Long-term trends of atmospheric organochlorine pollutants and polycyclic aromatic hydrocarbons over the southeastern Tibetan Plateau

Long-term monitoring in remote regions is essential for revealing pollution trends at the global scale but relevant studies remain limited. In the present study, a six-year continuous monitoring of atmospheric persistent organic pollutants (POPs) and polycyclic aromatic hydrocarbons (PAHs) was carried out at Lulang in the southeastern Tibetan Plateau (TP). Average concentrations of hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCB), dichlorodiphenyltrichloroethanes (DDTs), polychlorinated biphenyls (PCBs) and PAHs were 13.5, 8.9, 41.7, 1.8 pg/m3 and 6.2 ng/m3, respectively. Obvious seasonality was found for all the target compounds. HCHs, DDTs and PCBs had their highest concentrations in summer (monsoon season) and lowest in winter, which is consistent with the fluctuation of the Indian monsoon. Meanwhile, HCB and 5–6-ring PAHs showed opposite variations, possibly induced by local sources and the westerly flow in winter. Declining trends were observed for most of the chemicals, except o,p′-DDE, HCB and PCBs. A declining trend in the α/γ-HCH ratio indicated a shift from technical HCHs to lindane. An increasing trend in the o,p′/p,p′-DDT ratio suggested a likely shift from technical DDTs to dicofol. For PAHs, the contribution from high-temperature combustion has increased recently. The half-lives of α-HCH, γ-HCH, o,p′-DDT, p,p′-DDT were 6.1, 108, 77.6 and 14.2 years, respectively. The half-lives of γ-HCH, o,p′-DDT and p,p′-DDT were higher than those in the Arctic, indicating these compounds will persist in the TP for a longer period. The temporal trends in atmospheric POPs were possibly induced by emissions in India and likely driven by wind speed in Lulang. This study contributes toward a better understanding of the behavior and transport of POPs in the TP region.