Formation of hydrogen peroxide and depletion of oxalic acid in atmospheric water by photolysis of iron(III)-oxalato complexes

The generation of hydrogen peroxide (H₂O₂) and the depletion of oxalic acid by photochemical/chemical cycling of Fe(III)/Fe(II)-oxalato complexes in sunlight has been studied under the conditions typical for acidified atmospheric water. H₂O₂ is produced through the reduction of oxygen by intermediates formed from photoreactions of Fe(III)-oxalato complexes. The rate of H₂O₂ formation increases with sunlight intensity, and with oxalate and Fe(III) concentration within the concentration range used. This rate is 3.7 nM s⁻¹ when 1 μM Fe(III) and 5 μM oxalate at pH 4 is exposed to September noon sunlight. Speciation calculations based on the concentration range of Fe(III) and oxalic acid in atmospheric water indicate that Fe(III)-oxalato complexes are often the predominant species of dissolved Fe(III). The concentrations of Fe(III)-oxalato complexes are sufficiently large to make their photolysis a dominant source of in-cloud H₂O₂, O₂·-, HO₂, and OH radicals and a major sink for atmospheric oxalic acid.