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Products and mechanisms of the gas-phase reactions of OH radicals and O3 with 2-methyl-3-buten-2-ol
Atmospheric Environment 33 (1999) 2893}2905
Products and mechanisms of the gas-phase reactions of OH radicals and O with 2-methyl-3-buten-2-ol Alvaro Alvarado, Ernesto C. Tuazon, Sara M. Aschmann, Janet Arey, Roger Atkinson* Air Pollution Research Center, University of California, Riverside, CA 92521, USA Received 24 September 1998; accepted 22 January 1999
Abstract Products of the gas-phase reactions of OH radicals (in the presence of NO) and O with the biogenic organic compound 2-methyl-3-buten-2-ol have been investigated using gas chromatography with #ame ionization detection (GC-FID), combined gas chromatography}mass spectrometry (GC-MS), gas chromatography with Fourier transform infrared detection (GC-FTIR), in situ FT-IR spectroscopy and in situ atmospheric pressure ionization tandem mass spectrometry (API-MS/MS). Formaldehyde, 2-hydroxy-2-methylpropanal and acetone were identi"ed from both the OH radical and O reactions, glycolaldehyde and organic nitrate (s) were also observed from the OH radical reaction, and the OH radical formation yield from the O reaction was measured. The formaldehyde, 2-hydroxy-2-methyl propanal, glycolaldehyde, acetone and organic nitrate yields from the OH radical reaction were 0.29$0.03, 0.19$0.07, 0.61$0.09, 0.58$0.04 and 0.05$0.02, respectively, and the formaldehyde, 2-hydroxy-2-methylpropanal and OH radical formation yields from the O reaction were 0.29$0.03, 0.30$0.06 (0.47 from FT-IR measurements) and 0.19 (uncertain to a factor of 1.5), respectively. Acetone was also observed from the O reaction, but appeared to be formed from secondary reactions. Reaction mechanisms are presented and discussed. 1999 Elsevier Science Ltd. All rights reserved. Keywords: 2-Methyl-3-buten-2-ol; Hydroxyl radical; Ozone; Reaction products; Biogenic organic
1. Introduction Large quantities of volatile non-methane organic compounds (biogenic NMOC) are emitted into the atmosphere from vegetation (Guenther et al., 1995). The
* Corresponding author. Tel.: #1-909-787-4191; fax: #1909-787-5004. E-mail address: [email protected] (R. Atkinson) Present address: US Environmental Protection Agency, 1650 Arch St., Philadelphia, PA 19103, USA. Also at: Interdepartmental Graduate Program in Environmental Toxicology and Department of Environmental Sciences, University of California, Riverside, CA, USA.
Also at: Department of Chemistry, University of California, Riverside, CA, USA.
biogenic NMOC emitted from vegetation are isoprene, C H monoterpenes, C H sesquiterpenes, and a number of oxygenated compounds including methanol, 2-methyl-3-buten-2-ol [CH C(CH )(OH)CH"CH ], cis 3-hexen-1-ol [CH CH CH"CHCH CH OH], cis-3 hexenyl acetate [CH CH CH"CHCH CH OC(O)CH ], 6-methyl-5-hepten-2-one [(CH ) C"CHCH CH C(O)CH ] and linalool [(CH ) C"CHCH CH C(CH )(OH)CH" CH ] (Isidorov et al., 1985; Arey et al., 1991a}c, 1995; MacDonald and Fall, 1993; Goldan et al., 1993; Ciccioli et al., 1993; KoK nig et al., 1995; Kesselmeier et al., 1996). In the troposphere these biogenic NMOC can undergo photolysis and gas-phase reactions with OH radicals, NO radicals and O (Atkinson, 1994, 1997; Atkinson and Arey, 1998). The rate constants of these potential tropospheric transformation processes appear to be reasonably well understood for the majority of the NMOC
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 1 0 6 - 5
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identi"ed as biogenic emissions (Grosjean et al., 1993a, b, 1996; Grosjean and Grosjean, 1994, 1998; Atkinson, 1994, 1997; Rudich et al., 1995, 1996; Atkinson et al., 1995a, b; Hallquist et al., 1996; Fantechi et al., 1998a; Ferronato et al., 1998; Atkinson and Arey, 1998). However, the products and mechanisms of the reactions of OH radicals, NO radicals and O with many biogenic NMOC, in particular with monoterpenes, sesquiterpenes and oxygenates, are much less well understood (Atkinson, 1994; Atkinson and Arey, 1998), in part because of di$culties in identifying and quantifying multifunctional products such as hydroxycarbonyls which appear to comprise a large fraction of the reaction products (Yu et al., 1995; Kwok et al., 1995, 1996a; Shu et al., 1997; Aschmann et al., 1997a, 1998). In this work we have investigated the products of the gas-phase reactions of 2-methyl-3-buten-2-ol, a reactive oxygenated biogenic NMOC (Goldan et al., 1993), with OH radicals and O using a variety of analytical tech niques for product identi"cation and quanti"cation, including derivatization of hydroxy-containing products. In the troposphere, 2-methyl-3-buten-2-ol reacts with OH and NO radicals and O , with the OH radical and O reactions being the most important loss processes during daylight hours and nighttime, respectively (Atkinson and Arey, 1998).
2. Experimental Experiments were carried out in a 5800 l evacuable, Te#on-coated chamber containing a multiple-re#ection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FT-IR) spectrometer and with irradiation provided by a 24 kW xenon arc "ltered through a Pyrex pane to remove wavelengths (300 nm, and in two &7000}8000 l Te#on chambers with irradiation provided by two parallel banks of blacklamps. One of the Te#on chambers is interfaced to a PE SCIEX API III MS/MS direct air sampling atmospheric pressure ionization tandem mass spectrometer (API-MS). All three chambers are "tted with Te#on-coated fans to ensure rapid mixing of the reactants during their introduction into the chambers. All experiments were carried out at 298$2 K and 740 Torr total pressure of air. Hydroxyl radicals were generated by the photolysis of methyl nitrite (CH ONO) and ethyl nitrite (C H ONO) in air at wavelengths '300 nm (Atkinson et al., 1981): RCH ONO#hlPRCH O #NO,
(1)
RCH O #O PRCHO#HO ,
(2)
HO #NOPOH#NO ,
(3)
where R"H (methyl nitrite) or CH (ethyl nitrite). NO was added to the reactant mixtures to suppress the
formation of O and NO radicals (Atkinson et al., 1981). Because HCHO is the primary photolytic product of methyl nitrite (see above), the photolysis of ethyl nitrite in air was used as the OH radical source for the determination of HCHO formation yields in the 5800 l evacuable chamber (Tuazon and Atkinson, 1990). In the &7000}8000 l Te#on chambers with analyses by GCFID and API-MS, OH radicals were generated by the photolysis of CH ONO in air. 2.1. Evacuable chamber with analysis by FT-IR absorption spectroscopy The products of the reactions of 2-methyl-3-buten-2-ol with OH radicals and O were studied in the 5800 l evacuable chamber with analyses by in situ FT-IR absorption spectroscopy. For the OH radical reactions, two experiments were carried out, with initial concentrations of ethyl nitrite, NO and 2-methyl-3-buten-2-ol of 2.4;10 molecules cm\, (1.2}2.4);10 molecules cm\, and (3.9}4.7);10 molecules cm\, respectively. Intermittent irradiations of 1.0}2.0 min duration were carried out with analyses by FT-IR spectroscopy during the dark periods, and additional NO was added in 1.2;10 molecules cm\ aliquots (to give a total amount of NO added to the chamber of 3.6;10 molecules cm\) as needed to suppress the formation of O and hence NO radicals. Spectra were recorded with 64 co-added interferograms (scans) per spectrum (1.8 min measurement time), with a full-width at half-maximum (fwhm) resolution of 0.7 cm\ and a pathlength of 62.9 m. Calibrated IR spectra of 2-methyl-3-buten-2-ol were obtained by measuring known partial pressures of the gaseous compound into a 5.25 l Pyrex bulb with an MKS Baratron 0}100 Torr sensor, and #ushing the contents of the bulb into the 5800 l chamber with FT-IR analyses of the authentic compound. Reference IR spectra of certain of the products observed were available from previous IR calibrations in this laboratory (Tuazon and Atkinson, 1989), and the following absorption bands were used for the analyses of the reactant and products: 2-methyl-3-buten-2-ol, 943.8 cm\; formaldehyde, 1745.5 cm\; acetone, 1217.5 cm\; and glycolaldehyde, 1114.5 cm\. In the O reactions, two experiments were carried out in which cyclohexane was added at concentrations su$cient to scavenge *95% of the OH radicals formed from the O reaction with 2-methyl-3-buten-2-ol, and the initial 2-methyl-3-buten-2-ol and cyclohexane concentrations were (3.0}4.1);10 and 7.2;10 molecules cm\, respectively. Either one addition of O in O diluent (with an initial O concentration in the cham ber of 2.2;10 molecules cm\) or four O /O addi tions (with each addition corresponding to an O concentration in the chamber of (0.21}4.1);10 molecules cm\) were made to the chamber during an
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
experiment. IR spectra were taken during the experiments as described above. 2.2. Teyon chamber with analysis by GC-FID For the OH radical reaction experiments carried out in the 7900 l Te#on chamber, the initial reactant concentrations (in molecules cm\ units) were: CH ONO, &2.4;10; NO, &2.4;10; and 2 methyl-3-buten-2-ol, (2.3}2.7);10. Irradiations were carried out at 20% of the maximum light intensity for 2}10 min, resulting in up to 51% reaction of the initially present 2-methyl-3-buten-2-ol. Experiments were carried out to study the O reac tions in the absence and presence of cyclohexane as an OH radical scavenger, with the experiments in the presence of cyclohexane employing GC-FID analyses without derivatization. For the O experiments in the presence of cyclohexane (at su$cient concentrations to scavenge '95% of the OH radicals formed) and with analyses without derivatization, the initial 2-methyl-3buten-2-ol and cyclohexane concentrations were (3.7}3.8);10 molecules cm\ and (1.2}1.5);10 molecules cm\, respectively. For the O experiments carried out in the absence of cyclohexane and with GC analysis after derivatization, the initial 2-methyl-3-buten-2-ol concentrations were (2.1}3.1);10 molecules cm\. For both sets of experiments, 3 additions of 50 or 500 cm volume of O in O diluent (each addition corre sponding to an initial O concentration in the chamber of &6;10 or &6;10 molecules cm\) were made to the chamber during an experiment, with the 500 cm volume of O in O additions being used in the experi ments with the higher 2-methyl-3-buten-2-ol concentrations. The concentrations of 2-methyl-3-buten-2-ol and acetone were measured by GC-FID. Gas samples of 25 cm volume were collected from the chamber onto Tenax-TA solid absorbent, with subsequent thermal desorption at &2253C onto a 30 m DB-1701 megabore column in a Hewlett Packard (HP) 5710 GC held at !203C and then programmed to 2003C at 83C min\. For the experiments with product derivatization and GC analysis, 1,4-dichlorobenzene was added to the chamber to act as an internal standard to determine the volume sampled. 1,4-Dichlorobenzene is not expected to react with O (Atkinson and Carter, 1984) and reacts only slowly with the OH radical, with a room temperature rate constant of &4;10\ cm molecules\ s\ (Atkinson, 1989,1994). The 1,4-dichlorobenzene concentrations in the chamber (&1.2;10 molecules cm\) were measured by GC-FID with thermal desorption as described above. Gas samples of &3 l volume were collected from the chamber onto Tenax-TA solid absorbent, with subsequent elution with &0.5 ml diethyl ether inside a glove-bag "lled with dry nitrogen
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(to minimize reaction of the trimethylsilyl reagents with water vapor in competition with their reaction with the product species). The samples were then concentrated to &0.2 ml volume and sealed with a Mininert valve. Anisole was added to the extracts to allow the "nal volume of extract to be determined. Trimethylimidazole#N,Obis[trimethylsilyl]acetamide#trimethylchlorosilane (2 ll of a 3 : 3 : 2 mixture of these reagents) was added to the concentrated Tenax eluates, and the samples heated to 703C overnight. GC-FID analyses of the concentrated eluates were carried out after the derivatization, using the same DB-1701 megabore column used to analyze for 2-methyl-3-buten-2-ol (see above). GC-MS analyses were carried out using a 30 m;0.32 mm i.d. DB-1701 fused silica capillary column in a HP 5890 GC interfaced to a HP 5971A Mass Selective Detector operated in the scanning mode, and GC-FTIR analyses used a similar column in a HP 5890 GC interfaced to a HP 5965B FTIR detector. The concentration of the internal standard, 1,4dichlorobenzene, was measured before and after derivatization, while the concentrations of 2-methyl-3buten-2-ol were quanti"ed before derivatization. The hydroxy-containing product 2-hydroxy-2-methylpropanal [(CH ) C(OH)CHO] was measured after de rivatization as the trimethylsilyl derivative. The amount of 1,4-dichlorobenzene in the chamber, as measured by GC-FID analyses of 25}100 cm volume samples, allowed the volume of the &3 l samples collected on Tenax solid adsorbent with a pump to be calculated. The GC-FID response factor for the trimethylsilyl derivative of 2-hydroxy-2-methylpropanal was calculated relative to that for 2-methyl-3-buten-2-ol using the e!ective carbon number method (Scanlon and Willis, 1985). 2.3. Teyon chamber with analysis by API-MS For the reactions of OH radicals and O with 2 methyl-3-buten-2-ol with API-MS analyses, the chamber contents were sampled through a 25 mm diameter; 75 cm length Pyrex tube at a #ow rate of &20 l min\ 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 (Kwok et al., 1996a,b; Aschmann et al., 1997a,b). Use of the MS/MS mode with CAD allows the `daughter iona or `parent iona spectrum of a given ion peak observed in the MS scanning mode to be obtained (Kwok et al., 1996a,b; Aschmann et al., 1997a,b). The positive ion mode was used in all these API-MS and API-MS/MS analyses, with protonated water hydrates (H O>(H O) ) gener L ated by the corona discharge in the chamber diluent gas being responsible for the protonation of analytes. Ions are drawn by an electric potential from the ion source through the sampling ori"ce into the mass-analyzing "rst
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quadrupole or third quadrupole. For 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., 1997b). Neutral molecules and particles are prevented from entering the ori"ce by a #ow of high-purity nitrogen (`curtaina 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 homo- and heterodimers (Aschmann et al., 1997b). The initial concentrations of CH ONO, NO and 2 methyl-3-buten-2-ol in the OH radical reactions were &2.4;10 molecules cm\ each, and the reactant mixture was irradiated for 2 min at 20% of the maximum light intensity. For the O reactions, the initial concen tration of 2-methyl-3-buten-2-ol was 2.4;10 molecules cm\ and &1.2;10 molecules cm\ of O (in O diluent) was added to the chamber. 2.4. Chemicals The chemicals used, and their stated purities were: cyclohexane (high-purity solvent grade), American Burdick and Jackson; acetone (HPLC grade), Fisher Scienti"c; 2-methyl-3-buten-2-ol (99%), Aldrich Chemical Company; trimethylimidazole#N,O-bis[trimethylsilyl]acetamide#trimethylchlorosilane (3 : 3 : 2), Supelco Inc.; and NO (*99.0%), Matheson Gas Products. Methyl nitrite was prepared and stored as described previously (Taylor et al., 1980; Atkinson et al., 1981), and O in O diluent was generated as needed using a Welsbach T-408 ozone generator at pre-calibrated voltage and O #ow settings. 3. Results 3.1. Analyses by gas chromatography without derivatization GC-FID, GC-MS and GC-FTIR analyses of irradiated CH ONO} NO}2-methyl-3-buten-2-ol}air and of reacted O }2-methyl-3-buten-2-ol}cyclohexane (in ex cess)}air mixtures showed the formation of acetone from both the OH radical and O reactions. Cyclohexanone and cyclohexanol were also observed from the O reac tion, indicating that OH radicals are formed from the reaction of O with 2-methyl-3-buten-2-ol. Because OH radicals also react with the acetone product, for the OH radical reaction the measured concentrations of acetone were corrected to take into account secondary reactions with the OH radical as described previously (Atkinson et al., 1982). The multiplicative factors F to account for secondary reactions with the OH radical increase with the rate constant ratio
Fig. 1. Plots of the amounts (corrected for reaction with the OH radical) of acetone and 2-hydroxy-2-methylpropanal (analyzed as the trimethylsilyl derivative) against the amounts of 2methyl-3-buten-2-ol reacted with the OH radical in the presence of NO. Analyses by gas chromatography.
k(OH#product)/k(OH#reactant) and with the extent of reaction (Atkinson et al., 1982). The rate constants used for the reactions of the OH radical with 2-methyl3-buten-2-ol and acetone at 298 K were (in units of 10\ cm molecules\ s\) 64 (Rudich et al., 1995) and 0.219 (Atkinson, 1994), respectively, and the maximum value of the multiplicative factor F was (1.01 for acetone formation. As representative of the data obtained, the amounts of acetone formed (corrected for secondary reactions) are plotted against the amounts of 2-methyl-3-buten-2-ol reacted with the OH radical in the presence of NO in Fig. 1. The acetone formation yields from the OH radical and O reactions obtained by least-squares analysis of the data obtained are given in Table 1. The formation yields of cyclohexanone and cyclohexanol in the reaction of O with 2-methyl-3-buten-2-ol in the presence of ex cess cyclohexane were used to derive the yield of the OH radical from the reaction of O with 2-methyl-3-buten-2 ol, as described previously (Atkinson and Aschmann, 1993). Least-squares analysis of the amounts of cyclohexanone plus cyclohexanol formed against the amounts of 2-methyl-3-buten-2-ol reacted with O re sulted in a ratio of (cyclohexanone plus cyclohexanol formed)/(2-methyl-3-buten-2-ol reacted)"0.094$0.008, where the indicated error is the two least-squares standard deviation. Combining this ratio of (cyclohexanone plus cyclohexanol formed)/(2-methyl-3-buten-2-ol reacted) with a ratio of (cyclohexanone plus cyclohexanol formed)/(cyclohexane reacted with the OH
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
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Table 1 Products formed, and their formation yields, from the reactions of the OH radical (in the presence of NO) and O with 2-methyl-3 buten-2-ol Formation yield Product
GC
OH radical reaction Formaldehyde 2-Hydroxy-2-methylpropanal Glycolaldehyde
0.19$0.07
Acetone
0.59$0.06
FT-IR
Literature
Reference
0.29$0.03
0.35$0.04 0.093$0.033
Ferronato et al. (1998) Fantechi et al. (1998b)
0.61$0.09
0.50$0.05 0.280$0.028 0.52$0.05 0.141$0.002
Ferronato et al. (1998) Fantechi et al. (1998b) Ferronato et al. (1998) Fantechi et al. (1998b)
0.36$0.09 0.48, 0.57 0.30$0.02 0.23$0.06 0.125, 0.182
Grosjean and Grosjean (1995) Fantechi et al. (1998b) Grosjean and Grosjean (1995) Grosjean and Grosjean (1995) Fantechi et al. (1998b)
0.295, 0.50 0.44, 0.295
Fantechi et al. (1998b) Fantechi et al. (1998b)
0.57$0.05
Organic nitrate(s)
0.05$0.02
O reaction Formaldehyde
0.29$0.03
2-Hydroxy-2-methylpropanal Acetone OH radical HC(O)OH CO CO
0.30$0.06 0.42$0.04 0.15!0.48 0.19 > \
0.47 0.12$0.02
0.01!0.03
0.11$0.02 0.09$0.02
Errors are two least-squares standard deviations combined with the estimated overall uncertainties in the GC-FID or FT-IR calibration factors for 2-methyl-3-buten-2-ol and products of $5% each ($8% for the FT-IR analysis of glycolaldehyde), unless noted otherwise. O reactions were carried out in the presence of an OH radical scavenger unless noted otherwise. OH radicals generated by the photolysis of H O in the presence of low concentrations of NO (see text); not clear whether RO V radicals predominantly reacted with NO or not. Indicated errors are two least-squares standard deviations. See Table 2 for possible structures. In the presence of su$cient cyclohexane to scavenge &90% of the OH radicals formed. In the presence of su$cient methylcyclohexane to scavenge &90% of the OH radicals formed. In the absence of an OH radical scavenger. Referenced to spectra from the OH radical reactions where the formation yield of 2-hydroxy-2-methylpropanal is assumed to be identical to that for HCHO (see text). From two experiments carried out with initial 2-methyl-3-buten-2-ol and cyclohexane concentrations of (2.1}2.4);10 and 1.1;10 molecules cm\, respectively, and with three additions of aliquots of O in O . Plots of the amounts of acetone formed against the amount of 2-methyl-3-buten-2-ol reacted were curved, indicating that acetone is not a primary product. The range of yields shown is based on initial and "nal slopes of the experimental data.
Yield increased with the extent of O reaction.
radical)"0.50 (Atkinson and Aschmann, 1993) leads to the OH radical formation yield from the reaction of O with 2-methyl-3-buten-2-ol given in Table 1, with an estimated overall uncertainty of a factor of 1.5 (Atkinson and Aschmann, 1993).
3.2. Analysis by gas chromatography after derivatization For the reactions of 2-methyl-3-buten-2-ol with OH radicals (in the presence of NO) and O (in the absence
of an OH radical scavenger to avoid the formation of cyclohexanol, which would also potentially derivatize), treatment of the diethyl ether Tenax eluates with the trimethylsilyl reagent resulted in the conversion of 2methyl-3-buten-2-ol to a trimethylsilyl ether and to the formation of the trimethylsilyl ether derivative of one reaction product. GC-MS analyses of the derivatized diethyl ether eluates showed that the mass spectra of the trimethylsilyl ether derivatives of 2-methyl-3-buten-2-ol and its reaction product had high-mass ions at m/z 158 [M]> and 145 [(M-15)]>, respectively. Trimethylsilyl
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ethers analyzed by GC-MS give characteristic fragmentation patterns, with the molecular ion often not being evident and the highest-mass ion being the [M-CH ]> ion, which can be used to determine the molecular weight of the compound (Pierce, 1968). The trimethylsilyl derivative of the hydroxy-containing reaction product of 2methyl-3-buten-2-ol therefore has a molecular weight of 160. The mass spectra of both derivatized compounds also show the presence of ions at m/z 73 and 75, which correspond to the trimethylsilyl ether fragments (CH ) Si> and (CH ) Si"OH> (Pierce, 1968), respec tively. GC-FTIR spectra of the trimethylsilyl ether derivative formed from the OH radical and O reactions with 2-methyl-3-buten-2-ol were identical and exhibited IR absorption bands at 2700 and 2795 cm\ (attributed to an aldehydic C}H stretch), 1752 cm\ (attributed to a C"O stretch) and 847 cm\ ((CH ) Si}O}R stretch), thus indicating the presence of both an aldehydic }CHO group and a derivatized hydroxyl group. This reaction product from the 2-methyl-3-buten-2-ol reactions is therefore identi"ed as 2-hydroxy-2-methylpropanal [(CH ) C(OH)CHO]. The measured amounts of 2-hydroxy-2-methylpropanal formed from the reaction of the OH radical with 2-methyl-3-buten-2-ol were corrected for secondary reactions with the OH radical, using a calculated rate constant for the reaction of the OH radical with 2hydroxy-2-methylpropanal of 2.20;10\ cm molecules\ s\ (Kwok and Atkinson, 1995). As an example of the data obtained, Fig. 1 shows a plot of the amounts of 2-hydroxy-2-methylpropanal formed, corrected for reaction with the OH radical, against the amounts of 2methyl-3-buten-2-ol reacted with the OH radical in the presence of NO. The formation yields of 2-hydroxy-2methylpropanal from the OH radical and O reactions, obtained by least-squares analysis of the data, are given in Table 1. 3.3. Analyses by in situ FT-IR spectroscopy Selected products from the gas-phase reactions of 2methyl-3-buten-2-ol with the OH radical in the presence of NO and with O in the presence of su$cient cyclo hexane to scavenge '95% of the OH radicals formed were identi"ed and quanti"ed by in situ FT-IR absorption spectroscopy. The products observed and quanti"ed by spectral desynthesis were HCHO, glycolaldehyde [HOCH CHO], and acetone from the OH radical-in itiated reaction; and HCHO, acetone, HC(O)OH, CO, and CO from the O reaction. The formation yields of these products are given in Table 1, where the measured concentrations of HCHO, HOCH CHO and CH C(O)CH have again been corrected for secondary reactions using the respective OH radical reaction rate constants (in units of 10\ cm molecules\ s\) of 9.37
Fig. 2. Plots of the amounts of formaldehyde, acetone and glycolaldehyde (all corrected for reaction with the OH radical) against the amounts of 2-methyl-3-buten-2-ol reacted with the OH radical in the presence of NO. Combined data from two experiments; analyses by FT-IR spectroscopy.
(Atkinson, 1994), 9.9 (Niki et al., 1987a; Atkinson, 1989) and 0.219 (Atkinson, 1994), respectively. The corrections for secondary reactions were )5% for HCHO and HOCH CHO, and were negligible for acetone. Plots of the amounts of HCHO, HOCH CHO and CH C(O)CH formed, corrected for secondary reactions, against the amounts of 2-methyl-3-buten-2-ol reacted with the OH radical are shown in Fig. 2. We have previously shown that any contribution to HCHO formation from secondary reactions of CH CHO gener ated from the photolysis of ethyl nitrite is minor ((4% formation yield) in irradiated C H ONO}NO}1-al kene}air mixtures (Atkinson et al., 1995b), and the formation of HCHO from ethyl nitrite chemistry in the present experiments would be even less because of the higher reactivity of 2-methyl-3-buten-2-ol. Fig. 3A is the residual spectrum, obtained after subtraction of absorptions by the remaining reactants and known products, at the end of one of the C H ONO} NO}2-methyl-3-buten-2-ol}air irradiations which shows distinct R}ONO }type absorption bands at 848, 1283, and 1664 cm\. The amount of organic nitrate(s) formed was estimated from the integrated area of the well-isolated 1283 cm\ band (1260}1305 cm\ range), using the average absorption coe$cient of 1.2;10\ cm molecules\ for similar compounds (Tuazon and Atkinson, 1990), and the calculated RONO formation yield is included in Table 1 (distinct features due to ethyl nitrate formed from the photooxidation of ethyl nitrite allowed
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
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2-ol with O (see Tuazon et al., 1997 and references therein).
3.4. Analyses by API-MS
Fig. 3. (A) Residual product spectrum from an irradiated ethyl nitrite}NO}2-methyl-3-buten-2-ol}air mixture, showing the RONO absorptions at 848, 1283 and 1664 cm\. (B) Residual product spectrum from an O #2-methyl-3-buten-2-ol reaction. (C) Di!erence spectrum [(A) minus (B)] where absorptions assigned to 2-hydroxy-2-methylpropanal (*) have been canceled (see text). Gaps in the spectra correspond to regions of very strong absorption by NO formed (in A) and added cyclohexane (in B).
the contribution of ethyl nitrate to be taken into account and hence the organic nitrate(s) yield given in Table 1 is for organic nitrates other than ethyl nitrate). Also evident in Fig. 3A are absorption bands, marked by asterisks, which are believed to be due to 2-hydroxy2-methylpropanal and which are also present in the residual spectrum from the reaction of 2-methyl-3-buten2-ol with O (Fig. 3B). Fig. 3C was obtained by subtract ing Fig. 3B from Fig. 3A, with cancellation of absorptions attributed to 2-hydroxy-2-methylpropanal. By assuming that the formation yield of 2-hydroxy-2-methylpropanal from the OH radical reaction with 2-methyl-3-buten-2-ol is equal to that of the HCHO yield (see discussion) and taking into account secondary reactions of HCHO and 2-hydroxy-2-methylpropanal as discussed above, a quantitative IR spectrum of 2-hydroxy-2-methylpropanal was obtained and used to determine the formation yield of 2-hydroxy-2-methylpropanal in the reaction of O with 2-methyl-3-buten-2-ol (see Table 1). Fig. 3C also shows that another minor carbonyl product (C"O stretch band at 1733 cm\) is formed during the C H ONO}NO}2-methyl-3-buten-2-ol}air irradiation, while the 1095 cm\ band and other adjoining (negative) absorptions possibly indicate the existence of an ozonide during the reaction of 2-methyl-3-buten-
Experiments were carried out using API-MS for analysis of the products formed from the reactions of 2methyl-3-buten-2-ol with the OH radical in the presence of NO and with O in the absence of an OH radical scavenger. API-MS/MS `daughter iona and `parent iona spectra were obtained for ion peaks observed in the API-MS analyses. Product ion peaks were identi"ed based on the observation of homo- or hetero-dimer (and, in certain cases, trimer) ions (for example, [(M ) #H]>, [(M ) #H]> and [M #M #H]>, . . . . where P1 and P2 are products) in the API-MS/MS `parent iona spectra, and consistency of the API-MS/MS `daughter iona spectrum of a homo- or hetero-dimer ion with the `parent iona spectra of the various [M #H]> ion peaks. Water cluster ion peaks of the . product ions, [M#H#H O]>, were also occasionally observed. The products observed from the OH radical-initiated reaction are listed in Table 2 and the evidence for the formation of these products, in the form of API-MS and API-MS/MS data of molecular ions, dominant fragment ions, and the presence of homo- and heterodimers formed in the API-MS under the experimental conditions employed, is also summarized in Table 2. In agreement with the GC analyses and the in situ FT-IR spectroscopic analyses, acetone, glycolaldehyde and 2-hydroxy-2methylpropanal were observed. In addition, an organic nitrate of molecular weight 165 was observed. The APIMS/MS `daughter iona spectrum of the [M#H]> ion of this product is shown in Fig. 4, with the presence of an intense NO> fragment at 46 u being evident. This nitrate is attributed to (CH ) C(OH)CH(ONO )CH OH and/ or (CH ) C(OH)CH(OH)CH ONO formed from the reactions of the (CH ) C(OH)CH(OO )CH OH and (CH ) C(OH)CH(OH)CH OO radicals with NO (Atkin son, 1994, 1997). For the O reaction, less-de"nitive data were obtained from the API-MS analyses, although API-MS and APIMS/MS analyses indicated that acetone and 2-hydroxy2-methylpropanal were formed as products, in agreement with our GC analyses and in situ FT-IR spectroscopic analyses (Table 1).
4. Discussion 4.1. OH radical reaction The reaction of the OH radical with 2-methyl-3-buten2-ol is expected to proceed almost entirely by initial
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Table 2 Products formed from the OH radical-initiated reaction of 2-methyl-3-buten-2-ol in the presence of NO, and experimental observations from in situ API-MS and API-MS/MS analyses Product
API-MS data
Other evidence
Acetone M (MW 58)
Acetone observed by GC-FID and FT-IR analyses. MS/MS of 119 u ion peak
Glycolaldehyde M (MW 60)
M #H"59 M #M #H"117 M #M #H"119 M #M #H"145 + M #M #H"119
2-Hydroxy-2-methylpropanal M (MW 88)
M #M #H"254
(CH ) C(OH)CH(ONO )CH OH and/or (CH ) C(OH)CH(OH)CH ONO M (MW 165)
M #H"166 M #M #H"254 M #M #H#H O"270 + M #M #H"331
Glycolaldehyde observed by FT-IR analysis. MS/MS of 119 u ion peak Identi"ed by GC-MS after derivatization (see text). MS/MS of 254 u ion peak MS/MS of 166 u ion peak (Fig. 4). MS/MS of 254, 270 and 331 u ion peaks. Parents of 166 u at 224 [M #M #H], 252 [M #M #H], + 254, 270, 288 [M #M #2H O#H], 310 + [M #M #M #H], 331 and 338 + [M #M #M #H] u. + + -
M "2-methyl-3-butene-2-ol, molecular weight"86. + -
Fig. 4. API-MS/MS CAD spectrum of the 166 u ion peak, attributed to the [M#H]> ion of (CH ) C(OH)CH(OH)CH ONO and/or (CH ) C(OH)CH(ONO )CH OH, observed in the API-MS spectra of an irradiated CH ONO}NO}2-methyl-3-buten-2-ol}air mix ture. The 46 u fragment peak is attributed to NO>.
addition of the OH radical to the carbon atoms of the 'C"C( bond (Atkinson, 1989): OH#(CH ) C(OH)CH"CH P(CH ) C(OH)C HCH OH and (CH ) C(OH)CH(OH)C H .
(4)
These b-hydroxyalkyl radicals add O to form the corre sponding b-hydroxyalkyl peroxy radicals, which in the presence of NO react to form either the organic nitrate or NO plus the corresponding b-hydroxyalkoxy radical. For example, for the (CH ) C(OH)C HCH OH radical, (CH ) C(OH)C HCH OH#O P (CH ) C(OH)CH(OO )CH OH, (5)
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
(CH ) C(OH)CH(OO )CH OH#NO P(CH ) C(OH)CH(ONO )CH OH, * (CH ) C(OH)CH(O )CH OH#NO . P
(6a) (6b)
The resulting b-hydroxyalkoxy radicals (CH ) C(OH) CH(O )CH OH and (CH ) C(OH)CH(OH)CH O can decompose by C}C bond scission, react with O , or (for the (CH ) C(OH)CH(OH)CH O radical) isomerize via a six-membered transition state (Atkinson, 1997), as shown in Reaction Scheme 2 respectively. The nitrates observed by our API-MS and APIMS/MS analyses (Table 2), and assumed to be the organic nitrates quanti"ed by FT-IR spectroscopy (Table 1), are then (CH ) C(OH)CH(ONO )CH OH formed as shown in reactions (4)}(6a), and/or (CH ) C(OH) CH(OH)CH ONO formed from the second b-hydroxy alkyl peroxy radical shown in reaction (4) by reactions analogous to reactions (5) and (6a). The product data shown in Table 1 further show that the b-hydroxyalkoxy radicals predominantly decompose to form CH C(O)CH #HOCH CHO and (CH ) C(OH)CHO#HCHO. The formation yields of acetone as measured by GC-FID and FT-IR analyses are in excellent agreement, with an average acetone yield of 0.58$0.04, and the acetone yield is in excellent agreement with the measured formation yield of glycolaldehyde (0.61$0.09), as expected from Reaction Scheme 1 because these two products are co-products of the same decomposition pathway. However, while Reaction Schemes 1 and 2 predict that HCHO and (CH ) C(OH)CHO should be co-prod ucts, our measured formation yield of (CH ) C(OH)CHO is &35% lower than the yield of HCHO measured by in situ FT-IR spectroscopy (though in agreement within the combined uncertainties). This lower yield of 2-hydroxy-2-methylpropanal than expected suggests that the derivatization/GC-FID analysis procedure for hydroxy-containing compounds is (100% e$cient. Our data indicate that the OH radical adds predominantly to the terminal carbon atom and that the resulting (CH ) C(OH)CH(O )CH OH alkoxy radical decomposes preferentially to form acetone plus glycolaldehyde. The predicted rates of the O reaction and the two decomposition channels (Atkinson, 1997) for the (CH ) C(OH)CH(O )CH OH radical at 298 K and atmospheric pressure of air are 4.4;10 s\ for reaction with O , 4.4;10 s\ for decomposition to (CH ) C(OH)CHO#C H OH, and 2.0;10 s\ for de composition to HOCH CHO#(CH ) C OH. These predicted reaction rates agree with our observations and with our conclusion that the dominant decomposition channel of the (CH ) C(OH)CH(O )CH OH radical is to form glycolaldehyde plus acetone. Terminal OH radical addition therefore accounts for *60% of the overall OH radical reaction. The predicted rate of
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reaction with O and of decomposition of the (CH ) C(OH)CH(OH)CH O radical at 298 K and atmo spheric pressure of air are 4.9;10 and 3.2;10 s\, respectively (Atkinson, 1997), again consistent with our product data. The products observed and quanti"ed account for 92$6% of the overall products. The products of the gas-phase reaction of 2-methyl-3buten-2-ol with the OH radical have also been investigated by Ferronato et al. (1998) and Fantechi et al. (1998b). Ferronato et al. (1998) generated OH radicals by the photolysis of C-labeled CH ONO}NO}air mix tures and used in situ FT-IR spectroscopy for analysis, with the use of C-labeled CH ONO allowing the formation of HCHO from the reaction of OH radicals with 2-methyl-3-buten-2-ol to be measured. As shown in Table 1, our measured formation yields of HCHO, acetone and glycolaldehyde are in agreement, within the combined experimental uncertainties, with the formation yields reported by Ferronato et al. (1998). Ferronato et al. (1998) did not report data for 2-hydroxy-2-methylpropanal nor for organic nitrates, noting that no spectral features attributable to 2-hydroxy-2-methylpropanal could be identi"ed. In the study of Fantechi et al. (1998b), OH radicals were generated by the photolysis of H O in air in the presence of much lower concentrations of NO (the V initial H O , 2-methyl-3-buten-2-ol and NO concen V trations were (3.6}7.2);10 molecules cm\, (1.9}2.7); 10 molecules cm\, and (2.4}24);10 molecules cm\, respectively). Hence it is not clear whether organic peroxy (RO ) radicals reacted predominantly with NO in the experiments of Fantechi et al. (1998b) or whether RO #RO and/or RO #HO reactions pre dominated. The formation yields of HCHO, acetone and glycolaldehyde measured by Fantechi et al. (1998b) using in situ FT-IR spectroscopy are given in Table 1, and are seen to be signi"cantly lower, by factors of &2}4, than the formation yields measured in this work and by Ferronato et al. (1998). Using 2,4-dinitrophenyl hydrazine (DNPH) to derivatize carbonyl compounds, Fantechi et al. (1998b) also tentatively identi"ed 2-hydroxy-2methylpropanal as a product of the OH radical reaction. 4.2. O3 reaction Based on the understanding of O reactions with al kenes, O will initially add to the 'C"C( bond in 2-methyl-3-buten-2-ol to form a primary ozonide, which then rapidly decomposes to ([C H OO ]*# (CH ) C(OH)CHO) and (HCHO#[(CH ) C(OH) C HOO ]H) (Atkinson, 1994,1997), where [ ]H denotes an energy-rich biradical. The sum of the formation yields of the `primarya carbonyls (CH ) C(OH)CHO and HCHO should therefore be unity (Atkinson, 1997; Grosjean and Grosjean, 1997). Using the derivatization/GC-FID procedure to quantify 2-hydroxy-2-methylpropanal
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A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
Reaction Scheme 1.
Reaction Scheme 2.
(Table 1), the sum of the HCHO plus (CH ) C(OH)CHO formation yields of 0.59 from our present work is again consistent with the less than 100% e$ciency of the derivatization/GC-FID analysis procedure for 2-hydroxy-2-methylpropanal. Quanti"cation of 2-hydroxy-2methylpropanal by FT-IR spectroscopy, based on the expectation that the formation yields of HCHO and 2-hydroxy-2-methylpropanal from the OH radical reaction with 2-methyl-3-buten-2-ol are identical (Reaction Schemes 1 and 2 and discussion above), results in a sum of the `primarya carbonyls of 0.76. From the assumption that the 2-hydroxy-2-methylpropanal formation yield is equal to that of HCHO in the OH radical reaction and the ensuing estimate of the 2-hydroxy-2-methylpropanal formation yield in the O reaction, it may be deduced from the data in Table 1 that the derivatization method applied to 2-hydroxy-2-methylpropanal has an overall e$ciency of &65%.
The products of the gas-phase reaction of 2-methyl-3buten-2-ol with O , in the presence of an OH radical scavenger, have also been investigated by Grosjean and Grosjean (1995) and Fantechi et al. (1998b). Our measured HCHO and 2-hydroxy-2-methylpropanal yields are in general agreement with those reported by Grosjean and Grosjean (1995) (Table 1), but our HCHO yield is up to a factor of 2 lower than the two values reported by Fantechi et al. (1998b) using cyclohexane and methylcyclohexane as OH radical scavengers (Table 1). Fantechi et al. (1998b) also tentatively identi"ed 2-hydroxy-2-methylpropanal as a product of the O reaction, using DNPH as a derivatization agent. It should be noted that in the study of Grosjean and Grosjean (1995), the identi"cation of 2-hydroxy-2-methylpropanal was stated to be tentative and its quanti"cation was subject to signi"cant uncertainties because of a lack of an authentic standard for response factor calibration.
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
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We assume that the initial products of the decomposition of the primary ozonide are ([C H OO ]H#(CH ) C(OH)CHO) and (HCHO#[(CH ) C(OH)C HOO ]H), with the biradicals undergoing collisional stabilization in competition with a number of decomposition and/or rearrangement pathways (Atkinson, 1997). Our HCHO and 2-hydroxy-2-methylpropanal formation yields (Table 1) indicate that the primary ozonide decomposes to ([C H OO ]*#(CH ) C(OH)CHO) &50% of the time and to (HCHO#[(CH ) C(OH)C HOO ]H) &30% of the time. However, our quanti"cation of 2-hydroxy-2methylpropanal is subject to signi"cant uncertainties (as noted above, its analysis by derivatization/GC-FID results in a lower limit and analysis by in situ FT-IR spectroscopy involves a number of assumptions and uncertainties), and we suggest that the initial products of the decomposition of the primary ozonide are ([C H OO ]H#(CH ) C(OH)CHO) in &70% yield and (HCHO#[(CH ) C(OH)C HOO ]H) in &30% yield. The likely reactions of the [C H OO ]H biradical have been discussed previously, and include stabilization, decomposition to CO#H O and decomposition to OH#HC O (Atkinson, 1997). A major uncertainty concerns the reactions of the initially energy-rich [(CH ) C(OH)C HOO ]H biradical. This biradical cannot undergo isomerization to form the energy-rich hydroperoxide which then decomposes to form the OH radical (Niki et al., 1987b), and the nonavailability of this reaction channel is reasonably consistent with the low OH radical formation yield of &0.19 (which can be partially explained by OH radical formation from the [C H OO ]H biradical). Grosjean and Gros jean (1995) suggested that this biradical could decompose to ultimately form acetone:
[(CH ) C(OH)C HOO ]H biradical [for example, with water vapor, carbonyls, and 2-methyl-3-buten-2-ol (Neeb et al., 1996a, 1996b, 1997)].
[(CH ) C(OH)C HOO ]HPCO #H#(CH ) C OH `O CH C(O)CH #HO . (7) An alternative decomposition pathway could also lead to the formation of acetone:
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[(CH ) C(OH)C HOO ]HP[(CH ) C(OH)CH(OQ )OQ ]H P[(CH ) C(OQ )CH(O)OH]H CH C(O)CH #HC(O)OH. (8) Our FT-IR data in Table 1 do show similar acetone and CO yields, although there are other routes to CO formation in addition to reaction (7). The acetone formation yields measured in di!erent sets of experiments (Table 1) show much scatter, with one set of GC-FID analyses showing an acetone yield which increased with the extent of reaction, from an initial value of (0.15 to &0.48 at larger extents of reaction. The various acetone yields observed may then re#ect competition between various reaction pathways of the
4.3. Atmospheric implications As noted earlier, in the troposphere 2-methyl-3-buten2-ol reacts with OH and NO radicals and O , with the OH radical reaction being calculated to be the dominant loss process and with a lifetime of a few hours during daytime (Atkinson and Arey, 1998). We have accounted for the majority (92$6%) of the pathways involved in the OH radical reaction in the presence of NO, with acetone plus glycolaldehyde accounting for &60% of the reaction products. Hence emission of 2-methyl-3buten-2-ol from vegetation will lead to an in situ tropospheric source of acetone.
Acknowledgements The authors gratefully thank the National Science Foundation for supporting this research through Grant No. ATM-9414036, and thank the National Science Foundation (Grant No. ATM-9025361) and the University of California, Riverside, for funds for the purchase of the PE SCIEX API III MS/MS instrument, and thanks Drs. J.J. Orlando and G.S. Tyndall for communicating their results prior to publication. RA and JA thank the Agricultural Experiment Station, University of California, for partial salary support.
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A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905 Kwok, E.S.C., Atkinson, R., Arey, J., 1995. Observation of hydroxycarbonyls from the OH radical-initiated reaction of isoprene. Environmental Science and Technology 29, 2467}2469. Kwok, E.S.C., Aschmann, S.M., Arey, J., Atkinson, R., 1996a. Product formation from the reaction of the NO radical with isoprene and rate constants for the reactions of methacrolein and methyl vinyl ketone with the NO radical. International Journal of Chemical Kinetics 28, 925}934. Kwok, E.S.C., Arey, J., Atkinson, R., 1996b. Alkoxy radical isomerization in the OH radical-initiated reactions of C }C n-alkanes. Journal of Physical Chemistry 100, 214}219. MacDonald, R.C., Fall, R., 1993. Detection of substantial emissions of methanol from plants to the atmosphere. Atmospheric Environment 27A, 1709}1713. Neeb, P., Horie, O., Moortgat, G.K., 1996a. Gas-phase ozonolysis of ethene in the presence of hydroxylic compounds. International Journal of Chemical Kinetics 28, 721}730. Neeb, P., Horie, O., Moortgat, G.K., 1996b. Formation of secondary ozonides in the gas-phase ozonolysis of simple alkenes. Tetrahedron Letters 37, 9297}9300. Neeb, P., Sauer, F., Horie, O., Moortgat, G.K., 1997. Formation of hydroxymethyl hydroperoxide and formic acid in alkene ozonolysis in the presence of water vapour. Atmospheric Environment 31, 1417}1423. Niki, H., Maker, P.D., Savage, C.M., Hurley, M.D., 1987a. Fourier transform infrared study of the kinetics and mechanisms for the Cl-atom- and HO-radical-initiated oxidation of glycolaldehyde. Journal of Physical Chemistry 91, 2174}2178. Niki, H., Maker, P.D., Savage, C.M., Breitenbach, L.P., Hurley, M.D., 1987b. FTIR spectroscopic study of the mechanism for the gas-phase reaction between ozone and tetramethylethylene. Journal of Physical Chemistry 91, 941}946. Pierce, A.E., 1968. Silyation of Organic Compounds. Pierce Chemical Co., Rockford, IL, pp. 1}159.
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Rudich, Y., Talukdar, R., Burkholder, J.B., Ravishankara, A.R., 1995. Reaction of methylbutenol with hydroxyl radical: mechanism and atmospheric implications. Journal of Physical Chemistry 99, 12 188}12 194. Rudich, Y., Talukdar, R.K., Fox, R.W., Ravishankara, A.R., 1996. Rate coe$cients for reactions of NO with a few ole"ns and oxygenated ole"ns. Journal of Physical Chemistry 100, 5374}5381. Scanlon, J.T., Willis, D.E., 1985. Calculation of #ame ionization detector relative response factors using the E!ective Carbon Number concept. Journal of Chromatographic Science 23, 333}340. Shu, Y., Kwok, E.S.C., Tuazon, E.C., Atkinson, R., Arey, J., 1997. Products of the gas-phase reactions of linalool with OH radicals, NO radicals, and O . Environmental Science and Technology 31, 896}904. Taylor, W.D., Allston, T.D., Moscato, M.J., Fazakas, G.B., Kozlowski, R., Takacs, G.A., 1980. Atmospheric photodissociation lifetimes for nitromethane, methyl nitrite, and methyl nitrate. International Journal of Chemical Kinetics 12, 231}240. Tuazon, E.C., Atkinson, R., 1989. A product study of the gasphase reaction of methyl vinyl ketone with the OH radical in the presence of NO . International Journal of Chemical V Kinetics 21, 1141}1152. Tuazon, E.C., Atkinson, R., 1990. A product study of the gas-phase reaction of isoprene with the OH radical in the presence of NO . International Journal of Chemical V Kinetics 22, 1221}1236. Tuazon, E.C., Aschmann, S.M., Arey, J., Atkinson, R., 1997. Products of the gas-phase reactions of O with a series of methyl-substituted ethenes. Environmental Science and Technology 31, 3004}3009. Yu, J., Je!ries, H.E., Le Lacheur, R.M., 1995. Identifying airborne carbonyl compounds in isoprene atmospheric photooxidation products by their PFBHA oximes using gas chromatography/ion trap mass spectrometry. Environmental Science and Technology 29, 1923}1932.
Products and mechanisms of the gas-phase reactions of OH radicals and O with 2-methyl-3-buten-2-ol Alvaro Alvarado, Ernesto C. Tuazon, Sara M. Aschmann, Janet Arey, Roger Atkinson* Air Pollution Research Center, University of California, Riverside, CA 92521, USA Received 24 September 1998; accepted 22 January 1999
Abstract Products of the gas-phase reactions of OH radicals (in the presence of NO) and O with the biogenic organic compound 2-methyl-3-buten-2-ol have been investigated using gas chromatography with #ame ionization detection (GC-FID), combined gas chromatography}mass spectrometry (GC-MS), gas chromatography with Fourier transform infrared detection (GC-FTIR), in situ FT-IR spectroscopy and in situ atmospheric pressure ionization tandem mass spectrometry (API-MS/MS). Formaldehyde, 2-hydroxy-2-methylpropanal and acetone were identi"ed from both the OH radical and O reactions, glycolaldehyde and organic nitrate (s) were also observed from the OH radical reaction, and the OH radical formation yield from the O reaction was measured. The formaldehyde, 2-hydroxy-2-methyl propanal, glycolaldehyde, acetone and organic nitrate yields from the OH radical reaction were 0.29$0.03, 0.19$0.07, 0.61$0.09, 0.58$0.04 and 0.05$0.02, respectively, and the formaldehyde, 2-hydroxy-2-methylpropanal and OH radical formation yields from the O reaction were 0.29$0.03, 0.30$0.06 (0.47 from FT-IR measurements) and 0.19 (uncertain to a factor of 1.5), respectively. Acetone was also observed from the O reaction, but appeared to be formed from secondary reactions. Reaction mechanisms are presented and discussed. 1999 Elsevier Science Ltd. All rights reserved. Keywords: 2-Methyl-3-buten-2-ol; Hydroxyl radical; Ozone; Reaction products; Biogenic organic
1. Introduction Large quantities of volatile non-methane organic compounds (biogenic NMOC) are emitted into the atmosphere from vegetation (Guenther et al., 1995). The
* Corresponding author. Tel.: #1-909-787-4191; fax: #1909-787-5004. E-mail address: [email protected] (R. Atkinson) Present address: US Environmental Protection Agency, 1650 Arch St., Philadelphia, PA 19103, USA. Also at: Interdepartmental Graduate Program in Environmental Toxicology and Department of Environmental Sciences, University of California, Riverside, CA, USA.
Also at: Department of Chemistry, University of California, Riverside, CA, USA.
biogenic NMOC emitted from vegetation are isoprene, C H monoterpenes, C H sesquiterpenes, and a number of oxygenated compounds including methanol, 2-methyl-3-buten-2-ol [CH C(CH )(OH)CH"CH ], cis 3-hexen-1-ol [CH CH CH"CHCH CH OH], cis-3 hexenyl acetate [CH CH CH"CHCH CH OC(O)CH ], 6-methyl-5-hepten-2-one [(CH ) C"CHCH CH C(O)CH ] and linalool [(CH ) C"CHCH CH C(CH )(OH)CH" CH ] (Isidorov et al., 1985; Arey et al., 1991a}c, 1995; MacDonald and Fall, 1993; Goldan et al., 1993; Ciccioli et al., 1993; KoK nig et al., 1995; Kesselmeier et al., 1996). In the troposphere these biogenic NMOC can undergo photolysis and gas-phase reactions with OH radicals, NO radicals and O (Atkinson, 1994, 1997; Atkinson and Arey, 1998). The rate constants of these potential tropospheric transformation processes appear to be reasonably well understood for the majority of the NMOC
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 1 0 6 - 5
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identi"ed as biogenic emissions (Grosjean et al., 1993a, b, 1996; Grosjean and Grosjean, 1994, 1998; Atkinson, 1994, 1997; Rudich et al., 1995, 1996; Atkinson et al., 1995a, b; Hallquist et al., 1996; Fantechi et al., 1998a; Ferronato et al., 1998; Atkinson and Arey, 1998). However, the products and mechanisms of the reactions of OH radicals, NO radicals and O with many biogenic NMOC, in particular with monoterpenes, sesquiterpenes and oxygenates, are much less well understood (Atkinson, 1994; Atkinson and Arey, 1998), in part because of di$culties in identifying and quantifying multifunctional products such as hydroxycarbonyls which appear to comprise a large fraction of the reaction products (Yu et al., 1995; Kwok et al., 1995, 1996a; Shu et al., 1997; Aschmann et al., 1997a, 1998). In this work we have investigated the products of the gas-phase reactions of 2-methyl-3-buten-2-ol, a reactive oxygenated biogenic NMOC (Goldan et al., 1993), with OH radicals and O using a variety of analytical tech niques for product identi"cation and quanti"cation, including derivatization of hydroxy-containing products. In the troposphere, 2-methyl-3-buten-2-ol reacts with OH and NO radicals and O , with the OH radical and O reactions being the most important loss processes during daylight hours and nighttime, respectively (Atkinson and Arey, 1998).
2. Experimental Experiments were carried out in a 5800 l evacuable, Te#on-coated chamber containing a multiple-re#ection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FT-IR) spectrometer and with irradiation provided by a 24 kW xenon arc "ltered through a Pyrex pane to remove wavelengths (300 nm, and in two &7000}8000 l Te#on chambers with irradiation provided by two parallel banks of blacklamps. One of the Te#on chambers is interfaced to a PE SCIEX API III MS/MS direct air sampling atmospheric pressure ionization tandem mass spectrometer (API-MS). All three chambers are "tted with Te#on-coated fans to ensure rapid mixing of the reactants during their introduction into the chambers. All experiments were carried out at 298$2 K and 740 Torr total pressure of air. Hydroxyl radicals were generated by the photolysis of methyl nitrite (CH ONO) and ethyl nitrite (C H ONO) in air at wavelengths '300 nm (Atkinson et al., 1981): RCH ONO#hlPRCH O #NO,
(1)
RCH O #O PRCHO#HO ,
(2)
HO #NOPOH#NO ,
(3)
where R"H (methyl nitrite) or CH (ethyl nitrite). NO was added to the reactant mixtures to suppress the
formation of O and NO radicals (Atkinson et al., 1981). Because HCHO is the primary photolytic product of methyl nitrite (see above), the photolysis of ethyl nitrite in air was used as the OH radical source for the determination of HCHO formation yields in the 5800 l evacuable chamber (Tuazon and Atkinson, 1990). In the &7000}8000 l Te#on chambers with analyses by GCFID and API-MS, OH radicals were generated by the photolysis of CH ONO in air. 2.1. Evacuable chamber with analysis by FT-IR absorption spectroscopy The products of the reactions of 2-methyl-3-buten-2-ol with OH radicals and O were studied in the 5800 l evacuable chamber with analyses by in situ FT-IR absorption spectroscopy. For the OH radical reactions, two experiments were carried out, with initial concentrations of ethyl nitrite, NO and 2-methyl-3-buten-2-ol of 2.4;10 molecules cm\, (1.2}2.4);10 molecules cm\, and (3.9}4.7);10 molecules cm\, respectively. Intermittent irradiations of 1.0}2.0 min duration were carried out with analyses by FT-IR spectroscopy during the dark periods, and additional NO was added in 1.2;10 molecules cm\ aliquots (to give a total amount of NO added to the chamber of 3.6;10 molecules cm\) as needed to suppress the formation of O and hence NO radicals. Spectra were recorded with 64 co-added interferograms (scans) per spectrum (1.8 min measurement time), with a full-width at half-maximum (fwhm) resolution of 0.7 cm\ and a pathlength of 62.9 m. Calibrated IR spectra of 2-methyl-3-buten-2-ol were obtained by measuring known partial pressures of the gaseous compound into a 5.25 l Pyrex bulb with an MKS Baratron 0}100 Torr sensor, and #ushing the contents of the bulb into the 5800 l chamber with FT-IR analyses of the authentic compound. Reference IR spectra of certain of the products observed were available from previous IR calibrations in this laboratory (Tuazon and Atkinson, 1989), and the following absorption bands were used for the analyses of the reactant and products: 2-methyl-3-buten-2-ol, 943.8 cm\; formaldehyde, 1745.5 cm\; acetone, 1217.5 cm\; and glycolaldehyde, 1114.5 cm\. In the O reactions, two experiments were carried out in which cyclohexane was added at concentrations su$cient to scavenge *95% of the OH radicals formed from the O reaction with 2-methyl-3-buten-2-ol, and the initial 2-methyl-3-buten-2-ol and cyclohexane concentrations were (3.0}4.1);10 and 7.2;10 molecules cm\, respectively. Either one addition of O in O diluent (with an initial O concentration in the cham ber of 2.2;10 molecules cm\) or four O /O addi tions (with each addition corresponding to an O concentration in the chamber of (0.21}4.1);10 molecules cm\) were made to the chamber during an
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
experiment. IR spectra were taken during the experiments as described above. 2.2. Teyon chamber with analysis by GC-FID For the OH radical reaction experiments carried out in the 7900 l Te#on chamber, the initial reactant concentrations (in molecules cm\ units) were: CH ONO, &2.4;10; NO, &2.4;10; and 2 methyl-3-buten-2-ol, (2.3}2.7);10. Irradiations were carried out at 20% of the maximum light intensity for 2}10 min, resulting in up to 51% reaction of the initially present 2-methyl-3-buten-2-ol. Experiments were carried out to study the O reac tions in the absence and presence of cyclohexane as an OH radical scavenger, with the experiments in the presence of cyclohexane employing GC-FID analyses without derivatization. For the O experiments in the presence of cyclohexane (at su$cient concentrations to scavenge '95% of the OH radicals formed) and with analyses without derivatization, the initial 2-methyl-3buten-2-ol and cyclohexane concentrations were (3.7}3.8);10 molecules cm\ and (1.2}1.5);10 molecules cm\, respectively. For the O experiments carried out in the absence of cyclohexane and with GC analysis after derivatization, the initial 2-methyl-3-buten-2-ol concentrations were (2.1}3.1);10 molecules cm\. For both sets of experiments, 3 additions of 50 or 500 cm volume of O in O diluent (each addition corre sponding to an initial O concentration in the chamber of &6;10 or &6;10 molecules cm\) were made to the chamber during an experiment, with the 500 cm volume of O in O additions being used in the experi ments with the higher 2-methyl-3-buten-2-ol concentrations. The concentrations of 2-methyl-3-buten-2-ol and acetone were measured by GC-FID. Gas samples of 25 cm volume were collected from the chamber onto Tenax-TA solid absorbent, with subsequent thermal desorption at &2253C onto a 30 m DB-1701 megabore column in a Hewlett Packard (HP) 5710 GC held at !203C and then programmed to 2003C at 83C min\. For the experiments with product derivatization and GC analysis, 1,4-dichlorobenzene was added to the chamber to act as an internal standard to determine the volume sampled. 1,4-Dichlorobenzene is not expected to react with O (Atkinson and Carter, 1984) and reacts only slowly with the OH radical, with a room temperature rate constant of &4;10\ cm molecules\ s\ (Atkinson, 1989,1994). The 1,4-dichlorobenzene concentrations in the chamber (&1.2;10 molecules cm\) were measured by GC-FID with thermal desorption as described above. Gas samples of &3 l volume were collected from the chamber onto Tenax-TA solid absorbent, with subsequent elution with &0.5 ml diethyl ether inside a glove-bag "lled with dry nitrogen
2895
(to minimize reaction of the trimethylsilyl reagents with water vapor in competition with their reaction with the product species). The samples were then concentrated to &0.2 ml volume and sealed with a Mininert valve. Anisole was added to the extracts to allow the "nal volume of extract to be determined. Trimethylimidazole#N,Obis[trimethylsilyl]acetamide#trimethylchlorosilane (2 ll of a 3 : 3 : 2 mixture of these reagents) was added to the concentrated Tenax eluates, and the samples heated to 703C overnight. GC-FID analyses of the concentrated eluates were carried out after the derivatization, using the same DB-1701 megabore column used to analyze for 2-methyl-3-buten-2-ol (see above). GC-MS analyses were carried out using a 30 m;0.32 mm i.d. DB-1701 fused silica capillary column in a HP 5890 GC interfaced to a HP 5971A Mass Selective Detector operated in the scanning mode, and GC-FTIR analyses used a similar column in a HP 5890 GC interfaced to a HP 5965B FTIR detector. The concentration of the internal standard, 1,4dichlorobenzene, was measured before and after derivatization, while the concentrations of 2-methyl-3buten-2-ol were quanti"ed before derivatization. The hydroxy-containing product 2-hydroxy-2-methylpropanal [(CH ) C(OH)CHO] was measured after de rivatization as the trimethylsilyl derivative. The amount of 1,4-dichlorobenzene in the chamber, as measured by GC-FID analyses of 25}100 cm volume samples, allowed the volume of the &3 l samples collected on Tenax solid adsorbent with a pump to be calculated. The GC-FID response factor for the trimethylsilyl derivative of 2-hydroxy-2-methylpropanal was calculated relative to that for 2-methyl-3-buten-2-ol using the e!ective carbon number method (Scanlon and Willis, 1985). 2.3. Teyon chamber with analysis by API-MS For the reactions of OH radicals and O with 2 methyl-3-buten-2-ol with API-MS analyses, the chamber contents were sampled through a 25 mm diameter; 75 cm length Pyrex tube at a #ow rate of &20 l min\ 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 (Kwok et al., 1996a,b; Aschmann et al., 1997a,b). Use of the MS/MS mode with CAD allows the `daughter iona or `parent iona spectrum of a given ion peak observed in the MS scanning mode to be obtained (Kwok et al., 1996a,b; Aschmann et al., 1997a,b). The positive ion mode was used in all these API-MS and API-MS/MS analyses, with protonated water hydrates (H O>(H O) ) gener L ated by the corona discharge in the chamber diluent gas being responsible for the protonation of analytes. Ions are drawn by an electric potential from the ion source through the sampling ori"ce into the mass-analyzing "rst
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quadrupole or third quadrupole. For 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., 1997b). Neutral molecules and particles are prevented from entering the ori"ce by a #ow of high-purity nitrogen (`curtaina 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 homo- and heterodimers (Aschmann et al., 1997b). The initial concentrations of CH ONO, NO and 2 methyl-3-buten-2-ol in the OH radical reactions were &2.4;10 molecules cm\ each, and the reactant mixture was irradiated for 2 min at 20% of the maximum light intensity. For the O reactions, the initial concen tration of 2-methyl-3-buten-2-ol was 2.4;10 molecules cm\ and &1.2;10 molecules cm\ of O (in O diluent) was added to the chamber. 2.4. Chemicals The chemicals used, and their stated purities were: cyclohexane (high-purity solvent grade), American Burdick and Jackson; acetone (HPLC grade), Fisher Scienti"c; 2-methyl-3-buten-2-ol (99%), Aldrich Chemical Company; trimethylimidazole#N,O-bis[trimethylsilyl]acetamide#trimethylchlorosilane (3 : 3 : 2), Supelco Inc.; and NO (*99.0%), Matheson Gas Products. Methyl nitrite was prepared and stored as described previously (Taylor et al., 1980; Atkinson et al., 1981), and O in O diluent was generated as needed using a Welsbach T-408 ozone generator at pre-calibrated voltage and O #ow settings. 3. Results 3.1. Analyses by gas chromatography without derivatization GC-FID, GC-MS and GC-FTIR analyses of irradiated CH ONO} NO}2-methyl-3-buten-2-ol}air and of reacted O }2-methyl-3-buten-2-ol}cyclohexane (in ex cess)}air mixtures showed the formation of acetone from both the OH radical and O reactions. Cyclohexanone and cyclohexanol were also observed from the O reac tion, indicating that OH radicals are formed from the reaction of O with 2-methyl-3-buten-2-ol. Because OH radicals also react with the acetone product, for the OH radical reaction the measured concentrations of acetone were corrected to take into account secondary reactions with the OH radical as described previously (Atkinson et al., 1982). The multiplicative factors F to account for secondary reactions with the OH radical increase with the rate constant ratio
Fig. 1. Plots of the amounts (corrected for reaction with the OH radical) of acetone and 2-hydroxy-2-methylpropanal (analyzed as the trimethylsilyl derivative) against the amounts of 2methyl-3-buten-2-ol reacted with the OH radical in the presence of NO. Analyses by gas chromatography.
k(OH#product)/k(OH#reactant) and with the extent of reaction (Atkinson et al., 1982). The rate constants used for the reactions of the OH radical with 2-methyl3-buten-2-ol and acetone at 298 K were (in units of 10\ cm molecules\ s\) 64 (Rudich et al., 1995) and 0.219 (Atkinson, 1994), respectively, and the maximum value of the multiplicative factor F was (1.01 for acetone formation. As representative of the data obtained, the amounts of acetone formed (corrected for secondary reactions) are plotted against the amounts of 2-methyl-3-buten-2-ol reacted with the OH radical in the presence of NO in Fig. 1. The acetone formation yields from the OH radical and O reactions obtained by least-squares analysis of the data obtained are given in Table 1. The formation yields of cyclohexanone and cyclohexanol in the reaction of O with 2-methyl-3-buten-2-ol in the presence of ex cess cyclohexane were used to derive the yield of the OH radical from the reaction of O with 2-methyl-3-buten-2 ol, as described previously (Atkinson and Aschmann, 1993). Least-squares analysis of the amounts of cyclohexanone plus cyclohexanol formed against the amounts of 2-methyl-3-buten-2-ol reacted with O re sulted in a ratio of (cyclohexanone plus cyclohexanol formed)/(2-methyl-3-buten-2-ol reacted)"0.094$0.008, where the indicated error is the two least-squares standard deviation. Combining this ratio of (cyclohexanone plus cyclohexanol formed)/(2-methyl-3-buten-2-ol reacted) with a ratio of (cyclohexanone plus cyclohexanol formed)/(cyclohexane reacted with the OH
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
2897
Table 1 Products formed, and their formation yields, from the reactions of the OH radical (in the presence of NO) and O with 2-methyl-3 buten-2-ol Formation yield Product
GC
OH radical reaction Formaldehyde 2-Hydroxy-2-methylpropanal Glycolaldehyde
0.19$0.07
Acetone
0.59$0.06
FT-IR
Literature
Reference
0.29$0.03
0.35$0.04 0.093$0.033
Ferronato et al. (1998) Fantechi et al. (1998b)
0.61$0.09
0.50$0.05 0.280$0.028 0.52$0.05 0.141$0.002
Ferronato et al. (1998) Fantechi et al. (1998b) Ferronato et al. (1998) Fantechi et al. (1998b)
0.36$0.09 0.48, 0.57 0.30$0.02 0.23$0.06 0.125, 0.182
Grosjean and Grosjean (1995) Fantechi et al. (1998b) Grosjean and Grosjean (1995) Grosjean and Grosjean (1995) Fantechi et al. (1998b)
0.295, 0.50 0.44, 0.295
Fantechi et al. (1998b) Fantechi et al. (1998b)
0.57$0.05
Organic nitrate(s)
0.05$0.02
O reaction Formaldehyde
0.29$0.03
2-Hydroxy-2-methylpropanal Acetone OH radical HC(O)OH CO CO
0.30$0.06 0.42$0.04 0.15!0.48 0.19 > \
0.47 0.12$0.02
0.01!0.03
0.11$0.02 0.09$0.02
Errors are two least-squares standard deviations combined with the estimated overall uncertainties in the GC-FID or FT-IR calibration factors for 2-methyl-3-buten-2-ol and products of $5% each ($8% for the FT-IR analysis of glycolaldehyde), unless noted otherwise. O reactions were carried out in the presence of an OH radical scavenger unless noted otherwise. OH radicals generated by the photolysis of H O in the presence of low concentrations of NO (see text); not clear whether RO V radicals predominantly reacted with NO or not. Indicated errors are two least-squares standard deviations. See Table 2 for possible structures. In the presence of su$cient cyclohexane to scavenge &90% of the OH radicals formed. In the presence of su$cient methylcyclohexane to scavenge &90% of the OH radicals formed. In the absence of an OH radical scavenger. Referenced to spectra from the OH radical reactions where the formation yield of 2-hydroxy-2-methylpropanal is assumed to be identical to that for HCHO (see text). From two experiments carried out with initial 2-methyl-3-buten-2-ol and cyclohexane concentrations of (2.1}2.4);10 and 1.1;10 molecules cm\, respectively, and with three additions of aliquots of O in O . Plots of the amounts of acetone formed against the amount of 2-methyl-3-buten-2-ol reacted were curved, indicating that acetone is not a primary product. The range of yields shown is based on initial and "nal slopes of the experimental data.
Yield increased with the extent of O reaction.
radical)"0.50 (Atkinson and Aschmann, 1993) leads to the OH radical formation yield from the reaction of O with 2-methyl-3-buten-2-ol given in Table 1, with an estimated overall uncertainty of a factor of 1.5 (Atkinson and Aschmann, 1993).
3.2. Analysis by gas chromatography after derivatization For the reactions of 2-methyl-3-buten-2-ol with OH radicals (in the presence of NO) and O (in the absence
of an OH radical scavenger to avoid the formation of cyclohexanol, which would also potentially derivatize), treatment of the diethyl ether Tenax eluates with the trimethylsilyl reagent resulted in the conversion of 2methyl-3-buten-2-ol to a trimethylsilyl ether and to the formation of the trimethylsilyl ether derivative of one reaction product. GC-MS analyses of the derivatized diethyl ether eluates showed that the mass spectra of the trimethylsilyl ether derivatives of 2-methyl-3-buten-2-ol and its reaction product had high-mass ions at m/z 158 [M]> and 145 [(M-15)]>, respectively. Trimethylsilyl
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ethers analyzed by GC-MS give characteristic fragmentation patterns, with the molecular ion often not being evident and the highest-mass ion being the [M-CH ]> ion, which can be used to determine the molecular weight of the compound (Pierce, 1968). The trimethylsilyl derivative of the hydroxy-containing reaction product of 2methyl-3-buten-2-ol therefore has a molecular weight of 160. The mass spectra of both derivatized compounds also show the presence of ions at m/z 73 and 75, which correspond to the trimethylsilyl ether fragments (CH ) Si> and (CH ) Si"OH> (Pierce, 1968), respec tively. GC-FTIR spectra of the trimethylsilyl ether derivative formed from the OH radical and O reactions with 2-methyl-3-buten-2-ol were identical and exhibited IR absorption bands at 2700 and 2795 cm\ (attributed to an aldehydic C}H stretch), 1752 cm\ (attributed to a C"O stretch) and 847 cm\ ((CH ) Si}O}R stretch), thus indicating the presence of both an aldehydic }CHO group and a derivatized hydroxyl group. This reaction product from the 2-methyl-3-buten-2-ol reactions is therefore identi"ed as 2-hydroxy-2-methylpropanal [(CH ) C(OH)CHO]. The measured amounts of 2-hydroxy-2-methylpropanal formed from the reaction of the OH radical with 2-methyl-3-buten-2-ol were corrected for secondary reactions with the OH radical, using a calculated rate constant for the reaction of the OH radical with 2hydroxy-2-methylpropanal of 2.20;10\ cm molecules\ s\ (Kwok and Atkinson, 1995). As an example of the data obtained, Fig. 1 shows a plot of the amounts of 2-hydroxy-2-methylpropanal formed, corrected for reaction with the OH radical, against the amounts of 2methyl-3-buten-2-ol reacted with the OH radical in the presence of NO. The formation yields of 2-hydroxy-2methylpropanal from the OH radical and O reactions, obtained by least-squares analysis of the data, are given in Table 1. 3.3. Analyses by in situ FT-IR spectroscopy Selected products from the gas-phase reactions of 2methyl-3-buten-2-ol with the OH radical in the presence of NO and with O in the presence of su$cient cyclo hexane to scavenge '95% of the OH radicals formed were identi"ed and quanti"ed by in situ FT-IR absorption spectroscopy. The products observed and quanti"ed by spectral desynthesis were HCHO, glycolaldehyde [HOCH CHO], and acetone from the OH radical-in itiated reaction; and HCHO, acetone, HC(O)OH, CO, and CO from the O reaction. The formation yields of these products are given in Table 1, where the measured concentrations of HCHO, HOCH CHO and CH C(O)CH have again been corrected for secondary reactions using the respective OH radical reaction rate constants (in units of 10\ cm molecules\ s\) of 9.37
Fig. 2. Plots of the amounts of formaldehyde, acetone and glycolaldehyde (all corrected for reaction with the OH radical) against the amounts of 2-methyl-3-buten-2-ol reacted with the OH radical in the presence of NO. Combined data from two experiments; analyses by FT-IR spectroscopy.
(Atkinson, 1994), 9.9 (Niki et al., 1987a; Atkinson, 1989) and 0.219 (Atkinson, 1994), respectively. The corrections for secondary reactions were )5% for HCHO and HOCH CHO, and were negligible for acetone. Plots of the amounts of HCHO, HOCH CHO and CH C(O)CH formed, corrected for secondary reactions, against the amounts of 2-methyl-3-buten-2-ol reacted with the OH radical are shown in Fig. 2. We have previously shown that any contribution to HCHO formation from secondary reactions of CH CHO gener ated from the photolysis of ethyl nitrite is minor ((4% formation yield) in irradiated C H ONO}NO}1-al kene}air mixtures (Atkinson et al., 1995b), and the formation of HCHO from ethyl nitrite chemistry in the present experiments would be even less because of the higher reactivity of 2-methyl-3-buten-2-ol. Fig. 3A is the residual spectrum, obtained after subtraction of absorptions by the remaining reactants and known products, at the end of one of the C H ONO} NO}2-methyl-3-buten-2-ol}air irradiations which shows distinct R}ONO }type absorption bands at 848, 1283, and 1664 cm\. The amount of organic nitrate(s) formed was estimated from the integrated area of the well-isolated 1283 cm\ band (1260}1305 cm\ range), using the average absorption coe$cient of 1.2;10\ cm molecules\ for similar compounds (Tuazon and Atkinson, 1990), and the calculated RONO formation yield is included in Table 1 (distinct features due to ethyl nitrate formed from the photooxidation of ethyl nitrite allowed
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
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2-ol with O (see Tuazon et al., 1997 and references therein).
3.4. Analyses by API-MS
Fig. 3. (A) Residual product spectrum from an irradiated ethyl nitrite}NO}2-methyl-3-buten-2-ol}air mixture, showing the RONO absorptions at 848, 1283 and 1664 cm\. (B) Residual product spectrum from an O #2-methyl-3-buten-2-ol reaction. (C) Di!erence spectrum [(A) minus (B)] where absorptions assigned to 2-hydroxy-2-methylpropanal (*) have been canceled (see text). Gaps in the spectra correspond to regions of very strong absorption by NO formed (in A) and added cyclohexane (in B).
the contribution of ethyl nitrate to be taken into account and hence the organic nitrate(s) yield given in Table 1 is for organic nitrates other than ethyl nitrate). Also evident in Fig. 3A are absorption bands, marked by asterisks, which are believed to be due to 2-hydroxy2-methylpropanal and which are also present in the residual spectrum from the reaction of 2-methyl-3-buten2-ol with O (Fig. 3B). Fig. 3C was obtained by subtract ing Fig. 3B from Fig. 3A, with cancellation of absorptions attributed to 2-hydroxy-2-methylpropanal. By assuming that the formation yield of 2-hydroxy-2-methylpropanal from the OH radical reaction with 2-methyl-3-buten-2-ol is equal to that of the HCHO yield (see discussion) and taking into account secondary reactions of HCHO and 2-hydroxy-2-methylpropanal as discussed above, a quantitative IR spectrum of 2-hydroxy-2-methylpropanal was obtained and used to determine the formation yield of 2-hydroxy-2-methylpropanal in the reaction of O with 2-methyl-3-buten-2-ol (see Table 1). Fig. 3C also shows that another minor carbonyl product (C"O stretch band at 1733 cm\) is formed during the C H ONO}NO}2-methyl-3-buten-2-ol}air irradiation, while the 1095 cm\ band and other adjoining (negative) absorptions possibly indicate the existence of an ozonide during the reaction of 2-methyl-3-buten-
Experiments were carried out using API-MS for analysis of the products formed from the reactions of 2methyl-3-buten-2-ol with the OH radical in the presence of NO and with O in the absence of an OH radical scavenger. API-MS/MS `daughter iona and `parent iona spectra were obtained for ion peaks observed in the API-MS analyses. Product ion peaks were identi"ed based on the observation of homo- or hetero-dimer (and, in certain cases, trimer) ions (for example, [(M ) #H]>, [(M ) #H]> and [M #M #H]>, . . . . where P1 and P2 are products) in the API-MS/MS `parent iona spectra, and consistency of the API-MS/MS `daughter iona spectrum of a homo- or hetero-dimer ion with the `parent iona spectra of the various [M #H]> ion peaks. Water cluster ion peaks of the . product ions, [M#H#H O]>, were also occasionally observed. The products observed from the OH radical-initiated reaction are listed in Table 2 and the evidence for the formation of these products, in the form of API-MS and API-MS/MS data of molecular ions, dominant fragment ions, and the presence of homo- and heterodimers formed in the API-MS under the experimental conditions employed, is also summarized in Table 2. In agreement with the GC analyses and the in situ FT-IR spectroscopic analyses, acetone, glycolaldehyde and 2-hydroxy-2methylpropanal were observed. In addition, an organic nitrate of molecular weight 165 was observed. The APIMS/MS `daughter iona spectrum of the [M#H]> ion of this product is shown in Fig. 4, with the presence of an intense NO> fragment at 46 u being evident. This nitrate is attributed to (CH ) C(OH)CH(ONO )CH OH and/ or (CH ) C(OH)CH(OH)CH ONO formed from the reactions of the (CH ) C(OH)CH(OO )CH OH and (CH ) C(OH)CH(OH)CH OO radicals with NO (Atkin son, 1994, 1997). For the O reaction, less-de"nitive data were obtained from the API-MS analyses, although API-MS and APIMS/MS analyses indicated that acetone and 2-hydroxy2-methylpropanal were formed as products, in agreement with our GC analyses and in situ FT-IR spectroscopic analyses (Table 1).
4. Discussion 4.1. OH radical reaction The reaction of the OH radical with 2-methyl-3-buten2-ol is expected to proceed almost entirely by initial
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Table 2 Products formed from the OH radical-initiated reaction of 2-methyl-3-buten-2-ol in the presence of NO, and experimental observations from in situ API-MS and API-MS/MS analyses Product
API-MS data
Other evidence
Acetone M (MW 58)
Acetone observed by GC-FID and FT-IR analyses. MS/MS of 119 u ion peak
Glycolaldehyde M (MW 60)
M #H"59 M #M #H"117 M #M #H"119 M #M #H"145 + M #M #H"119
2-Hydroxy-2-methylpropanal M (MW 88)
M #M #H"254
(CH ) C(OH)CH(ONO )CH OH and/or (CH ) C(OH)CH(OH)CH ONO M (MW 165)
M #H"166 M #M #H"254 M #M #H#H O"270 + M #M #H"331
Glycolaldehyde observed by FT-IR analysis. MS/MS of 119 u ion peak Identi"ed by GC-MS after derivatization (see text). MS/MS of 254 u ion peak MS/MS of 166 u ion peak (Fig. 4). MS/MS of 254, 270 and 331 u ion peaks. Parents of 166 u at 224 [M #M #H], 252 [M #M #H], + 254, 270, 288 [M #M #2H O#H], 310 + [M #M #M #H], 331 and 338 + [M #M #M #H] u. + + -
M "2-methyl-3-butene-2-ol, molecular weight"86. + -
Fig. 4. API-MS/MS CAD spectrum of the 166 u ion peak, attributed to the [M#H]> ion of (CH ) C(OH)CH(OH)CH ONO and/or (CH ) C(OH)CH(ONO )CH OH, observed in the API-MS spectra of an irradiated CH ONO}NO}2-methyl-3-buten-2-ol}air mix ture. The 46 u fragment peak is attributed to NO>.
addition of the OH radical to the carbon atoms of the 'C"C( bond (Atkinson, 1989): OH#(CH ) C(OH)CH"CH P(CH ) C(OH)C HCH OH and (CH ) C(OH)CH(OH)C H .
(4)
These b-hydroxyalkyl radicals add O to form the corre sponding b-hydroxyalkyl peroxy radicals, which in the presence of NO react to form either the organic nitrate or NO plus the corresponding b-hydroxyalkoxy radical. For example, for the (CH ) C(OH)C HCH OH radical, (CH ) C(OH)C HCH OH#O P (CH ) C(OH)CH(OO )CH OH, (5)
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
(CH ) C(OH)CH(OO )CH OH#NO P(CH ) C(OH)CH(ONO )CH OH, * (CH ) C(OH)CH(O )CH OH#NO . P
(6a) (6b)
The resulting b-hydroxyalkoxy radicals (CH ) C(OH) CH(O )CH OH and (CH ) C(OH)CH(OH)CH O can decompose by C}C bond scission, react with O , or (for the (CH ) C(OH)CH(OH)CH O radical) isomerize via a six-membered transition state (Atkinson, 1997), as shown in Reaction Scheme 2 respectively. The nitrates observed by our API-MS and APIMS/MS analyses (Table 2), and assumed to be the organic nitrates quanti"ed by FT-IR spectroscopy (Table 1), are then (CH ) C(OH)CH(ONO )CH OH formed as shown in reactions (4)}(6a), and/or (CH ) C(OH) CH(OH)CH ONO formed from the second b-hydroxy alkyl peroxy radical shown in reaction (4) by reactions analogous to reactions (5) and (6a). The product data shown in Table 1 further show that the b-hydroxyalkoxy radicals predominantly decompose to form CH C(O)CH #HOCH CHO and (CH ) C(OH)CHO#HCHO. The formation yields of acetone as measured by GC-FID and FT-IR analyses are in excellent agreement, with an average acetone yield of 0.58$0.04, and the acetone yield is in excellent agreement with the measured formation yield of glycolaldehyde (0.61$0.09), as expected from Reaction Scheme 1 because these two products are co-products of the same decomposition pathway. However, while Reaction Schemes 1 and 2 predict that HCHO and (CH ) C(OH)CHO should be co-prod ucts, our measured formation yield of (CH ) C(OH)CHO is &35% lower than the yield of HCHO measured by in situ FT-IR spectroscopy (though in agreement within the combined uncertainties). This lower yield of 2-hydroxy-2-methylpropanal than expected suggests that the derivatization/GC-FID analysis procedure for hydroxy-containing compounds is (100% e$cient. Our data indicate that the OH radical adds predominantly to the terminal carbon atom and that the resulting (CH ) C(OH)CH(O )CH OH alkoxy radical decomposes preferentially to form acetone plus glycolaldehyde. The predicted rates of the O reaction and the two decomposition channels (Atkinson, 1997) for the (CH ) C(OH)CH(O )CH OH radical at 298 K and atmospheric pressure of air are 4.4;10 s\ for reaction with O , 4.4;10 s\ for decomposition to (CH ) C(OH)CHO#C H OH, and 2.0;10 s\ for de composition to HOCH CHO#(CH ) C OH. These predicted reaction rates agree with our observations and with our conclusion that the dominant decomposition channel of the (CH ) C(OH)CH(O )CH OH radical is to form glycolaldehyde plus acetone. Terminal OH radical addition therefore accounts for *60% of the overall OH radical reaction. The predicted rate of
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reaction with O and of decomposition of the (CH ) C(OH)CH(OH)CH O radical at 298 K and atmo spheric pressure of air are 4.9;10 and 3.2;10 s\, respectively (Atkinson, 1997), again consistent with our product data. The products observed and quanti"ed account for 92$6% of the overall products. The products of the gas-phase reaction of 2-methyl-3buten-2-ol with the OH radical have also been investigated by Ferronato et al. (1998) and Fantechi et al. (1998b). Ferronato et al. (1998) generated OH radicals by the photolysis of C-labeled CH ONO}NO}air mix tures and used in situ FT-IR spectroscopy for analysis, with the use of C-labeled CH ONO allowing the formation of HCHO from the reaction of OH radicals with 2-methyl-3-buten-2-ol to be measured. As shown in Table 1, our measured formation yields of HCHO, acetone and glycolaldehyde are in agreement, within the combined experimental uncertainties, with the formation yields reported by Ferronato et al. (1998). Ferronato et al. (1998) did not report data for 2-hydroxy-2-methylpropanal nor for organic nitrates, noting that no spectral features attributable to 2-hydroxy-2-methylpropanal could be identi"ed. In the study of Fantechi et al. (1998b), OH radicals were generated by the photolysis of H O in air in the presence of much lower concentrations of NO (the V initial H O , 2-methyl-3-buten-2-ol and NO concen V trations were (3.6}7.2);10 molecules cm\, (1.9}2.7); 10 molecules cm\, and (2.4}24);10 molecules cm\, respectively). Hence it is not clear whether organic peroxy (RO ) radicals reacted predominantly with NO in the experiments of Fantechi et al. (1998b) or whether RO #RO and/or RO #HO reactions pre dominated. The formation yields of HCHO, acetone and glycolaldehyde measured by Fantechi et al. (1998b) using in situ FT-IR spectroscopy are given in Table 1, and are seen to be signi"cantly lower, by factors of &2}4, than the formation yields measured in this work and by Ferronato et al. (1998). Using 2,4-dinitrophenyl hydrazine (DNPH) to derivatize carbonyl compounds, Fantechi et al. (1998b) also tentatively identi"ed 2-hydroxy-2methylpropanal as a product of the OH radical reaction. 4.2. O3 reaction Based on the understanding of O reactions with al kenes, O will initially add to the 'C"C( bond in 2-methyl-3-buten-2-ol to form a primary ozonide, which then rapidly decomposes to ([C H OO ]*# (CH ) C(OH)CHO) and (HCHO#[(CH ) C(OH) C HOO ]H) (Atkinson, 1994,1997), where [ ]H denotes an energy-rich biradical. The sum of the formation yields of the `primarya carbonyls (CH ) C(OH)CHO and HCHO should therefore be unity (Atkinson, 1997; Grosjean and Grosjean, 1997). Using the derivatization/GC-FID procedure to quantify 2-hydroxy-2-methylpropanal
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Reaction Scheme 1.
Reaction Scheme 2.
(Table 1), the sum of the HCHO plus (CH ) C(OH)CHO formation yields of 0.59 from our present work is again consistent with the less than 100% e$ciency of the derivatization/GC-FID analysis procedure for 2-hydroxy-2-methylpropanal. Quanti"cation of 2-hydroxy-2methylpropanal by FT-IR spectroscopy, based on the expectation that the formation yields of HCHO and 2-hydroxy-2-methylpropanal from the OH radical reaction with 2-methyl-3-buten-2-ol are identical (Reaction Schemes 1 and 2 and discussion above), results in a sum of the `primarya carbonyls of 0.76. From the assumption that the 2-hydroxy-2-methylpropanal formation yield is equal to that of HCHO in the OH radical reaction and the ensuing estimate of the 2-hydroxy-2-methylpropanal formation yield in the O reaction, it may be deduced from the data in Table 1 that the derivatization method applied to 2-hydroxy-2-methylpropanal has an overall e$ciency of &65%.
The products of the gas-phase reaction of 2-methyl-3buten-2-ol with O , in the presence of an OH radical scavenger, have also been investigated by Grosjean and Grosjean (1995) and Fantechi et al. (1998b). Our measured HCHO and 2-hydroxy-2-methylpropanal yields are in general agreement with those reported by Grosjean and Grosjean (1995) (Table 1), but our HCHO yield is up to a factor of 2 lower than the two values reported by Fantechi et al. (1998b) using cyclohexane and methylcyclohexane as OH radical scavengers (Table 1). Fantechi et al. (1998b) also tentatively identi"ed 2-hydroxy-2-methylpropanal as a product of the O reaction, using DNPH as a derivatization agent. It should be noted that in the study of Grosjean and Grosjean (1995), the identi"cation of 2-hydroxy-2-methylpropanal was stated to be tentative and its quanti"cation was subject to signi"cant uncertainties because of a lack of an authentic standard for response factor calibration.
A. Alvarado et al. / Atmospheric Environment 33 (1999) 2893}2905
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We assume that the initial products of the decomposition of the primary ozonide are ([C H OO ]H#(CH ) C(OH)CHO) and (HCHO#[(CH ) C(OH)C HOO ]H), with the biradicals undergoing collisional stabilization in competition with a number of decomposition and/or rearrangement pathways (Atkinson, 1997). Our HCHO and 2-hydroxy-2-methylpropanal formation yields (Table 1) indicate that the primary ozonide decomposes to ([C H OO ]*#(CH ) C(OH)CHO) &50% of the time and to (HCHO#[(CH ) C(OH)C HOO ]H) &30% of the time. However, our quanti"cation of 2-hydroxy-2methylpropanal is subject to signi"cant uncertainties (as noted above, its analysis by derivatization/GC-FID results in a lower limit and analysis by in situ FT-IR spectroscopy involves a number of assumptions and uncertainties), and we suggest that the initial products of the decomposition of the primary ozonide are ([C H OO ]H#(CH ) C(OH)CHO) in &70% yield and (HCHO#[(CH ) C(OH)C HOO ]H) in &30% yield. The likely reactions of the [C H OO ]H biradical have been discussed previously, and include stabilization, decomposition to CO#H O and decomposition to OH#HC O (Atkinson, 1997). A major uncertainty concerns the reactions of the initially energy-rich [(CH ) C(OH)C HOO ]H biradical. This biradical cannot undergo isomerization to form the energy-rich hydroperoxide which then decomposes to form the OH radical (Niki et al., 1987b), and the nonavailability of this reaction channel is reasonably consistent with the low OH radical formation yield of &0.19 (which can be partially explained by OH radical formation from the [C H OO ]H biradical). Grosjean and Gros jean (1995) suggested that this biradical could decompose to ultimately form acetone:
[(CH ) C(OH)C HOO ]H biradical [for example, with water vapor, carbonyls, and 2-methyl-3-buten-2-ol (Neeb et al., 1996a, 1996b, 1997)].
[(CH ) C(OH)C HOO ]HPCO #H#(CH ) C OH `O CH C(O)CH #HO . (7) An alternative decomposition pathway could also lead to the formation of acetone:
References
[(CH ) C(OH)C HOO ]HP[(CH ) C(OH)CH(OQ )OQ ]H P[(CH ) C(OQ )CH(O)OH]H CH C(O)CH #HC(O)OH. (8) Our FT-IR data in Table 1 do show similar acetone and CO yields, although there are other routes to CO formation in addition to reaction (7). The acetone formation yields measured in di!erent sets of experiments (Table 1) show much scatter, with one set of GC-FID analyses showing an acetone yield which increased with the extent of reaction, from an initial value of (0.15 to &0.48 at larger extents of reaction. The various acetone yields observed may then re#ect competition between various reaction pathways of the
4.3. Atmospheric implications As noted earlier, in the troposphere 2-methyl-3-buten2-ol reacts with OH and NO radicals and O , with the OH radical reaction being calculated to be the dominant loss process and with a lifetime of a few hours during daytime (Atkinson and Arey, 1998). We have accounted for the majority (92$6%) of the pathways involved in the OH radical reaction in the presence of NO, with acetone plus glycolaldehyde accounting for &60% of the reaction products. Hence emission of 2-methyl-3buten-2-ol from vegetation will lead to an in situ tropospheric source of acetone.
Acknowledgements The authors gratefully thank the National Science Foundation for supporting this research through Grant No. ATM-9414036, and thank the National Science Foundation (Grant No. ATM-9025361) and the University of California, Riverside, for funds for the purchase of the PE SCIEX API III MS/MS instrument, and thanks Drs. J.J. Orlando and G.S. Tyndall for communicating their results prior to publication. RA and JA thank the Agricultural Experiment Station, University of California, for partial salary support.
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