Iron (Fe) speciation in size-fractionated aerosol particles in the Pacific Ocean: The role of organic complexation of Fe with humic-like substances in controlling Fe solubility
Authors: Sakata Kohei , Kurisu Minako , Takeichi Yasuo , Sakaguchi Aya , Tanimoto Hiroshi , Tamenori Yusuke , Matsuki Atsushi , Takahashi Yoshio 
Published: March 2022

Abstract. Atmospheric deposition is one of the dominant sources of dissolved Fe on the ocean surface. Atmospheric processes are recognized as controlling fractional Fe solubility (Fesol%) in marine aerosol particles, but the impact of these processes on Fesol% remains unclear. One of the reasons for this is the lack of field observations focusing on the relationship between Fesol% and Fe species in the marine aerosol particles. In particular, the effects of organic ligands on the Fesol% have not been well investigated through observational studies. In this study, Fe species in size-fractionated aerosol particles in the Pacific Ocean were determined by X-ray absorption fine structure (XAFS) spectroscopy. The internal mixing states of Fe with organic carbons were investigated using scanning transmission X-ray microscopy (STXM). The effects of atmospheric processes on Fesol% in the marine aerosol particles were investigated based on these speciation results. Iron in size-fractionated aerosol particles was mainly derived from mineral dust regardless of aerosol diameter because the enrichment factor of Fe was almost 1 in both coarse (PM1.3-10.2) and fine aerosol particles (PM1.3). About 80 % of total Fe (insoluble + labile Fe) was present in coarse aerosol particles (PM1.3-10.2), whereas labile Fe was mainly present in fine aerosol particles (PM1.3). The Fesol% in PM1.3-10.2 was not well increased (2.56±2.53 %, 0.00–8.50 %, n = 20) by the atmospheric processes because mineral dust was not acidified beyond the buffer capacity of calcite. By contrast, mineral dust in PM1.3 was acidified beyond the buffer capacity of calcite. As a result, Fesol% in PM1.3 (0.202–64.7 %, n = 10) is an order of magnitude higher than those in PM1.3-10.2. The PM1.3 contained ferric organic complexes with humic-like substances (Fe(III)-HULIS, but not included Fe-oxalate complexes), of which abundance correlated with Fesol%. The Fe(III)-HULIS was formed during transport in the Pacific Ocean since the Fe(III)-HULIS was not found in aerosol particles in Beijing and Japan. The pH estimations of mineral dust in PM1.3 revealed that Fe was solubilized by proton-promoted dissolution under highly acidic conditions (pH < 3.0), whereas Fe(III)-HULIS was stabilized under moderately acidic conditions (pH: 3.0–6.0). Since the observed labile Fe concentration could not be reproduced by proton-promoted dissolution under moderately acidic conditions, the pH of mineral dust was increased after proton-promoted dissolution. The cloud process in the marine atmosphere increased the pH of mineral dust because the dust particles were covered with organic carbons and Na. At this stage, the precipitation of ferrihydrite was suppressed by Fe(III)-HULIS because of its high water solubility. Thus, the organic complexation of Fe with HULIS plays a significant role in the stabilization of Fe initially solubilized by proton-promoted dissolution.