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 O 3(aq) and OH(aq). The radical OH(aq) is produced rapidly by the aqueous‐phase reactions O 2− + O 3 and H 2 O 2 + h ν and is removed primarily by oxidation of H 2 C(OH)2, H 2 O 2, 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 p H 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 H 2 C(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 p H; clouds with p H 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 HSO 5− (peroxymonosulfate). Peroxymonosulfate appears to be stable in remote clouds and could contribute a large fraction of total cloud water sulfur.