Photon Factory Activity Report 2019 #37 (2020)
BL-4A, BL-12C, BL-13A.BL-19A/2018G575, 2018S1001, 2019G093

Speciation and isotope ratios of Fe in biomass burning aerosols
and anthropogenic aerosols
Minako KURISU,1,2 Kohei SAKATA,3 Chihiro MIYAMOTO,1 and Yoshio TAKAHASHI1
1
The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
2
Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho,Yokosuka, 237-0061, Japan
3
Center for Global Environmental Research, National Institute Environmental Studies, 16-2Onogawa, Tsukuba, Ibaraki 305-8506, Japan.
1. Introduction
Iron (Fe) in combustion aerosols emitted mainly byhuman activities exerts various effects on the environmentand human health. The deficiency of dissolved Fe limits
phytoplankton growth in some areas in the open ocean.
Since supply of Fe to the surface ocean stimulates primary
production and affects carbon and nitrogen cycles, manystudies have been conducted regarding Fe cycle in theocean. The main sources of Fe to the surface ocean are
natural aerosols (mainly aeolian dust), dissolution ofcoastal sediment, and hydrothermal vents, whereas Fe incombustion aerosols has been recognized as anotherpossible Fe source due to its high solubility to seawater. In
addition, biomass burning is an important source of soluble
Fe transported to the open ocean. Iron isotope ratios can beused as a tracer because Fe emitted by combustion canyield low Fe isotope ratios due to isotope fractionation
during evaporation processes. However, data on Fe isotope
ratios of various aerosol particles such as naturallyoccurring aerosols, volcanic ash, biomass burning aerosols
and anthropogenic aerosols emitted during combustion
processes are lacking. Moreover, speciation of Fe inaerosols is also linked to the isotopic fractionation, sinceFe emitted by the combustion processes mainly take Fe
oxides and Fe hydroxide forms. Thus, combination of
speciation of Fe in aerosols and determination of Fe
isotope ratio is important to identify and quantify the Fe inaerosols supplied by various natural and anthropogenic
processes.
2. Fe in aerosols emitted by biomass burning
Biomass burning is an important source of soluble Fetransported to the open ocean; however, its exact
contribution remains unclear. Iron isotope ratios (δ56Fe)
can be used as a tracer because Fe emitted by combustion
can yield low Fe isotope ratios due to isotope fractionation
during evaporation processes. However, data on Fe isotope
ratios of aerosol particles emitted during biomass burning
are lacking. We collected size-fractionated aerosol samples
before, during, and after a biomass burning event andcompared their Fe isotope ratios. On the basis of theconcentrations of several elements and Fe species
determined by X-ray absorption fine structure (XAFS)
spectroscopy and micro-XAFS conducted at BL-12C and
BL-4A, respectively, we found that Fe emitted during theevent mainly comprised suspended soil particles in all thesize fractions. Iron isotope ratios of fine particles before
and after the event were low due to the influence of otheranthropogenic combustion sources, but they were closer tothe crustal value during the event because of the influenceof Fe from suspended soil. In this study, functional group
analysis of carbon in each aerosol particle by scanningtransmission X-ray microscopy (STXM) coupled by X-ray
absorption near-edge structure (XANES) measured at BL13A/BL-19A was also used to estimate temperature duringformation of biomass burning processes.
3. Fe in aerosols emitted from Fe steel plant
On the other hand, Fe in anthropogenic aerosols
emitted from a steel plant was also investigated in our study,
and its formation process of anthropogenic Fe with low
δ56Fe was discussed. The samples contained a large
amount of submicron spherical Fe oxide particles in finesize fraction, suggesting that the particles were emitted byhigh-temperature evaporation. Fine particles yielded muchlower δ56Fe values (as low as= −3.34±0.36‰) than coarse
particles (on average 0.09±0.19‰), indicating that Fe
isotope fractionation occurred during evaporation in thesteel plant. The Fe speciation emitted from the plant wasalso examined by X-ray absorption fine structure (XAFS)
spectroscopy and micro-XAFS conducted at BL-12C andBL-4A, respectively, it was found that Fe species weremainly wüstite (FeO), magnetite (Fe3O4), ferrihydrite,
maghemite (β-Fe2O3), and hematite (α-Fe2O3), but naturalminerals of Fe-containing aluminosilicates (e.g., biotiteand illite) were scarce, suggesting that Fe were mainly
from the steel plant. Based on the isotope mixing plot andresults of extraction experiment, the estimated δ56Fe value
for evaporated Fe was −4.1‰. From mass balancecalculation using −4.1‰ as the representative δ56Fe value
for evaporated Fe, its contribution was approximately
43%±8% of the total soluble Fe in aerosols in a part of theNorthwest Pacific, implying the large contribution ofanthropogenic Fe.
References[1] M. Kurisu and Y. Takahashi, Atmosphere 10, 76 (2019).
[2] M. Kurisu et al., ACS Earth Space Chem. 3, 588 (2019).
* ytakaha@eps.s.u-tokyo.ac.jp