Mechanisms for the release of halogens from sea‐salt particles by free radical reactions

Abstract

A possible source of gas phase bromine in the Arctic winter and early spring is the sea‐salt aerosol. In this paper, chemical mechanisms for the release of photochemically active bromine from sea‐salt are examined. The first of these is oxidation of bromide to elemental bromine by peroxymonosulfuric acid (Caro's acid) produced by the free radical chain oxidation of S(IV). The chain reaction could be initiated in the dark by NO 2 or, following polar sunrise, by the reaction of ozone with Superoxide. Although the yield of Caro's acid at 298 K is small, the yield may increase at the low temperatures encountered in the Arctic. This could result in the conversion of a large fraction of the initial sea‐salt alkalinity to Caro's acid. Caro's acid is known to oxidize bromide to Br 2. Since this mechanism requires low temperatures and high SO 2 concentrations, it is only effective during the winter and early spring and should not oxidize significant amounts of halides in the global marine boundary layer. A second possible mechanism is the free radical oxidation of aqueous bromide to bromine by OH and HO 2. This may be effective at moderate p H and may contribute to Br cycling on a global scale. The evaluation of both mechanisms is highly uncertain because of incomplete physical‐chemical data. In addition to these primary release mechanisms, there are possible autocatalytic cycles for the release of bromine from sea‐salt. These involve the gas phase production of either BrO or HOBr and the return of these species to the sea‐salt particles where they initiate additional oxidation of bromide. The efficiency of these cycles should depend critically on the relative amounts of HOBr and HBr produced by the gas phase chemistry. These mechanisms do not appear to be effective as sources of photochemically active Cl.