Rate constants for the gas-phase reactions of a series of alkylnaphthalenes with the OH radical

Naphthalene and its C1- and C2-alkyl derivatives are semivolatile polycyclic aromatic hydrocarbons expected to be in the gas phase in ambient atmospheres and subject to degradation by gas-phase reactions with the hydroxyl (OH) radical. Using a relative rate method, rate constants for the gas-phase reactions of OH radicals with a series of alkylnaphthalenes have been measured at 298 ± 2 K and at atmospheric pressure of air. The compounds studied include naphthalene, 1- and 2-methylnaphthalene (1-, 2-MN), 1- and 2-ethylnaphthalene (1-, 2-EN), and the 10 dimethyl-naphthalene isomers (1,2-; 1,3-; 1,4-; 1,5-; 1,6-; 1,7-; 1,8-; 2,3-; 2,6-; and 2,7-DMN). Using 1,2,3- and 1,3,5-trimethylbenzene as reference compounds, the rate constant obtained for the OH radical reaction of naphthalene was (2.39 ± 0.09) × 10-11 cm3 molecule-1 s-1. Relative to naphthalene, the rate constants measured for the alkylnaphthalenes were (in units of 10-11 cm3 molecule-1 s-1, where the errors given are two standard deviations and include the above uncertainty in the naphthalene rate constant) 1-MN, 4.09 ± 0.20; 2-MN, 4.86 ± 0.25; 1-EN, 3.64 ± 0.41; 2-EN, 4.02 ± 0.55; 1,2-DMN, 5.96 ± 0.55; 1,3-DMN, 7.49 ± 0.39; 1,4-DMN, 5.79 ± 0.36; 1,5-DMN, 6.01 ± 0.35; 1,6-DMN, 6.34± 0.36; 1,7-DMN, 6.79 ± 0.45; 1,8-DMN, 6.27 ± 0.43; 2,3-DMN, 6.15 ± 0.47; 2,6-DMN, 6.65 ± 0.35; 2,7-DMN, 6.87 ± 0.43. These data show that, under atmospheric conditions, the DMN isomers react most rapidly with the OH radical with calculated lifetimes of 1.9-2.4 h, followed by the MNs and ENs with lifetimes of 2.9-3.8 h and naphthalene with a lifetime of 5.8 h. These differences in reactivity were confirmed by the nighttime/daytime concentration ratios of the alkylnaphthalenes measured in ambient Riverside, CA, samples.