Enrichment in More Soluble Ferric Iron (Fe(III)_S) during Dust Transport by Nitric Acid Uptake

Iron (Fe) redox cycling in dust aerosols impacts the supply of bioavailable Fe to marine ecosystems and the formation of reactive oxygen species. It has been suggested that photochemical processes taking place on mineral particles could reduce large fractions of Fe(III) to Fe(II), which is a more bioavailable form of Fe. Moreover, soluble Fe(III) is also bioavailable. Here, we carried out atmospheric observations and laboratory simulations to show that dust aging processes influenced Fe(II)/Fe(III) redox cycling and enhanced the Fe solubility during long-range transport. Our results show one contrast phenomenon: increasing Fe solubility accompanied by more soluble Fe(III) enrichment in aged dust aerosols. The increasing NO_X-to-SO₂ ratios and thus the increasing availability of HNO₃ enhance soluble Fe(III)-to-Fe(II) ratios. The dissolution experiment of Fe-containing dust demonstrated that high nitrate concentrations suppressed soluble Fe(II) formation at low pH. This enrichment is likely due to a surface-mediated redox mechanism involving nitric acid and Fe(II) when dust plumes are mixed with urban air pollutants. This mechanism is potentially the major mechanism for enhancing Fe solubility in aged dust aerosols and should be included by modeling to better understand the effect of Fe(II)/Fe(III) redox cycling on ocean biological productivity and atmospheric oxidation capacity.