Revolatilisation of soil-accumulated pollutants triggered by the summer monsoon in India

Persistent organic pollutants that have accumulated in soils can be remobilised by volatilisation in response to chemical equilibrium with the atmosphere. Clean air masses from the Indian Ocean, advected with the onset of the summer monsoon, are found to reduce concentrations of hexachlorocyclohexane (HCH), dichlorodiphenyltrichloroethane (DDT) and its derivatives, endosulfan and polychlorinated biphenyls (PCBs) in air at a mountain site (all in the range 5-20 pgm^-3/ by 77 %, 70 %, 82%and 45 %, respectively. The analysis of fugacities in soil and air suggest that the arrival of summer monsoon triggers net volatilisation or enhances ongoing revolatilisation of the now-banned chemicals HCH and PCBs from background soils in southern India. The response of the air-soil exchange was modelled using a regional air pollution model, WRF-Chem PAH/POP. The results suggest that the air is increasingly polluted during transport by the south-westerly monsoon winds across the subcontinent. Using a multidecadal multimedia mass balance model, it is found that air-surface exchange of HCH and DDT have declined since the ban of these substances from agriculture, but remobilisation of higher chlorinated PCBs may have reached a historical high, 40 years after peak emission.