On the role of OH and and HO2 radicals in the troposphere

According to a photochemical model proposed by Levy, the effective number densities of hydroxyl radicals in the troposphere are of the order of 2.5 × 10⁶ molecules cm³. It is shown here that such high OH 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 OH number density suggested by the observations is at the most one fifth of that predicted by Levy. It appears that the reconversion of HO₂ to OH radicals is not as effective as assumed in the model. Calculations are presented for the loss of radicals to aerosol particles. It is shown that aerosol scavenging of OH radicals is negligible compared to consumption by gas reactions, but that aerosol scavenging of HO₂ radicals might provide an important loss process for this species.