Atmospheric pathways of phosphorous to the Bay of Bengal: contribution from anthropogenic sources and mineral dust

The continental outflow from the Indo-Gangetic Plain and Southeast Asia, during the late NE monsoon (January–March), dominates the transport of chemical constituents to the marine atmospheric boundary layer (MABL) of the Bay of Bengal (BoB). During the rest of the year, prevailing wind regimes and meteorological conditions do not favour the atmospheric transport of continental products. Here we report on the spatio-temporal variability of inorganic phosphorous (P_Inorg= ) in the MABL and its dry-deposition flux to the surface BoB. On the basis of the abundance of P_Inorg in PM_2.5 (0.1–0.8 nmol m⁻³) and PM₁₀ (0.3–2.8 nmol m⁻³), we document its dominant occurrence in the coarse mode (D_a≥2.5 µm). The analytical data also provide evidence for the chemical processing of mineral dust by acidic species and mobilisation of P_Inorg during the long-range atmospheric transport. However, significantly high P_Inorg/non-sea-salt Ca²⁺ ratios over the BoB suggest dominant contribution from anthropogenic sources (fertilisers and biomass burning emissions). P_Inorg concentration over the Arabian Sea is about 4 to 5 times lower and is primarily associated with the mineral dust from desert regions. The dry-deposition flux of P_Inorg to the BoB varies by one order of magnitude (0.5–5.0 µmol P m⁻² d⁻¹; Av: 0.02 Tg P yr⁻¹). These results have implications to the air-sea deposition of phosphorous over oceanic regions downwind of the pollution sources and impact on the biogeochemistry of surface waters.