2:15 PM - 2:30 PM
[ACG41-03] Enhancement of Iron Solubility by Organic Matters in Sea Spray Aerosol
Keywords:Sea spray aerosol, Iron (Fe), Water soluble organic carbon, X-ray absorption fine structure, Scanning transmission X-ray microscope
Methods: Size-fractionated sampling of marine aerosols were conducted during a research cruise of R/V Hakuho-Maru (KH-14-6, GEOTRACES, latitudinal cruise of the Western Pacific). Custom-built PTFE filters were employed because filter backgrounds of trace metals were 10-2 to 10-3 times lower in the custom-built PTFE filters than commercial cellulose filters. Trace metal concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS). Macroscopic Fe speciation experiments were conducted by X-ray absorption fine structure (XAFS) spectroscopy. In addition, Fe species and their mixing states in individual aerosol were determined by a nanoscopic speciation technique of scanning transmission X-ray microscope.
Results and Discussion: Total Fe concentrations in aerosols were higher in coarse fractions than in fine fractions. In contrast, the concentrations and extractabilities of labile Fe were higher in the fine fractions. The size-distributions of Fe extractability were similar to those of WSOC. These results indicated that the dissolution of Fe from aerosol particles were related with WSOC. Iron species in aerosol particles larger than 0.69 μm were typical crustal materials: biotite and Fe-(hydro)oxides. On the other hand, aerosol particles finer than 0.69 μm contained Fe(III)-organic complexes of citric acids and deferoxamine (DFO). Extraction experiments with Fe speciation showed that the spectrum of total Fe can be explained by insoluble Fe (residue of extraction) and Fe(III)-organic complexes. This result means that Fe(III)-organic complexes preferentially dissolved in the aqueous-phase. In addition, nanoscopic Fe speciation experiments revealed that Fe in mineral dust co-existed with OC and Na (tracer of SSA) on the aerosol surface. Citric acid and DFO are simulated materials of siderophore in the surface ocean (Barbeau, 2006). Therefore, siderophore-like organic matters from seawater play an important role in increasing atmospheric Fe solubility. Enhancement of the atmospheric Fe solubility by WSOC is important to regulate the Earth’s climate not only in the present but also in the past because this process can be explained by only natural materials.