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 = PO43-) in the MABL and its dry-deposition flux to the surface BoB. On the basis of the abundance of P-Inorg in PM2.5 (0.1 = 0.8 nmol m(-3)) and PM10 (0.3 = 2.8 nmolm(-3)), we document its dominant occurrence in the coarse mode (D-a >= 2.5 mu m). The analytical data also provide evidence for the chemical processing of mineral dust by acidic species and mobilisation of PInorg 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). PInorg 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 PInorg to the BoB varies by one order of magnitude (0.5-5.0 mu mol Pm-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.