Characterization of NMHCs in downtown urban center Kathmandu and rural site Nagarkot in Nepal

The total of 28 different C2–C6 non-methane hydrocarbons including isoprene emitted from natural and anthropogenic sources in urban and rural sites of Kathmandu, capital of Nepal, were characterized for the first time in Nepal, in November 1998. Thirty-eight whole air samples were analyzed by using GC/FID. Ethene, acetylene and C4–C5 alkanes were identified as the source signature in Kathmandu urban ambient air. Hydrocarbon emissions from vehicular exhaust and gasoline evaporation sources were confirmed to outweigh natural gas and biogenic sources. Comparison of NMHCs profile normalized by acetylene between Kathmandu and Tokyo, Japan showed prominent difference in C2–C5 alkanes relative to acetylene. Significant amount of ethene at the rural site was assigned as emitted from biogenic sources because of its significant correlation with isoprene (R2=0.52). Predominant amount of isoprene (average 278 pptv) observed at the urban site were assigned to be emitted from vehicular exhaust as it exhibited high correlation with other anthropogenically emitted hydrocarbons (R2=0.72 with acetylene). Photochemical aging analysis showed that the mixing ratio variation of urban air transporting towards the rural site took place through its OH radical initiated oxidation and the observed alkanes in the rural site were mostly from the transported urban air.