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

Abstract

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 ~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 nssCa2+ (dust tracer), indicating heterogeneous reactive uptake on mineral aerosol surface. Despite high concentrations, the deposition fluxes of NH4+ (~9.4 � 7.1 ?mol m?2 d?1) and WSON (7.4 � 7.0 ?mol m?2 d?1) are lower than NO3? (27 � 11 ?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 yr?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 ?mol m?2 d?1) can account for <5.3% of fixed?carbon by a primary production in the Arabian Sea.