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

Abstract

Abstract. Atmospheric deposition is one of the main sources of dissolved iron (Fe) in the ocean surfaces. Atmospheric processes are recognized as controlling fractional Fe solubility (Fesol%) in marine aerosol particles. However, 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 marine aerosol particles. In particular, the effects of organic ligands on Fesol% have not been thoroughly investigated in observational studies. In this study, Fe species in size-fractionated aerosol particles in the Pacific Ocean were determined using X-ray absorption fine structure (XAFS) spectroscopy. The internal mixing states of Fe and organic carbon were investigated using scanning transmission X-ray microscopy (STXM). The effects of atmospheric processes on Fesol% in marine aerosol particles were investigated based on the 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 (PM>1.3) and fine aerosol particles (PM1.3). Approximately 80 % of the total Fe (insoluble + labile Fe) was present in PM>1.3, whereas labile Fe was mainly present in PM1.3. The Fesol% in PM>1.3 was not significantly 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. In 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) was an order of magnitude higher than that in PM>1.3. The PM1.3 contained ferric organic complexes with humic-like substances (Fe(III)-HULIS, but not Fe-oxalate complexes), and the abundance correlated with Fesol%. Iron(III)-HULIS was formed during transport in the Pacific Ocean because Fe(III)-HULIS was not found in aerosol particles in Beijing and Japan. The pH estimations of mineral dust in PM1.3 established 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 increased after proton-promoted dissolution. The cloud process in the marine atmosphere increases the mineral dust pH because the dust particles are covered with organic carbon and Na. The precipitation of ferrihydrite was suppressed by Fe(III)-HULIS owing to its high water solubility. Thus, the organic complexation of Fe with HULIS plays a significant role in the stabilization of Fe that was initially solubilized by proton-promoted dissolution.

Role of Dust and Iron Solubility in Sulfate Formation during the Long-Range Transport in East Asia Evidenced by 17O-Excess Signatures

“…A, although the anthropogenic emission of TMIs is accurately considered based on the latest emission data set, the modeled estimates of solubilities of anthropogenic and dust Fe and Mn remain highly uncertain in cloud water, as pointed out previously. 14 , 29 , 31 , 33 The increased Fe solubilities have been indicated based on global model comparisons with multiple field campaigns over the Northwest Pacific 57 and also observed toward the downstream region of East Asia, 58 , 59 mainly because fine mineral aerosols can be acidified due to air pollution. Thus, for Exp.…”

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

“…9b). In addition, non-magnetic aluminosilicates of coal combustions and MSWI could not increase the EF of Fe in fine aerosol particles because the EF of Fe in these ashes was almost 1 (Sakata et al, 2022;Li et al, 2022). Therefore, non-magnetic aluminosilicates derived from coal combustion and MSWI were unlikely the dominant source of aerosol particles with high Fesol% and [d-Fe]/[d-Al] ratios.…”

“…6e and 6f). Enrichment of d-Fe and d-Al in size-fractionated aerosol samples has been reported not only in the urban atmosphere (Fang et al, 2017;Hsieh et al, 2022) but also in the marine atmosphere (McDaniel et al, 2019;Baker et al, 2020;Sakata et al, 2022). These previous studies have reported that the higher d-Fe and d-Al in coarse aerosol particles were caused by either or both (i) contamination of anthropogenic aerosol with high Fesol% and Alsol% and (ii) solubilization of Fe and Al in fine aerosol particles by atmospheric processes.…”

“…In the present study, we focused on biotite, illite, and Fe-rich chlorite as the representative mineral species of aluminosilicates in aerosol particles because these aluminosilicates were detected in aerosol particles collected in East Asia (Takahashi et al, 2011;Kurisu et al, 2016a;Sakata et al, 2022). When Fe and Al in these aluminosilicates were leached by proton-promoted dissolution (pH 1.0-7.0) under oxic conditions, the [d-Fe]/[d-Al] ratios of biotite, illite, and Fe-rich chlorite was 0.427-0.930 (average: 0.776 ± 0.152), 0.156-0.689 (average: 0.404 ± 0.189), and 0.142-1.03 (average: 0.699 ± 0.303), respectively (Fig.…”

“…In the chemical aging of Fe-bearing particles during transport from East Asia to the Pacific Ocean, atmospheric processing after passing over Japan has a small effect on the Fesol% of marine aerosol particles (Buck et al, 2013;Sakata et al, 2022).…”