On the role of OH and and HO 2 radicals in the troposphere

Abstract

According t o a photochemical model proposed by Levy, the effective number densities of hydroxyl radicals in the troposphere are of the order of 2.5 x loE molecules cm3. It is shown here that such high O H number densities are inconsistent with two independent sets of observational data: The distribution of CO mixing ratios with latitude in the northern and southern hemisphere; and with the rate of nitrate precipitation in the northern hemisphere. The upper limit of the effective O H number density suggested by the observations is at the most one fifth of that predicted by Levy. I t appears that the reconversion of HO, to OH radicals is not as effective as assumed in the model. Calculations are presented for the loss of radicals t o aerosol particles. It is shown that aerosol scavenging of OH radicals is negligible compared t o consumption by gas reactions, but that aerosol scavenging of HO, radicals might provide an important loss process for this species.