Photochemical formation of peroxides in the boundary layer

Abstract

Boundary layer gas phase concentrations of H2O2 and CH3OOH are calculated using a multilevel Lagrangian photochemical model. A range of emission conditions and resultant pollutant levels are considered, such as occur in the northeastern United States during summer months. The dependence of peroxide concentrations on chemical rate constants and meteorological variables is analyzed in terms of a budget equation for the formation and removal of radicals in the atmosphere. For representative summer conditions (solar intensity, humidity, and ambient O3 level) the production of radicals, primarily from photolysis of O3, occurs significantly faster than the average NOx emission rate in the northeastern United States. A necessary condition of this imbalance is that only a fraction of the radicals introduced into the atmosphere can be removed by reactions with NO and NO2. The remaining radicals are removed primarily by recombination reactions forming peroxides. Variations in radical production, such as might reasonably be caused by changes in precursor concentration, latitude, season, cloud cover and/or humidity, can reverse the sense of the inequality between NOx emissions and radical production and lead to an order of magnitude decrease in the amount of peroxides formed.