Environ. Sci. Technol. 2005, 39, 5302-5310
Reactions of Chlorine Atoms with a Series of Aromatic Hydrocarbons L I N W A N G , † J A N E T A R E Y , * ,†,‡ A N D R O G E R A T K I N S O N * ,†,‡,§ Air Pollution Research Center, University of California, Riverside, California 92521
Aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), are present in urban and rural atmospheres. Reactions of PAHs with Cl atoms may occur in the marine boundary layer and in coastal regions. To assess the importance of these reactions and to investigate whether any unique chlorine-containing products are formed from these reactions, we have measured the rate constants for the gas-phase reactions of Cl atoms with toluene-d8, 1,3,5-trimethylbenzene (1,3,5-TMB), naphthalene, 1-methylnaphthalene-d10 (1-MN-d10), 1- and 2-methylnaphthalene (1- and 2-MN), 1- and 2-ethylnaphthalene (1- and 2-EN), and the dimethylnaphthalenes (DMNs) at 296 ( 2 K. A relative rate technique was used, and, using our measured rate constant for the reaction of Cl atoms with 1,3,5TMB of 2.42 × 10-10 cm3 molecule-1 s-1, the rate constants (in units of 10-10 cm3 molecule-1 s-1) are as follows: naphthalene, e0.0091 ( 0.0003; 1-MN, 1.21 ( 0.16; 2-MN, 1.05 ( 0.13; 1-EN, 2.12 ( 0.35; 2-EN, 1.38 ( 0.27; 1,2DMN, 3.61 ( 0.68; 1,3-DMN, 2.90 ( 0.22; 1,4-DMN, 2.93 ( 0.30; 1,5-DMN, 2.31 ( 0.19; 1,6-DMN, 2.15 ( 0.20; 1,7DMN, 3.05 ( 0.34; 1,8-DMN, 3.07 ( 0.44; 2,3-DMN, 2.93 ( 0.49; 2,6-DMN, 2.34 ( 0.18; and 2,7-DMN, 2.00 ( 0.22, where the indicated errors are two standard deviations and do not include the uncertainty in the rate constant for 1,3,5-TMB. The measured deuterium isotope effects for the toluene-d8 and 1-MN-d10 reactions indicate that the reactions proceed by initial H- (or D-) atom abstraction. The products identified and quantified from the toluene and 1-MN reactions using gas chromatography and in situ direct air sampling atmospheric pressure ionization tandem mass spectrometry were benzaldehyde (84% ( 7% yield) and benzyl alcohol (11% ( 2% yield) from toluene and 1-naphthaldehyde (∼36%, lower limit to yield) and 1-naphthyl alcohol (∼12%, lower limit to yield) from 1-MN. These products confirm that H-atom abstraction is the dominant, if not sole, reaction pathway for the alkylbenzenes and alkylnaphthalenes, consistent with the 100-fold lower rate constant measured for naphthalene compared to the alkylnaphthalenes and with the measured deuterium isotope effects. * To whom correspondence should be addressed. For J. Arey: Telephone: (951) 827-3502; Fax: (951) 827-5004; E-mail: janet.arey@ ucr.edu. For R. Atkinson: Telephone: (951) 827-4191; Fax: (951) 827-5004; E-mail:
[email protected]. † Also with Environmental Toxicology Graduate Program, University of California, Riverside. ‡ Also with Department of Environmental Sciences, University of California, Riverside. § Also with Department of Chemistry, University of California, Riverside. 5302
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 14, 2005
Introduction Aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs) such as naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and dimethylnaphthalenes, are present in gasoline and diesel fuels (1) and are emitted in both gasoline- and diesel-fueled vehicle exhaust (1-3). In the troposphere, gaseous volatile and semivolatile organic compounds can be transformed by gas-phase reactions with hydroxyl (OH) radicals, nitrate (NO3) radicals, ozone (O3) and, possibly, chlorine (Cl) atoms (4). Although reactions with OH radicals, NO3 radicals, and O3 are generally considered to be the major transformation processes of gaseous organic compounds in the troposphere, the importance of Cl atom reactions in the Arctic lower troposphere during springtime has become apparent (5-7). It has been postulated that Cl atom reactions may also be significant in the marine boundary layer and in coastal regions (8-11). Many estimates of tropospheric Cl atom concentrations have been made to evaluate the importance of Cl atom reactions as a loss process for certain volatile organic compounds in the troposphere. Computer modeling of atmospheric tetrachloroethene concentration data, using OH radical concentrations derived from ambient CH3CCl3 data, led to an estimated average global tropospheric Cl atom concentration of
