Mineral and anthropogenic aerosols in Arabian Sea-atmospheric boundary layer: Sources and spatial variability

The chemical characteristics of aerosols in the Arabian Sea-atmospheric boundary layer (AABL) have been studied during the spring inter-monsoon (April and May 2006) based on the analysis of water-soluble constituents (Na+, NH4+, K+, Mg2+ Ca2+ Cl-, NO3-, and SO42-), crustal elements (Al, Fe, and Ca), and carbonaceous species (EC and OC). The total suspended particulate (TSP) abundance ranged from 8.2 to 46.9 mu g m(-3) (average (Av) = 24.7 +/- 10.4 mu g m(-3)) during 22 days cruise covering a latitudinal transect from 9 degrees N to 22 degrees N. The water-soluble species account for 35% of TSP; with dominant contribution of Ca2+ and SO42- followed by Na+ and minor contributions of K+, Mg2+, Cl-, and NO3-. The abundances of Ca2+ and SO42- do not exhibit any noticeable latitudinal distribution pattern but the non-sea-salt (nss) component constitutes similar to 85-90% of their total concentration, indicating dominant transport from continental sources. Furthermore, a significant linear positive correlation among nss-Ca2+ and nss-SO42-, and nss-Ca2+ /nSO(4)(2-) molar ratio averaging around 0.61 (range: 0.20-1.16) suggests uptake of anthropogenic SO42- by mineral dust (CaCO3). The chemical reaction favoring this neutralization of nss-SO42- is also evident from the abundance pattern of water-soluble Ca2+ nearly equal to the total Ca content measured in the aerosols. Using Al as a proxy, the mineral dust in AABL ranged from 2.7 to 23.7 mu g m(-3); with relatively high abundance occurring over the south Arabian Sea. On average, mineral dust accounts for 44% of the TSP and Fe/Al weight-ratio exhibit characteristic narrow range: 0.40-0.59. The impact of carbonaceous species (EC and OC) is nowhere pronounced in the AABL. The dry-deposition fluxes of Al, Fe, and bioavailable Fe to the surface Arabian Sea are estimated to be 665, 395, and 40 mu g m(-2) d(-1), respectively. (C) 2008 Elsevier Ltd. All rights reserved.