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

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 (Feinline-formula_sol %) 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] inline-formula $M2inlinescrollmathml/$ 8pt14ptsvg-formulamathimg527256ea34e0af356380afd605ccefc0 acp-23-9815-2023-ie00001.svg8pt14ptacp-23-9815-2023-ie00001.png  [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 70 % of the total Fe in total suspended particulates (TSPs) was present in coarse aerosol particles, whereas about 70 % of d-Fe in TSPs was mainly found in fine aerosol particles. The average Feinline-formula_sol % in fine aerosol particles (11.4 inline-formula± 7.0 %) was higher than that of coarse aerosol particles (2.19 inline-formula± 2.27 %). In addition, the average ratio of [d-Fe] inline-formula $M6inlinescrollmathml/$ 8pt14ptsvg-formulamathimg073414a2b77546d8d5847ae97897d626 acp-23-9815-2023-ie00002.svg8pt14ptacp-23-9815-2023-ie00002.png  [d-Al] in coarse aerosol particles (0.408 inline-formula± 0.168) was lower than that in fine aerosol particles (1.15 inline-formula± 0.80). The range of [d-Fe] inline-formula $M9inlinescrollmathml/$ 8pt14ptsvg-formulamathimg880d1b22cfae9b4167ff115d05c6894c acp-23-9815-2023-ie00003.svg8pt14ptacp-23-9815-2023-ie00003.png  [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] inline-formula $M10inlinescrollmathml/$ 8pt14ptsvg-formulamathimg3af55808dad7e355d8e0b0b2a0272ce7 acp-23-9815-2023-ie00004.svg8pt14ptacp-23-9815-2023-ie00004.png  [d-Al] ratios of fine aerosol particles ranged from 0.386 to 4.67, and [d-Fe] inline-formula $M11inlinescrollmathml/$ 8pt14ptsvg-formulamathimg165b352473919034209a9d51d0eaf41d acp-23-9815-2023-ie00005.svg8pt14ptacp-23-9815-2023-ie00005.png  [d-Al] ratios greater than 1.50 cannot be explained by proton- and ligand-promoted dissolutions (1.00 inline-formula< [d-Fe] inline-formula $M13inlinescrollmathml/$ 8pt14ptsvg-formulamathimg8550e2e9970f84100ffbfa4da4f4f543 acp-23-9815-2023-ie00006.svg8pt14ptacp-23-9815-2023-ie00006.png  [d-Al] inline-formula< 1.50). The [d-Fe] inline-formula $M15inlinescrollmathml/$ 8pt14ptsvg-formulamathimg7572a9d7afeaa92ba0e8bb6f686362bd acp-23-9815-2023-ie00007.svg8pt14ptacp-23-9815-2023-ie00007.png  [d-Al] ratio correlated with the enrichment factor of Fe in fine aerosol particles (inline-formular: 0.505), which indicates that anthropogenic Fe with a high [d-Fe] inline-formula $M17inlinescrollmathml/$ 8pt14ptsvg-formulamathimga3a809672b156f3719eee3cbaf593ee5 acp-23-9815-2023-ie00008.svg8pt14ptacp-23-9815-2023-ie00008.png  [d-Al] ratio was the source of d-Fe in fine aerosol particles. The high [d-Fe] inline-formula $M18inlinescrollmathml/$ 8pt14ptsvg-formulamathimg62d2c8208bbdf49afb8db19c9f7b6b50 acp-23-9815-2023-ie00009.svg8pt14ptacp-23-9815-2023-ie00009.png  [d-Al] ratio was attributed to anthropogenic Fe oxides emitted from high-temperature combustions. Finally, the fraction of anthropogenic Fe oxides to d-Fe in TSPs was calculated based on the [d-Fe] inline-formula $M19inlinescrollmathml/$ 8pt14ptsvg-formulamathimg2cb9305d3133b5ddd344bed6f97e59c4 acp-23-9815-2023-ie00010.svg8pt14ptacp-23-9815-2023-ie00010.png  [d-Al] ratio of aerosols and their emission source samples. As a result, the fraction of anthropogenic Fe oxides to d-Fe in TSPs varied from 1.48 % to 80.7 %. A high fraction was observed in summer when air masses originated from industrial regions in Japan. By contrast, approximately 10 % of d-Fe in the TSPs collected in spring and during Asian dust events was derived from anthropogenic Fe oxides when air masses were frequently transported from East Asia to the Pacific Ocean. Thus, mineral dust was the dominant source of d-Fe in Asian outflow to the Pacific Ocean.