The free radical chemistry of cloud droplets and its impact upon the composition of rain

Abstract

Calculations are presented that simulate the free radical chemistries of the gas phase and aqueous phase within a warm cloud during midday. It is demonstrated that in the presence of midday solar fluxes the heterogeneous scavenging of OH and HO 2 from the gas phase by cloud droplets can represent a major source of free radicals to cloud water, provided the accommodation or sticking coefficient for these species impinging upon water droplets is ≥10 −4 . The aqueous‐phase HO 2 radicals are found to be converted to H 2 O 2 by aqueous‐phase chemical reactions at a rate that suggests that this mechanism could produce a significant fraction of the H 2 O 2 found in cloud droplets. The rapid oxidation of sulfur species dissolved in cloudwater by this free‐radical‐produced H 2 O 2 as well as by aqueous‐phase OH radicals could conceivably have a significant impact upon the chemical composition of rain.