A study of the NO_x dependence of isoprene oxidation

A large set of isoprene and isoprene oxidation product concentration data from four North American sites was examined to assess the NO_x dependence of the daytime oxidation of isoprene. Sites that represent a wide range of NO_x (50 ppt to 30 ppb) were studied and include the Dickson, Tennessee, and Cornelia Fort Air Park sites during the 1999 Southern Oxidants Study, the Pellston, Michigan, site during the 1998 PROPHET summer intensive, and the Kejimkujik National Park site during the Atlantic 1996 study. Knowledge of NO_x and HO_x concentrations were critical for this evaluation. While NO_x data are readily available at all sites, HO_x data are limited. We employed a simple 10-reaction HO_x model to calculate steady state OH radical concentrations as a function of [NO_x] to enable analysis of the data from all sites. Here, we use methyl vinyl ketone (MVK) concentrations to quantify the extent of isoprene-OH oxidation. Making use of the MVK/isoprene ratio, we show that the rate of production of isoprene oxidation products at various North American sites, although highly variable, exhibits the crossover from NO_x-dependent to VOC-dependent conditions at ε18 ppb [NO_x], in agreement with what is calculated from HO_x measurements and our calculations.