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

Abstract

The atmospheric deposition of iron (Fe) promotes primary production in the surface ocean, which results in the enhanced uptake of carbon dioxide into surface seawater. Given that microorganisms in seawater utilize dissolved Fe (d-Fe) as a nutrient, the bioavailability of Fe in aerosol particles depends on its solubility. However, the factors controlling fractional Fe solubility (Fesolg%) in aerosol particles have not been fully understood. This study performed annual observations of the total and dissolved metal concentrations in size-fractionated (seven fractions) aerosol particles at Higashi-Hiroshima, Japan. The feasibility of the molar concentration ratio of d-Fe relative to dissolved Al ([d-Fe]g/g[d-Al]) as an indicator of sources of d-Fe in aerosol particles was investigated 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-and ligand-promoted dissolutions). Approximately 70g% of the total Fe in total suspended particulates (TSPs) was present in coarse aerosol particles, whereas about 70g% of d-Fe in TSPs was mainly found in fine aerosol particles. The average Fesolg% in fine aerosol particles (11.4g±g7.0g%) was higher than that of coarse aerosol particles (2.19g±g2.27g%). In addition, the average ratio of [d-Fe]g/g[d-Al] in coarse aerosol particles (0.408g±g0.168) was lower than that in fine aerosol particles (1.15g±g0.80). The range of [d-Fe]g/g[d-Al] ratios in the coarse aerosol particles (0.121-0.927) was similar to that obtained by proton-promoted dissolution of mineral dust (0.1-1.0), which indicates that the d-Fe in coarse aerosol particles was derived from mineral dust. The [d-Fe]g/g[d-Al] ratios of fine aerosol particles ranged from 0.386 to 4.67, and [d-Fe]g/g[d-Al] ratios greater than 1.50 cannot be explained by proton-and ligand-promoted dissolutions (1.00g