Atmospheric chemistry in East Asia determines the iron solubility of aerosol particles supplied to the North Pacific Ocean

Abstract. The deposition of dissolved iron (d-Fe) from East Asian aerosols to the North Pacific Ocean modulates primary productivity in surface waters, facilitating uptake of atmospheric carbon dioxide by the ocean, thereby impacting global climate. Since the microorganisms in the surface seawater utilize d-Fe as a micronutrient, bioavailability of aerosol Fe depends on its solubility (Fe_sol%). Although Fe_sol% is influenced by both emission sources and atmospheric processing, their effects on Fe_sol% are not fully understood. We assessed the factors controlling Fe_sol% in size-fractionated aerosol particles collected along the Sea of Japan coast for one year (July 2019–June 2020). Approximately 70 % of d-Fe in East Asian aerosols was present in fine aerosol particles (<1.3 µm), with Fe_sol% ranging from 4.1 % to 94.9 %. Anthro-Fe accounted for about 50 % of d-Fe in fine aerosol particles during periods outside the COVID-19 lockdown, but its contribution was negligible during the lockdown. The Fe_sol% in fine aerosol particles correlated with the abundance of water-soluble Fe species (Fe(II, III)-sulfates and Fe(III)-oxalate). These water-soluble Fe species were detected in both mineral dust and anthropogenic aerosols in fine aerosol particles. Dissolution models optimized for Fe in mineral dust and anthropogenic aerosols showed that Fe in both aerosol types dissolved by proton-promoted dissolution under acidic conditions (pH < 2.0). Subsequently d-Fe dissolved from aerosols was stabilized by the formation of Fe(III)-oxalate in the aqueous phase. Thus, comprehensive understandings of the chemical alteration processes of East Asian aerosols are essential for accurately quantifying their Fe_sol% upon transport to the North Pacific.