Monocyclic Aromatic Hydrocarbons in Kathmandu During the Winter Season

Mixing ratios of seven monocyclic aromatic hydrocarbons, as well as NO₂, SO₂ and O₃, were measured by long path differential optical absorption spectroscopy (DOAS) at a suburban site in Kathmandu, Nepal, during Jan.–Feb. 2003. The results showed average benzene (3.9?±?1.8 ppbv), toluene (13.3?±?7.1 ppbv), and sum of xylene isomers (42.2?±?15.7 ppbv) mixing ratios in Kathmandu. The xylenes concentrations were higher than in the large cities that have been studied. The observed ratio of toluene to benzene (2.9?±?1.8) reflected the small fraction of vehicles with catalytic converters in the Kathamndu. Analysis of the late afternoon time series of aromatics, NO _x , and wind data indicated that road traffic was one of the main sources of aromatics in the urban air. In addition, the correlations between aromatics, SO₂, NO _x , and PM₁₀ during the night strongly suggested that fossil and biomass fuel burning made an important contribution to air pollution in the Kathmandu valley. Aromatic pollution was further strengthened by daily recurring stable meteorological conditions and the surrounding topography. The chemical reaction of aromatics with free radicals during the daytime could also be deduced. High ratios of phenol/benzene and para-cresol/toluene could not be explained by chemical processes, and suggested direct emission of phenol and para-cresol in the Kathmandu atmosphere.