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).






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Shrestha, P, AP Barros and A Khlystov (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

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Ana P. Barros

Edmund T. Pratt, Jr. School Distinguished Professor Emeritus 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 completed and M.Sc. degree in Environmental Science Engineering at the OHSU/OGI School of Science and Engineering. She earned a Ph.D. in Civil and Environmental Engineering from the University of Washington, Seattle in 1993. Her graduate studies were supported in part by fellowships from the Portuguese Foundation for Science and Technology (FCT/JNICT), and NASA’s Graduate Fellowship program.  She is registered with the Portuguese Order of Professional Engineers since 1986. 

 Dr. Barros was in the engineering faculty at the University of Porto, Penn State University, and Harvard University before joining Duke University in 2004. Her primary research interests are in Hydrology, Hydrometeorology and Environmental Physics with a focus on water-cycle processes in the coupled land-atmosphere-biosphere system particularly in regions of complex terrain, the study of multi-scale interface phenomena in complex environments across the Earth Sciences, remote sensing of the environment (precipitation, clouds, soil moisture, and vegetation), climate predictability, extreme events and risk assessment of natural hazards. Prof. Barros is especially proud of having involved dozens of students in undergraduate research, a great majority of which continued their studies to earn graduate degrees in science and engineering.  Over recent years her work has focused on precipitation processes including microphysics and dynamics or orographic precipitation, and land-atmosphere interactions in mountainous regions from the Himalayas to the Andes and the Southern Appalachians including Land-Use Land-Cover Change impacts on regional climate. Her research relies on intensive field and laboratory experiments, large–scale computational modeling, nonlinear data analysis and environmental informatics.

Prof. Barros served in the Space Studies Board of the National Research Council, and in several committees of the Water Science and Technology Board and the Board of Atmospheric Sciences and Climate including the Climate Research Committee, and she was a member of the US National Committee for the International Hydrology Program (IHP) of the UNESCO.  She served as an elected member of the Council of the American Meteorological Society, and serves and served in several committees within the American Meteorological Society and the American Geophysical Union. Prof. Barros was a member of the NOAA’s Climate and Global Change Program and serves or has served in several working groups at NASA, NSF and DOE-ARM. She has been an active member of several professional organizations including the IEEE, ASCE, AGU, AMS, AAAS, EGS, ASEE, and AWRA, and she currently serves in the ASCE committee on Climate Change and Adaptation. Prof. Barros was the Chief Editor of the Journal of Hydrometeorology for five years, and she was an AE of the Journal of Hydrology until 2015 among other editor posts. She has been a Senior Fellow who the Energy and Climate Partnership of the Americas (ECPA) since 2011. 

Prof. Barros received Early Career Investigator awards from NSF and NASA in 1995 and 1996. She was a George W. Merck Faculty Fellow at Harvard University 1999-2003, and Packard Fellow nominee from Penn State University. She received the Prize Foundation A. Almeida in Engineering in 1985, and the Lorenz G. Straub Award for her Ph.D. thesis in 1993, and the NASA GSFC Robert H. Goddard Award (GPM GV Team), Category of Exceptional Achievement in Science in 2014 for contributions in support of the Global Precipitation Measurement Mission.  Dr. Barros is a Member of the ASCE, Senior Member of the IEEE, a Fellow of the American Meteorological Society, and a Fellow of the American Geophysical Union.  She was the AMS Sigma-Xi Distinguished Lecturer 2014-2015.

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