THE OXIDATION OF ISOPRENE IN THE TROPOSPHERE: MECHANISM AND MODEL CALCULATIONS

Abstract

A chemical mechanism describing the oxidation of isoprene by OH and O3 in the troposphere has been developed and incorporated into a one-dimensional steady-state photochemical model. Calculations have been performed for continental conditions at two latitudes, 15°N and 45°N. At 45°N latitude, the effects of anthropogenic hydrocarbon emissions on the vertical profiles of NOx (NO + NO2) and HNO3 overshadow the effects of isoprene emissions; at 15°N, the reduced anthropogenic emissions and the increased isoprene emissions produce increases of 26 and 4% in the column contents of NOx and HNO3, respectively. The integrated columns of CO at 45°N and 15°N latitude increased by 10 and 31%, respectively, when isoprene chemistry was included, but these increases are much smaller than those that would have been obtained if all the carbon in isoprene had been photochemically transformed into CO. It appears that the fate of a significant quantity of isoprene is in the formation of longer carbon-chain (R > CH3) oxygenated organic species. The longer carbon-chain alkylhydroperoxides and alkylperoxyacids appear to be important free radical sinks in the tropics given the lower NOx concentrations generally found in the tropics.