Acidification of the coastal Bay of Bengal by aerosols deposition

Polluted aerosol transport from South Asia containing oxides of nitrogen and sulphur and their deposition on surface may acidify coastal waters. To test this hypothesis, we have conducted experiments involving (a) variability in aerosol composition at a coastal station (Visakhapatnam, central east coast of India) during 2013–2014 (monthly) and 2015–2016 (weekly observations), and (b) simultaneous observations of aerosols over land and adjacent coastal water in winter of 2013 and 2016. The annual average composition of aerosols during this study was dominated by SO42− (48%) followed by NO3− (15%). Sulphate (~12 μg m−3) exhibited high concentrations in Fall Intermonsoon (FIM) and winter monsoon (WM), whereas higher nitrate (3–4.5 μg m−3) concentrations were observed during summer monsoon (SM) and FIM. The mean [NO3−/SO42−] ratio of 0.32 suggests that atmospheric aerosol over the study region is contributed by transportation of fossil fuel burning emissions from the subcontinent by high-altitude large-scale wind circulation. The concentrations of NO2 and SO2 varied from 17.2 to 34.3 and 11.5 to 16.4 µg m−3, respectively with mean [SO2/NO2] ratio of 0.57 and [SO42−/SO42−+SO2] ratio of 0.33 indicates coal burning (power plants/industries) and fossil fuel burning may be the major source of atmospheric dust in the study region. Comparison of total cations and anion concentrations indicate aerosols are acidic in FIM and SM and mixed nature (acidic/basic) in WM but near neutral in spring Intermonsoon (SIM). Simultaneous experiments revealed that about 5–45% of the atmospheric aerosols were deposited within 10 km from the coast. The in vitro experiments indicated that the deposition of atmospheric aerosols resulted in a measurable decrease in pH of surface seawater and displayed significant relationship between decrease in pH and concentration of NO3−/SO42−, but it was weaker with NO2/SO2 suggesting former ions contribute significantly in lowering pH of coastal waters than latter. The impact of decrease in pH on acid–base equilibrium, carbonate chemistry and gas exchanges need to be assessed.