Measurement report: Stoichiometry of dissolved iron and aluminum as an indicator of the factors controlling the fractional solubility of aerosol iron: Results of the annual observations of size-fractionated aerosol particles in Japan
Authors: Sakata Kohei , Sakaguchi Aya , Yamakawa Yoshiaki , Miyamoto Chihiro , Kurisu Minako , Takahashi Yoshio 
Published: March 2023

Abstract. Atmospheric deposition of iron (Fe) in aerosol particles is enhanced primary production on the ocean surface, resulting in promoting the uptake of carbon dioxide into the surface seawater. Atmospheric deposition of iron (Fe) promotes primary production in the surface ocean, resulting in enhanced uptake of carbon dioxide into surface seawater. Since microorganisms in seawater utilize dissolved Fe (d-Fe) as a nutrient, the bioavailability of Fe in aerosol particles depends on its solubility. However, factors controlling fractional Fe solubility (Fesol%) in aerosol particles have not been fully understood. This study performed annual observations of Fesol% in size-fractionated (seven fractions) aerosol particles at Higashi-Hiroshima, Japan. In particular, the feasibility of the molar concentration ratio of d-Fe relative to dissolved Al ([d-Fe]/[d-Al]) as an indicator of the sources of d-Fe in aerosol particles because this ratio is likely dependent on the emission sources of Fe (e.g., mineral dust, fly ash, and anthropogenic Fe oxides) and their dissolution processes (proton-promoted and ligand-promoted dissolutions). Approximately 70 % of total Fe and dissolved Fe was present in coarse and fine aerosol particles, respectively, and the average Fesol% in fine aerosol particles (11.4 ± 6.97 %) was higher than that of coarse aerosol particles (2.19 ± 2.27 %). In addition, the average ratio of [d-Fe]/[d-Al] in coarse aerosol particles (0.408 ± 0.168) was lower than that in fine aerosol particles (1.15 ± 0.803). The range of [d-Fe]/[d-Al] ratios in the coarse aerosol particles (0.121–0.927) was similar to that obtained by proton-promoted dissolutions of mineral dust (0.1–1.0), indicating that d-Fe in coarse aerosol particles were derived from mineral dust. The [d-Fe]/[d-Al] ratios of aerosol particles ranged from 0.386 to 4.67, and [d-Fe]/[d-Al] ratios greater than 1.5 cannot be explained by proton-promoted dissolution and ligand-promoted dissolution (1.0 < [d-Fe]/[d-Al] < 1.5). The [d-Fe]/[d-Al] ratio correlated with the enrichment factor of Fe in fine aerosol particles (r: 0.505), indicating that anthropogenic Fe with a high [d-Fe]/[d-Al] ratio was the source of d-Fe in fine aerosol particles. The high [d-Fe]/[d-Al] ratio was attributed to Fe-oxides emitted from high-temperature combustions (high-temp-FeOx). Finally, the fraction of high-temp-FeOx to d-Fe in total suspended particulate (TSP) was calculated based on the [d-Fe]/[d-Al] ratio of aerosols and their emission source samples. As a result, the fraction of high-temp-FeOx to d-Fe in TSP varied from 1.48 to 80.7 %. The high fraction was found in summer when air masses originated from industrial regions in Japan. By contrast, approximately 10 % of d-Fe in the TSP samples collected in spring and during Asian dust events was derived from high-temp-FeOx, when air masses were frequently transported from East Asia to the Pacific Ocean. Thus, mineral dust is the dominant source of d-Fe in Asian outflow to the Pacific Ocean.