Impact of anthropogenic sources on aerosol iron solubility over the Bay of Bengal and the Arabian Sea

Ambient aerosols collected from the marine atmospheric boundary layer of the Bay of Bengal and the Arabian Sea have been studied to assess the fractional solubility of aerosol iron, defined as Fe-ws (%) = Fe-ws/Fe-Tot x 100; where Fe-Tot is total aerosol iron and Fe-ws is water soluble iron. The mass concentration of Fe-Tot over the two oceanic regions is not significantly different. However, the fractional solubility is 1-2 orders of magnitude higher over the Bay of Bengal (1.4-24%) compared to that over the Arabian Sea (0.02-0.4%). The spatio-temporal variability in Fe-ws (%) is attributed to differences in the nature of the mineral dust over the two oceanic regions. The Arabian Sea receives coarse dust from desert regions; whereas transport of alluvial dust from the Indo-Gangetic Plain is a dominant source to the Bay of Bengal. The poor fractional solubility (< 1%) of Fe from mineral dust, hitherto overestimated in the literature, is documented for the Arabian Sea. A significant linear relationship (P-value < 0.001) between Fe-ws (%), Fe-Tot and nss-SO4 (2-) over the Bay of Bengal provides evidence for the chemical processing of mineral dust. Furthermore, the role of anthropogenic sources (biomass burning and fossil-fuel combustion) in enhancing the Fe-ws (%) is discernible from the chemical composition of fine mode (PM2.5) aerosols over the Bay of Bengal. The potential impact of these Fe-dust depositions on phytoplankton carbon fixation and surface ocean biogeochemistry is discussed.