Aerosol iron solubility over Bay of Bengal: Role of anthropogenic sources and chemical processing

The chemical composition (involving water-soluble inorganic constituents, crustal elements and carbonaceous species) of size-segregated aerosols (PM10 and PM2.5), collected from the marine atmospheric boundary layer (MABL) of the Bay of Bengal (BoB) during 27th Dec' 08–28th Jan' 09, has been studied to ascertain the factors controlling the spatio-temporal variability in the fractional solubility of aerosol iron. Based on the air-mass back-trajectory (AMBT) analyses and chemical proxies, continental outflow from the two major source regions has been identified, viz: (1) outflow from the Indo-Gangetic Plain (IGP) sampled over north-BoB (N-BoB); and (2) south-east Asian (SEA) outflow over south-BoB (S-BoB). A significant linear relationship among fractional Fe solubility [WS-Fe (%)] and nss-SO42− over N-BoB (characterized by higher abundance of aerosol iron (FeA) and SO42−) provides evidence for the acid processing of mineral dust during atmospheric transport from IGP. The enhancement in the solubility of aerosol constituents is also evident from a linear increase in nss-Ca2+ with nss-SO42−. In contrast, a temporal shift in the winds, representing the outflow from south-east Asia and aerosol composition over south-BoB, exhibit enhanced fractional solubility of aerosol Fe (range: 11.4 to 49.7%) associated with the lower abundance of dust (< 100 ng m− 3 of FeA) and nss-SO42− (< 15 µg m− 3). These observations suggest the dominance of combustion sources (biomass burning and fossil-fuel) in dictating the aerosol iron solubility over south Bay of Bengal. The impact of the anthropogenic sources is also ascertained based on the covariance of WS-Fe with K+ and OC (organic carbon); as well as enrichment factor of heavy metals (Pb and Cd) associated with the outflow from south-east Asia.