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

Abstract

The chemical composition (involving water-soluble inorganic constituents, crustal elements and carbonaceous species) of size-segregated aerosols (PM₁₀ and PM_2.5), 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-SO₄^2- over N-BoB (characterized by higher abundance of aerosol iron (Fe_A) and SO₄^2-) 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-SO₄^2-. 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 Fe_A) and nss-SO₄^2- (< 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.