Aerosol NO3- and Pb-210 distribution over the central-eastern Arabian Sea and their air-sea deposition fluxes

The concentrations of NO3-, SO42- and Pb-210 have been measured in the mari ne boundary layer over the central-eastern Arabian Sea to investigate the s ources of NO3- to this region and its deposition flux to the sea surface. B ulk aerosol samples were collected during April-May (intermonsoon) 1994, Ju ly-August (monsoon) 1995, 1996 and February-March (winter) 1995, 1997. The NO3- and (210P)b concentrations, during the intermonsoon and winter, ranged from 0.4 to 4.1 mu g m(-3) and (12.3 to 70.3) x 10(-3) dpm m(-3), respecti vely; with systematically higher concentrations during the winter. Their co ncentrations were the lowest, 0.2 to 0.8 mu g m(-3) and (4.0 to 17.6) x 10( -3) dpm m-3, respectively, during monsoon. The seasonal and spatial distrib utions of NO3- and Pb-210 in the aerosols show a significant positive corre lation and bring to light the dominant role of continental sources in the c hemistry of aerosols over the Arabian Sea. The NO3- deposition flux to the Arabian Sea surface is determined to be 0.9 mg m(-2) d(-1) based on the NO3 -/Pb-210 ratio (43 mu g dpm(-1)) and the measured Pb-210 deposition flux of 20 dpm m(-2) d(-)1. The atmospheric NO3- flux to the mixed layer is insign ificant compared to that fixed by primary production in this region and tha t supplied to the mixed layer from the base of the euphotic zone. The annua l mean Pb-210 concentration (25 x 10(-3) dpm m(-3)) and its atmospheric dep osition flux (20 dpm m(-2) d(-1)) yield an effective deposition velocity of similar to 0.9 cm s(-1) for Pb-210 aerosols; similar to that used for deri ving NO3- and NH4+ deposition fluxes. The seasonal trend for SO42- concentr ations is guile similar to that of NO3- with the non-sea-sail component of SO42- being generally higher during the winter and the intermonsoon periods .