Aerosol NO^-₃ and ²¹⁰Pb distribution over the central-eastern Arabian Sea and their air-sea deposition fluxes

Abstract

The concentrations of NO^-₃, SO^2-₄ and ²¹⁰Pb have been measured in the marine boundary layer over the central-eastern Arabian Sea to investigate the sources of NO^-₃ to this region and its deposition flux to the sea surface. Bulk aerosol samples were collected during April - May (intermonsoon) 1994, July - August (monsoon) 1995, 1996 and February - March (winter) 1995, 1997. The NO^-₃ and ²¹⁰Pb concentrations, during the intermonsoon and winter, ranged from 0.4 to 4.1 µg m^-3 and (12.3 to 70.3) × 10^-3 dpm m^-3, respectively; with systematically higher concentrations during the winter. Their concentrations were the lowest, 0.2 to 0.8 µg m^-3 and (4.0 to 17.6) × 10^-3 dpm m^-3, respectively, during monsoon. The seasonal and spatial distributions of NO^-₃ and ²¹⁰Pb in the aerosols show a significant positive correlation and bring to light the dominant role of continental sources in the chemistry of aerosols over the Arabian Sea. The NO^-₃ deposition flux to the Arabian Sea surface is determined to be 0.9 mg m^-2 d^-1 based on the NO^-₃/²¹⁰Pb ratio (43 µg/dpm^-1) and the measured ²¹⁰Pb deposition flux of 20 dpm m^-2 d^-1. The atmospheric NO^-₃ flux to the mixed layer is insignificant compared to that fixed by primary production in this region and that supplied to the mixed layer from the base of the euphotic zone. The annual mean ²¹⁰Pb concentration (25 × 10^-3 dpm m^-3) and its atmospheric deposition flux (20 dpm m^-2 d^-1) yield an effective deposition velocity of ~0.9 cm s^-1 for ²¹⁰Pb aerosols; similar to that used for deriving NO^-₃ and NH⁺₄ deposition fluxes. The seasonal trend for SO^2-₄ concentrations is quite similar to that of NO^-₃, with the non-sea-salt component of SO^2-₄ being generally higher during the winter and the intermonsoon periods.