Mechanisms of the gas-phase reactions of aromatic hydrocarbons and PAHs with OH and NO3 radicals

Chemical removal for the simple monocyclic aromatic hydrocarbons ( benzene and the C-1-C-3 alkylbenzenes) and naphthalene and the C-1-C-2 alkylnaphthalenes in the atmosphere is by reaction with hydroxyl ( OH) radicals. Naphthalene and the C-1-C-2 alkylnaphthalenes may also be removed, but to a much lesser extent, by reaction with nitrate ( NO3) radicals. While rate constants for the gas-phase reactions of OH radicals and NO3 radicals with many of the simple monocyclic aromatic hydrocarbons and with naphthalene and the C-1-C-2 alkylnaphthalenes have been measured, the detailed mechanisms of these OH radical- and NO3 radical-initiated reactions in the atmosphere are less well understood, especially for naphthalene and the alkylnaphthalenes. Here we present the available data on the reaction mechanisms occurring under laboratory conditions and attempt to reconcile these data with ambient atmospheric measurements. The OH radical reactions with benzene and alkylbenzenes and the OH radical and NO3 radical reactions with naphthalene and alkylnaphthalenes proceed mainly by initial addition of the OH or NO3 radical to the aromatic ring( s) at room temperature and below. The NO3 radical reactions with alkylbenzenes proceed by H-atom abstraction from the C-H bonds of the alkyl substituent group( s), while the NO3 radical reactions of naphthalene and the alkylnaphthalenes proceed by addition and with rates proportional to the NO2 concentration. The OH-monocyclic aromatic adducts react with O-2 under atmospheric conditions, while the OH-naphthalene/alkylnaphthalene and NO3-naphthalene/alkylnaphthalene adducts appear to undergo significant reaction with NO2 under urban atmospheric conditions.