Evaluation of aerosol iron solubility over Australian coastal regions based on inverse modeling: implications of bushfires on bioaccessible iron concentrations in the Southern Hemisphere

Mineral dust is the major source of external micro-nutrients such as iron (Fe) to the open ocean. However, large uncertainties in model estimates of Fe emissions and aerosol-bearing Fe solubility (i.e., the ratio of labile Fe (L_Fe) to total Fe (T_Fe)) in the Southern Hemisphere (SH) hampered accurate estimates of atmospheric delivery of bioavailable Fe to the Southern Ocean. This study applied an inverse modeling technique to a global aerosol chemistry transport model (IMPACT) in order to optimize predictions of mineral aerosol Fe concentrations based on recent observational data over Australian coastal regions (110°E–160°E and 10°S–41°S). The optimized (a posteriori) model did not only better capture aerosol T_Fe concentrations downwind from Australian dust outbreak but also successfully reproduced enhanced Fe solubility (7.8 ± 8.4%) and resulted in much better agreement of L_Fe concentrations with the field measurements (1.4 ± 1.5 vs. 1.4 ± 2.3 ng Fe m^–3). The a posteriori model estimates suggested that bushfires contributed a large fraction of L_Fe concentrations in aerosols, although substantial contribution from missing sources (e.g., coal mining activities, volcanic eruption, and secondary formation) was still inferred. These findings may have important implications for the projection of future micro-nutrient supply to the oceans as increasing frequency and intensity of open biomass burning are projected in the SH.