Characteristics and sources of polycyclic aromatic hydrocarbons in atmospheric aerosols in the Kathmandu Valley, Nepal

The Kathmandu Valley in the foothills of the Himalayas, where the capital city of Nepal is located, has one of the most serious air pollution problems in the world. In this study, total suspended particle (TSP) samples collected over a year (April 2013-March 2014) in the Kathmandu Valley were analyzed for determining the concentrations of 15 priority particle-bound polycyclic aromatic hydrocarbons (PAHs). The TSP and PAN concentrations were extremely high, with annual average concentration being 199 +/- 124 mu g/m(3) and 155 +/- 130 ng/m(3), respectively, which are comparable to those observed in Asian cities such as Beijing and Delhi. The TSP and PAH concentrations varied considerably, with the seasonal average concentration being maximal during the post-monsoon season followed by, in descending order, the winter, pre-monsoon, and monsoon seasons. In the winter and pre-monsoon seasons, ambient TSP and PAN concentrations increased because of emissions from brick kilns and the use of numerous small generators. Moreover, in the pre-monsoon season, forest fires in the surrounding regions influenced the TSP and PAH concentrations in the valley. PAHs with 4 to 6 rings constituted a predominant proportion (92.3-93.3%) of the total PAHs throughout the year. Evaluation of diagnostic molecular ratios indicated that the atmospheric PAHs in the Kathmandu Valley originated mainly from diesel and biomass combustion. The toxic equivalent quantity (TEQ) of particle phase PAHs ranged between 2.74 and 81.5 ng TEQ/m(3), which is considerably higher than those reported in other South Asian cities, and 2-80 times higher than the World Health Organization guideline (1 ng TEQ/m3). This suggests that ambient PAH levels in the Kathmandu Valley pose a serious health risk to its approximately 3.5 million residents. (C) 2015 Elsevier B.V. All rights reserved.