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

Abstract

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 (N_Inorg, N_Org), their spatial variability and dry-deposition fluxes have been studied in two size fractions (PM_2.5 and PM₁₀) collected during Jan-2009 from the MABL of Bay of Bengal. The mass concentration of N_Inorg (NH₄⁺ + NO₃^-, range: 18 to 565 nmol m^{- 3}) dominates the total soluble nitrogen (N_Tot = N_Inorg + N_Org,) in the fine mode (PM_2.5), and occurs mainly as NH₄⁺ (range: 16 to 561 nmol m^{- 3}). The mass-ratio of N_Inorg in PM_2.5 to PM₁₀ centers around 0.85, suggesting that contribution of coarse mode NO₃ ^-is relatively insignificant. The dominant contribution of N_Inorg (as NH₄⁺) to N_Tot 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 N_Org also dominates the fine fraction and accounts for no more than 40% of N_Tot, with relatively high concentrations along coastal regions. A significant linear relationship among N_Org, 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 (N_Inorg + N_Org) to the Bay of Bengal ranges from 2 to 167 µ mol 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.