Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season

摘要

Aerosol particle number size distribution and chemical composition weremeasured at two low altitude sites, one urban and one relatively pristinevalley, in Central Nepal during the 2009 pre-monsoon season (May–June). Thisis the first time that aerosol size distribution and chemical compositionwere measured simultaneously at lower elevations in the middle Himalayanregion in Nepal. The aerosol size distribution was measured using a ScanningMobility Particle Sizer (SMPS, 14–340 nm), and the chemicalcomposition of the filter samples collected during the field campaign wasanalyzed in the laboratory. Teflon membrane filters were used for ionchromatography (IC) and water-soluble organic carbon and nitrogen analysis.Quartz fiber filters were used for organic carbon and elemental carbonanalysis. Multi-lognormal fits to the measured aerosol size distributionindicated a consistent larger mode around 100 nm which is usually the oldest,most processed background aerosol. The smaller mode was located around 20 nm,which is indicative of fresh but not necessarily local aerosol. The diurnalcycle of the aerosol number concentration showed the presence of two peaks(early morning and evening), during the transitional periods of boundarylayer growth and collapse. The increase in number concentration during thepeak periods was observed for the entire size distribution. Although thepossible contribution of local emissions in size ranges similar to thelarger mode cannot be completely ruled out, another plausible explanation isthe mixing of aged elevated aerosol in the residual layer during the morningperiod as suggested by previous studies. Similarly, the evening timeconcentration peaks when the boundary layer becomes shallow concurrent withincrease in local activity. A decrease in aerosol number concentration wasobserved during the nighttime with the development of cold (downslope)mountain winds that force the low level warmer air in the valley to rise.The mountain valley wind mechanisms induced by the topography along with thevalley geometry appear to have a strong control in the diurnal cycle of theaerosol size distribution. During the sampling period, the chemicalcomposition of PM was dominated by organic matter at both sites. Organiccarbon (OC) comprised the major fraction (64–68%) of the aerosolconcentration followed by ionic species (24–26%, mainly SO and NH). Elemental Carbon (EC) compromised 7–10% of thetotal composition and 27% of OC was found to be water soluble at bothsites. The day-to-day variability observed in the time series of aerosolcomposition could be explained by the synoptic scale haze that extended tothe sampling region from the Indian Gangetic Plain (IGP), and rainfalloccurrence. In the presence of regional scale haze during dry periods, themean volume aerosol concentration was found to increase and so did theaerosol mass concentrations.