Chemistry of OH in remote clouds and its role in the production of formic acid and peroxymonosulfate

Abstract

The chemistry of OH in a remote nonprecipitating tropical cloud is studied with a coupled gas‐phase and aqueous‐phase chemical model. The model takes into account the radial dependence of the concentrations of short‐lived aqueous‐phase species, in particular O3(aq) and OH(aq). The radical OH(aq) is produced rapidly by the aqueous‐phase reactions O2− + O3 and H2O2 + hν and is removed primarily by oxidation of H2C(OH)2, H2O2, and HCOO−. Gas‐droplet transfer of OH must be modeled as a reversible process, that is, the droplets cannot be assumed to be diffusion‐limited OH(g) sinks. A strong OH(aq) concentration gradient exists between the surface and the interior of the droplets. The concentration of OH(aq) is strongly dependent on pH but is only weakly dependent on the sticking coefficient, the droplet radius, or the liquid water content of the cloud. Formic acid is rapidly produced by the aqueous‐phase reaction H2C(OH)2 + OH, but HCOO− is in turn rapidly oxidized by OH(aq). The HCOOH concentration in cloud is shown to be strongly dependent on cloud water pH; clouds with pH greater than 5 are not efficient HCOOH sources. A novel mechanism is proposed for the oxidation of S(IV) by OH(aq). The main product is predicted to be HSO5− (peroxymonosulfate). Peroxymonosulfate appears to be stable in remote clouds and could contribute a large fraction of total cloud water sulfur.