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

Abstract. 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 suspended particles (TSP) from six sites along two south–north transects across the central Himalayas. Spatially, the annual average TSP and PAH (especially 5- and 6-ring) concentrations were found to decrease noticeably along both transects. The concentration levels of TSP and PAHs at Lumbini were found to be the highest (TSP: 209 ± 113 μg/m³; PAHs: 91.6 ± 54.6 ng/m³) which are comparable to those in some South Asian cities, but three and thirteen times higher than those at Nyalam (TSP: 59.1 ± 62.0 μg/m³; PAHs: 5.57 ± 3.36 ng/m³), respectively.The dry deposition fluxes also had a decreasing trend pattern from the southern to northern side of the Himalayas. Moreover, annual TSP and PAH concentrations exhibited a logarithmic decreasing pattern with increasing elevation especially in the non-monsoon seasons (TSP: y = −57.3lnx+552, R² = 0.952; PAHs: y = −26.8lnx+229, R² = 0.948). The TSP and PAH concentrations showed a clear seasonal variation, with the minimum concentrations (TSP: 47.9 μg/m³; PAHs: 16.8 ng/m³) around the mid-monsoon season and the maximum concentrations (TSP: 442 μg/m³; PAHs: 192 ng/m³) in the winter season at Lumbini. While at the remote sites (e.g. Nyalam and Zhongba), these pollutants were relatively constant throughout the year with relatively higher abundance during the pre-monsoon season. For example, approximately 80 % of samples have PAH concentrations lower than 10 ng/m³ at Nyalam and Zhongba. And just a few samples with higher PAH concentrations, however, not more than 21 ng/m³, were observed during the pre-monsoon season. Both IndP/(IndP+BghiP) and Fla/(Fla+Pyr) ratios suggested that atmospheric PAHs from the Nepal sites were mainly associated with emission of biomass, coal burning and petroleum combustion. A similar composition pattern with relatively uniform contributions of 4 groups to total PAHs was found between the two sides of the Himalayas (e.g. Jomsom, Zhongba, and Nyalam), suggesting that the northern side of the Himalayas may be affected by anthropogenic emissions from the IGP due to long-range transportation as well as the unique mountain/valley breeze system which bring pollution from the IGP into Tibet across the high Himalayas.