Atmospheric dry deposition of inorganic and organic nitrogen to the Bay of Bengal: Impact of continental outflow
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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 sub(Inorg), N sub(Org)), their spatial variability and dry-deposition fluxes have been studied in two size fractions (PM sub(2.5) and PM sub(10)) collected during Jan-2009 from the MABL of Bay of Bengal. The mass concentration of NInorg (NH sub(4) sup(+) + NO sub(3) sup(−), range: 18 to 565 nmol m sup(−3)) dominates the total soluble nitrogen (N sub(Tot) = N sub(Inorg) + N sub(Org,)) in the fine mode (PM sub(2.5)), and occurs mainly as NH sub(4) sup(+) (range: 16 to 561 nmol m sup(−3)). The mass-ratio of NInorg in PM sub(2.5) to PM sub(10) centers around 0.85, suggesting that contribution of coarse mode NO sub(3) sup(−) is relatively insignificant. The dominant contribution of N sub(Inorg) (as NHsub(4) sup(+)) to N sub(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 sub(Org) also dominates the fine fraction and accounts for no more than 40% of N sub(Tot), with relatively high concentrations along coastal regions. A significant linear relationship among N sub(Org), NH sub(4) sup(+), nss-K sup(+) and EC (p-value less than 0.001) suggest their common source from biomass burning emissions and large-scale application of fertilizers (urea). The dry-deposition flux of nitrogen (N sub(Inorg) + N sub(Org)) to the Bay of Bengal ranges from 2 to 167 mu mol m sup(−2) d sup(−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.
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