Low-intensity radiolysis study of free-radical reactions in cloudwater: H sub 2 O sub 2 production and destruction. [Gamma radiation]

Reactions in cloudwater can be important pathways for chemical transformation of atmospheric trace gases. One such reaction is the oxidation of dissolved sulfur dioxide by hydrogen peroxide. H{sub 2}O{sub 2} is formed by the disproportionation of hydroperoxyl and superoxide radicals, O{sub 2}({minus}I). The authors report measurements of the rate of H{sub 2}O{sub 2} production from O{sub 2}({minus}I) radicals generated by low-intensity cobalt-60 radiolysis of synthetic cloudwater solutions and actual precipitation samples. The authors results, employing O{sub 2}({minus}I) production rates comparable to those expected upon transfer of HO{sub 2} from interstitial cloud air to cloudwater, confirm model predictions that H{sub 2}O{sub 2} production if frequently the major fate of O{sub 2}({minus}I) radicals. However, there is evidence of significant reaction between S(IV) and O{sub 2}({minus}I), with a rate coefficient of (3 {plus minus} 2) {times} 10{sup 4} at pH 4.96. In addition, the presence of 1 {mu}M dissolved iron decreases the H{sub 2}O{sub 2} yield, principally because of destruction of H{sub 2}O{sub 2} by Fe(II).