Effect of input pathways and altitudes on spatial distribution of polycyclic aromatic hydrocarbons in background soils, the Tibetan Plateau.

Limited data on the spatial distribution of polycyclic aromatic hydrocarbons (PAHs) in Tibetan soils have been reported. This study measured the concentrations of PAHs in background soils across Tibet including 44 surface soils (0-10 cm) and 14 subsurface soils (10-20 cm) and assessed the effect of input pathways and altitudes on spatial distribution of PAHs. The concentrations of 15 US EPA priority PAHs (∑15-PAHs, naphthalene excluded) in all surface soils ranged from 1.50 to 29.88 ng/g dry weight with a mean concentration of 6.09 ng/g. The levels of PAHs in soils from southeast of Tibet were higher than those from northwest, which might be attributed to both more local emission as a result of relative higher population density and contributions of long-range atmosphere transport (LRAT) processes from Indian subcontinent in the south and Sichuan in the east. For the northwest Tibet, the westerly wind originated from the western boundaries between India and Nepal but not local emission might play an important role in input of PAHs, since there were slight anthropogenic activities. A general decrease of ∑15-PAHs with depth suggested their atmospheric input, while surface runoff might also play an important role in input of PAHs to soils from canyon and valley. The abundances of more volatile three-ring PAHs positively correlated with altitudes while less volatile ones unrelated (four rings) or inversely correlated (five and six rings) with altitudes, indicating cold condensation effect. The results of PAH isomer ratios suggested mixed sources of petroleum evaporation and combustion of petroleum and biomass.