Formation of Nitro Products from the Gas-Phase OH Radical-Initiated Reactions of Toluene, Naphthalene, and Biphenyl: Effect of NO2 Concentration

Aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), are released into the atmosphere principally during incomplete combustion and account for similar to 20% of nonmethane organic compounds in urban air. Reaction with OH radicals is the dominant atmospheric chemical loss process for aromatic hydrocarbons, leading mainly to the formation of an OH-aromatic or OH-PAH adduct which then reacts with O-2 and/or NO2. For OH-monocyclic aromatic adducts, reaction with O-2 dominates under atmospheric conditions; however, no data are available concerning the relative importance of reactions of OH-PAH adducts with O-2 and NO2. We have measured formation yields of 3-nitrotoluene, 1- and 2-nitronaphthalene, and 3-nitrobiphenyl from the OH radical-initiated reactions of toluene, naphthalene, and biphenyl as a function of NO2 concentration. Our data show that the OH-aromatic adduct reactions with O-2 and NO2 are of equal importance in the atmosphere at NO2 mixing ratios of similar to 3.3 ppmV for toluene, similar to 0.06 ppmV for naphthalene, and similar to 0.6 ppmV for biphenyl. Ambient concentrations of toluene, naphthalene, and biphenyl and their nitrated products measured at a site in the Los Angeles air basin are consistent with our laboratory measurements.