Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season
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Aerosol particle number size distribution and chemical composition were measured at two low altitude sites, one urban and one relatively pristine valley, in Central Nepal during the 2009 pre-monsoon season (May-June). This is the first time that aerosol size distribution and chemical composition were measured simultaneously at lower elevations in the middle Himalayan region in Nepal. The aerosol size distribution was measured using a Scanning Mobility Particle Sizer (SMPS, 14-340 nm), and the chemical composition of the filter samples collected during the field campaign was analyzed in the laboratory. Teflon membrane filters were used for ion chromatography (IC) and watersoluble organic carbon and nitrogen analysis. Quartz fiber filters were used for organic carbon and elemental carbon analysis. Multi-lognormal fits to the measured aerosol size distribution indicated 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 diurnal cycle of the aerosol number concentration showed the presence of two peaks (early morning and evening), during the transitional periods of boundary layer growth and collapse. The increase in number concentration during the peak periods was observed for the entire size distribution. Although the possible contribution of local emissions in size ranges similar to the larger mode cannot be completely ruled out, another plausible explanation is the mixing of aged elevated aerosol in the residual layer during the morning period as suggested by previous studies. Similarly, the evening time concentration peaks when the boundary layer becomes shal-low concurrent with increase in local activity. A decrease in aerosol number concentration was observed 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 the valley geometry appear to have a strong control in the diurnal cycle of the aerosol size distribution. During the sampling period, the chemical composition of PM2.5 was dominated by organic matter at both sites. Organic carbon (OC) comprised the major fraction (64-68%) of the aerosol concentration followed by ionic species (24- 26%, mainly SO2-4 and NH +4 ). Elemental Carbon (EC) compromised 7-10% of the total composition and 27% of OC was found to be water soluble at both sites. The day-to-day variability observed in the time series of aerosol composition could be explained by the synoptic scale haze that extended to the sampling region from the Indian Gangetic Plain (IGP), and rainfall occurrence. In the presence of regional scale haze during dry periods, the mean volume aerosol concentration was found to increase and so did the aerosol mass concentrations. © 2010 Author(s).
Published Version (Please cite this version)10.5194/acp-10-11605-2010
Publication InfoBarros, Ana; Khlystov, Andrey; & Shrestha, P (2010). Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season. Atmospheric Chemistry and Physics, 10(23). pp. 11605-11621. 10.5194/acp-10-11605-2010. Retrieved from http://hdl.handle.net/10161/4617.
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Edmund T. Pratt, Jr. School Professor of Civil and Environmental Engineering
Professor Ana Barros was born in Africa, grew up in Angola and Portugal, and has lived almost all of her adult life in the United States. She attended the Faculty of Engineering of the University of O’Porto where she obtained a summa cum laude Diploma in Civil Engineering with majors in Structures and Hydraulics in 1985, and a M.Sc. degree in Ocean Engineering in 1988 with a thesis focusing on numerical modeling of sediment transport in estuaries and coastal regions. In 1990, Dr. Barros