Characteristics and Sources of Carbonaceous, Ionic, and Isotopic Species of Wintertime Atmospheric Aerosols in Kathmandu Valley, Nepal

摘要

To investigate the air pollution from aerosols in Kathmandu during winter, bulk aerosol samples were collected during winter 2007–2008 to characterize carbonaceous and ionic species and carbon and nitrogen isotopes. This study illustrates the applications of carbon and nitrogen isotope data for characterizing aerosols and their implications for identifying sources that were inconsistent with the results for the carbonaceous and ionic aerosols. Mean concentrations of organic carbon (OC), elemental carbon (EC), and water soluble organic carbon (WSOC) in Kathmandu during the period were 20.02 ± 6.59 (1σ), 4.48 ± 1.17, and 10.09 ± 3.64 µgC/m3, respectively. Elemental carbon and OC were correlated (R2 = 0.56), likely indicating common sources for both species, as well as for the precursors that led to the formation of secondary organic carbon (SOC). The mean estimated SOC contribution to OC was 31%, suggesting that local emission is more important than transport and processing during winter in Kathmandu. On average, 50% of the OC was water soluble, and the correlation of SOC with WSOC (R2 = 0.66) suggests that the majority of SOC and some primary organic carbon (POC) were water soluble in Kathmandu. The mean δ13C of -25.74 ± 0.19‰ observed in aerosols of Kathmandu confirms consistent anthropogenic sources such as fossil fuel combustion. Heavier carbon also was observed to be associated with the water-soluble fraction of OC in aerosols. The mean δ15N of 9.45 ± 0.87‰ suggests the limited influence of biomass burning and its strong correlation with crustal cations Ca2+ (R2 = 0.74, p 0.05) and Mg2+ (R2 = 0.71, p 0.05) indicates distant sources. Principal component analysis revealed four major sources/pathways for particles: local and vehicular emissions, secondary gas-to-particle conversion, aqueous processing, and dust transport, each explaining ~39, 23, 11, and 9% of the variance.