Formation of -Hydroxycarbonyls from the OH Radical-Initiated

radicals, the reactions of the OH radical with trans-2-butene, trans-3-hexene, and R-pinene in the absence of NO lead to the formation of the expected...
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Environ. Sci. Technol. 2000, 34, 1702-1706

Formation of β-Hydroxycarbonyls from the OH Radical-Initiated Reactions of Selected Alkenes

constants and the concentrations of HO2 radicals, RO˙ 2 radicals, and NO (3, 4):

SARA M. ASCHMANN, JANET AREY,† AND R O G E R A T K I N S O N * ,†,‡ Air Pollution Research Center, University of California, Riverside, California 92521

CH3CH(OH)CH(OO˙ )CH3 + RO˙ 2 f CH3CH(OH)CH(O˙ )CH3 + RO˙ + O2 (4a)

CH3CH(OH)CH(OO˙ )CH3 + HO2 f CH3CH(OH)CH(OOH)CH3 + O2 (3)

β-Hydroxycarbonyls can be formed from the gas-phase reactions of alkenes with the OH radical, both in the presence and in the absence of NO. To date, because of analytical difficulties, few data have been reported for the formation of this class of compound from the reactions of the OH radical with alkenes. We have determined that β-hydroxyketones can be readily analyzed by gas chromatography, and in this work we have shown that in 1 atm of air the β-hydroxyalkoxy radicals formed in the reactions of the OH radical with trans-2-butene, trans-3-hexene, 1-butene, and R-pinene in the presence of NO primarily decompose rather than react with O2. Rate constant ratios kd/kO2 (or lower limits thereof), where kd and kO2 are respectively the rate constants for the decomposition and the reaction with O2 of the intermediate β-hydroxyalkoxy radicals, have been obtained for the reactions of the CH3CH(O˙ )CH(OH)CH3, CH3CH2CH(O˙ )CH2OH, and CH3CH2CH(O˙ )CH(OH)CH2CH3 radicals at 296 ( 2 K and atmospheric pressure. Using the O3 reactions with the alkenes to generate OH radicals, the reactions of the OH radical with trans-2-butene, trans-3-hexene, and R-pinene in the absence of NO lead to the formation of the expected β-hydroxycarbonyls and (at least for trans-2-butene) the R,β-diol.

Introduction Alkenes are emitted into the atmosphere from both biogenic and anthropogenic sources (1, 2). In the troposphere, alkenes react with OH radicals, NO3 radicals, and O3, with the OH radical reaction often dominating over the O3 reaction during daylight hours (3, 4). The OH radical reactions proceed mainly by initial addition to the carbon atoms of the >CdC< bond(s) (3), resulting in the formation of β-hydroxyalkyl radicals which then rapidly add O2 to form the corresponding β-hydroxyalkyl peroxy radicals (3). For example, for the 2-butenes:

OH + CH3CHdCHCH3 f CH3CH(OH)C˙ HCH3

(1)

CH3CH(OH)C˙ HCH3 + O2 f CH3CH(OH)CH(OO˙ )CH3 (2) In the troposphere, the hydroxyalkyl peroxy radicals will react with HO2 radicals, organic peroxy (RO˙ 2) radicals, and NO with the dominant reaction depending on the reaction rate * Corresponding author phone: (909)787-4191; fax: (909)787-5004; e-mail: [email protected]. † Also at the Department of Environmental Sciences and Interdepartmental Graduate Program in Environmental Toxicology. ‡ Also at the Department of Chemistry. 1702

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CH3CH(OH)CH(OO˙ )CH3 + RO˙ 2 f R CH3CH(OH)C(O)CH3 + (1 - R) CH3CH(OH)CH(OH)CH3 + RO˙ 2 products + O2 (4b)

The β-hydroxyalkoxy radicals formed in reactions 4a and/or 5b react with O2 (if an R-H is present), decompose by C-C bond scission, and isomerize through a six-membered transition state (if feasible) (3-5). For the CH3CH(OH)CH(O˙ )CH3 radical formed from the 2-butenes, only reaction with O2 and decomposition are possible:

CH3CH(OH)CH(O˙ )CH3 + O2 f CH3CH(OH)C(O)CH3 + HO2 (6) CH3CH(OH)CH(O˙ )CH3 f CH3C˙ HOH + CH3CHO

(7)

with the CH3C˙ HOH radical reacting rapidly with O2 to form a second molecule of acetaldehyde (3, 4):

CH3C˙ HOH + O2 f CH3CHO + HO2

(8)

β-Hydroxycarbonyls can therefore be formed from the reactions of the OH radical with alkenes in both the presence of NO (through the reaction sequence eqs 1, 2, 5b, and 6) and the absence of NO (through the reaction sequence eqs 1, 2, 4a, and 6 or the reaction sequence eqs 1, 2, and 4b). Previous product studies of the reactions of the OH radical with alkenes in the presence of NO show that for ethene the intermediate HOCH2CH2O˙ radical both decomposes and reacts with O2 (6-8) and that for propene, 1-butene, 2methylpropene, cis- and trans-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene the intermediate β-hydroxyalkoxy radicals primarily decompose (9-13). Orlando et al. (8) have shown that for the reaction of the OH radical with ethene in the presence of NO, ∼25% of the HOCH2CH2O˙ radicals formed from the exothermic reaction of the HOCH2CH2OO˙ radical with NO are chemically activated and undergo “prompt” decomposition.A similar situation also appears to apply to the β-hydroxyalkoxy radicals formed from propene (13) in the presence of NO. For the OH radical-initiated reactions of 1-butene through 1-octene, isomerization of the intermediate β-hydroxyalkoxy radicals can also occur and becomes increasingly important with increasing carbon number of the 1-alkene (14, 15). However, apart from the recent study of Vereecken et al. (13) of the propene reaction, previous product studies of the reactions of 1-butene and the methyl-substituted ethenes have only reported the formation of the β-hydroxyalkoxy radical decomposition products (plus in some cases the organic nitrates formed from the RO˙ 2 + NO reactions) (9-12, 16-18), and in these cases the measurement uncer10.1021/es991125a CCC: $19.00

 2000 American Chemical Society Published on Web 03/24/2000

tainties do not rule out the formation of β-hydroxycarbonyls from the O2 reaction channel occurring to a small extent (up to ∼10%). Few product studies of the reactions of the OH radical with alkenes in the absence of NO have been carried out (7, 12, 19). Only in the product study of Barnes et al. (7) of the ethene reaction were the HOCH2CHO and HOCH2CH2OH products arising from the RO˙ 2 + RO˙ 2 reactions (directly through reaction 4b and, for HOCH2CHO, through reaction 4a followed by the reaction of the HOCH2CH2O˙ radical with O2) quantified, by in situ Fourier transform infrared spectroscopy. In this work, we have used gas chromatography to identify and quantify β-hydroxyketones formed from the reactions of the OH radical with trans-2-butene, trans-3-hexene, 1-butene, and R-pinene in the presence and absence of NO.

Experimental Methods Experiments were carried out at 296 ( 2 K and 740 ( 5 Torr total pressure of air or O2-N2 in a 7900-L Teflon chamber with analysis by gas chromatography with flame ionization detection (GC-FID) and combined gas chromatographymass spectrometry (GC-MS). When needed, irradiation was provided by two parallel banks of black lamps. The chamber is fitted with a Teflon-coated fan, which ensured rapid mixing of reactants during their introduction into the chamber. The chamber diluent gas was either purified air at ∼5% relative humidity or an O2-air mixture with an O2 partial pressure of 660 ( 30 Torr. OH Radical-Initiated Reactions in the Presence of NO. Hydroxyl radicals were generated in the presence of NO by the photolysis of methyl nitrite in air at wavelengths >300 nm (20), and NO was added to the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals (20):

CH3ONO + hν f CH3O˙ + NO

(9)

CH3O˙ + O2 f HCHO + HO2

(10)

HO2 + NO f OH + NO2

(11)

The initial reactant concentrations (in molecule cm-3 units) were as follows: CH3ONO, ∼2.4 × 1014; NO, ∼2.4 × 1014; and alkene, (2.24-3.02) × 1013. Experiments were carried out at an O2 partial pressure of 660 ( 30 Torr, apart from one experiment with 1-butene, which was carried out in air (i.e., at an O2 partial pressure of 155 Torr). Irradiations were carried out at 20% of the maximum light intensity for 2.5-10 min, resulting in up to 68% reaction of the initially present alkene. The concentrations of the reactant alkene and reaction products were measured during the experiments by GC-FID. For the analyses of 1-butene and trans-2-butene, gas samples were collected in a 100 cm3 volume all-glass gas-tight syringe and transferred via a gas sampling loop and valve onto a 30 m DB-5 megabore column in a Hewlett-Packard (HP) 5890 GC, initially held at -25 °C and then temperature programmed to 200 °C at 8 °C min-1. For analyses of trans-3hexene, R-pinene, and β-hydroxycarbonyls, gas samples of 100 cm3 volume were collected from the chamber onto TenaxTA solid adsorbent, with subsequent thermal desorption at ∼225 °C onto a DB-1701 megabore column in an HP 5710 GC, initially held at 0 °C and then temperature programmed to 200 °C at 8 °C min-1. In addition, gas samples were collected onto Tenax-TA solid adsorbent for GC-MS analyses, with thermal desorption onto a 60 m HP-5 fused silica capillary column in an HP 5890 GC interfaced to a HP 5970 mass selective detector operating in the scanning mode. GC-FID response factors for the alkenes, 3-hydroxy-2-butanone, 1-hydroxy-2-butanone, and 4-hydroxy-3-hexanone were measured as described previously (14). Relative to the

measured GC-FID response factors for trans-3-hexene, R-pinene, n-decane, n-butylcyclohexane, and 3,4-diethylhexane, the measured response factors for 3-hydroxy-2butanone, 4-hydroxy-3-hexanone, and 1-hydroxy-2-butanone agreed with the calculated effective carbon numbers (21) to within 20%. The GC-FID response factor for 2-hydroxy3-pinanone was therefore calculated relative to that measured for R-pinene using their effective carbon numbers (21). NO and the initial NO2 concentrations were measured using a Thermo Environmental Instruments, Inc., Model 42 NONO2-NOx chemiluminescence analyzer. OH Radical-Initiated Reactions in the Absence of NO. OH radicals were generated in the absence of added NOx by the reaction of O3 with the alkene being studied (12). The initial alkene concentrations were (2.18-4.50) × 1013 molecule cm-3, and 2-4 additions of 50 cm3 volume of O3 in O2 diluent (each corresponding to an initial O3 concentration in the chamber of ∼5 × 1012 molecule cm-3) were added to the chamber during an experiment. Analyses for reactants and products were carried out as described above, with the time between the O3 addition and the sampling for GC-FID analyses being such that the majority of the O3 had reacted away. The chemicals used and their stated purities were as follows: 2,3-butanediol (98%), trans-3-hexene (99+%), 1-hydroxy-2-butanone (95%), 3-hydroxy-2-butanone, 2-hydroxy3-pinanone (99%), and R-pinene (99+%), Aldrich Chemical Co.; 4-hydroxy-3-hexanone (95+%), TCI America; NO (g99.0%), 1-butene (g99.0%), and trans-2-butene (g95%), Matheson Gas Products. Methyl nitrite was prepared and stored as decribed previously (20).

Results and Discussion OH Radical Reactions in the Presence of NO. Irradiations of CH3ONO-NO-trans-2-butene-O2 (660 ( 30 Torr)-N2 (80 ( 30 Torr) and CH3ONO-NO-trans-3-hexene-O2 (660 ( 30 Torr)-N2 (80 ( 30 Torr) mixtures showed no evidence for the formation of 3-hydroxy-2-butanone from trans-2butene or of 4-hydroxy-3-hexanone from trans-3-hexene, and only upper limits to the concentrations of these β-hydroxycarbonyls could be obtained from the GC-FID analyses. However, irradiations of CH3ONO-NO-R-pineneO2 (660 ( 30 Torr)-N2 (80 ( 30 Torr), CH3ONO-NO1-butene-O2 (660 ( 30 Torr)-N2 (80 ( 30 Torr), and CH3ONO-NO-1-butene-air mixtures showed the presence of small GC peaks at the correct retention times for the formation of 2-hydroxy-3-pinanone from R-pinene and of 1-hydroxy-2-butanone from 1-butene, and these GC peaks increased in area with the extents of reaction. β-Hydroxycarbonyls also react with the OH radical (22). Secondary reactions of the β-hydroxycarbonyls with the OH radical were taken into account as described previously (23), using rate constants for the OH radical reactions (in units of 10-12 cm3 molecule-1 s-1) of trans-2-butene, 64.8 (3); trans3-hexene, 67 [estimated (24)]; R-pinene, 54.2 (3); 1-butene, 31.7 (3); 3-hydroxy-2-butanone, 10.3 (22); 4-hydroxy-3hexanone, 15.1 (22); 2-hydroxy-3-pinanone, 8.3 [estimated (24)]; and 1-hydroxy-2-butanone, 7.7 (22). Corrections to take into account secondary reactions of the β-hydroxycarbonyls with the OH radical increase with the rate constant ratio k(OH + hydroxycarbonyl)/k(OH + alkene) and with the extent of reaction (23) and were e11%, e17%, e8%, and e16% for the trans-2-butene, trans-3-hexene, R-pinene, and 1-butene reactions, respectively. The β-hydroxycarbonyl formation yields (or upper limits thereof) obtained from the experimental data are given in Table 1. Our formation yields for the formation of β-hydroxycarbonyls from the reactions of the OH radical with trans2-butene, 1-butene, and R-pinene in the presence of NO are consistent with previous product studies (no product studies VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Formation Yields of β-Hydroxycarbonyls from Reactions of the OH Radical with Selected Alkenes, in the Presence of NO, at 296 ( 2 K and Atmospheric Pressure

alkene

β-hydroxycarbonyl

O2 pressure (Torr)

trans-2-butene 3-hydroxy-2-butanone 660 ( 30 trans-3-hexene 4-hydroxy-3-hexanone 660 ( 30 1-butene 1-hydroxy-2-butanone 660 ( 30 155 ( 2 R-pinene 2-hydroxy-3-pinanone 660 ( 30

molar yielda e0.0030 e0.0040 0.024 ( 0.014 0.005 ( 0.005 0.025 ( 0.010

a Indicated uncertainties are the two least-squares standard deviations combined with the estimated uncertainties in the GC-FID response factors for 1-butene and 1-hydroxy-2-butanone of (5% each and for 2-hydroxy-3-pinanone, relative to that of R-pinene, of (20%.

of the OH radical-initiated reaction of trans-3-hexene have been reported to date). For the reaction of trans-2-butene, Niki et al. (9) observed the formation of acetaldehyde with a molar formation yield of 2.1 (i.e., quantitative conversion of trans-2-butene to 2 molecules of acetaldehyde). For the 1-butene reaction, Atkinson et al. (10) observed the formation of propanal with a yield of 0.94 ( 0.12, and Kwok et al. (15) using atmospheric pressure ionization mass spectrometry (API-MS) observed the formation of propanal and a C4dihydroxycarbonyl (the latter being attributed to isomerization of the CH3CH2CH(OH)CH2O˙ radical and being estimated to have a yield of ∼4% of the propanal yield). In previous studies of the R-pinene reaction, the gaseous products observed have been pinonaldehyde and formaldehyde (25-28) [with GC-FID analyses of pinonaldehyde giving a formation yield of 28 ( 5% (27)], with the API-MS study of Aschmann et al. (29) indicating the formation of dihydroxycarbonyls, hydroxynitrates, and dihydroxynitrates. In these previous studies of trans-2-butene (9), 1-butene (10, 15), and R-pinene (24-29), no formation yields of β-hydroxycarbonyls or upper limits thereof were reported and hence formation yields of these β-hydroxycarbonyls can only be derived by difference using the measured yields of the decomposition products (acetaldehyde, propanal, and pinonaldehyde, respectively) and any isomerization products. As noted in the Introduction, in the presence of sufficient concentrations of NO such that organic peroxy (RO˙ 2) radicals react dominantly with NO, then β-hydroxycarbonyls arise from the reaction of the intermediate β-hydroxyalkoxy radicals with O2 in competition with decomposition and (if feasible) isomerization. For the trans-2-butene and trans3-hexene reactions, only one β-hydroxyalkoxy radical is formed in each case, and hence our upper limits to the β-hydroxycarbonyl formation yields allow more rigorous lower limits to the rate constant ratio kd/kO2 (and the rate constant kd if the rate constant kO2 is known) to be obtained than previously, where kd and kO2 are the rate constants for β-hydroxyalkoxy radical decomposition and reaction with O2, respectively. In deriving the rate constant ratios kd/kO2, the occurrence of reaction 5a (formation of β-hydroxynitrates from the RO˙ 2 + NO reaction) and any isomerization reactions of the β-hydroxyalkoxy radicals need to be taken into account. On the basis of the data of Muthuramu et al. (17) and O’Brien et al. (18) for analogous hydroxyalkyl peroxy radicals, β-hydroxynitrate formation is estimated to account for e5% of the total reactions of the CH3CH(OO˙ )CH(OH)CH3 and CH3CH2CH(OO˙ )CH(OH)CH2CH3 radicals with NO, and we estimate (14, 15) that isomerization of the CH3CH2CH(O˙ )CH(OH)CH2CH3 radical accounts for ∼5% of the competing decomposition pathway at room temperature. Lower limits to the rate constant ratios kd/kO2 of g6.5 × 1021 and >4.5 × 1021 molecule cm-3 are then obtained at 296 ( 2 K for the 1704

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CH3CH(O˙ )CH(OH)CH3 and CH3CH2CH(O˙ )CH(OH)CH2CH3 radicals, respectively. If it is assumed that these β-hydroxyalkoxy radicals react with O2 with a rate constant identical to those for alkoxy radicals of structure R2CHO˙ [8.0 × 10-15 cm3 molecule-1 s-1 at 298 K (3)], then lower limits to the rate constants for decomposition of the CH3CH(O˙ )CH(OH)CH3 and CH3CH2CH(O˙ )CH(OH)CH2CH3 radicals are >5.2 × 107 and >3.6 × 107 s-1, respectively, at 296 ( 2 K. In the case of the 1-butene reaction, OH radical addition can occur at the internal or terminal carbon atom. The formation yield of the CH3CH2C˙ HCH2OH radical from the addition reaction is not well-known (30, 31), although it is probably in the range of 0.35-0.65, and we assume a yield of 0.5. Our formation yield for the β-hydroxyketone CH3CH2C(O)CH2OH [note that the hydroxyaldehyde CH3CH2CH(OH)CHO formed from internal OH radical addition to 1-butene was not observed, probably because it does not elute from a gas chromatography column without derivatization, as we have previously observed for (CH3)2C(OH)CHO (32)] allows a rate constant ratio kd/kO2 for the CH3CH2CH(O˙ )CH2OH radical to be derived. Assuming that organic nitrate formation from the reactions of NO with CH3CH2CH(OO˙ )CH2OH and CH3CH2CH(OH)CH2OO˙ radicals is e5% (17, 18), then our data given in Table 1 leads to a rate constant ratio of kd/kO2 for the CH3CH2CH(O˙ )CH2OH radical of ∼4 × 1020 molecule cm-3 at 296 ( 2 K. OH radical addition to R-pinene can occur at either the 2- or 3-position, with the relative importance of these addition channels not presently being known. Therefore, we cannot obtain a rate constant ratio kd/kO2 for the β-hydroxyalkoxy radical leading (via O2 reaction) to 2-hydroxy-3-pinanone, although this rate constant ratio is probably similar to that derived above for the CH3CH2CH(O˙ )CH2OH radical. The lower limit to the rate constant ratio kd/kO2 obtained here for the CH3CH(O˙ )CH(OH)CH3 radical is a factor of 2 higher than the value empirically estimated by Atkinson (5), and the approximate value of the rate constant ratio kd/kO2 obtained here for the CH3CH2CH(O˙ )CH2OH radical is a factor of ∼30 higher than that empirically estimated by Atkinson (5), both suggesting that the estimation method of Atkinson (5) underestimates the decomposition rates of β-hydroxyalkoxy radicals. This conclusion is consistent with the underestimate of the decomposition rate of the HOCH2CH2O˙ radical formed from ethene [by a factor of 25 taking into account the “prompt” decomposition of this radical when formed from the exothermic HOCH2CH2OO˙ + NO reaction (8)] (5). As noted above, when formed from the exothermic ROO˙ + NO reaction ∼25% of the HOCH2CH2O˙ radical undergoes prompt decomposition (8), and prompt decomposition is calculated to occur for the CH3CH(O˙ )CH2OH radical formed from propene (13). Prompt decomposition of a substantial fraction of the more complex β-hydroxyalkoxy radicals formed from the RO˙ 2 + NO reactions studied here is therefore expected. OH Radical Reactions in the Absence of NO. For these experiments, OH radicals were generated by the reaction of O3 with the alkene being studied (trans-2-butene, trans-3hexene, and R-pinene), and hence the alkenes were consumed by reaction with both O3 and OH radicals. Because of the complexity of these reaction systems, only a limited study was carried out for each of these three alkenes. GCFID analyses of reacted O3-alkenesair mixtures showed the formation of 3-hydroxy-2-butanone and 2,3-butanediol from trans-2-butene and of 4-hydroxy-3-hexanone from trans3-hexene. These results were confirmed by GC-MS analyses. A peak at the correct retention time for the formation of 2-hydroxy-3-pinanone from R-pinene was observed in the GC-FID analyses, but was not confirmed by GC-MS. The measured yields of these products during the O3 reactions are given in Table 2.

TABLE 2. Formation Yields of Selected Products from Reactions of O3 with Selected Alkenes at 296 ( 2 K and Atmospheric Pressure of Air alkene

product

measured yielda

yield from OH reactionb

trans-2-butene

3-hydroxy-2-butanonec 2,3-butanediolc 4-hydroxy-3-hexanonec 2-hydroxy-3-pinanoned pinonaldehyded

0.066 ( 0.005 0.055 ( 0.005 0.10 ( 0.01 ∼0.02 ∼0.20

0.17 0.14 0.27 ∼0.05 e

trans-3-hexene R-pinene

a Indicated uncertainties are the two least-squares standard deviations combined with the estimated uncertainties in the GC-FID response factors for the alkene and products of (5% each. b Assuming an OH radical formation yield from the O3 reactions of: trans-2-butene, 0.64 (3); trans-3-hexene, 0.60 (estimated by analogy with trans-2-butene); and R-pinene, 0.75 (3); and assuming that all of the OH radicals formed react with the alkene. c Identified by matching of GC retention times in GC-FID and GC-MS analyses and mass spectrum with those of an authentic standard. d Identified by matching of GC retention time with that of an authentic standard. e Pinonaldehyde is also formed from the O3 reaction with R-pinene (3).

SCHEME 1

The reactions of O3 with the alkenes studied here do not lead to the formation of the observed β-hydroxycarbonyls (3). Hence the formation yields of these products from the reactions of the OH radical with the alkenes studied can be derived using measured or estimated OH radical formation yields from the O3 reactions with the alkenes (3) and are given in Table 2 (the OH radical formation yields from the O3 reactions used in these calculations are given in the footnotes to Table 2). Clearly, the reactions of the OH radical with these alkenes [and especially trans-2-butene for which both CH3C(O)CH(OH)CH3 and CH3CH(OH)CH(OH)CH3 were identified and quantified] in the absence of NO leads to the formation of β-hydroxycarbonyls. β-Hydroxycarbonyl and R,β-diol formation is expected from reactions of the β-hydroxyalkyl peroxy radicals with organic peroxy radicals (including the selfreaction) [reaction 4b plus reaction 4a followed by reaction 6 for the β-hydroxycarbonyl and reaction 4b for the R,βdiol]. These reactions are shown in Scheme 1 for the trans2-butene reaction system, with the products of interest being shown in boxes. The occurrence of these reactions has been observed by Barnes et al. (7) for the HOCH2CH2OO˙ radical and by Boyd et al. (33) for the (CH3)2C(OH)CH2OO˙ radical. While further analysis of the data given in Table 2 is unwarranted given the complexity of the reaction systems employed, the occurrence of reaction 4b leading to formation of β-hydroxcarbonyls and diols is evident. Atmospheric Implications. Because the dominant reaction of β-hydroxyalkoxy radicals in the lower troposphere is by decomposition rather than by reaction with O2 to form

β-hydroxycarbonyls (see also refs 8 and 13), the major pathway to β-hydroxcarbonyl formation in the lower troposphere is from the reactions of β-hydroxyalkyl peroxy [>C(OH)CH(OO˙ )R] and β-acylalkyl peroxy [RC(O)C(OO˙ )1 day (22), measurements of ambient atmospheric concentrations of this class of multifunctional compound will be of interest.

Acknowledgments We gratefully thank the U.S. Environmental Protection Agency, Office of Research and Development, for supporting this research through Assistance Agreement R-825252-01-0. While this research has been supported by the U.S. Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.

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Received for review September 30, 1999. Revised manuscript received January 9, 2000. Accepted January 27, 2000. ES991125A