Using a passive air sampler to monitor air-soil exchange of organochlorine pesticides in the pasture of the central Tibetan Plateau.

Air-soil exchange is a key process controlling the fate of persistent organic pollutants (POPs). However, the "sink effect" of soil for POPs in Tibetan pasture has not been clear. In NamCo, in the central Tibetan Plateau (TP) where the land is covered by grass, a modified passive air sampler (PAS) (thickness: 2cm) was tested. Using the PAS, the atmospheric gaseous phase organochlorine pesticides (OCPs) at 11 heights from close-to-surface (2cm) to 200cm above ground, in summer and in winter, were measured. Concentrations of OCPs in summer were higher than those in winter. Both in summer and winter, atmospheric concentrations of OCPs decreased with decreasing height from 200 to 2cm, indicating that OCPs were being deposited from air to soil. Air deposition of OCPs was possibly driven by wind speed. Furthermore, based on air OCPs at 0-3cm near the surface, the interface exchange of OCPs between air and soil was studied by the fugacity method. The results showed that pastural soil in the TP was a "sink" of OCPs even in summer. The mean deposition fluxes of α-HCH, γ-HCH and o,p'-DDT were 0.72, 0.24 and 0.54pg/h/m2, respectively, and it was estimated that the level of these pollutants in the soil will double every 24, 66 and 206years, respectively. This study will contribute to the further understanding of global cycling of POPs in different land covers.