Atmospheric deposition fluxes of ⁷Be, ²¹⁰Pb and chemical species to the Arabian Sea and Bay of Bengal

Abstract

Aerosol samples collected close to the air-sea interface, between February 1997 to February 1999, over the Arabian Sea and Bay of Bengal were analyzed to determine the atmospheric dry-deposition of Fe (dust inputs), anthropogenic constituents (NO₃⁻, SO₄²⁻) and environmental nuclides (⁷Be, ²¹⁰Pb). In general, aerosol ²¹⁰Pb concentrations showed a good correlation with ⁷Be, suggesting the long-range transport of ²¹⁰Pb from the continents (via upper troposphere) and similarities in the processes governing their deposition through the marine boundary layer (MBL). The relatively low ⁷Be/²¹⁰Pb ratios observed over the Bay of Bengal, during February 1999, are dominated by aerosol transport from the continental surface sources. The dry deposition fluxes of ²¹⁰Pb and ⁷Be, to these two oceanic regions, average around 245 and 1860 Bq m⁻² y⁻¹, respectively. The non-sea-salt (nss) SO₄²⁻ (range: 1.7 to 9.4 µg m⁻³) and NO₃⁻ (range: 0.6 to 4.1 μg m⁻³) are uncorrelated in the MBL, presumably because continental pollution sources for SO₄²⁻ overwhelm the transport of NO₃⁻ from crustal dust and biomass burning. The oceanic biogenic emission (DMS) constitutes a very minor source for nss-SO₄²⁻. The enhanced concentrations of aerosol NO₃⁻ and Fe observed over the Arabian Sea are attributed to dust storm activities from the adjacent desert areas (Saudi Arabia and Thar). Significant scatter in the linear regression analyses indicate that the aerosol composition along the coastal tracks is different from those transported to the open ocean. On average, dry deposition fluxes of N-NO₃ and non-marine SO₄²⁻ are 150 and 1225 mg m⁻² y⁻¹, respectively. In contrast, dry deposition of Fe over the Arabian Sea (255 mg m⁻² y⁻¹) far exceeds that over the Bay of Bengal (93 mg m⁻² y⁻¹). Thus, dust from desert regions appears to be a potential source of micronutrients (Fe) to the surface Arabian Sea.