Dicarbonyl Products of the OH Radical-Initiated ... - ACS Publications

Dec 31, 2008 - SULEKHA CHATTOPADHYAY,. ROLAND D. CUSICK, AND. ROGER ATKINSON* ,†,‡. Air Pollution Research Center University of California...
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Environ. Sci. Technol. 2009, 43, 683–689

Dicarbonyl Products of the OH Radical-Initiated Reaction of a Series of Aromatic Hydrocarbons J A N E T A R E Y , * ,† G E N E V I E V E O B E R M E Y E R , SARA M. ASCHMANN, SULEKHA CHATTOPADHYAY, ROLAND D. CUSICK, AND R O G E R A T K I N S O N * ,†,‡ Air Pollution Research Center University of California Riverside, California 92521

Received July 10, 2008. Accepted November 8, 2008.

Aromatic hydrocarbons are important constituents of vehicle exhaust and of nonmethane organic compounds in ambient urban air. We used a derivatization technique with methane positive chemical ionization gas chromatography/mass spectrometry to investigate the carbonyl products formed from the OH radicalinitiated reactions of toluene, the xylenes, and the trimethylbenzenes. Characteristic diderivatized molecular ions of dicarbonyl products were obtained. Consistent with previous studies, the 1,2-dicarbonyls glyoxal, methylglyoxal, and biacetyl were observed, as were all but one of the possible unsaturated 1,4dicarbonyl coproducts. Unsaturated 1,4-diketones had formation yields similar to their potential coproduct 1,2dicarbonyls. However, apart from HC(O)CHdCHCHO, unsaturated 1,4-dialdehydes and keto-aldehydes were generally observed in lower yield than their potential 1,2-dicarbonyl coproducts.

Introduction Aromatic hydrocarbons are important constituents of gasoline and diesel fuels and of exhaust from vehicles powered by these fuels (1-3), and they comprise ∼20% of the total nonmethane organic compounds present in ambient urban air (4). In the troposphere, aromatic hydrocarbons react dominantly with OH radicals, by H-atom abstraction from the C-H bonds of the alkyl substituent group(s) and by OH radical addition to the aromatic ring to form a hydroxycyclohexadienyl-type radical (hereafter referred to as an “OHaromatic adduct”) (4, 5). Under atmospheric conditions OH radical addition dominates, accounting for >90% of the overall reaction for toluene, xylenes, and trimethylbenzenes (TMBs) (4, 6, 7). The OH-aromatic adducts react with O2 and with NO2, but not with NO (8), with the O2 reaction including reversible addition of O2 to the OH-aromatic adduct to form an OHaromatic-O2 peroxy radical (9, 10). Measured rate constants for reactions of the OH-aromatic adducts with NO2 and for the irreversible reaction with O2 show that for benzene, toluene, and p-xylene the OH-aromatic adducts react almost exclusively with O2 in the troposphere, with the O2 and NO2 reactions being of equal importance at atmospheric pressure of air at an NO2 concentration of (3-13) × 1013 molecule * Address correspondence to either author. Phone: (951) 8273502(J.A.); (951)-827-4191(R.A.). E-mail: [email protected] (J.A.); [email protected] (R.A.). † Also Department of Environmental Sciences. ‡ Also Department of Chemistry. 10.1021/es8019098 CCC: $40.75

Published on Web 12/31/2008

 2009 American Chemical Society

cm-3 (mixing ratios of ∼1.2-5 ppmv) (8). Possible reactions of an OH-toluene adduct with O2 are shown in Scheme 1, based on the studies of Bohn and Zetzsch (9), Bohn (10), Klotz et al. (11), Volkamer et al. (12), and Raoult et al. (13), leading to the formation of o-cresol and methylglyoxal and HC(O)CHdCHCHO and/or glyoxal and CH3C(O)CHd CHCHO after initial OH radical addition at the 2-position in toluene. The reaction of NO with the peroxy radical to ultimately form a diunsaturated 1,6-dicarbonyl (HC(O)C(CH3)dCHCHdCHCHO in Scheme 1) is only significant at high NO levels (>1 ppmv) (11). OH radical addition at the other positions in toluene can lead, by analogous reaction pathways, to m-cresol, p-cresol, glyoxal and CH3C(O)CHdCHCHO or HC(O)C(CH3)dCHCHO, and methylglyoxal and HC(O)CHdCHCHO. To date, there is no quantitative evidence for the formation under atmospherically relevant conditions of first-generation products from the reactions of the OH-aromatic adducts with O2 other than phenols, 1,2dicarbonyls and unsaturated 1,4-dicarbonyls, although formation of diunsaturated 1,6-dicarbonyls and unsaturated epoxy-dicarbonyls have been reported from derivatization with combined gas chromatography/mass spectrometry (GC/ MS) (14, 15) and in situ chemical ionization mass spectrometry (16, 17) studies. Furthermore, apart from 3-hexene-2,5-dione [CH3C(O)CHdCHC(O)CH3] for which standards of the Z- and Eisomers have been synthesized and which elutes from GC columns without prior derivatization (18, 19), identification and quantification of the unsaturated 1,4-dicarbonyls has been difficult and to date generally only low yields of the unsaturated 1,4-dicarbonyls have been reported under atmospheric conditions (20-23) (again, apart from 3-hexene2,5-dione and possibly 3-methyl-3-hexene-2,5-dione (21)), as shown in Table 1. One may therefore conclude that either (a) many of the expected unsaturated 1,4-dicarbonyls are not formed or are formed in significantly lower yields than anticipated, (b) that these unsaturated 1,4-dicarbonyls are formed but react rapidly in the gas-phase (through photolysis and/or OH radical reaction) or heterogeneously (and cyclized furanones have been observed in a number of studies (21)), or (c) that they are formed in yields similar to those of their likely coproducts and that the lower reported yields are due to analytical deficiencies. Even if it is assumed that the unsaturated 1,4-dicarbonyls have formation yields identical to their presumed coproduct 1,2-dicarbonyls, the aromatic aldehydes and benzyl nitrates formed from the H-atom abstraction pathway and the phenols, 1,2-dicarbonyls and unsaturated 1,4-dicarbonyls formed from the OH radical addition pathway would only account for ∼60-70% of the overall reaction products for toluene, xylenes, and TMBs (28). Clearly, more work is required to identify products in an isomer-specific manner, and to determine their formation pathways and yields under atmospheric conditions, as well as any dependence of their yields on NO and NO2 concentrations. Initially, we used solid-phase microextraction (SPME) fibers precoated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) for on-fiber derivatization of carbonyl compounds (29) to investigate carbonyl-containing products formed from the OH radical-initiated reactions of toluene, o-, m-,and p-xylene and 1,2,3-, 1,2,4-, and 1,3,5TMB (30). We report here the results of an alternative derivatization technique, namely sampling onto an XAD-4 resin denuder system (31) precoated with PFBHA (32). The major benefit of the denuder system over the SPME system is that the denuder system allows collection of large volumes of air, whereas the SPME system is based on the adsorption VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 1. Possible Pathways Involved in the Reaction of One of the OH-Toluene Adducts with O2

equilibrium. Also, by waiting overnight after sampling before extracting the products, we observed that monoderivatives of dicarbonyls were seldom seen, in contrast to coated-SPME sampling. Thus, using this derivatization method with methane positive chemical ionization gas chromatography/ mass spectrometry (PCI-GC/MS), characteristic diderivatized molecular ions of the dicarbonyl products were obtained.

Experimental Methods Experiments were carried out in a ∼7000 L volume Teflon chamber at 296 ( 2 K and ∼735 Torr of dry purified air. The chamber is equipped with two parallel banks of blacklamps for irradiation, and a Teflon-coated fan to ensure rapid mixing of reactants during their introduction into the chamber. OH radicals were generated by the photolysis of CH3ONO in air at wavelengths >300 nm, and NO was included in the reactant mixtures to suppress the formation of O3 and of NO3 radicals (16, 19, 29). The initial concentrations of CH3ONO, NO and aromatic hydrocarbon were ∼2.4 × 1013 molecule cm-3 each, and irradiations were carried out at a light intensity corresponding to an NO2 photolysis rate of J(NO2) ) 0.062 min-1 for times ranging from 3.75 min for 1,3,5-TMB to 30 min for toluene and resulting in 19-27% consumption of the initially present aromatic hydrocarbon. The aromatic hydrocarbons examined were toluene; o-, m-, and p-xylene; and 1,2,3-, 1,2,4-, and 1,3,5-TMB, and replicate reactions of p-xylene were conducted as the first and last in the experimental series. The concentrations of the aromatic hydrocarbons were measured by GC with flame ionization detection, using thermal desorption of 100 cm3 volume gas samples collected onto Tenax-TA solid adsorbent (19). After each reaction, the carbonyl products were analyzed as their oxime derivatives by sampling for 1.5 h through a 2.5 µm aluminum cyclone (URG-2000-30EH, UCR, Chapel Hill, NC) at a flow rate of 15 L min-1 onto a 5 channel, 400 mm length denuder (URG2000-30B5, URG, Chapel Hill, NC) coated with ground XAD-4 resin (31). Prior to each sample, the XAD resin-coated denuder was further coated with ∼100 mg of PFBHA hydrochloride dissolved in 5 mL of methanol. After coating with PFBHA, the denuder was dried with nitrogen gas. After sampling, the denuder was stored overnight and extracted the next day with three 50 mL aliquots of CH2Cl2 (g95% of the products being in the first 50 mL aliquot). A 4 mL aliquot of the first extract was concentrated to ∼300 µL for analysis of the unsaturated 1,4- and 1,2-dicarbonyls. The remaining extract was spiked with a known quantity of biphenyl as an internal standard and then rotovapped and returned to 10 mL volume for analysis of the 1,2-dicarbonyls on a relative basis. The extracts were analyzed by PCI-GC/MS using methane as the CI gas in an Agilent 5975 Inert XL mass selective detector 684

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with a 60 m DB-17 capillary column (250 µm i.d., 0.25 µm phase) operated in the scanning mode. Each carbonyl group derivatized to an oxime added 195 mass units to the compound’s molecular weight (m.w.). Methane-CI gave protonated molecular ions ([M+H]+) and smaller adduct ions at [M+29]+ and [M+41]+. The NO and initial NO2 concentrations were measured using a Thermo Environmental Instruments, Inc., model 42 NO-NO2-NOx analyzer, noting that CH3ONO contributes ∼100% to the “NO2” signal. The estimated NO2 concentrations at the end of the irradiation (assuming that ([NO] + [NO2]) remained constant during the irradiations (33)) were in the range (1.0-2.1) × 1013 molecule cm-3 (0.4-0.9 ppmv). The chemicals used, and their stated purities, were 2,3butanedione [biacetyl] (99%), O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (98+%), toluene (99+%), 1,2,3-trimethylbenzene (90%), 1,2,4-trimethylbenzene (98%), 1,3,5-trimethylbenzene (98%), o-xylene (97%), m-xylene (99+%), and p-xylene (99+%), Aldrich; and NO (g99.0%), Matheson Gas Products. Methyl nitrite was synthesized (34) and stored at 77 K under vacuum.

Results and Discussion Reactions were carried out with the extent of reaction being kept in the range 19-27% to minimize secondary reactions of dicarbonyl products, many of which are significantly more reactive than their precursor aromatic hydrocarbon (4). In addition to the mono-oximes of benzaldehyde, tolualdehydes, and dimethylbenzaldehydes from toluene, the xylenes, and the TMBs, respectively, a series of dicarbonyls were observed as their dioximes. The 1,2-dicarbonyls glyoxal, methylglyoxal, and biacetyl were identified by matching GC retention times and mass spectra of their oximes with standards either introduced directly into the chamber (biacetyl) or generated in situ from the OH radical-initiated reactions of 3-methyl2-butenal (for glyoxal) (35) and methacrolein (for methylglyoxal) (36) and sampled using the same procedures used for the aromatic hydrocarbon reactions. There are four dioximes of methylglyoxal (syn,syn; syn,anti; anti,syn; and anti,anti) and, because of symmetry, three each for glyoxal and biacetyl. For the unsaturated 1,4-dicarbonyls, there are up to eight dioximes, with six for those with a center of symmetry such as HC(O)CHdCHCHO and 3-hexene-2,5dione. Schemes 1 and 2, and analogous schemes for the other aromatic hydrocarbons, show that glyoxal, methylglyoxal, and biacetyl are the possible 1,2-dicarbonyl products from toluene, the xylenes, and the TMBs, and the possible unsaturated1,4-dicarbonylcoproductsoftheseareHC(O)CHd CHCHO, two m.w. 98 dicarbonyls, four m.w. 112 dicarbonyls, and two m.w. 126 dicarbonyls (see Table 2). As noted above, our sampling and analysis procedure produced almost exclusively diderivatives of the dicarbonyls. Figure 1 shows methane PCI-GC/MS ion chromatograms for the [M+H]+ ion of the diderivatized molecular weight 98 species (m/z ) 489) from the OH radical-initiated reactions and illustrates how the m.w. 98 coproducts, CH3C(O)CHdCHCHO and HC(O)C(CH3)dCHCHO, were identified by their characteristic spectra, consisting mainly of [M+H]+ (m/z ) 489) and adduct ions and [M+H-198]+ fragment ions. o-Xylene and 1,2,3-TMB can produce only CH3C(O)CHdCHCHO, as a coproduct to methylglyoxal and biacetyl, respectively. As seen from Figure 1, seven peaks were resolved for this m.w. 98 isomer. Similarly, p-xylene and 1,2,4-TMB give HC(O)C(CH3)dCHCHO as the coproduct to methylglyoxal and biacetyl, respectively, and four peaks were resolved for this m.w. 98 isomer. As seen in Figure 1, ion traces for toluene and m-xylene show the presence of both isomers, and neither is seen in the 1,3,5-TMB reaction.

TABLE 1. Literature Formation Yields of 1,2- and Unsaturated 1,4-Dicarbonyls from the OH Radical-Initiated Reactions of Aromatic Hydrocarbons (TMB = Trimethylbenzene) aromatic

1,2-dicarbonyl

molar yield (%)

benzene toluene

(CHO)2 (CHO)2

29 ( 10 ; 32.0 ( 5 , 39 23.8 ( 2.5d; 30.6 ( 6b, 37 ( 2c

m-xylene

CH3C(O)CHO CH3C(O)CHO

37 ( 2c 42 ( 5e; 31.9 ( 0.9f; 40 ( 1.8g

(CHO)2 CH3C(O)CHO (CHO)2 CH3C(O)CHO

39.4 ( 11g; 31.9 ( 5b 12 ( 2e; 10.5 ( 3.4f; 21.7 ( 7.6g 6.6 ( 1.1g 37 ( 1i; 35.7 ( 1.7f; 44 ( 7.4g

CH3C(O)C(O)CH3 CH3C(O)CHO

11.4 ( 2.4g; 10.2h 64 ( 3i; 60.2 ( 3.3f; 90 ( 25g

p-xylene 1,2,4-TMB

1,3,5-TMB

a

b

c

potential unsaturated 1,4-dicarbonyl coproducts

molar yield (%)

HC(O)CHdCHCHO CH3C(O)CHdCHCHO HC(O)C(CH3)dCHCHO HC(O)CHdCHCHO CH3C(O)CHdCHCHO HC(O)C(CH3)dHCHO CH3C(O)CHdC(O)CH3 HC(O)C(CH3)dCHCHO CH3C(O)C(CH3)dCHC(O)CH3 CH3C(O)CHdCHC(O)CH3 CH3C(O)C(CH3))CHCHO CH3C(O)CHdC(CH3)CHO HC(O)C(CH3)dC(CH3)CHO HC(O)C(CH3)dCHCHO CH3C(O)CHdC(CH3)CHO

10.3 ( 3.1a, 16c 3.1 ( 0.4d, >13.8 ( 1.5c not reported,d not observedc 13 ( 7c 12 ( 1.2g not reportedg 22.1 ( 3.2g; 32.3h 7.1 ( 3.5g 7.9 ( 4g 16.1 ( 1.2g; 30.9h not reportedg not reportedg not reportedg 4.5 ( 2.2g 5 ( 1.5g

a Berndt and Bo¨ge (22). b Volkamer et al. (12, 24). c Go´mez Alvarez et al. (23). Data for benzene are averages from two experiments with yields of: glyoxal, 42 ( 3% and 36 ( 2%; and 1,4-butendial, 17 ( 9% and 15 ( 6%. d Smith et al. (20). e Bandow and Washida (25). f Tuazon et al. (26). g Smith et al. (21). h Bethel et al. (19), extrapolated to low NO2 concentration. i Bandow and Washida (27).

TABLE 2. Ring-Opened Dicarbonyls Observed and Their Assignments xylene trimethylbenzene ring-opened product

m.w. toluene o- m- p- 1,2,3- 1,2,4- 1,3,5-

(CHO)2 CH3C(O)CHO CH3C(O)C(O)CH3 HC(O)CH)CHCHO CH3C(O)CH)CHCHO HC(O)C(CH3))CHCHO CH3C(O)C(CH3))CHCHO CH3C(O)CH)C(CH3)CHO CH3C(O)CH)CHC(O)CH3 HC(O)C(CH3)dC(CH3)CHOb CH3C(O)C(CH3)dC(CH3)CHO CH3C(O)C(CH3))CHC(O)CH3

58 72 86 84 98 98 112 112 112 112 126 126

X X X X X

X X X X X X X X X X X X X X X

X X X

X X X

X

X X

X Xa Xa Xa

X

Xc X

a

See Figure 2 for relative abundances of m.w. 112 isomers from 1,2,4-TMB. b A possible product from o-xylene and 1,2,4-TMB (see Scheme 2 and text). c Observed, but much less abundant than assumed glyoxal coproduct.

Figure 2 shows the ion chromatograms for m/z 503, the protonated molecular ion of diderivatized m.w. 112 species, and the adduct ion [M+C2H5]+ of diderivatized m.w. 84 1,4butendial. Four peaks (asterisks) for the m.w. 84 species, HC(O)CHdCHCHO, from toluene are seen in the top ion trace; the mass spectrum of each of these peaks showed an abundant [M+H]+ ion at m/z 475. Note that HC(O)CHd CHCHO is also formed from o-xylene (off-scale peaks marked with asterisks). For 1,2,3-TMB, only one m.w. 112 isomer can be formed, and the eight peaks attributed to CH3C(O)C(CH3)dCHCHO are marked with arrows with circles and seven of these peaks are also identified in the o-xylene trace. m-Xylene and 1,3,5-TMB each show three peaks attributed to the sole possible isomer, CH3C(O)CHd C(CH3)CHO (dashed arrows). A small amount of this isomer was also found in the 1,2,4-TMB reaction (two peaks are marked, the earliest peak coeluted with the other isomer). The five peaks in the p-xylene reaction are attributed to the sole possible isomer, CH3C(O)CHdCHC(O)CH3, and this is the major isomer observed in the 1,2,4-TMB reaction. A very small amount of CH3C(O)C(CH3)dCHCHO was also observed in the 1,2,4-TMB reaction (the peaks could only be seen when the Y-axis scale was expanded). The peak marked with † on

the 1,2,3-TMB trace is from the [M+H-H2O]+ fragment of a diderivatized m.w. 130 species. Note that the fourth possible m.w. 112 isomer, HC(O)C(CH3)dC(CH3)CHO, that could be formed in the o-xylene and 1,2,4-TMB reactions was not observed. As shown in Scheme 2, for the o-xylene (1,2dimethylbenzene) reaction HC(O)C(CH3)dC(CH3)CHO could be formed after OH radical addition at the 4-position; however, the preferred allylic radical (labeled (A) in Scheme 2) would lead to formation of CH3C(O)C(CH3)dCHCHO. Similarly for 1,2,4-TMB, HC(O)C(CH3)dC(CH3)CHO could be formed after initial OH radical addition at the 4- or 5-positions, but again, the allylic radical would be expected to favor formation of CH3C(O)C(CH3)dCHCHO and CH3C(O)C(CH3)dCHC(O)CH3, respectively. Diderivatized m.w. 126 dicarbonyls were observed only from the reactions of 1,2,3- and 1,2,4-TMB, with only a very small amount of the m.w. 126 dicarbonyl from 1,2,3-TMB, attributed to CH3C(O)C(CH3)dC(CH3)CHO, being observed. Dioximes of m.w. 102 (all apart from the 1,3,5-TMB reaction), 116 (m-xylene and 1,2,4- and 1,3,5-TMB reactions), and 130 (1,2,3-TMB reaction) were observed, with abundant [M+H-H2O]+ fragment ions, and these are attributed to C4-, C5- and C6-hydroxydicarbonyls (29), respectively. These hydroxydicarbonyls are anticipated second-generation products from the unsaturated 1,4-dicarbonyls (37). However, we saw no evidence for the formation of the previously postulated and/or qualitatively observed diunsaturated 1,6dicarbonyls such as CH3C(O)CHdCHCHdCHCHO formed from toluene, and its isomers and homologues, nor of unsaturated epoxy-1,6-dicarbonyls or epoxycyclohexenones (14-17, 38).

While our data for the dicarbonyls are not quantitative in that calibrations of the coated denuder/PCI-GC/MS sampling and analysis system were not carried out for any of the dicarbonyls observed here (many of which are not commercially available), because the sampling and analysis method was consistent for all of the experiments (including VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Methane PCI-GC/MS ion chromatograms for the [M+H]+ ion of the diderivatized m.w. 98 species (m/z ) 489) from the OH radical-initiated reactions of aromatics (see Table 2 for structures). The full mass spectra of each peak showed an abundant [M+H]+ and smaller adduct ions, [M+29]+ and [M+41]+, as well as a characteristic fragment at [M+H-198]+. There are seven distinct peaks (plain arrows) for CH3C(O)CHdCHCHO as seen for o-xylene and 1,2,3-TMB, where this is the only possible m.w. 98 dicarbonyl. There are four major peaks (dashed arrows) for HC(O)C(CH3)dCHCHO as seen for p-xylene and 1,2,4-TMB where this is the only possible m.w. 98 dicarbonyl. Both m.w. 98 isomers are formed from toluene and m-xylene. The peaks in 1,2,4-TMB and 1,3,5-TMB marked by an asterisk are [M+H-H2O]+ fragments of a diderivatized m.w. 116 species attributed to a C5-hydroxydicarbonyl (see text). for the replicate p-xylene reactions) we have placed our measured PCI-GC/MS ion peak area counts on an absolute basis using literature yield data for the 1,2-dicarbonyls. For the 1,2-dicarbonyls glyoxal and methylglyoxal, corrections for secondary reactions with OH radicals were made as described previously (35, 36) using the literature OH radical reaction rate constants for the aromatic hydrocarbons and 1,2-dicarbonyls (5, 6). Since the extents of reaction were approximately the same in all experiments, corrections for OH radical reaction depended mainly on the rate constant ratios k(OH + 1,2-dicarbonyl)/k(OH + aromatic hydrocarbon), with maximum corrections of 26% for glyoxal and 37% for methylglyoxal (for the toluene reaction) and with no corrections needed for biacetyl. Photolysis of the 1,2dicarbonyls by blacklamps for the irradiation times used here was negligible (19, 39). The glyoxal formation yields shown in Figure 3 (black bars, top panel) are relative to an assigned value of 31.9% 686

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FIGURE 2. Methane PCI-GC/MS ion chromatograms (m/z ) 503) for the [M+H]+ ions of the diderivatized m.w. 112 species and an adduct ion [M+C2H5]+ of the diderivatized m.w. 84 species (see Table 2 for structures). Four peaks (asterisks) for the m.w. 84 HC(O)CHdCHCHO from toluene are seen in the top ion trace; the mass spectrum of each of these peaks showed an abundant [M+H]+ ion at m/z 475. HC(O)CHdCHCHO was also formed from o-xylene (off-scale peaks marked with asterisks). For 1,2,3-TMB, only one m.w. 112 isomer can be formed, and the eight peaks attributed to CH3C(O)C(CH3)dCHCHO are marked with arrows with circles on top and seven of these peaks are also identified on the o-xylene trace. m-Xylene and 1,3,5-TMB each show three peaks attributed to the sole possible isomer, CH3C(O)CHdC(CH3)CHO (dashed arrows). A small amount of this isomer was also found in the 1,2,4-TMB reaction (two peaks are marked, the earliest peak coelutes with the other isomer). The five peaks in the p-xylene reaction are attributed to the sole possible isomer, CH3C(O)CHdCHC(O)CH3, and this is the major isomer observed in the 1,2,4-TMB reaction. A very small amount of CH3C(O)C(CH3)dCHCHO was also observed in the 1,2,4-TMB reaction (the peaks can only be seen when the y-axis scale is expanded). The peak marked with † on the 1,2,3-TMB trace is from the [M+H-H2O]+ fragment of a diderivatized m.w. 130 species attributed to a C6-hydroxydicarbonyl (see text). from the p-xylene reaction (24). The methylglyoxal formation yields (black bars, middle panel of Figure 3) are obtained using an average response factor relative to assigned values of 36.4% from the 1,2,4-TMB reaction (26, 27) and 62.1% from the 1,3,5-TMB reaction (26, 27). The biacetyl formation yields (black bars, lower panel of Figure 3) are obtained using an average response factor relative to assigned values of 18.5% from the o-xylene reaction (39) and 45% from the 1,2,3-TMB reaction (27). As seen from Supporting Information Table S3, our resulting 1,2-dicarbonyl yields are consistent with literature data (some of which were used to place our data on an absolute basis).

SCHEME 2. Possible Pathways in the o-Xylene Reaction Leading to Glyoxal, (CHO)2, Plus m.w. 112 Unsaturated 1,4-Dicarbonyls

The yields of potential unsaturated 1,4-dicarbonyl coproducts are based on a second set of analyses where all dicarbonyls were quantified on the basis of the PCI-GC/MS area counts of the [M+H]+ ion peaks and placed on a yield basis relative to the area counts of the 1,2-dicarbonyls. The

yields range over a factor of 2.9 (see Supporting Information Table S4) with the lowest derived yields using the methylglyoxal area counts for comparison and the highest yields using biacetyl for comparison. For all of these unsaturated 1,4-dicarbonyls, corrections were made for secondary reac-

FIGURE 3. Glyoxal formation yields from the aromatics (black bars, top panel) are relative to a value of 31.9% from the p-xylene reaction (24). The methylglyoxal formation yields (black bars, middle panel) are relative to values of 36.4% from the 1,2,4-TMB reaction and 62.1% from the 1,3,5-TMB reaction (26, 27). The biacetyl formation yields (black bars, lower panel) are relative to values of 18.5% from the o-xylene reaction (39) and 45% from the 1,2,3-TMB reaction (27); see text for additional details. The yields of potential coproducts are based on a second set of analyses where all carbonyls were quantified on the basis of the GC/MS area counts of the [M+H]+ ion peaks and placed on a yield basis relative to the area counts of the 1,2-dicarbonyls. The yields range over a factor of 2.9 (see Supporting Information Table S4) with the lowest being from the use of the methylglyoxal area counts for comparison and the highest yields using biacetyl for comparison. The bars show the average and the error bar shows the upper values. Black bars refer to the 1,2-dicarbonyls, blue to HC(O)CHdCHCHO, pink to CH3C(O)CHdCHCHO, tan to HC(O)C(CH3)dCHCHO, red to CH3C(O)C(CH3)dCHCHO, bright yellow to CH3C(O)CH)C(CH3)CHO, green to CH3C(O)CH)CHC(O)CH3, and turquoise to CH3C(O)C(CH3)dCHC(O)CH3 (see also Supporting Information Table S4). VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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tions with OH radicals, using rate constants (cm3 molecule-1 s-1) for the reactions of OH radicals of: 1,4-butendial, 5.29 × 10-11 (4); m.w. 98 1,4-dicarbonyls, 5.67 × 10-11 (4); and m.w. 112 1,4-dicarbonyls, 6.56 × 10-11 (4), with multiplicative correction factors ranging from 1.13 for m.w. 112 dicarbonyls from 1,3,5-TMB to 2.60 and 2.75 for HC(O)CHdCHO and m.w.98dicarbonyls,respectively,fromtoluene.ForHC(O)CHd CHCHO and CH3C(O)CHdCHCHO, corrections were also made for photolysis using values of J(1,4-dicarbonyl)/J(NO2) ) 0.20 (40, 41), corresponding to J(1,4-dicarbonyl) ) 0.0125 min-1. The additional correction factors for photolysis (over those for OH radical reaction) of these two 1,4-dicarbonyls decreased as the irradiation times decreased, and ranged from 1.05 for formation from 1,2,4-TMB to 1.13 for formation from toluene. Photolytic loss of 3-hexene-2,5-dione by blacklamps has been shown to be of no importance for the light intensity and durations employed here (rather, Z-/Eisomerization occurs) (18, 19). However, for the other 1,4dicarbonyls no photolysis rates are available and hence no corrections could be reliably made. The resulting derived yields of the unsaturated 1,4-dicarbonyls are shown in Figure 3 and compared with literature data for the unsaturated 1,4dicarbonyls and corresponding data from the same studies for 1,2-dicarbonyls in Supporting Information Table S4. It should be noted that two experiments were carried out for p-xylene (the first and last in the experimental series), and the glyoxal and methylglyoxal yields from these two experiments agreed to within 10-15%. So although semiquantitative, our data when placed on an absolute basis should give a reasonable idea of the relative amounts of coproducts formed, although the present inability to correct many of the unsaturated 1,4-dicarbonyls for photolysis may lead to an underestimate of the formation yield for a number of the unsaturated 1,4-dicarbonyls. In Figure 3, the bars show the average and the error bars the upper values. The black bars refer to 1,2-dicarbonyls, the blue to HC(O)CHdCHCHO, the pink to CH3C(O)CHd CHCHO, the tan to HC(O)C(CH3)dCHCHO, the red to CH3C(O)C(CH3)dCHCHO, the bright yellow to CH3C(O)CHdC(CH3)CHO, the green to CH3C(O)CHdCHC(O)CH3, and the turquoise to CH3C(O)C(CH3)dCHC(O)CH3. The yields are also listed in Supporting Information Table S4. The data shown in Figure 3 (and listed in Supporting Information Table S4) suggest that at least in some cases the unsaturated 1,4dicarbonyls appear to be coproducts to the 1,2-dicarbonyls, being formed in similar yield. This is the case for the toluene reaction (glyoxal + CH3C(O)CHdCHCHO or HC(O)C(CH3)d CHCHO and methylglyoxal + HC(O)CHdCHCHO) and for certain sets of 1,2-dicarbonyl + unsaturated 1,4-dicarbonyl products from the o-, m-, and p-xylene and 1,2,4-TMB reactions. More specifically, the unsaturated 1,4-diketones are observed with formation yields similar to their potential coproduct 1,2-dicarbonyls, as presently and previously (19, 24) observed for formation of 3-hexene-2,5-dione and glyoxal from the p-xylene reaction and observed here for formation of 3-hexene-2,5-dione and methylglyoxal and for glyoxal and CH3C(O)C(CH3)dCHC(O)CH3 from the 1,2,4-TMB reaction (Figure 3 and Supporting Information Table S4). However, apart from HC(O)CHdCHCHO, unsaturated 1,4dialdehydes and 1,4-keto-aldehydes are generally observed in lower yield than their potential 1,2-dicarbonyl coproducts. Possible reasons for these lower measured yields of unsaturated 1,4-dialdehydes and 1,4-keto-aldehydes include their rapid photolysis and/or losses to the chamber walls with polymerization.

Acknowledgments We thank the California Air Resources Board (Contract No. 03-319), the CalEPA Office of Environmental Health Hazard Assessment (Contract No. 06-E0016), and the U.S. Environ688

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mental Protection Agency (Grant R833752) for supporting this research. While this work has been supported by these Agencies, the results and content of this publication do not necessarily reflect the views and opinion of any of these agencies. Dr. Douglas Lane of Environment Canada is thanked for supplying the finely ground XAD and for helpful discussions regarding the denuder coating procedures.

Supporting Information Available Two tables and additional references. This material is available free of charge via the Internet at http://pubs.acs.org.

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