Dry-deposition of inorganic and organic nitrogen aerosols to the Arabian Sea: Sources, transport and biogeochemical significance in surface waters

Air-to-sea deposition of water-soluble total nitrogen (WSTN) can influence the primary productivity in the coastal oceans. Here, we assessed the concentrations of aerosol inorganic (WSIN: NH4+ + NO3-) and WSTN over the Arabian Sea during winter season (SS379:6-24 December 2018). The mean concentrations of NH4+ (109 +/- 83 nmol m(-3)) overwhelm that of NO3- (32 +/- 13 nmol m(-3)) and water-soluble organic nitrogen (WSON: WSTN-WSIN: 86 +/- 81 nmol m(-3)), and contributing to similar to 50 +/- 31% of WSTN mass. Significant linear relationships of WSON with water-soluble organic carbon and NH4+ with non-sea-salt (nss)-K+ is observed suggesting their common origin from biomass burning and fertilizers. Backward air mass trajectories and satellite-based fire counts further revealed their provenance in the Indo-Gangetic Plain and southern India. The concentration of NO3- moderately correlated with nss-Ca2+ (dust tracer), indicating heterogeneous reactive uptake on mineral aerosol surface. Despite high concentrations, the deposition fluxes of NH4+ (similar to 9.4 +/- 7.1 mu mol m(-2) d(-1)) and WSON (7.4 +/- 7.0 mu mol m(-2) d(-1)) are lower than NO3- (27 +/- 11 mu mol m(-2) d(-1)) because of their predominant fine nature (i.e., strong correlation with nss-SO42). We also constrained the total annual atmospheric deposition rates of WSIN (0.94 Tg y(-1)) and WSON (0.08 Tg yr(-1)) to the Arabian Sea during the continental outflow (November-April). The maximum dry-deposition of WSON to the Arabian Sea (0.24 Tg yr(-1)) is twice that of the riverine supply (0.11 Tg yr(-1)), highlighting the significance of aeolian sources. By using Redfield Stoichiometry, the WSTN deposition (21-73 mu mol m(-2) d(-1)) can account for <5.3% of fixed-carbon by a primary production in the Arabian Sea.