Polycyclic aromatic hydrocarbons (PAHs) in aerosols over the centralHimalayas along two south-north transects

Our understanding of the transport of polycyclic aromatic hydrocarbons (PAHs) from the Indo–Gangetic Plains(IGP) to the Himalayas remains limited. Concentrations of PAHs were therefore measured in total suspendedparticles (TSP) from six sites along two south–north transects across the central Himalayas. Spatially, the annualaverage TSP and PAH (especially 5- and 6-ring) concentrations were found to decrease noticeably along bothtransects. The dry deposition fluxes had similar distribution pattern with the ambient PAH levels. Moreover, annualTSP and PAH concentrations exhibited a logarithmic decreasing pattern with increasing elevation especially inthe non-monsoon seasons (TSP: y=-57.3lnx+552, R2=0.952; PAHs: y=-26.8lnx+229, R2=0.948). The TSP andPAH concentrations showed a clear seasonal variation, with the minimum concentrations around the mid-monsoonseason and the maximum concentrations in winter season at Lumbini and Pokhara. While at other remote sites thesepollutants were slightly higher during the non-monsoon season than those in the monsoon season. The diagnosticratio suggested that atmospheric PAHs from the Nepal sites were mainly associated with emission of biomass,coal burning and petroleum combustion. A similar composition pattern was found between the two sides of theHimalayas, suggesting that the northern side of the Himalayas may be affected by anthropogenic emissions fromthe IGP due to long-range transportation as well as the unique mountain/valley breeze system which bring pollutionfrom the IGP into Tibet across the high Himalayas.