Chemical characteristics of aerosols in MABL of bay of Bengal and Arabian sea during spring inter-monsoon: a comparative study

Abstract

The chemical composition of aerosols in the Marine Atmospheric Boundary Layer (MABL) of Bay of Bengal (BoB) and Arabian Sea (AS) has been studied during the spring and inter-monsoon (March-May 2006) based on the analysis of water soluble constituents (Na⁺, NH⁴⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-, NO₃^- and SO₄^2-), crustal elements (Al, Fe, and Ca) and carbonaceous species (EC, OC). The total suspended particulates (TSP) ranged from 5.2 to 46.6 μg m^-3 and 8.2 to 46.9 μg m^-3 during the sampling transects in the BoB and AS respectively. The water-soluble species, on average, accounted for 44% and 33% of TSP over BoB and AS respectively, with dominant contribution of SO₄^2- over both the oceanic regions. However, distinct differences with respect to elevated abundances of NH₄⁺ in the MABL of BoB and that of Na⁺ and Ca²⁺ in AS are clearly evident. The non-sea-salt component of SO₄^2- ranging from 82 to 98% over BoB and 35 to 98% over AS; together with nss-Ca²⁺/nss-SO₄^2- equivalent ratios 0.12 to 0.5 and 0.2 to 1.16, respectively, provide evidence for the predominance of anthropogenic constituents and chemical transformation processes occurring within MABL. The concentrations of OC and EC average around 1.9 and 0.4 μg m^-3 in BoB and exhibit a decreasing trend from north to south; however, abundance of these carbonaceous species are not significantly pronounced over AS. The abundance of Al, used as a proxy for mineral aerosols, varied from 0.2 to 1.9 μg m^-3 over BoB and AS, with a distinctly different spatial pattern - decreasing north to south in BoB in contrast to an increasing pattern in the Arabian Sea.