Formation and Reaction of Hydroxycarbonyls from the Reaction of OH

GC-MS (12, 19), direct air sampling atmospheric pressure ionization tandem mass spectrometry (API-MS/MS) (13, 24), and proton-transfer reaction mass ...
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Environ. Sci. Technol. 2005, 39, 4091-4099

Formation and Reaction of Hydroxycarbonyls from the Reaction of OH Radicals with 1,3-Butadiene and Isoprene J I L L I A N B A K E R , † J A N E T A R E Y , * ,‡ A N D R O G E R A T K I N S O N * ,‡,§ Air Pollution Research Center, University of California, Riverside, California 92521

1,3-Butadiene and isoprene (2-methyl-1,3-butadiene) are emitted into the atmosphere in vehicle exhaust and, in the case of isoprene, from vegetation. We have investigated the formation and further reaction of products of their hydroxyl radical-initiated reactions using atmospheric pressure ionization mass spectrometry (API-MS) and solid-phase microextraction fibers precoated with O-(2,3,4,5,6pentafluorobenzyl)hydroxylamine for on-fiber derivatization of carbonyl compounds, with subsequent analysis by thermal desorption and gas chromatography with flame ionization detection (SPME/GC-FID) or MS detection. Products attributed as HOCH2CHdCHCHO and HOCH2CHdCHCH2ONO2 (and isomers) from 1,3-butadiene; HOCD2CD)CDCDO and HOCD2CD)CDCD2ONO2 (and isomers) from 1,3butadiene-d6; HOCH2C(CH3)dCHCHO and/or HOCH2CHd C(CH3)CHO, and HOCH2C(CH3)dCHCH2ONO2 (and isomers) from isoprene; and HOCD2C(CD3)dCDCDO and/or HOCD2CD)C(CD3)CDO, and HOCD2C(CD3)dCDCD2ONO2 (and isomers) from isoprene-d8 were observed as their NO2adducts in the API-MS analyses. The hydroxycarbonyls were observed from SPME/GC-FID analyses of the 1,3butadiene and isoprene reactions as their oximes, together with acrolein, glycolaldehyde, and glyoxal from the 1,3butadiene reaction. A rate constant for the reaction of OH radicals with 4-hydroxy-2-butenal of (5.7 ( 1.4) × 10-11 cm3 molecule-1 s-1 at 298 ( 2 K was derived, and formation yields of acrolein and 4-hydroxy-2-butenal from the +15 1,3-butadiene reaction of 58 ( 10% and 25-10 %, respectively, were determined. Analogous experiments showed that the two C5-hydroxycarbonyls formed from isoprene have rate constants for their reactions with OH radicals of (1.0 ( 0.3) × 10-10 cm3 molecule-1 s-1 and (4 ( 2) × 10-11 cm3 molecule-1 s-1 and a combined yield of ∼15%, although isomer-specific identification of the hydroxycarbonyls was not achieved.

Introduction 1,3-Butadiene [CH2dCHCHdCH2] and isoprene [2-methyl1,3-butadiene; CH2dC(CH3)CHdCH2] are emitted into the atmosphere from several sources, including vehicle exhaust * Address correspondence to either author. Phone: (951)827-4191 (R.A.); (951)827-3502 (J.A.). E-mail: [email protected] (R.A.); [email protected] (J.A.). † Also Environmental Sciences Graduate Program. ‡ Also Department of Environmental Sciences. § Also Department of Chemistry. 10.1021/es047930t CCC: $30.25 Published on Web 04/30/2005

 2005 American Chemical Society

(1-3), and in particular, isoprene is emitted from vegetation in large quantities (∼450 million tonnes (carbon) per year worldwide) (4). Isoprene plays a major role in the chemistry of the lower troposphere, including, for example, in the eastern United States (5-7). For both 1,3-butadiene and isoprene, reaction with hydroxyl (OH) radicals is their dominant chemical loss process during daytime, with lifetimes of 1-2 h at a OH radical concentration of 2 × 106 molecule cm-3 (8, 9). The major readily identifiable products observed from the OH radical-initiated reactions in the presence of NO are acrolein and formaldehyde from 1,3-butadiene (10-14) and methyl vinyl ketone, methacrolein, and formaldehyde from isoprene (14-20). However, these products account for only 55-70% of the reaction products formed (13, 14, 16-18, 20), and hydroxynitrates from the reactions of hydroxyperoxy radicals with NO account for only an additional ∼7-11% of the products from the 1,3-butadiene reaction (13, 14) and ∼4.4-13% of the products from the isoprene reaction (14, 16, 21). Unsaturated 1,4-hydroxyaldehydes, HOCH2CHd CHCHO from 1,3-butadiene and HOCH2C(CH3)dCHCHO and HOCH2CHdC(CH3)CHO from isoprene, have been postulated as reaction products (22, 23), and products attributed to these species have been observed by derivatization/ GC-MS (12, 19), direct air sampling atmospheric pressure ionization tandem mass spectrometry (API-MS/MS) (13, 24), and proton-transfer reaction mass spectrometry (PTR-MS) (20) from the 1,3-butadiene (12, 13) and isoprene (19, 20, 24) reactions. Scheme 1 shows the proposed reactions for 1,3-butadiene in the presence of NO. Theoretical studies of the analogous 1,2- and 1,4-hydroxyalkoxy radicals formed in the isoprene reaction (25-30) indicate that, as shown in Scheme 1, at room temperature and atmospheric pressure the •OCH2CH(OH)CHdCH2 and HOCH2CH(O•)CHdCH2 radicals decompose, while the HOCH2CHdCHCH2O• radical dominantly isomerizes via a six-member transition state. The reactions shown in Scheme 1, and analogous reactions for the isoprene system, lead to the products observed, noting that the formation of HOCH2CHdCHCHO from 1,3-butadiene and of HOCH2C(CH3)dCHCHO and HOCH2CHdC(CH3)CHO from isoprene have largely been inferred from molecular weight information obtained from derivatization/GC-MS (12, 19), API-MS/MS (13, 24), and PTR-MS (20) analyses, and Zhao et al. (20) have recently obtained a total yield of the molecular weight 100 C5-hydroxycarbonyls from the isoprene reaction of 19.3 ( 6.1%. In this work, we have used solid-phase microextraction fibers precoated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) for on-fiber derivatization of carbonyl compounds (31-34) with subsequent analysis by gas chromatography with flame ionization detection (GC-FID) and combined gas chromatography-mass spectrometry (GC-MS) to investigate the formation and reaction of hydroxycarbonyls from the OH radical-initiated reactions of 1,3-butadiene and isoprene. In addition, analyses were also carried out by atmospheric pressure ionization tandem mass spectrometry (API-MS) in the presence of NO2 to observe hydroxycarbonyls and hydroxynitrates as their NO2- adducts (33, 35).

Experimental Methods Experiments were carried out at 298 ( 2 K and 740 Torr total pressure of air at ∼5% relative humidity in a ∼7500 L Teflon chamber interfaced to a PE SCIEX API III MS/MS direct air sampling, atmospheric pressure ionization tandem mass spectrometer (API-MS), with irradiation provided by two VOL. 39, NO. 11, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 1

parallel banks of blacklamps. The chamber is fitted with a Teflon-coated fan to ensure rapid mixing of reactants during their introduction into the chamber. OH radicals were generated by the photolysis of methyl nitrite (CH3ONO) in air at wavelengths >300 nm (32-35), and NO was added to the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals. Analyses by API-MS. In the experiments with API-MS analyses, the chamber contents were sampled through a 25-mm diameter × 75-cm length Pyrex tube at ∼20 L min-1 directly into the API mass spectrometer source. The operation of the API-MS in the MS (scanning) and MS/MS [with collision-activated dissociation (CAD)] modes has been described elsewhere (33, 35). Use of the MS/MS mode with CAD allows the “product ion” or “precursor ion” spectrum of a given ion peak observed in the MS scanning mode to be obtained (35). Both positive and negative ion modes were used in this work. In the positive ion mode, protonated water clusters, H3O+(H2O)n, formed from a corona discharge in the chamber diluent air (at ∼5% relative humidity) are the reagent ion, and a range of oxygenated species can be observed in this mode of operation (35). In the negative ion mode, the superoxide ion (O2-), its hydrates [O2(H2O)n]-, and O2 clusters [O2(O2)n]- are the major reagent negative ions in the chamber pure air. Other reagent ions, for example, NO2- and NO3-, are formed through reactions between the primary reagent ions and neutral molecules such as NO2, and instrument tuning and operation were designed to induce cluster formation. To ensure that NO2- dominated as the reagent ion (35), NO2 was added postreaction in the present study, and analytes were then detected as adducts, [NO2‚M]-, formed between the neutral analyte (M) and NO2- (33, 35). Previous work in this laboratory (33, 35) indicates that the use of NO2- reagent ions allows primarily hydroxy-compounds to be detected (for example, hydroxycarbonyls and hydroxynitrates). In the positive or negative ion modes, ions are then drawn by an electric potential from the ion source through the sampling orifice into the mass-analyzing first quadrupole or third quadrupole. Neutral molecules and particles are prevented from entering the orifice by a flow of high-purity nitrogen (“curtain” gas). The initial concentrations (molecule 4092

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cm-3 units) were CH3ONO and NO, ∼1.2 × 1013 each, and 1,3-butadiene, 1,3-butadiene-d6, isoprene, or isoprene-d8, (1.24-1.93) × 1013, and a single irradiation was carried out at 20% of the maximum light intensity for 45-75 s. After the irradiation, 2.4 × 1013 molecule cm-3 of NO2 was added to the chamber to ensure that NO2- adducts dominated over O2- adducts (35), and 1.2 × 1012 molecule cm-3 of 5-hydroxy2-pentanone (isoprene and isoprene-d8 reactions) or 4-hydroxy-3-hexanone (1,3-butadiene and 1,3-butadiene-d6 reactions) was added to the chamber in an attempt to quantify the hydroxycarbonyl reaction products. Analyses of 1,3butadiene, 1,3-butadiene-d6, isoprene, and isoprene-d8 were carried out with gas samples collected in a 100 cm3 all-glass, gastight syringe and were introduced via a gas sampling loop onto a 30-m DB-5 megabore column initially held at -25 °C and then temperature programmed to 250 °C at 8 °C min-1. SPME Sampling with GC Analyses. In these experiments, the initial reactant concentrations (in molecule cm-3 units) were CH3ONO, ∼2.4 × 1013; NO, ∼2.4 × 1013; 1,3-butadiene, (1.35-1.58) × 1013; or isoprene, (1.18-1.30) × 1013, except for a single experiment with 1,3-butadiene with initial concentrations (in molecule cm-3 units) of CH3ONO, NO, and 1,3-butadiene of 2.4 × 1013, 4.8 × 1013, and 5.27 × 1013, respectively. Irradiations were carried out at 20% of the maximum light intensity for 1-9 min for 1,3-butadiene and 0.7-9 min for isoprene, resulting in up to 75% and 82% reaction of 1,3-butadiene and isoprene, respectively. 3-Pentanone, at a concentration of ∼5 × 1012 molecule cm-3, was added to the chamber after the irradiation as an internal standard for quantification of the hydroxycarbonyl products (33, 34). For the analysis of the 3-pentanone internal standard, 100 cm3 gas samples were collected from the chamber onto Tenax-TA solid adsorbent with subsequent thermal desorption at ∼250 °C onto a 30-m DB-1701 megabore column held at -40 °C and then temperature programmed to 250 °C at 8 °C min-1. Analyses of 1,3-butadiene and isoprene were carried out as described above. The hydroxycarbonyl products (and the 3-pentanone internal standard) were analyzed using on-fiber derivatization with SPME (31-34) and employing one of two 65-µm poly(dimethylsiloxane)/divinylbenzene PDMS/DVB fibers (either a “standard” partially cross-linked fiber or a StableFlex highly cross-linked “flexible” fiber, both from Supelco, Bellefonte, PA). The fibers were coated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) by a 30-min headspace extraction over a vigorously stirred aqueous solution of PFBHA hydrochloride. The PFBHA coating of the fiber was carried out under nitrogen gas to minimize any acetone contamination from laboratory air. The coated fiber was then exposed to the reactants in the chamber for 5 min, with the chamber mixing fan on, to form a carbonyl oxime (33, 34). For GC-FID analyses, the exposed fiber was then removed from the chamber and thermally desorbed in the injection port of the GC at 250 °C onto a 30-m DB-1701 megabore column held at 40 °C and then temperature programmed at 8 °C min-1 to 260 °C. Product peak identification was carried out by GC-MS using a Varian 2000 GC/MS/MS with isobutane chemical ionization and a DB-1701 column, using a similar temperature program to that for the GC-FID analyses. The NO and initial NO2 concentrations were measured using a Thermo Environmental Instruments, Inc., Model 42 chemiluminescence NO-NO2-NOx analyzer. SPME/GC-FID Response Factors. Derivatization/GC-FID response factors for the oximes of acrolein, methacrolein, methyl vinyl ketone, trans-2-pentenal, 2-methyl-2-pentenal, 5-hydroxy-2-pentanone, and glycolaldehyde were measured relative to that for the oxime of 3-pentanone for both fiber types used here, by introducing measured amounts of authentic standards (together with 3-pentanone) into the

FIGURE 1. API-MS negative ion mode analysis post-reaction, after adding ∼2.4 × 1013 molecule cm-3 of NO2 (resulting in NO2- and, in some instances, also NO3-, adducts). Upper mass spectrum: OH radical reaction of 1,3-butadiene. Lower mass spectrum: OH radical reaction of 1,3-butadiene-d6. The ion peak at 162 u in both spectra is the NO2- adduct of the 4-hydroxy-3-hexanone internal standard, and that at 153 u in the 1,3-butadiene-d6 spectrum is an NO3- adduct of the molecular weight 91 product. See text for product assignments. Teflon chamber and conducting two to four replicate analyses with the coated SPME fiber, as described previously (33). Known concentrations of glycolaldehyde were prepared in situ from irradiation of CH3ONO-NO-2-methyl-3-buten2-ol-air mixtures (33), calculated using our previously measured glycolaldehyde yield of 58 ( 4% [a weighted average of the formation yields of glycolaldehyde and its coproduct acetone (36)] and taking into account the small loss of glycolaldehyde because of its reaction with OH radicals (37). Chemicals. The chemicals used, and their stated purities, were acrolein (90%), isoprene (99%), methacrolein (95%), 2-methyl-3-buten-2-ol (98%), 2-methyl-2-pentenal (97%), methyl vinyl ketone (99%), 3-pentanone (99%), and trans2-pentenal (95%), Aldrich Chemical Co.; 4-hydroxy-3-hexanone and 5-hydroxy-2-pentanone (96%), TCI Americas; 1,3butadiene-d6 (98% D6) and isoprene-d8 (98% D8), Cambridge Isotope Laboratories, Inc.; and 1,3-butadiene (99%) and NO (g99.0%), Matheson Gas Products. Methyl nitrite was prepared and stored as described previously (33-35).

Results and Discussion API-MS Analyses of the 1,3-Butadiene, 1,3-Butadiene-d6, Isoprene, and Isoprene-d8 Reactions. In the positive ion mode, the API-MS spectra of irradiated CH3ONO-NO-air mixtures of 1,3-butadiene, 1,3-butadiene-d6, isoprene, and isoprene-d8 were virtually identical to those observed previously for 1,3-butadiene and 1,3-butadiene-d6 (13) and isoprene and isoprene-d8 (24). The API-MS and API-MS/MS spectra from our present and previous (13, 24) studies showed the formation of products of molecular weight 56 (acrolein), 86, and 133 from 1,3-butadiene; 60 (acrolein-d4), 91 and 139 from 1,3-butadiene-d6; 70 (methyl vinyl ketone or methacrolein) and 100 from isoprene; and 76 (methyl vinyl ketoned6 or methacrolein-d6) and 107 from isoprene-d8.

FIGURE 2. API-MS negative ion mode analysis post-reaction, after adding ∼2.4 × 1013 molecule cm-3 of NO2 (resulting in NO2- and, in some instances, also NO3-, adducts). Upper mass spectrum: OH radical reaction of isoprene. Lower mass spectrum: OH radical reaction of isoprene-d8. The ion peak at 148 u in both spectra is the NO2- adduct of the 5-hydroxy-2-pentanone internal standard, and those at 209 u (upper spectrum) and 217 u (lower spectrum) are the NO3- adducts of the molecular weight 147 and 155 products, respectively. See text for product assignments. In the negative ion mode in the presence of g2.4 × 1013 molecule cm-3 of NO2 (NO2 being formed from the reactions of NO with HO2 and organic peroxy radicals, in addition to that added after the reaction), API-MS and API-MS/MS analyses of irradiated CH3ONO-NO-air mixtures of 1,3butadiene, 1,3-butadiene-d6, isoprene, and isoprene-d8 showed the presence of NO2- adduct peaks of products of molecular weight 86 and 133 from 1,3-butadiene, 91 and 139 from 1,3-butadiene-d6, 100 and 147 from isoprene, and 107 and 155 from isoprene-d8, as shown in Figures 1 and 2. The products of molecular weight 86, 91, 100, and 107 are attributed to hydroxycarbonyls, HOCH2CHdCHCHO from 1,3-butadiene, HOCD2CD)CDCDO from 1,3-butadiene-d6, HOCH2C(CH3)dCHCHO and/or HOCH2CHdC(CH3)CHO from isoprene, and HOCD2C(CD3)dCDCDO and/or HOCD2CD)C(CD3)CDO from isoprene-d8 (13, 24), noting that if ODcontaining products are formed, rapid OD/OH exchange occurs under our experimental conditions (38). The products of molecular weight 133, 139, 147, and 155 are attributed to hydroxynitrates, HOCH2CHdCHCH2ONO2 and isomers from 1,3-butadiene, HOCD2CD)CDCD2ONO2 and isomers from 1,3-butadiene-d6, HOCH2C(CH3)dCHCH2ONO2 and isomers from isoprene, and HOCD2C(CD3)dCDCD2ONO2 and isomers from isoprene-d8 (13, 24). Figures 1 and 2 show representative negative ion API-MS spectra, with 5-hydroxy-2-pentanone or 4-hydroxy-3-hexanone being present as an internal standard for possible quantification of the unsaturated 1,4hydroxycarbonyls formed from 1,3-butadiene and isoprene. Making the assumption that the signal intensities of the NO2adducts of hydroxycarbonyls are proportional to the hydroxycarbonyl concentration (35), then the derived formation yields of the hydroxycarbonyls from 1,3-butadiene, 1,3butadiene-d6, isoprene, and isoprene-d8 were significantly greater than 100%, implying that under our experimental conditions the efficiency of formation of NO2- adducts of the unsaturated hydroxycarbonyls is much greater than for VOL. 39, NO. 11, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Measured SMPE/GC-FID Response Factors response factor relative to oxime of 3-pentanonea standard fiber set Ab

carbonyl 5-hydroxy-2-pentanone glycolaldehyde acrolein methacrolein methyl vinyl ketone trans-2-pentenal 2-methyl-2-pentenal

flexible fiber

15.0d 18.8d 1.15

10.0 2.1

set Bb

set Cc

14.3 16.3 16.3 21.7 31.6, 32.7 1.86 1.77 1.67, 1.59 0.37 0.28 0.21 0.20 8.3 9.3 9.2 2.1 1.3

a The overall uncertainties in the relative SPME/GC-FID response factors are estimated to be ∼(20%. Note the good agreement of independent measurements of the response factors (carried out within 4 days of each other) for glycolaldehyde (set B) and for acrolein (set C). b Used in this work, response factors measured in 03/2003 (set A) and 04/2003 (set B). c Different flexible fiber, response factors measured in 10/2003 (fiber not used in the reactions). d From Reisen et al. (33); this fiber was used in the first series of experiments with 1,3-butadiene.

the hydroxycarbonyl internal standards. Therefore, while showing the formation of hydroxycarbonyls and hydroxynitrates from the OH radical-initiated reactions of 1,3butadiene and isoprene and their deuterated analogues, the API-MS analyses were not useful for quantification of these unsaturated 1,4-hydroxycarbonyls. SPME with On-Fiber Derivatization and GC Analyses. 1,3-Butadiene Reaction, GC-MS. GC-MS analyses of irradiated CH3ONO-NO-1,3-butadiene-air mixtures sampled with precoated SPME fibers showed the presence of monooximes of molecular weight 56, 60, and 86 products and a dioxime of a molecular weight 58 product. Comparison of the GC retention time and mass spectrum with an authentic standard showed the molecular weight 56 product to be acrolein [CH2dCHCHO], and the molecular weight 86 carbonyl-containing product is attributed to 4-hydroxy-2butenal [HOCH2CHdCHCHO] (see Scheme 1). The molecular weight 60 product was identified as glycolaldehyde [HOCH2CHO] by comparison with glycolaldehyde formed in situ from the OH radical-initiated reaction of 2-methyl3-buten-2-ol (33, 36, 39). The molecular weight 58 product is attributed to glyoxal but was not quantified. As discussed in greater detail below, glycolaldehyde and glyoxal are second-generation products. SPME/GC-FID Response Factors. Experiments were carried out in which a measured concentration of 3-pentanone was included in the reactant mixture as a slowly reacting internal standard [3-pentanone is a factor of ∼30 less reactive than 1,3-butadiene toward OH radicals (9)], with the concentrations of 1,3-butadiene and 3-pentanone being measured by GC-FID as described above and with the concentrations of the oximes of 3-pentanone, acrolein, glycolaldehyde, and 4-hydroxy-2-butenal being measured by GC-FID after on-fiber derivatization. As listed in Table 1, derivatization/GC-FID response factors for the oximes of acrolein and glycolaldehyde were measured relative to that for the oxime of 3-pentanone (using authentic standards in the gas-phase; see Experimental Section) for both fiber types used here, the standard 65-µm PDMS/DVB SPME fiber (termed “standard” hereafter) used in the Reisen et al. (33) study and a 65-µm PDMS/DVB StableFlex SPME fiber (hereafter termed “flexible”). Reisen et al. (33) measured response factors relative to 3-pentanone for the oximes of 33 saturated carbonyl compounds (aldehydes, ketones, and hydroxycarbonyls) and concluded that the response factor for hydroxycarbonyls for which authentic standards are not available can be estimated by assuming that replacement of a CH3 group by a OH group enhances the response by a 4094

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FIGURE 3. Plots of the amounts of acrolein measured by SPME/ GC-FID, corrected for reactions with OH radicals (see text), against the amounts of 1,3-butadiene reacted with the OH radical. b, standard fiber; 0, flexible fiber. factor of 5.1 (to within an uncertainty of a factor of ∼2) (33). Therefore, a relative response factor for 2-pentenal was measured (see Table 1) and used to estimated the relative response factor for the oximes of 4-hydroxy-2-butenal. Relative response factors for a subset of carbonyls and hydroxycarbonyls were measured for both the flexible fiber and the standard fiber and the results are also given in Table 1. As evident from Table 1, the relative response factors for the flexible fiber compared to those for the standard fiber were generally similar, including for the 1,4-hydroxycarbonyl, 5-hydroxy-2-pentanone. Additionally, compound structure similarly affected the responses on both fibers. For example, comparing the response of 2-methyl-2-pentenal with that of 2-pentenal, there was significantly reduced response of 2-methyl-2-pentenal on both fibers, presumably because of steric interference to the oxime formation from the methyl group adjacent to the carbonyl group. For glycolaldehyde, the relative response factor of the flexible fiber seemed to increase as the fiber aged. It was determined that, although there are differences between the standard and flexible fibers, the relative response factors for the flexible fiber were not sufficiently different from those for the standard fiber (33) to warrant changing the estimation method (33) for carbonyls for which standards are not available. For acrolein and glycolaldehyde, measured response factors, which were fiber and fiber-age appropriate, were used for quantification. Because Reisen et al. (33) found that, with the on-fiber derivatization/GC-FID sampling analysis conditions employed here, there appeared to be an upper limit to the response, the estimated relative response factor used for 4-hydroxy-2-butenal with both SPME fibers was 25. Acrolein Yield. Figure 3 shows a plot of the amounts of acrolein formed, corrected for secondary reaction with OH radicals, against the amounts of 1,3-butadiene reacted with the OH radical. Corrections for secondary reaction of acrolein with OH radicals were carried out as described previously (40), using rate constants for reactions of OH radicals with acrolein and 1,3-butadiene of 1.99 × 10-11 cm3 molecule-1 s-1 (41, 42) and 6.66 × 10-11 cm3 molecule-1 s-1 (9), respectively. The multiplicative correction factors increase with the extent of reaction (40) and were e1.28. Least-squares analyses of the data shown in Figure 3 lead to acrolein formation yields of 64 ( 9% and 55 ( 4% using the standard and flexible fibers, respectively, where the indicated errors

are two least-squares standard deviations. Combined with estimated additional uncertainties in the GC-FID response factors for 1,3-butadiene and acrolein (the latter as its oxime) of (5% and (20%, respectively, our acrolein yields are 64 ( 16% and 55 ( 12%, respectively, with a weighted average of 58 ( 10%. These acrolein yields obtained using the on-fiber derivatization and GC-FID analysis method are in excellent agreement with our previous yields of 61 ( 5% for collection onto Tenax-TA solid adsorbent with thermal desorption and GC-FID analysis and 55 ( 5% using in situ Fourier transform infrared spectroscopic analysis (13). 4-Hydroxy-2-butenal Yield and OH Radical Reaction Rate Constant. As described previously (34), for the reactions

OH + 1,3-butadiene f R 4-hydroxy-2-butenal (+ other products) (1) OH + 4-hydroxy-2-butenal f β glycolaldehyde (+ other products) (2) OH + glycolaldehyde f products

(3)

then (34)

[4-hydroxy-2-butenal]t ) R[1,3-butadiene]tok1 -k1[OH]t - e-k2[OH]t) (I) (e (k2 - k1) where R is the yield of 4-hydroxy-2-butenal in reaction 1, k1 and k2 are the rate constants for reactions 1 and 2, respectively, [4-hydroxy-2-butenal]t is the concentration of 4-hydroxy2-butenal at time t, [1,3-butadiene]to is the initial concentration of 1,3-butadiene, and [OH] is the OH radical concentration. Equation I holds even if the OH radical concentration is not constant (34), in which case [OH]t is replaced by ∫[OH]dt, and the sole assumption for formulating eq I is that reactions 1 and 2 are the only loss processes for 1,3-butadiene and the hydroxycarbonyl, respectively. Because ln([1,3butadiene]to/[1,3-butadiene]t) ) k1∫[OH]dt (or k1[OH]t if the OH radical concentration is constant), then eq I becomes

[hydroxycarbonyl]t ) A(e-x - e-Bx)

(II)

where A ) R[1,3-butadiene]tok1/(k2 - k1), B ) k2/k1, and x ) ln([1,3-butadiene]to/[1,3-butadiene]t). For a given experiment, or a series of experiments with the same initial concentration of 1,3-butadiene, A is therefore constant, and the hydroxycarbonyl concentration at time t depends on the values of x and B. The value of ln([1,3-butadiene]to/[1,3-butadiene]t) at which the hydroxycarbonyl concentration is a maximum, [hydroxycarbonyl]max, depends only on the rate constant ratio k2/k1, being given by ln(k2/k1)/[(k2/k1) - 1] ) ln B/(B - 1). Measurement of the hydroxycarbonyl concentration as a function of the extent of reaction during OH radical-initiated reactions of 1,3-butadiene therefore allows the rate constant ratio k2/k1, and hence the rate constant k2, to be determined. As shown by Baker et al. (34), the variation of the value of ln([1,3-butadiene]to/[1,3-butadiene]t) at which [4-hydroxy2-butenal]t is a maximum is most sensitive to changes in the rate constant ratio for values of k2/k1 ∼ 1, and derivation of k2/k1 becomes difficult for values of k2/k1 < 0.2 and k2/k1 > 2. Figure 4 shows plots of the measured concentrations of 4-hydroxy-2-butenal (using an estimated relative response factor of 25) against ln([1,3-butadiene]to/[1,3-butadiene]t) [in effect, against the extent of reaction]. The hydroxycarbonyl concentrations have been scaled to a constant initial 1,3butadiene concentration of 1.44 × 1013 molecule cm-3. Consistent with eqs I and II, the data for the single experiment

FIGURE 4. Plots of eq I for formation of 4-hydroxy-2-butenal from the OH radical-initiated reaction of 1,3-butadiene, together with calculated curves from eq I for various values of k2/k1 and the molar yield of 4-hydroxy-2-butenal. The different symbols denote different experiments. Open symbols, standard fiber; filled symbols, flexible fiber. with an initial 1,3-butadiene concentration of 5.17 × 1013 molecule cm-3 then becomes indistinguishable from the remaining experiments using the same SPME fiber with initial 1,3-butadiene concentrations of (1.35-1.58) × 1013 molecule cm-3. Figure 4 shows that the two fibers give different absolute concentrations of 4-hydroxy-2-butenal, but the shape of the curves is essentially identical. The data in Figure 4 show that for both fibers the 4-hydroxy-2-butenal concentration maximizes at ln([1,3-butadiene]to/[1,3-butadiene]t) ) 1.05-1.10, corresponding to a rate constant ratio of k2/k1 ∼ 0.85. While there is a certain amount of run-to-run variation in the measured amount of 4-hydroxy-2-butenal for a specific fiber, the experimental data in Figure 4 are well fit (solid lines) with a rate constant ratio k2/k1 ) 0.85 and a 4-hydroxy-2butenal formation yield of 0.305 (standard fiber) and 0.19 (flexible fiber). For the data obtained using the standard fiber, calculations are also shown for a rate constant ratio of k2/k1 ) 0.65 and yield of 0.27 and for a rate constant ratio of k2/k1 ) 1.05 and a yield of 0.335 (increasing the rate constant ratio requires an increase in the yield and vice versa). Similar results would be obtained on varying the rate constant ratio for the data obtained with the flexible fiber. The fits of the experimental data to the calculated curves lead to the rate constant ratio

k2(OH + 4-hydroxy-2-butenal)/k1(OH + 1,3-butadiene) ) 0.85 ( 0.2 at 298 ( 2 K This rate constant ratio can be placed on an absolute basis using a rate constant for the reaction of OH radicals with 1,3-butadiene of 6.66 × 10-11 cm3 molecule-1 s-1 (9), resulting in

k2(OH + 4-hydroxy-2-butenal) ) (5.7 ( 1.4) ×

10-11 cm3 molecule-1 s-1 at 298 ( 2 K

where the indicated error does not include the uncertainties in the rate constant for the reaction of OH radicals with 1,3butadiene (expected to be approximately (10%). The estimated 298 K rate constant k2, using the method of Kwok and Atkinson (43) updated as described by Bethel et al. (44) and Papagni et al. (45), is 5.8 × 10-11 cm3 molecule-1 s-1, with OH radical addition to the CdC bond and H-atom abstraction VOL. 39, NO. 11, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Plots of the amounts of 4-hydroxy-2-butenal measured by SPME/GC-FID, corrected for reactions with OH radicals (see text), against the amounts of 1,3-butadiene reacted with the OH radical. O, standard fiber; 0, flexible fiber. The indicated errors of the yields are two least-squares standard deviations. from the CHO group being estimated to account for 66% and 29%, respectively, of the overall reaction and with H-atom abstraction from the CH2OH group accounting for the remaining 5% of the reaction. This rate constant ratio k2/k1 can be used to correct the measured 4-hydroxy-2-butenal concentrations for reaction with OH radicals (40), and plots of the amounts of 4-hydroxy2-butenal formed, corrected for secondary reactions, against the amounts of 1,3-butadiene reacted are shown in Figure 5. Good straight lines are observed, and least-squares analyses lead to 4-hydroxy-2-butenal formation yields of 0.307 ( 0.030 for the standard fiber and 0.201 ( 0.014 for the flexible fiber, where the indicated errors are two least-squares standard deviations (the slight differences between these yields and those derived from fitting the data in Figure 4 with the same rate constant ratio of k2/k1 ) 0.85 are due to the small nonzero intercepts in the least-squares fits of the data in Figure 5). As expected, there is good consistency between the two methods of data presentation and analysis. In particular, Figure 5 shows that using a rate constant ratio k2/k1 ) 0.85 to correct the measured 4-hydroxy-2-butenal concentrations for secondary reactions (with the multiplicative correction factor being up to a value of 1.95) leads to good linear plots of 4-hydroxy-2-butenal formed against the amounts of 1,3butadiene reacted, indicating the correctness of the rate constant ratio derived using the method shown in Figure 4. Glycolaldehyde and Glyoxal Formation. Glycolaldehyde was also observed in these CH3ONO-NO-1,3-butadieneair irradiations. Glycolaldehyde is a product of the reaction

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of OH radicals with acrolein (42, 46), with reported yields of 25 ( 10% (42) and 32 ( 4% (46). However, under our experimental conditions, the amounts of glycolaldehyde formed from secondary reactions of acrolein with OH radicals are calculated to account for