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: July 2022

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.