Environ. Sci. Technol. 2010, 44, 3825–3831
Products and Mechanisms of the Gas-Phase Reactions of OH Radicals with 1-Octene and 7-Tetradecene in the Presence of NO SARA M. ASCHMANN, ERNESTO C. TUAZON, JANET AREY,† AND R O G E R A T K I N S O N * ,†,‡ Air Pollution Research Center, University of California, Riverside, California 92521
Received February 17, 2010. Revised manuscript received April 7, 2010. Accepted April 8, 2010.
Products of the gas-phase reactions of OH radicals with 1-octene and 7-tetradecene have been investigated at 296 ( 2 K and atmospheric pressure of air, using gas chromatography, direct air sampling atmospheric pressure ionization tandem mass spectrometry, and in situ Fourier transform infrared spectroscopy. We observe the hydroxynitrate(s) formed from reaction of the hydroxyalkylperoxy radicals with NO; heptanal and 4-hydroxyhexanal from decomposition of the 1,2hydroxyoctoxy and 7-hydroxy-8-tetradecoxy radicals; and dihydroxynitrates and dihydroxycarbonyls formed after isomerization of the intermediate 1,2-hydroxyalkoxy radicals. Formic acid formation was also observed from the 1-octene reaction, in ∼3% yield. In the presence of ∼1.5 × 1014 molecules cm-3 of NO, the respective molar formation yields of heptanal and 4-hydroxyhexanal were 28 ( 3% and 4% from 1-octene, and 86 ( 11% and 6% from 7-tetradecene. The 4-hydroxyhexanal yield increased with increasing NO concentration, and we attribute 4-hydroxyhexanal + HC(O)OH formation to a minor pathway of the RC•HOH + O2 reaction in the presence of NO. The reaction pathways occurring are discussed.
Introduction Alkenes comprise ∼10% of nonmethane volatile organic compounds present in urban air (1). In the troposphere, alkenes react with OH radicals, NO3 radicals and O3 (1-3), with daytime reaction with OH radicals being an important loss process (1). The OH radical reactions proceed by initial addition to CdC bonds and by H-atom abstraction from C-H bonds in the alkyl side-chains (2, 3), with the addition pathway dominating for eC14 acyclic alkenes (4-7). Our current understanding of OH radical-initiated degradation reactions of alkenes under conditions where organic peroxy (RO2•) radicals react exclusively with NO (1-3, 8) is shown in Scheme 1 for OH radical addition to an alkene with a gC3 alkyl substituent group. A key step after initial addition to the CdC bond is the fate of the 1,2-hydroxyalkoxy radicals, which can react with O2, decompose by C-C bond scission, and/or isomerize through a 6-member transition state (8). For 1,2-hydroxy* Corresponding author phone: (951) 827-4191; e-mail: ratkins@ mail.ucr.edu. † Also Department of Environmental Sciences. ‡ Also Department of Chemistry. 10.1021/es100550n
2010 American Chemical Society
Published on Web 04/26/2010
alkoxy radicals other than HOCH2CH2O• formed from ethene (9, 10), decomposition and/or isomerization dominate over reaction with O2, which can be neglected at room temperature (8). Product studies of gC4 1-alkenes indicate that at room temperature decomposition and isomerization of the intermediate 1,2-hydroxyalkoxy radicals are competitive (11-14). The alkoxy radical decompositions lead to the formation of two sets of carbonyl + RR’C•OH radical (R, R’ ) H or alkyl), while their isomerization reactions lead to dihydroxynitrates and dihydroxycarbonyls (2, 3, 12-14) (see Scheme 1). Orlando et al. (15, 16) observed the formation of formic acid, HC(O)OH, in low yield from the reactions of OH radicals with a number of alkenes, and suggested that in the presence of NO the reaction of RC•HOH radicals with O2 leads to HC(O)OH + R• as well as RCHO + HO2 (Scheme 1). This minor pathway to form HC(O)OH + R• is in competition with formation of RCHO + HO2 and, while potentially significant in experiments conducted at high NO concentrations, HC(O)OH + R• formation is of no importance under atmospheric conditions (17). Kinetic data for the reactions of OH radicals with eC14 1-alkenes (6), 2-methyl-1-alkenes (7) and E-2-alkenes (7) suggest that while H-atom abstraction is of increasing importance as the carbon number of the alkyl substituent group increases, it accounts for e30-40% of the overall OH radical reaction at room temperature (6, 7). In this work we have investigated products formed from the gas-phase reactions of OH radicals with 1-octene and 7-tetradecene, which were chosen for study because heptanal is a known product of the 1-octene reaction (11-13) and for 7-tetradecene the hydroxyalkoxy radical decomposition pathway is expected to lead to heptanal (see Scheme 1). The observed reaction products and derived mechanism for OH + 7-tetradecene are relevant to understanding secondary organic aerosol formation from the OH radical-initiated reactions of larger (>C8) alkenes, including 7-tetradecene (18).
Experimental Methods Reactions were carried out with analysis of reactants and products using gas chromatography with flame ionization detection (GC-FID), combined gas chromatography-mass spectrometry (GC-MS), and direct air sampling atmospheric pressure ionization tandem mass spectrometry (API-MS). OH radicals were generated in the presence of NO by the photolysis of CH3ONO in air at wavelengths >300 nm (6, 7, 19), and NO was included in the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals. Experiments were carried out in ∼7000 L Teflon chambers, equipped with two parallel banks of blacklamps for irradiation, at 296 ( 2 K and 735 Torr total pressure of dry purified air, with one of these chambers being interfaced to a PE SCIEX API III MS/MS (13, 14, 19). NO and the initial NO2 concentrations were monitored using a chemiluminescence NO-NO2-NOx analyzer. In addition, in situ Fourier transform infrared (FT-IR) spectroscopy was used to measure HC(O)OH formation (19) from the 1-octene reaction. These experiments were carried out at 299 ( 2 K and 760 Torr total pressure of synthetic air in a 5870 L Teflon-coated, evacuable chamber equipped with a multiple reflection optical system interfaced to a Mattson Galaxy 5020 FT-IR spectrometer. Irradiation was provided by a 24 kW xenon arc lamp, filtered through a 6 mm thick Pyrex pane to remove wavelengths
