Abstract
Observations show that the fractional solubility
of Fe (FS-Fe, percentage of dissolved to total Fe) in
dust aerosol increases considerably from 0.1% in regions
of high dust mass concentration to 80% in remote regions
where concentrations are low. Here, we combined laboratory
geochemical measurements with global aerosol model
simulations to test the hypothesis that the increase in FS-Fe
is due to physical size sorting during transport. We determined
the FS-Fe and fractional solubility of Al (FS-Al) in
size-fractionated dust generated from two representative soil
samples collected from known Saharan dust source regions
using a customized dust re-suspension and collection system.
The results show that the FS-Fe is size-dependent and
ranges from 0.1–0.3% in the coarse size fractions (>1 μm)
to 0.2–0.8% in the fine size fractions (<1 μm). The FSAl
shows a similar size distribution to that of the FS-Fe. The
size-resolved FS-Fe data were then combined with simulated
dust mass concentration and size distribution data from a
global aerosol model, GLOMAP, to calculate the FS-Fe of
dust aerosol over the tropical and subtropical North Atlantic
Ocean. We find that the calculated FS-Fe in the dust aerosol
increases systematically from 0.1% at high dust mass concentrations
(e.g., >100 μgm−3) to 0.2% at low concentrations
(<100 μgm−3) due to physical size sorting (i.e., particle
gravitational settling). These values are one to two orders
of magnitude smaller than those observed on cruises
across the tropical and sub-tropical North Atlantic Ocean under
an important pathway of Saharan dust plumes for similar dust mass concentrations. Even when the FS-Fe of submicrometer size fractions (0.18–0.32 μm, 0.32–0.56 μm, and 0.56–1.0 μm) in the model is increased by a factor of 10 over the measured values, the calculated FS-Fe of the dust is still more than an order of magnitude lower than that measured in the field. Therefore, the physical sorting of dust particles alone is unlikely to be an important factor in the observed inverse relationship between the FS-Fe and FS-Al and the atmospheric mineral dust mass concentrations. The results
suggest that processes such as chemical reactions and/or mixing with combustion particles are the main mechanisms to cause the increased FS-Fe in long-range transported dust aerosols.
suggest that processes such as chemical reactions and/or mixing with combustion particles are the main mechanisms to cause the increased FS-Fe in long-range transported dust aerosols.
Original language | English |
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Pages (from-to) | 8459-8469 |
Journal | Atmospheric Chemistry and Physics |
Volume | 11 |
DOIs | |
Publication status | Published - 2011 |