Contribution of combustion Fe in marine aerosols over the northwestern Pacific estimated by Fe stable isotope ratios
Authors: Kurisu Minako , Sakata Kohei , Uematsu Mitsuo , Ito Akinori , Takahashi Yoshio 
Published: October 2021

Abstract. The source apportionment of aerosol iron (Fe), including
natural and combustion Fe, is an important issue because aerosol Fe can
enhance oceanic primary production in the surface ocean. Based on our
previous finding that combustion Fe emitted by evaporation processes has Fe
isotope ratios (δ56Fe) that are approximately 4 ‰ lower than those of natural Fe, this study aimed to
distinguish aerosol Fe sources over the northwestern Pacific using two
size-fractionated marine aerosols. The δ56Fe values of fine and
coarse particles from the eastern or northern Pacific were found to be
similar to each other, ranging from 0.0 ‰ to 0.4 ‰. Most
of them were close to the crustal average, suggesting the dominance of
natural Fe. On the other hand, particles from the direction of East Asia
demonstrated lower δ56Fe values in fine particles (−0.5 ‰ to
−2.2 ‰) than in coarse particles (on average
−0.02 ± 0.12 ‰). The correlations between the
δ56Fe values and the enrichment factors of lead and vanadium
suggested that the low δ56Fe values obtained were due to the
presence of combustion Fe. The δ56Fe values of the soluble
component of fine particles in this region were lower than the total,
indicating the preferential dissolution of combustion Fe. In addition, we
found a negative correlation between the δ56Fe value and the
fractional Fe solubility in air masses from the direction of East Asia.
These results suggest that the presence of combustion Fe is an important
factor in controlling the fractional Fe solubility in air masses from the
direction of East Asia, whereas other factors are more important in the
other areas. By assuming typical δ56Fe values for combustion
and natural Fe, the contribution of combustion Fe to the total
(acid-digested) Fe in aerosols was estimated to reach up to 50 % of fine
and 21 % of bulk (coarse + fine) particles in air masses from the
direction of East Asia, whereas its contribution was small in the other
areas. The contribution of combustion Fe to the soluble Fe component
estimated for one sample was approximately twice as large as the total,
indicating the importance of combustion Fe as a soluble Fe source despite
lower emissions than the natural. These isotope-based estimates were
compared with those estimated using an atmospheric chemical transport model
(IMPACT), in which the fractions of combustion Fe in fine particles,
especially in air masses from the direction of East Asia, were consistent
with each other. In contrast, the model estimated a relatively large
contribution from combustion Fe in coarse particles, probably because of the
different characteristics of combustion Fe that are included in the model
calculation and the isotope-based estimation. This highlights the importance
of observational data on δ56Fe for size-fractionated aerosols
to scale the combustion Fe emission by the model. The average deposition
fluxes of soluble Fe to the surface ocean were 1.4 and 2.9 nmol m−2 d−1 from combustion and natural aerosols, respectively, in air masses
from the direction of East Asia, which suggests that combustion Fe could be an
important Fe source to the surface seawater among other Fe sources.
Distinguishing Fe sources using the δ56Fe values of marine
aerosols and seawater is anticipated to lead to a more quantitative
understanding of the Fe cycle in the atmosphere and surface ocean.