Origin, transport and deposition of aerosol iron to Australian coastal waters

Australia is a major source of Fe-laden dust to the anemic marine phytoplankton in the Southern Ocean and to Southern Hemisphere (SH) low latitudes diazotrophic bacteria. However, the paucity of observations and laboratory experiments on SH aerosols biases model predictions of atmospheric Fe deposition to the southern oceans and the subsequent response of ocean productivity. As a result of an extensive shipboard aerosol sampling effort, this study presents laboratory measurements of aerosol Fe concentrations, solubilities and fluxes and analysis of chemical tracers, highlighting the large heterogeneity between aerosol Fe sources in 5 coastal regions around Australia. While dust-sourced high Fe loadings and low Fe solubilities (5%) aerosols dominate the atmospheric burden of the western coasts of Australia, much lower Fe concentrations but greater Fe solubilities (10.5% and 13%) were measured in aerosols along the east coast which was attributed to solubility-enhancing atmospheric reactions with anthropogenic pollutants. Surprisingly high aerosol Fe solubilities (>20%) in northern Australia aerosols were associated with direct emissions or atmospheric reactions with bushfire emissions at tropical latitudes, which accounted for 49% of the total (sum) atmospheric dry deposition flux of labile Fe measured across the continent's surrounding seawaters in this study.