, was also not found. In addition, other than 4nitro-m-xylene, no products were observed (3,5-dimethylphenol or nitro-3,5-dimethylphenols)that would suggest a significant formation of the 5-OH adduct. This finding parallels similar results from toluene CSTR experiments (25),where very little m-cresol was detected, while high o- and p-nitrophenol yields were found. These data suggest that the effect resulting in low mcresol production in toluene systems may be augmented in m-xylene experimentsdue to the second m-methyl group (such results do not occur in the o-xylene data, Table 11). Aromatic meta reinforcement of the electron-donating influences of methyl groups results in ring activation for electrophilic substituents at the positions ortho and para to those groups (the 2-, 4-, and 6-positions) and deactivates the remaining positions (42). Thus, the rate of 5-OH adduct formation might be larger than indicated by O(3P) addition rates or 3,5-dimethylphenol yields, but the formation of 4-nitro-m-xylene may be highly preferred over that of 3,5-dimethylphenol. That is, for the 1-or 5-OH Environ. Sci. Technol., Vol. 21, No. 4, 1987

345

adducts, k+No2/kpHENoL may be greater than those for the 2- and 4-OH adducts. This difference was found to be nearly a factor of 20 in toluene CSTR experiments (25) and is very large but cannot be determined for m-xylene because no 3,5-dimethylphenol was measured. The m-xylene product yields suggest that phenols often form OH adducts at ring sites ortho and para to the methyl groups, but when m-OH adducts are expected, addition of adjacent nitro groups is favored. Addition of an oxygen molecule (below) should also be enhanced at those sites, but there is no product data from which to discern this. Thus, although it will be shown that nitration reactions are not very important under atmospheric conditions, the collective xylene and toluene data do suggest that orientation of substituents on aromatic rings causes complexity beyond the current bounds of explicit chemical representation in photochemical models. (c) Addition of Oxygen. The third primary reaction of the OH-xylene adducts is probably the reversible addition of an oxygen molecule (7). Again, more than one O2 adduct isomer can form because the original radical location would have been delocalized across the ring. The relative significance of each structure is not known. However, an esti'mate of the importance of O2addition to xylene-OH adducts can be made by assuming that these O2addition reactions account for all OH adduct carbon not reacting to form measured nitroxylenes or dimethylphenols. A ratio similar to those above may then be defined as

PHENOL k+Otff

1 C [dimethylphenols]/f,dd[Axylene] -

(C[dimethylphenols] + C[nitroxylenes]) ( 5 )

The OH adduct carbon is accounted for in the denominator by subtracting the detected nitroxylene and dimethylphenol yields from the addition fraction of the reacted xylene (fad&. This is the fraction of reacted xylene that did not react through the abstrction pathway or O(3P) reaction [O(3P)loss was only about 1.0%]. Equation 5 is shown as a possible inequality. This is because the right since all unside probably overestimates kPHENOL/k+Otff detected product mass, including dimethylphenol reaction products that were not nitrodimethylphenols, is included in the [Axylene] term. On the basis of all available CSTR data, the average value of kpHENOL/k+0zeff was determined to be greater than or equal to 0.28 f 0.11 for m-xylene and 0.16 f 0.08 for o-xylene. The effect of lower NO2 concentrations was estimated by replacing the C[nitroxylenes] term in eq 5 with

NO^

[NO,]

PHENOL

LO21

C [nitroxylenes] = --C [dimethylphenols] (6) (from eq 4). Calculations show that, as [NO,] drops below about 0.2 ppm, this lumped term becomes insignificant, resulting in kpHENOL/k+Otff > 0.27 f 0.11 for m-xylene and 0.15 f 0.06 for o-xylene as the limit when [NO,] approaches zero. It has been proposed that 0, adduct radicals undergo cyclization by creating O2 bridges across the ring though the structure, and the subsequent reaction mechanisms are unknown. Results from the previous toluene CSTR study (25) provided evidence for the existence of the [2.2.2] bicyclic radical in the formation of methyl-p-benzoquinone. Each xylene system formed a dimethyl-p-benzoquinone isomer, possibly through the following reactions: 346

Envlron. Sci. Technol., Vol. 21, No. 4, 1987

HO' 'H

As shown, the dimethyl-p-benzoquinone products appear to support the existence of [2.2.2] bicyclic radicals for both xylene photooxidation systems. Other types of bicyclic radicals could also occur (7, 16), but the reactions after bridging may not lead to stable, easily detected and identified compounds such as dimethyl-p-benzoquinones. Comparison of dimethyl-p-benzoquinone yields in Tables I1 and I11 shows that the 2,6 isomer, from m-xylene, formed in larger yields than the 2,3 isomer for o-xylene. probably (1) By reaction 8,2,6-dimethyl-p-benzoquinone originates with one of the most likely OH adducts, (2) is formed from bridging of the least sterically hindered O2 adduct, (3) has a tertiary bicyclic radical, and (4) need only sever a C-H bond to form the second carbonyl bond. In the reaction of o-xylene,one possible high-yield OH adduct would be forced to sever a C-CH3 bond to complete formation of the p-benzoquinone structure. By the mechanism in reaction 7, cleavage of the C-CH, bond would probably lead to methyl-p-benzoquinone (not detected in o-xylene CSTR experiments). 2,3-Dimethyl-p-benzoquinone has also been detected in UNC outdoor smog chamber experiments with o-xylene and NO, (5). The atmospheric chemistry of p-benzoquinone-type compounds is not known, although the quinoid structure suggests some atmospheric photooxidation is likely by analogy with similar species containing conjugated carbonyl and olefin bonds. Reaction of 2,6-dimethyl-p-benzoquinone did occur in batch photolysis experiments with HONO, although the loss rate was not determined. However, no products, including a-dicarbonyl species, were detected in these experiments. (d) Aromatic Ring Cleavage. Besides forming dimethyl-p-benzoquinones,xylene bicyclic radicals may react further, eventually resulting in ring cleavage and formation of some stable oxygenated fragments. The most prominent m-xylene fragmentation products are glyoxal and methylglyoxal (15,18, 19,43), while glyoxal, methylglyoxal, and biacetyl result from o-xylene cleavage (13-18, 43). The molar yields of glyoxal and methylglyoxal from the m-xylene experiments, corrected for OH reaction and photolysis losses (44),averaged 8.6 and 37.5%, The mean ratio of glyoxal to methylglyoxal was 0.23 f 0.02 molecule/molecule, slightly lower than previous results [0.32 (15),0.27 (18),and 0.39 (19)].Average yields for o-xylene fragments were 11.2, 34.0, and 15.8 molar percent for glyoxal, methylglyoxal, and biacetyl. The mean ratios of glyoxal and biacetyl to methylglyoxal were 0.33 and 0.46. Bandow and co-workers (15) reported very similar values of 0.36 and 0.43, while Shepson et al. (14), in a 5-min CH,ONO/NO/air irradiation, found these values to be 0.29 and 0.73 (data uncorrected for reaction loss). In six irradiations that averaged over 400 min, Takagi et al. (13) found molar ratios to be 6.31 and 1.92, demonstrating the higher reactivity of methylglyoxal (44) and the need for experiments of short duration when determining fractionation yields.

The combined a-dicarbonyl molar percent yields for m-xylene in the studies discussed above were 55 (15),40.5 (It?), and 36.9 (19). Bandow et al. (15) reported an average total yield of 41% for o-xylene experiments. Shepson and co-workers (14) found a total yield of only 23.5% in their o-xylene experiment but identified an additional 6.1 molar percent as longer chain, conjugated, oxygenated fragmentation products. In this study the total molar yield of dicarbonyls for the m-xylene CSTR experiments was about 46%; and for o-xylene, the combined molar yield was about 619%. Similar calculations yielded a value of only about 20% for earlier toluene CSTR experiments (25). Although it is expeected that addition of O2to OH adducts precedes formation of a-dicarbonyl products, the aromatic ring decomposition process is not well understood. Considering the values for kpHENOL/k+02eff and kabe/(kabs kadd)that were determined earlier, the O2addition pathway should account for 70% or less of the reacted xylene molecules for both xylene isomers. In toluene experiments this value was estimated to be about 60% (25). Comparison of these values to the a-dicarbonyl molar yields (above) indicates that, at best, only 1mol of a-dicarbonyl molecules would be expected from the fragmentation of an equal amount of OH-02-xylene adducts. For toluene, where molar yields were only about 20770,even less carbon was detected as a-dicarbonyl fragments. Apparently, a significant number of the OH adducts expected to add an oxygen molecule do not yield these highly reactive products. Whether the missing carbon exists in undetected fragmentation species (a small percentage of the reacted carbon is probably CO, C02, and formaldehyde) or in stable compounds formed prior to ring cleavage is not clear. However, the observed a-dicarbonyl yields from this and earlier studies indicate that, on a carbon basis, much product mass is unaccounted for in all of the systems studied and the missing mass is not likely to exist in the measured a-dicarbonyl species. Compared to the dimethylphenol and nitroxylene formation reactions, the oxygen addition-ring fragmentation sequence requires extra reaction steps prior to formation of the products attributed to that pathway (Figures 2 and 3). Because of these additional reactions and the various isomeric products in each, unsurmised low-yield channels could accumulate a significant portion of the reacted carbon as undetected products. The dimethyl-p-benzoquinone yields indicate that ring fragmentation is not instantaneous and that the O2 adduct or the [2.2.2] bicyclic radical may be more stable than currently described in photochemical kinetics models. Further, if there is sufficient time for internal rearrangement of a [2.2.2] bicyclic radical, the possibility of reactions of the resulting radical could be high. Besides further reactions of the dimethyl-p-benzoquinones, some possible reactions of the O2 adduct and subsequent species include (1)other types of internal rearrangements, (2) addition of further O2 molecules, (3) NO-to-NOz conversion and formation of organic and organic nitrate products, (4) unimolecular decomposition, and (5)reaction with other species such as H 0 2 or R02 radicals. Some mechanisms for these processes have been proposed (7, 14, 20, 26). Significant reaction in these pathways would be difficult to detect because many products with unknown molecular structures could occur in low yields, limiting the molecules available for ring decomposition. Such a scheme fits the results from the CSTR and other studies, suggesting that, contrary to the mechanistic descriptions in current photochemical models, much of the carbon involved in the reaction of oxygen

+

molecule addition to the OH adduct forms products, which are not a-dicarbonyl compounds. Conclusions and Recommendations For the aromatic species under study, various competitive processes result in different product distributions as the structural and energetic characteristics of the reacting ring are varied. In the case of the OH abstraction pathway, however, both the yields and chemistry were similar among the aromatic hydrocarbons studied. This is partly because the reactions occurred at the methyl group rather than upon the aromatic ring, thus, extending the distance over which different structural effects must be transmitted. In the OH addition pathway it was found that NO2 addition to the OH adduct would be of minor importance in the atmosphere because the reaction rates were comparatively small. The xylene systems formed about half the hydroxyalkylbenzene products (dimethylphenols) as did toluene (mostly o-cresol). It was postulated that the second xylene methyl group could promote additional mass through the O2addition reaction for a number of reasons. This competition with dimethylphenol formation probably led to the lower hydroxyalkylbenzene yields (lower kPHENOL/k+OJ found in the xylene systems. As noted, a-dicarbonyl yields for xylene systems were considerably larger than those from toluene experiments, even though nearly the same number of molecules were expected to pass through the O2 addition reactions. This may indicate that the degree of ring methylation can also influence the yields of dicarbonyl species. Additional methyl groups might result in more ring fragmentation, while alternate pathways may be more important for toluene adducts. Clearly, if these trends exist, they have only been demonstrated over a small range of compounds. The relative number and orientation of methyl groups must also lead to different product distributions. The high methylglyoxal yields in m-xylene experiments are probably responsible for some of the increased reactivity in m-xylene smog chamber systems. Because it has a high reaction rate with OH and can readily photolyze in the lower troposphere, methylglyoxal can most rapidly contribute radicals to a reactive system. Although o-xylene a-dicarbonyl yields are larger, more time is required for much of that product mass to enter radical cycles because biacetyl does not readily react with OH. Upon photolysis, immediate biacetyl radical contribution may be hindered by peroxyacyl radical reaction with NO2 in the formation of PAN. Toluene, besides having lower a-dicarbonyl yields, produces a large amount of glyoxal, which photolyzes slowly and has a lower OH reaction rate compared to that of methylglyoxal.,Hence, a major difference in the reactivity of the alkylbenzene species is no doubt associated with the reactivity of these fragments (of which only some have been identified). To explore this process further, dicarbonyl yields from similar species must be obtained, including isomers with larger alkyl groups of both electron-donating and steric-shielding effects. Additional investigation of species with different functional groups and group orientations, such as p-xylene, trimethylbenzenes, ethylbenzene, and methylethylbenzenes, should lead to a better understanding of the effect of structural characteristics. Also, since the reactions following oxygen addition to the OH adduct radical remain the least understood segment of alkylbenzene oxidation chemistry, this area should be the focus of future investigation. Finally, because some reaction products (such as alkylphenols) have been shown to have high yields or significant secondary reactions, the gas-phase chemistry of these compounds should be investigated. Environ. Sci. Technol., Vol. 21, No. 4, 1987

347

Acknowledgments We gratefully acknowledge m a n y useful discussions with Harvey Jeffries of the University of N o r t h Carolina. We also thank Marcia Dodge of the U.S. Environmental Protection Agency for her support and Daryl Sharp, Jean Perry, and Maurice Jackson for providing a n array of analytical skills. Registry No. OH, 3352-57-6; NO,, 11104-93-1; m-xylene, 108-38-3; o-xylene, 95-47-6; carbon monoxide, 630-08-0; formaldehyde, 50-00-0;glyoxal, 107-22-2;methylglyoxal, 78-98-8; biacetyl, 431-03-8; peroxyacetyl nitrate, 2278-22-0; m-methylbenzyl nitrate, 62285-55-6;m-tolualdehyde, 620-23-5; 2-nitro-m-xylene, 81-20-9; 4-nitro-m-xylene, 89-87-2; 5-nitro-m-xylene, 99-12-7; 2,6-dimethyl-p-benzoquinone, 527-61-7; 2,4-dimethylphenol, 105-67-9; 6-nitro-2,4-dimethylphenol, 14452-34-7;2,6-dimethyl2423-71-4; 3,5-diphenol, 576-26-1; 4-nitro-2,6-dimethylphenol, methylphenol, 108-68-9; o-methylbenzyl nitrate, 29510-54-1; otolualdehyde, 529-20-4; 3-nitro-o-xylene,83-41-0;4-nitro-o-xylene, 526-86-3; 2,3-dimethyl99-51-4; 2,3-dimethyl-p-benzoquinone, 19499-93-5; 3,4phenol, 526-75-0; 4-nitro-2,3-dimethylphenol, dimethylphenol, 95-65-8; 2-nitro-3,4-dimethylphenol, 3114-62-3; 5-nitro-3,4-dimethylphenol, 65151-58-8; 6-nitro-3,4-dimethylphenol, 18087-10-0. L i t e r a t u r e Cited Jeffries, H. E.; Sexton, K. G.; Morris, T. P.; Jackson, M.; Goodman, R. G.; Kamens, R. .; Holleman, M. S. Outdoor Smog Chamber Experiments Using Automobile Exhaust; U.S. Enviornmental Protection Agency: Research Triangle Park, NC, April 1985; EPA-600/3-85-032. Black, F. M.; High, L. E.; Lang, J. M. J. Air. Pollut. Control ASSOC. 1980, 30, 1216-1221. Grosjean, D.; Fung, K. J. Air Pollut. Control Assoc. 1984, 34,537-543. Lonneman, W. A.; Kopczynski, S. L.; Darley, P. E.; Sutterfield, F. D. Environ. Sci. Technol. 1974, 8 , 229-236. Jeffries, H. E.; Sexton, K. G.; Kamens, R. M.; Holleman, M. S. Outdoor Smog Chamber Experiments to Test Photochemical Models: Phase ZI; U.S. Environmental Protection Agency: Research Triangle Park, NC, April 1985; EPA-600/3-85-029. Whitten, G. Z. Environ. Znt. 1983, 9, 447-463. Atkinson, R.; Lloyd, A. C. J. Phys. Chem. Ref. Data 1984, 13, 315-444. Atkinson, R.; Darnall, K. R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., Jr. Adu. Photochem. 1979, 11, 375-488. Nicovich, J. M.; Thompson, R. L.; Ravishankara, A. R. J. Phys. Chem. 1981,85, 2913-2916. Pate, C. T.; Atkinson, R.; Pitts, J. N., Jr. J . Enuiron. Sci. Health, Part A 1976, A l l , 1-10, Atkinson, R.; Pitts, J. N., Jr. J. Phys. Chem. 1974, 78, 1780-1784. Atkinson, R.; Carter, W. P. L.; Plum, C. N.; Winer, A. M.; Pitts, J. N., Jr. Int. J. Chem. Kinet. 1984, 16, 887-898. Takagi, H.; Washida, N.; Akimoto, H.; Nagasawa, K.; Usui, Y.; Okuda, M. J. Phys. Chem. 1980,84,478-483. Shepson, P. B.; Edney, E. 0.;Corse, E. W. J. Phys. Chem. 1984,88,4122-4126. Bandow, H.; Washida, N. Bull. Chem. SOC.Jpn. 1985,58, 2541-2548. Darnall, K. R.; Atkinson, R.; Pitts, J. N., Jr. J. Phys. Chem. 1979,83, 1943-1946. Atkinson, R.; Carter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983,87, 1605-1610. Tuazon, E. C.; Mac Leod, H.; Atkinson, R.; Carter, W. P. L. Environ. Sci. Technol. 1986, 20, 383-387.

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Received for review February 3, 1986. Revised manuscript received September 29,1986. Accepted October 30,1986. Although the research described in this article has been funded in part by the U.S. Environmental Protection Agency through Cooperative Agreements R809954 entitled “Experimental Study of Aerosol Formation Mechanisms in a Controlled Atmosphere” and R808881 entitled “OutdoorSmog Chamber Experiments to Test Photochemical Models”, it has not been subject to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.