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: September 2023

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
(Fesol %) 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] / [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 Fesol % in fine aerosol particles (11.4 ± 7.0 %) 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 ± 0.80). 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 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] / [d-Al] ratios of fine aerosol particles ranged from 0.386 to 4.67,
and [d-Fe] / [d-Al] ratios greater than 1.50 cannot be explained by proton-
and ligand-promoted dissolutions (1.00 < [d-Fe] / [d-Al] < 1.50). The [d-Fe] / [d-Al] ratio correlated with the enrichment factor of Fe
in fine aerosol particles (r: 0.505), which indicates 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 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] / [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.