Atmospheric dry deposition of inorganic and organic nitrogen to the Bay of Bengal: Impact of continental outflow

The continental outflow from south and south-east Asia, persisting during January to March, dominates the widespread dispersal of pollutants over tropical Bay of Bengal. With a view to assess the impact of anthropogenic sources on surface ocean biogeochemistry, concentrations of water-soluble inorganic and organic nitrogen (NInorg, NOrg), their spatial variability and dry-deposition fluxes have been studied in two size fractions (PM2.5 and PM10) collected during Jan-2009 from the MABL of Bay of Bengal. The mass concentration of NInorg (NH4+ + NO3−, range: 18 to 565 nmol m− 3) dominates the total soluble nitrogen (NTot = NInorg + NOrg,) in the fine mode (PM2.5), and occurs mainly as NH4+ (range: 16 to 561 nmol m− 3). The mass-ratio of NInorg in PM2.5 to PM10 centers around 0.85, suggesting that contribution of coarse mode NO3− is relatively insignificant. The dominant contribution of NInorg (as NH4+) to NTot is also evident based on the data from earlier cruises (Feb–Mar-2001, Feb-2003 and Mar–Apr-2006) conducted in the Bay of Bengal. Water-soluble NOrg also dominates the fine fraction and accounts for no more than 40% of NTot, with relatively high concentrations along coastal regions. A significant linear relationship among NOrg, NH4+, nss-K+ and EC (p-value < 0.001) suggest their common source from biomass burning emissions and large-scale application of fertilizers (urea). The dry-deposition flux of nitrogen (NInorg + NOrg) to the Bay of Bengal ranges from 2 to 167 μmol m− 2 d− 1. The upper estimate of N-deposition is somewhat comparable with the model based fluxes, and can support up to 13% of the Primary Production in the Bay of Bengal.

► First field based data on soluble nitrogen in ambient aerosols over Bay of Bengal.
► Mass concentration of aerosol inorganic nitrogen is dominated by NH4+.
► Inorganic N account for nearly 85% of total soluble nitrogen (inorganic + organic).
► Both NH4+ and organic N exist in fine mode (PM2.5).
► Average N-deposition to Bay of Bengal accounts for ≤ 5% of surface Primary Productivity.