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 seawaters, facilitating uptake of atmospheric carbon dioxide by the ocean, thereby affecting the global climate. Given that microorganisms in surface seawater utilize d-Fe as a micronutrient, the bioavailability of aerosol Fe depends on its fractional solubility (Fesol%). Although emission sources and atmospheric processing influence Fesol%, their effects on Fesol% are incompletely understood. We assessed the factors controlling Fesol% in size-fractionated aerosol particles collected along the coast of the Sea of Japan over 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 Fesol% ranging from 4.1 % to 94.9 %. Anthro-Fe accounted for approximately 50 % of d-Fe in fine aerosol particles during pre- and post-lockdown of COVID-19, but its contribution was negligible during the COVID-19 lockdown period. Fesol% in fine aerosol particles was correlated with the abundance of water-soluble Fe species (Fe-sulfates and Fe-oxalate). These water-soluble Fe species were detected in mineral dust and anthropogenic aerosols in fine aerosol particles. Dissolution models optimized for Fe in mineral dust and anthropogenic aerosols demonstrated that Fe in both aerosols dissolved through 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. Therefore, a comprehensive understanding of the chemical alteration processes of East Asian aerosols is essential for accurately quantifying their Fesol% upon transport to the North Pacific.