Transport of semivolatile organic compounds to the Tibetan Plateau: Monthly resolved air concentrations at Nam Co

A flow-through sampler was deployed to record the seasonal variability of the atmospheric concentrations of semivolatile organic compounds (SOCs) at a remote research station located close to Nam Co Lake on the Tibetan plateau. Between October 2006 and February 2008, fifteen consecutive one month-long samples, with air volumes ranging from 4,500 to 16,000 m(3), were taken and analyzed for polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs). Separate analysis of three polyurethane plugs in series in combination with frontal chromatographic theory allows for the correction of the break-through observed for the most volatile SOCs. The concentrations of Sigma 56PCB in air range from 0.10 to 2.6 pg.m(-3) and are among the lowest values ever reported. Levels of OCPs at Nam Co are generally also very low, particularly during wintertime. The concentrations of hexachlorocyclohexanes (HCHs), endosulfans, and various dichlorodiphenyltrichloroethane (DDT) related substances display a distinct seasonal variability consistent with the monsoon. Back-trajectory analysis reveals that higher OCP levels during summer correlate with air mass origin south of the Himalayas. A high alpha/gamma-HCH ratio and a non-racemic composition of alpha-HCH during July/August suggest that evaporation from Nam Co Lake contributes to the relatively high concentrations of alpha-HCH (averaging ca. 91 pg.m(-3)) recorded in the summertime atmosphere.