PHOTOREDUCTION OF IRON(III) IN MARINE MINERAL AEROSOL SOLUTIONS

Although there have been a number of studies of the solubility of Fe in marine mineral aerosols, there have been few studies of the oxidation states of the soluble iron fraction and of the factors that affect the solubility of iron in aerosol solutions. In this Paper we present measurements of the concentrations of total Fe (including particulate), total soluble Fe and total soluble Fe(II) in marine aerosol particles. Only 1% of the total Fe and 7.5% of the soluble Fe was in the Fe(II) form. Photolysis experiments were performed with solutions extracted from aerosol filter samples and with solutions of Fe(III) in acidic sodium chloride. In both systems, Fe(II) concentrations increased rapidly when the solutions were exposed to sunlight and they attained steady state within an hour. However, in all cases Fe(II) is only a minor component even when conditions are favorable for photolysis. Fe(II) formation is hindered at the low pH that is believed to be characteristic of marine aerosol solutions. Solutions with added oxalate yielded greatly increased concentrations of Fe(II); this is probably related to the fact that Fe(III) oxalate complexes have strong ligand to metal charge transfer bands in the tropospheric solar UV-visible region. However, the presence of oxalate also leads to the formation of H2O2; when the radiation level decreased, the Fe(II) was partially or totally oxidized back to Fe(III) due to reactions with H2O2. The photoreduction of Fe(III) to Fe(II) did not appear to significantly increase the dissolution of Fe(III) from the dust mineral matrix.