Iron(II)-catalyzed photochemical decomposition of oxalic acid and generation of H2O2 in atmospheric liquid phases

Although the iron(III) -catalyzed decomposition of organic acids and formation of H2O2 in sunlit atmospheric water droplets has been well documented, field measurements have shown that a large fraction of the dissolved iron in fogs, clouds, and rainwaters is present as reduced Fe(II) species. In this study, we report on Fe(II)-initiated photochemical depletion of oxalic acid and formation of H2O2 in atmospheric water droplets. Our results show that the Fe(II)/Fe(III) couple is involved in the photocatalytic decomposition of oxalic acid and formation of H2O2 with Fe(III) being the more active catalyst species. The initial oxidation state of iron is not critical for the catalytical processes, since the photochemical cycling between Fe(II) and Fe(III) occurs rapidly in the atmospheric droplets.