Mineral and anthropogenic aerosols in Arabian sea-atmospheric boundary layer: sources and spatial variability

Abstract

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 SO₄^2-), 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 µg m^-3 (average (Av)=24.7±10.4 µg m^-3) during 22 days cruise covering a latitudinal transect from 9°N to 22°N. The water-soluble species account for 35% of TSP; with dominant contribution of Ca²⁺ and SO₄^2- followed by Na⁺ and minor contributions of K⁺, Mg²⁺, Cl^-, and NO₃^-. The abundances of Ca²⁺ and SO₄^2- do not exhibit any noticeable latitudinal distribution pattern but the non-sea-salt (nss) component constitutes 85-90% of their total concentration, indicating dominant transport from continental sources. Furthermore, a significant linear positive correlation among nss-Ca²⁺ and nss-SO₄^2-, and nss-Ca²⁺/nss-SO₄^2- molar ratio averaging around 0.61 (range: 0.20-1.16) suggests uptake of anthropogenic SO₄^2- by mineral dust (CaCO₃). The chemical reaction favoring this neutralization of nss-SO₄^2- 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 µg 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 µg m^-2 d^-1, respectively.