Products of reaction of OH radicals with A-pinene

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D14, 4191, 10.1029/2001JD001098, 2002 Products of reaction of OH radicals with A-pinene Sara M. Aschmann, Roger Atkinson, 1,2 and Janet Arey 1 Air Pollution Research Center, University of California at Riverside, Riverside, California, USA Received 16 July 2001; revised 10 January 2002; accepted 11 January 2002; published 18 July 2002. [1] Products of the gas-phase reaction of a-pinene with OH radicals in the presence of NO have been investigated using gas chromatography with flame ionization detection to quantify pinonaldehyde and in situ atmospheric pressure ionization mass spectrometry in the negative ion mode to quantify selected other products as their NO 2 adducts by utilizing C 6 -dihydroxycarbonyls and C 6 -hydroxynitrates formed in situ from the reaction of OH radicals with 1-hexene as an internal standard. The products quantified, and their molar formation yields, were: pinonaldehyde, 28 ± 5%; molecular weight 184 product (dihydroxycarbonyl), 19% (with an estimated uncertainty of a factor of 2); molecular weight 200 product, 11% (with an estimated uncertainty of a factor of 2). Together with a very approximate yield from our API-MS analyses for the formation of organic nitrates (1%) and literature data for acetone (plus coproducts), 65 70% of the reaction products and pathways are accounted for. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere composition and chemistry; 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties; KEYWORDS: hydroxyl radical, alpha-pinene, atmospheric reactions 1. Introduction [2] Large amounts of volatile nonmethane organic compounds are emitted into the atmosphere from vegetation [Guenther et al., 1995, 2000], and these biogenic organic compounds include isoprene, a series of C 10 monoterpenes, and a number of oxygenated species [Guenther et al., 1995, 2000; Fuentes et al., 2000]. The monoterpene hydrocarbons emitted from vegetation include a- and b-pinene, camphene, 3-carene, limonene, myrcene, and sabinene. In the atmosphere, these monoterpenes react with OH radicals, NO 3 radicals and O 3 [Atkinson, 1997, 2000; Atkinson and Arey, 1998] and rate constants for these gas-phase reactions have been reported [Atkinson, 1997; Atkinson and Arey, 1998]. Combining the literature rate constants with estimated ambient atmospheric concentrations of OH radicals and O 3 indicates that during daylight hours reaction with the OH radical will be important, and be the dominant loss process for several monoterpenes, including a-pinene. However, the products and the detailed chemical mechanisms of these monoterpene reactions are much less well understood [Atkinson, 1997, 2000]. [3] For example, for the reaction studied here, namely the OH radical-initiated reaction of a-pinene (the most-studied monoterpene) [Arey et al., 1990; Hatakeyama et al., 1991; Grosjean et al., 1992; Hakola et al., 1994; Aschmann et al., 1 Also at Interdepartmental Graduate Program in Environmental Toxicology and Department of Environmental Sciences, University of California at Riverside, Riverside, California, USA. 2 Also at Department of Chemistry, University of California at Riverside, Riverside, California, USA. Copyright 2002 by the American Geophysical Union. 0148-0227/02/2001JD001098 1998; Vinckier et al., 1998; Ruppert et al., 1999; Reissell et al., 1999; Nozière et al., 1999a; Orlando et al., 2000; Van den Bergh et al., 2000; Larsen et al., 2001; Vanhees et al., 2001; Peeters et al., 2001; Wisthaler et al., 2001], there are significant discrepancies in the products observed and their formation yields. At atmospheric pressure of air and in the presence of sufficient NO that organic peroxy radicals react dominantly with NO, Hatakeyama et al. [1991] and Nozière et al. [1999a] used in situ Fourier transform infrared (FTIR) spectroscopic analysis and reported pinonaldehyde formation yields of 56 ± 4% and 87 ± 20%, respectively, while studies from this laboratory using gas chromatography with flame ionization detection (GC-FID) obtained pinonaldehyde formation yields of 29% [Arey et al., 1990] and 28 ± 5% [Hakola et al., 1994]. The recent study of Wisthaler et al. [2001] used proton-transfer-reaction mass spectrometry to determine a pinonaldehyde yield of 34 ± 9%, in reasonable agreement with the gas chromatographic data [Arey et al., 1990; Hakola et al., 1994]. Acetone has also been observed and quantified as a product of the a-pinene reaction in a number of studies [Grosjean et al., 1992; Vinckier et al., 1998; Aschmann et al., 1998; Reissell et al., 1999; Nozière et al., 1999a; Orlando et al., 2000; Larsen et al., 2001; Vanhees et al., 2001; Wisthaler et al., 2001]. In addition, Aschmann et al. [1998] used in situ atmospheric pressure ionization tandem mass spectrometry (API-MS) and observed the formation of products of molecular weight 184, 200, 215, and 231, with those of molecular weight 184, 215, and 231 being attributed to dihydroxycarbonyl(s), hydroxynitrate(s), and dihydroxynitrate(s), respectively. Obviously, there is a marked disagreement in the measured formation yield of pinonaldehyde between the studies of Hatakeyama et al. [1991] and Nozièreet al. [1999a] employing FTIR for analysis and those using GC-FID and proton- ACH 6-1

ACH 6-2 ASCHMANN ET AL.: PRODUCTS OF OH RADICAL WITH a-pinene transfer-reaction mass spectrometry for quantification [Arey et al., 1990; Hakola et al., 1994; Wisthaler et al., 2001]. [4] If correct, the high pinonaldehyde yield observed by Nozière et al. [1999a], combined with their observation of organic nitrates in 18 ± 9% yield, would suggest that the formation of dihydroxycarbonyls and molecular weight 200 species observed by Aschmann et al. [1998] must be very minor. Because the previous GC-FID measurements of pinonaldehyde carried out by Arey et al. [1990] and Hakola et al. [1994] did not use authentic samples of pinonaldehyde for calibration of the GC-FID system, but rather used the effective carbon number concept [Scanlon and Willis, 1985], we have remeasured the pinonaldehyde formation yield from the reaction of OH radicals with a-pinene in the presence of NO by GC-FID using a synthesized sample of pinonaldehyde for calibration purposes. In addition, we have quantified the molecular weight 184, 200, 215, and 231 species observed by Aschmann et al. [1998] using in situ atmospheric pressure ionization mass spectrometry (API-MS). 2. Experimental [5] Experiments were carried out at 296 ± 2 K and 740 Torr total pressure of purified air diluent (at 5% relative humidity) in two 7500-L environmental chambers, each equipped with two parallel banks of blacklamps for irradiation and a Teflon-coated fan for rapid mixing of reactants during their introduction into the chamber [Hakola et al., 1994; Aschmann et al., 1998]. One of the chambers was interfaced to a PE SCIEX API-III MS/MS direct air sampling, atmospheric pressure ionization tandem mass spectrometer [Aschmann et al., 1998]. Two sets of experiments were carried out, one set using GC-FID for quantification of pinonaldehyde, and the second set of experiments using the API-MS instrument for quantification of selected product species that do not appear to be amenable to analysis by gas chromatography without prior derivatization. In both cases, OH radicals were generated by the photolysis of methyl nitrite (CH 3 ONO) in air at wavelengths >300 nm [Atkinson et al., 1981], and NO was added to the reactant mixtures to suppress the formation of O 3, and hence of NO 3 radicals. 2.1. Analyses by GC-FID [6] Irradiations of CH 3 ONO/NO/a-pinene/air mixtures were carried out, with initial CH 3 ONO, NO, and a-pinene concentrations of (2.2 2.4) 10 14, (1.6 2.2) 10 14, and (1.0 2.2) 10 13 molecule cm 3, respectively. These irradiations were at 20% of the maximum light intensity for 1 6 min, resulting in up to 53% consumption of the initially present a-pinene. The concentrations of a-pinene and pinonaldehyde were measured during the experiments by GC-FID. Gas samples of 100 cm 3 volume were collected from the chamber onto Tenax-TA solid adsorbent, with subsequent thermal desorption (with the heating element at 250 C) onto a 30-m DB-1701 megabore column held at 0 C and then temperature programmed to 240 C at 8 C min 1. The GC-FID response factors for a-pinene and pinonaldehyde were measured by introducing known amounts of the compounds into the chamber (by flushing measured amounts of the liquids from a 1-L Pyrex bulb with a stream of N 2 gas) and conducting several replicate GC- FID analyses (the chamber volume was determined by introducing a measured amount of trans-2-butene and analyzing its concentration using a precalibrated GC). [7] Good overall collection efficiency and recovery of the gas-phase pinonaldehyde sampled on the Tenax-TA adsorbent was verified by measuring the GC-FID response factors for pinonaldehyde and a-pinene from four independent GC-FID calibrations which resulted in a measured effective carbon number (ECN) of pinonaldehyde which was 0.90 ± 0.13 (2 standard deviations) of that calculated using the approach of Scanlon and Willis [1985]. The ECNs are proportional to the FID responses, and for the compounds relevant to this study are calculated as the number of carbons atoms in the compound, not counting those which are part of carbonyl groups, and with a minor correction to account for the carbon-carbon double bond in a-pinene; thus the calculated ECN for a-pinene is 9.9 and that for pinonaldehyde is 8.0 [Scanlon and Willis, 1985]. The presence of impurities in the pinonaldehyde sample (the purity level of the pinonaldehyde as determined by GC analyses was >90% [Alvarado et al., 1998]), and/or incomplete transfer of pinonaldehyde from the bulb into the chamber would result in a lower measured ECN for pinonaldehyde. Thus the good agreement (within 10%) of the measured and calculated ECN factors for the pinonaldehyde calibration sample, analyzed by an identical technique to that used during the OH radical experiments, supports the validity of our current, as well as previous [Arey et al., 1990; Hakola et al., 1994], pinonaldehyde quantifications. 2.2. Analyses by API-MS [8] Irradiations of CH 3 ONO/NO/a-pinene/1-hexene air mixtures were also carried out 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). 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 previously [Aschmann et al., 1997, 1998]. 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 [Aschmann et al., 1997, 1998]. [9] Both positive and negative ion modes were used in this work. In the positive ion mode, protonated water hydrates (H 3 O + (H 2 O) n ) generated by the corona discharge in the chamber diluent air were responsible for the protonation of analytes [Aschmann et al., 1997, 1998]. In the negative ion mode, adducts were formed between molecules and the negative ions generated by the negative corona around the discharge needle. The superoxide ion (O 2 ), its hydrates [O 2 (H 2 O) n ], and O 2 clusters [O 2 (O 2 ) n ] are the major reagent negative ions in the chamber pure air. Other reagent ions, for example, NO 2 and NO 3, are formed through reactions between the primary reagent ions and neutral molecules such as NO 2. Ions are drawn by an electric potential from the ion source through the sampling orifice into the mass-analyzing first quadrupole or third quadrupole.

ASCHMANN ET AL.: PRODUCTS OF OH RADICAL WITH a-pinene ACH 6-3 Figure 1. Mechanism for the formation of first-generation products from the reaction of OH radicals with 1-hexene in the presence of NO, based on the product studies of Atkinson et al. [1995], Kwok et al. [1996], and O Brien et al. [1998]. Note that OH addition could also occur at the 2-position leading to an analogous sequence of reactions. The products observed are shown in boxes. In these experiments, the API-MS instrument was operated under conditions that favored the formation of dimer ions in the ion source region [Aschmann et al., 1997, 1998]. Neutral molecules and particles are prevented from entering the orifice by a flow of high-purity nitrogen ( curtain gas ), and as a result of the declustering action of the curtain gas on the hydrated ions, the ions that are mass analyzed are mainly protonated molecular ions ([M + H] + ) and their protonated homodimers and heterodimers [Aschmann et al., 1997, 1998] in the positive ion mode and mainly O 2 or NO 2 adducts in the negative ion mode. Because NO 2 is generated from the oxidation of NO during the reactions (by HO 2 and organic peroxy radicals), it is preferable in the negative ion mode to measure the products as NO 2 adducts [Arey et al., 2001]. Therefore, in each experiment sufficient NO 2 was added to the chamber after the reaction was completed (with NO 2 concentrations (2 3) 10 13 molecule cm 3 ) so that NO 2 adducts of the hydroxycarbonyls and hydroxynitrates dominated over the corresponding O 2 adducts [Arey et al., 2001]. The C 6 -dihydroxycarbonyls (of molecular weight 132) formed in situ from the OH radical-initiated reaction of 1-hexene [Kwok et al., 1996] (see Figure 1) were used as an internal standard for the quantification of hydroxycarbonyls, while the C 6 -hydroxynitrates (of molecular weight 163) also formed in situ from the 1-hexene reaction [O Brien et al., 1998] (Figure 1) were used as an internal standard for the quantification of hydroxynitrates. As previously noted by Arey et al. [2001], using NO 2 adducts the API-MS is an order of magnitude more sensitive to hydroxynitrates than to hydroxycarbonyls. [10] The initial reactant concentrations (in molecule cm 3 units) were: CH 3 ONO, (4.5 45) 10 12 (and (4.5 5.1) 10 12 in all but one experiment); NO, (4.6 45) 10 12 (and (4.6 4.8) 10 12 in all but one experiment); a-pinene, (1.90 2.42) 10 12 ; and 1-hexene, (2.48 2.69) 10 12. For each experiment, the reactant mixture was irradiated for 3 min at 20% of the maximum light intensity, resulting in 27 47% (27 29% in all but one experiment) and 18 38% (18 22% in all but one experiment) consumption of the initially present a-pinene and 1-hexene, respectively, as measured by GC-FID as described above for the pinonaldehyde yield measurements. In two of these experiments, 9.6 10 11 molecule cm 3 of 5-hydroxy-2-pentanone was added to the chamber after the irradiation to serve as an additional internal standard, both for the 1-hexene products as well as the a-pinene products. 2.3. Chemicals [11] The chemicals used, and their stated purities, were 1- hexene (99%) and a-pinene (99+%), Aldrich Chemical Company; 5-hydroxy-2-pentanone, TCI America; and NO (99.0%), Matheson Gas Products. Pinonaldehyde was a

ACH 6-4 ASCHMANN ET AL.: PRODUCTS OF OH RADICAL WITH a-pinene synthesized (as described by Glasius et al. [1997]) sample shown to be >90% purity from GC-FID analyses, with the impurities being the solvents methyl tert-butyl ether and tetrahydrofuran [Alvarado et al., 1998]. Methyl nitrite was prepared and stored as described previously [Atkinson et al., 1981], and NO 2 was prepared as needed by reaction of NO with excess O 2. 3. Results [12] In agreement with previous studies, GC-FID analyses showed pinonaldehyde to be a reaction product. Because pinonaldehyde also reacts with OH radicals [Glasius et al., 1997; Hallquist et al., 1997; Alvarado et al., 1998; Nozière et al., 1999b], secondary reaction with OH radicals was taken into account as described previously [Atkinson et al., 1982], using rate constants for the reactions of OH radicals with a-pinene and pinonaldehyde of 5.37 10 11 cm 3 molecule 1 s 1 [Atkinson, 1997] and 4.8 10 11 cm 3 molecule 1 s 1 [Alvarado et al., 1998], respectively. The multiplicative correction factor F to take into account secondary reaction increases with the extent of reaction, and in these experiments the maximum value of F was 1.44. For the light intensity and irradiation times used in these experiments, photolysis of pinonaldehyde was negligible (<1%) [Alvarado et al., 1998]. Figure 3. API-MS spectrum showing the NO 2 adducts of products formed from an irradiated CH 3 ONO/NO/a-pinene/ 1-hexene/air mixture. The NO 2 adduct ion peaks identified: as products from 1-hexene are C 6 -dihydroxycarbonyl at 178 u, C 6 -hydroxynitrate at 209 u, and C 6 -dihydroxynitrate at 225 u. The NO 2 adduct ion peaks identified as products from a-pinene are: C 10 -dihydroxycarbonyl at 230 u, C 10 O 4 H 16 product at 246 u, C 10 -hydroxynitrate at 261 u, and C 10 -dihydroxynitrate at 277 u. The ion peak at 214 u is attributed to the NO 2 adduct of pinonaldehyde. Figure 2. Plot of the amounts of pinonaldehyde formed, corrected for reactions with OH radicals (see text), against the amounts of a-pinene reacted with the OH radical. Circles, analyses by GC-FID; circles and squares, this work, with independent calibrations of the GC-FID response factor for pinonaldehyde using an authentic standard; triangles, previous data from Arey et al. [1990] and Hakola et al. [1994], with the GC-FID response factor for pinonaldehyde being calculated from the a-pinene and pinonaldehyde ECNs and the measured GC-FID response factor for a-pinene. The line is a least squares fit to all the data shown. [13] A plot of the amounts of pinonaldehyde formed, corrected for reaction with OH radicals, against the amounts of a-pinene reacted is shown in Figure 2. Data from two sets of experiments with independent calibrations of the GC-FID response factors for a-pinene and pinonaldehyde are plotted, together with our previous data using calculated GC-FID response factors for pinonaldehyde derived from the measured a-pinene response factor and the calculated ECNs of a-pinene and pinonaldehyde and reevaluated for secondary reaction of pinonaldehyde using the rate constants given above. Figure 2 shows that the various sets of experiments are in good agreement, with least squares analyses resulting in a pinonaldehyde yield of 0.272 ± 0.046 from the two independent sets of experiments with measured GC-FID response factors, and 0.285 ± 0.044 from all of the data shown in Figure 2, where the indicated errors are two least squares standard deviations combined with estimated uncertainties in the relative GC-FID response factors for pinonaldehyde and a-pinene of ±10%. We therefore cite a pinonaldehyde formation yield from the reaction of OH radicals with a-pinene, in the presence of NO, of 0.28 ± 0.05. [14] API-MS and API-MS/MS analyses of irradiated CH 3 ONO/NO/a-pinene/air mixtures using H 3 O + (H 2 O) n as the reagent ions resulted in spectra identical to those presented and discussed previously [Aschmann et al., 1998]. In the negative ion mode, using NO 2 as the reagent ion, API-MS spectra of irradiated CH 3 ONO/NO/a-pinene/ air mixtures (and after addition of NO 2 ) showed the

ASCHMANN ET AL.: PRODUCTS OF OH RADICAL WITH a-pinene ACH 6-5 Table 1. Measured Product Formation Yields (molar) From the Reactions of OH Radicals With a-pinene and 1-Hexene in the Presence of NO a Product 1-Hexene Products as Internal Standard 5-Hydroxy-2-Pentanone as Internal Standard a-pinene reaction Pinonaldehyde 0.28 ± 0.05 b MW 184 product 0.19 c,d,e 0.18 d,f MW 200 product 0.11 c,d,e 0.08 d,f MW 215 product 0.006 d,e,g MW 231 product 0.004 d,e,g 1-hexene reaction C 6 -dihydroxycarbonyl 0.485 (assumed) h 0.40 d,i a See Figure 4 for suggested products. b Indicated errors are two least squares standard deviations combined with an estimated uncertainty in the relative GC-FID response factors for pinonaldehyde and a-pinene of ±10%. c Values are quantified from the signal intensities of the 230 and 246 u NO 2 adducts relative to that for the 178 u NO 2 adduct of the C 6 - dihydroxycarbonyls formed in situ from 1-hexene, based on a molar formation yield of the C 6 -dihydroxycarbonyls of 0.485 (see text). d Estimated overall uncertainty is a factor of 2. e No corrections were made for secondary reactions of OH radicals, because both the C 6 -dihydroxycarbonyl(s) from 1-hexene and the molecular weight 184, 200, 215, and 231 products from a-pinene react and any corrections would approximately cancel out. f No corrections were made for secondary reaction with the OH radical. g Values were quantified from the signal intensities of the 261 and 277 u NO 2 adducts relative to that for the 209 u NO 2 adduct of the C 6 - hydroxynitrates formed in situ from 1-hexene, based on a molar formation yield of the C 6 -hydroxynitrates of 0.055 [O Brien et al., 1998]. h See text. i Correction was shown for secondary reactions with the OH radical was made (10 23%), using a rate constant for reaction of OH radicals with the C 6 -dihydroxycarbonyl(s) of 3.0 10 11 cm 3 molecule 1 s 1, based on the data of Bethel et al. [2001] for four diols. presence of prominent ion peaks at 230, 246, 261 and 277 unified atomic mass units (u), corresponding to products of molecular weight 184, 200, 215, and 231 and consistent with products previously identified by positive ion API- MS analyses [Aschmann et al., 1998]. In addition, a less intense ion peak at 214 u was observed, corresponding to pinonaldehyde which we have previously observed to form an NO 2 adduct, as well as at 160, 176, and 188 u (corresponding to products of molecular weight 114, 130, and 142). Analogous API-MS spectra of irradiated CH 3 ONO/NO/a-pinene/1-hexene/air mixtures (Figure 3) showed additional ion peaks at 178, 209, and 225 u, corresponding to NO 2 adducts of 1-hexene products of molecular weight 132 (the C 6 -dihydroxycarbonyls CH 3 CH(OH)CH 2 CH 2 C(O)CH 2 OH and CH 3 CH 2 CH- (OH)CH 2 CH(OH)CHO [Atkinson et al., 1995; Kwok et al., 1996]), 163 (the C 6 -hydroxynitrates CH 3 CH 2 CH 2 CH 2 CH- (ONO 2 )CH 2 OH and CH 3 CH 2 CH 2 CH 2 CH(OH)CH 2 ONO 2 [O Brien et al., 1998]) and 179 (presumably C 6 -dihydroxynitrates) (see Figure 1). [15] The 178 u NO 2 adducts of the C 6 -dihydroxycarbonyl(s) formed from 1-hexene were used as an internal standard for quantification of the molecular weight 184 and 200 products observed as their NO 2 adducts at 230 and 246 u, respectively, and the 209 u NO 2 adducts of the C 6 -hydroxynitrate(s) were used as an internal standard for quantification of the molecular weight 215 and 231 products observed as their NO 2 adducts at 261 and 277 u, respectively. A formation yield of the C 6 -hydroxycarbonyls from the 1-hexene reaction of 0.485 was used, based on the measured formation yields of pentanal (0.46 ± 0.07 [Atkinson et al., 1995]) and C 6 -hydroxynitrates (0.055 ± 0.010 [O Brien et al., 1998]), and assuming that these are the sole first-generation products formed from the OH radical-initiated reaction of 1-hexene in the presence of NO [Kwok et al., 1996]. Note that 1,2-hydroxycarbonyl formation from reactions of the intermediate 1,2-hydroxyalkoxy radicals with O 2 has been shown to be of no importance under the conditions employed here [Kwok et al., 1996; Aschmann et al., 2000], and formation of dihydroxynitrates is presumably less important than of hydroxynitrates (see intensity of the 209 and 225 u ion peaks in Figure 3). The resulting formation yields of the molecular weight 184, 200, 251, and 231 species from a-pinene, from five experiments, are given in Table 1. Note that an API-MS/ MS product ion spectrum of the protonated molecular weight 184 product [see Aschmann et al., 1998] is quite different from the API-MS/MS product ion spectrum of Figure 4. Proposed structures of the molecular weight 184, 200, 215, and 231 u products [Aschmann et al., 1998] arising after initial OH radical addition to the 2-position of a-pinene. Analogous products may be formed after OH radical addition to the 3-position.

ACH 6-6 ASCHMANN ET AL.: PRODUCTS OF OH RADICAL WITH a-pinene the isomeric protonated cis-pinonic acid [Baker et al., 2002]. This shows that pinonic acid is not a significant product of the OH radical-initiated reaction of a-pinene in the presence of NO; the suggested identity of the molecular weight 184 and other products [Aschmann et al., 1998] are shown in Figure 4 for initial OH radical addition to the 2-position of a-pinene (analogous products may be formed after initial addition to the 3-position). [16] In two experiments, 5-hydroxy-2-pentanone was added to the chamber after the experiment to serve as an additional internal standard [Arey et al., 2001], also enabling an estimate of the formation yield of the C 6 -dihydroxycarbonyl(s) from 1-hexene and of the molecular weight 184 and 200 products from a-pinene to be made. These yields are also given in Table 1. 4. Discussion [17] In our laboratory, the accuracy of using calculated ECNs to predict the GC-FID responses of a range of volatile organic compounds and their reaction products for which standards are available has been verified [Aschmann et al., 2000; Bethel et al., 2001; Arey et al., 2001]. Therefore, the good agreement of the measured response factor for an authentic standard of pinonaldehyde and the response factor predicted from the calculated ECNs of pinonaldehyde and a-pinene indicates that our sampling and analysis procedures for pinonaldehyde were quantitative. The data in Figure 2 show no evidence for a decrease in the yield of pinonaldehyde with increasing extent of reaction, nor did variation of the initial a-pinene concentration by a factor of 2.2 have an observable effect on the pinonaldehyde formation yield, in contrast to the study of Nozière et al. [1999a] in which the pinonaldehyde yield was observed to decrease with increasing initial a-pinene concentration over the range (0.529 6.26) 10 13 molecule cm 3. [18] Our pinonaldehdye formation yield of 28 ± 5% is significantly lower than those reported by Hatakeyama et al. [1991] and Nozière et al. [1999a] of 56 ± 4% and 87 ± 20%, respectively, using in situ FTIR analysis, but is in agreement with the yield of 34 ± 9% recently measured by Wisthaler et al. [2001] using proton-transfer-reaction mass spectrometry. Reasons for the discrepancies with the FTIR studies could include incorrect calibration of the IR absorption band(s) of pinonaldehyde (in the study of Hatakeyama et al. [1991] an estimated absorption cross section was used because of the lack of a pinonaldehyde standard, and use of the cross section used by Nozière et al. [1999a] increases the yield to 78.5%) and/or the presence of other products containing aldehydic CHO group(s) (note that the IR absorption cross-section for pinonaldehyde at the aldehydic CHO group absorption position used by Nozière et al. [1999a] was taken from an earlier study [Hallquist et al., 1997]). The correctness of the lower pinonaldehyde formation yields measured here and by Wisthaler et al. [2001] is further suggested by the pinonaldehyde yield of 35.7% predicted from the theoretical study of Peeters et al. [2001], using quantitative structure-activity relationships and quantum chemistry calculations. [19] The presence of dihydroxycarbonyls as products of a-pinene and 1-hexene shows that isomerization of an initially formed hydroxyalkoxy radical must occur in each case [Kwok et al., 1996; Aschmann et al., 1998] (see Figure 1 for the 1-hexene reaction). Although the lack of authentic standards increases the uncertainty in our quantifications of the dihydroxycarbonyl and hydroxynitrate products from a-pinene, our API-MS quantifications of the molecular weight 184 dihydroxycarbonyl and molecular weight 215 hydroxynitrate products used internal standards of similar chemical structure (a C 6 -dihydroxycarbonyl and a C 6 -hydroxynitrate, respectively). Our quantification of the molecular weight 200 and 231 products from a-pinene (anticipated to be trihydroxycarbonyls and/or hydroxytricarbonyls, and dihydroxynitrate(s), respectively [Aschmann et al., 1998]), are likely to be more approximate because the internal standards used have different numbers/ combinations of functional groups. In this regard, particularly gratifying is the reasonable agreement between the measured formation yield of C 6 -dihydroxycarbonyl(s) from 1-hexene using 5-hydroxy-2-pentanone as an internal standard for API-MS analysis (Table 1) with the formation yield estimated from a mass balance based on the other known products [Atkinson et al., 1995]. [20] Our present and previous [Aschmann et al., 1998] data show that pinonaldehyde is not the dominant product formed from a-pinene, and that other products, including those of molecular weight 184 (dihydroxycarbonyls), 200, 215 (hydroxynitrates) and 231 (dihydroxynitrates), are also formed, some in appreciable yield (Table 1). The products for which yields are given in Table 1 account for 60% of the reaction products. Inclusion of the formation of acetone (and its various coproducts), with reported acetone yields in the range 5 11% [Reissell et al., 1999; Nozière et al., 1999a; Orlando et al., 2000; Wisthaler et al., 2001], increases the products (and reaction pathways) accounted for to 65 70% (with an estimated range of 50 108%). Our hydroxynitrate and dihydroxynitrate yield of 1% (Table 1) is an order of magnitude lower than the organic nitrates yield of 18 ± 9% reported by Nozière et al. [1999a] (and much lower than expected based on the hydroxynitrate yields from a series of C 2 C 6 alkenes [O Brien et al., 1998]). It is possible that these hydroxy- and dihydroxynitrate products partitioned into the particle phase or decayed to the chamber wall and hence were not analyzed by API-MS. [21] The observation of dihydroxynitrate (molecular weight 231) and dihydroxycarbonyl (molecular weight 184) products from the reaction of the OH radical with a- pinene shows that isomerization of a first-generation hydroxyalkoxy radical must occur. Furthermore, the yield data given here, although uncertain to a factor of 2 because of a lack of authentic standards, demonstrates that the isomerization reaction is competitive with the decomposition reaction leading to pinonaldehyde. [22] Acknowledgments. The authors gratefully thank the National Science Foundation (grant ATM-9809852) for support of this research. References Alvarado, A., J. Arey, and R. Atkinson, Kinetics of the gas-phase reactions of OH and NO 3 radicals and O 3 with the monoterpene reaction products pinonaldehyde, caronaldehyde, and sabinaketone, J. Atmos. Chem., 31, 281 297, 1998. Arey, J., R. Atkinson, and S. M. Aschmann, Product study of the gas-phase reactions of monoterpenes with the OH radical in the presence of NO x, J. Geophys. Res., 95, 18,539 18,546, 1990.

ASCHMANN ET AL.: PRODUCTS OF OH RADICAL WITH a-pinene ACH 6-7 Arey, J., S. M. Aschmann, E. S. C. Kwok, and R. Atkinson, Alkyl nitrate, hydroxyalkyl nitrate, and hydroxycarbonyl formation from the NO x -air photooxidations of C 5 C 8 n-alkanes, J. Phys. Chem. A, 105, 1020 1027, 2001. Aschmann, S. M., A. A. Chew, J. Arey, and R. Atkinson, Products of the gas-phase reaction of OH radicals with cyclohexane: Reactions of the cyclohexoxy radical, J. Phys. Chem. A, 101, 8042 8048, 1997. Aschmann, S. M., A. Reissell, R. Atkinson, and J. Arey, Products of the gas phase reactions of the OH radical with a- and b-pinene in the presence of NO, J. Geophys. Res., 103, 25,553 25,561, 1998. Aschmann, S. M., J. Arey, and R. Atkinson, Formation of b-hydroxycarbonyls from the OH radical-initiated reactions of selected alkenes, Environ. Sci. Technol., 34, 1702 1706, 2000. Atkinson, R., Gas-phase tropospheric reactions of volatile organic compounds, 1, Alkanes and alkenes, J. Phys. Chem. Ref. Data, 26, 215 290, 1997. Atkinson, R., Atmospheric chemistry of VOCs and NO x, Atmos. Environ., 34, 2063 2101, 2000. Atkinson, R., and J. Arey, Atmospheric chemistry of biogenic organic compounds, Acc. Chem. Res., 31, 574 583, 1998. Atkinson, R., W. P. L. Carter, A. M. Winer, and J. N. Pitts Jr., An experimental protocol for the determination of OH radical rate constants with organics using methyl nitrite as an OH radical source, J. Air Pollut. Control Assoc., 31, 1090 1092, 1981. Atkinson, R., S. M. Aschmann, W. P. L. Carter, A. M. Winer, and J. N. Pitts Jr., Alkyl nitrate formation form the NO x -air photooxidations of C 2 C 8 n-alkanes, J. Phys. Chem., 86, 4563 4569, 1982. Atkinson, R., E. C. Tuazon, and S. M. Aschmann, Products of the gasphase reactions of a series of 1-alkenes and 1-methylcyclohexene with the OH radical in the presence of NO, Environ. Sci. Technol., 29, 1674 1680, 1995. Baker, J., S. M. Aschmann, J. Arey, and R. Atkinson, Reactions of stabilized Criegee intermediates from the gas-phase reactions of O 3 with selected alkenes, Int. J. Chem. Kinet., 34, 73 85, 2002. Bethel, H. L., R. Atkinson, and J. Arey, Kinetics and products of the reactions of selected diols with the OH radical, Int. J. Chem. Kinet., 33, 310 316, 2001. Fuentes, J. D., et al., Biogenic hydrocarbons in the atmospheric boundary layer: A review, Bull. Am. Meteorol. Soc., 81, 1537 1575, 2000. Glasius, M., A. Calogirou, N. R. Jensen, J. Hjorth, and C. J. Nielsen, Kinetic study of gas-phase reactions of pinonaldehyde and structurally related compounds, Int. J. Chem. Kinet., 29, 527 533, 1997. Grosjean, D., E. L. Williams II, and J. H. Seinfeld, Atmospheric oxidation of selected terpenes and related carbonyls: Gas-phase carbonyl products, Environ. Sci. Technol., 26, 1526 1533, 1992. Guenther, A., et al., A global model of natural volatile organic compound emissions, J. Geophys. Res., 100, 8873 8892, 1995. Guenther, A., C. Geron, T. Pierce, B. Lamb, P. Harley, and R. Fall, Natural emissions of non-methane volatile organic compounds, carbon monoxide, and oxides of nitrogen from North America, Atmos. Environ., 34, 2205 2230, 2000. Hakola, H., J. Arey, S. M. Aschmann, and R. Atkinson, Product formation from the gas-phase reactions of OH radicals and O 3 with a series of monoterpenes, J. Atmos. Chem., 18, 75 102, 1994. Hallquist, M., I. Wängberg, and E. Ljungström, Atmospheric fate of carbonyl oxidation products originating from a-pinene and 3 -carene: Determination of rate of reaction with OH and NO 3 radicals, UV absorption cross sections, and vapor pressures, Environ. Sci. Technol., 31, 3166 3172, 1997. Hatakeyama, S., K. Izumi, T. Fukuyama, H. Akimoto, and N. Washida, Reactions of OH with a-pinene and b-pinene in air: Estimate of global CO production from the atmospheric oxidation of terpenes, J. Geophys. Res., 96, 947 958, 1991. Kwok, E. S. C., R. Atkinson, and J. Arey, Isomerization of b-hydroxyalkoxy radicals formed from the OH radical-initiated reactions of C 4 C 8 1-alkenes, Environ. Sci. Technol., 30, 1048 1052, 1996. Larsen, B. R., D. di Bella, M. Glasius, R. Winterhalter, N. R. Jensen, and J. Hjorth, Gas-phase OH oxidation of monoterpenes: Gaseous and particulate products, J. Atmos. Chem., 38, 231 276, 2001. Nozière, B., I. Barnes, and K.-H. Becker, Product study and mechanisms of the reactions of a-pinene and of pinonaldehyde with OH radicals, J. Geophys. Res., 104, 23,645 23,656, 1999a. Nozière, B., M. Spittler, L. Ruppert, I. Barnes, K. H. Becker, M. Pons, and K. Wirtz, Kinetics of the reactions of pinonaldehyde with OH radicals and with Cl atoms, Int. J. Chem. Kinet., 31, 291 301, 1999b. O Brien, J. M., E. Czuba, D. R. Hastie, J. S. Francisco, and P. B. Shepson, Determination of the hydroxy nitrate yields from the reactions of C 2 C 6 alkenes with OH in the presence of NO, J. Phys. Chem. A, 102, 8903 8908, 1998. Orlando, J. J., B. Nozière, G. S. Tyndall, G. E. Orzechowska, S. E. Paulson, and Y. Rudich, Product studies of the OH- and ozone-initiated oxidation of some monoterpenes, J. Geophys. Res., 105, 11,561 11,572, 2000. Peeters, J., L. Vereecken, and G. Fantechi, The detailed mechanism of the OH-initiated atmospheric oxidation of a-pinene: A theoretical study, Phys. Chem. Chem. Phys., 3, 5489 5504, 2001. Reissell, A., C. Harry, S. M. Aschmann, R. Atkinson, and J. Arey, Formation of acetone from the OH radical- and O 3 -initiated reactions of a series of monoterpenes, J. Geophys. Res., 104, 13,869 13,879, 1999. Ruppert, L., K. H. Becker, B. Nozière, and M. Spittler, Development of monoterpene oxidation mechanisms: Results from laboratory and smog chamber studies, in Proceedings of the EUROTRAC Symposium 98 on Transport and Chemical Transformation in the Troposphere, vol. 1, edited by P. M. Borrell and P. Borrell, pp. 63 68, WIT Press, Southampton, England, 1999. Scanlon, J. T., and D. E. Willis, Calculation of flame ionization detector relative response factors using the effective carbon number concept, J. Chromatog. Sci., 23, 333 340, 1985. Van den Bergh, V., I. Vanhees, R. De Boer, F. Compernolle, and C. Vinckier, Identification of the oxidation products of the reaction between a-pinene and hydroxyl radicals by gas and high-performance liquid chromatography with mass spectrometric detection, J. Chromatogr. A, 896, 135 148, 2000. Vanhees, I., V. Van den Bergh, V. R. Schildermans, R. De Boer, F. Compernolle, and C. Vinckier, Determination of the oxidation products of the reaction between a-pinene and hydroxyl radicals by high-performance liquid chromatography, J. Chromatogr. A, 915, 75 83, 2001. Vinckier, C., F. Compernolle, A. M. Saleh, N. Van Hoof, and I. Van Hees, Product yields of the a-pinene reaction with hydroxyl radicals and the implication on the global emission of trace compounds in the atmosphere, Fresenius Environ. Bull., 7, 361 368, 1998. Wisthaler, A., N. R. Jensen, R. Winterhalter, W. Lindinger, and J. Hjorth, Measurements of acetone and other gas phase product yields from the OH-initiated oxidation of terpenes by proton-transfer-reaction mass spectrometry (PTR-MS), Atmos. Environ., 35, 6181 6191, 2001. J. Arey, S. M. Aschmann, and R. Atkinson, Air Pollution Research Center, University of California at Riverside, 205 Fawcett Laboratory, Riverside, CA 92521, USA. (janet.arey@ucr.edu; aschmann@ucracl.ucr. edu; ratkins@mail.ucr.edu)