SUNLIGHT-INITIATED PARTIAL INHIBITION OF THE DISSOLVED IRON(III)-CATALYZED OXIDATION OF S(IV) SPECIES BY MOLECULAR-OXYGEN IN AQUEOUS-SOLUTION

The dissolved Fe(III)-catalysed oxidation of S(IV) species [SO2(aq), HSO3-/HOSO2-, SO3(2-)] by molecular oxygen is partially inhibited by sunlight, relative to dark controls, under condition that are realistic for tropospheric clouds and fogs {initial [S(IV)] = 4.0-68 muM, total [Fe(III)] = 38-750 nM, pH = 2.8-4.6}. Due to the unusual effect of sunlight on this reaction, empirical rate expressions for the Fe(III)-catalysed oxidation of S(IV) by O2 should not be used to predict daytime rates of H2SO4 formation in atmospheric hydrometeors via this pathway.