10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Spatial Variation in Aerosol Chemical Composition and Source Contribution in Nepal from Aerosol Mass Spectrometry Measurements
BENJAMIN WERDEN, Michael Giordano, J. Doug Goetz, Khadak Mahata, Narayan Babu Dhital, Nita Khanal, Amit Bhujel, Sagar Adhikari, Siva Praveen Puppala, Maheswar Rupakheti, Prakash Bhave, Robert J. Yokelson, Elizabeth Stone, Arnico Panday, Peter DeCarlo, Drexel University
Abstract Number: 1525 Working Group: Remote/Regional Atmospheric Aerosol
Abstract The Kathmandu Valley, an urbanized basin in Nepal, is home to over 3 Million people, and is one of the fastest growing metropolitan areas in South Asia. It is subject to extreme pollution events due to numerous unmitigated localized pollution sources and regional transport from the Indo-Gangetic Plain (IGP). Over 10% of Nepali fatalities are from lung disorders, making it the most common cause of death in the country. Previous field work has studied gas species, wintertime VOCs and PM in the valley. The Nepal Ambient Measurement and Source Testing Experiment [NAMaSTE] of 2015 is the first deployment of a High-Resolution Particle Time of Flight Aerosol Mass Spectrometer (HR-PToF-AMS) in Nepal. Recent measurements in 2017-2018 provide context for spatial analysis of aerosol composition differences and their source contributions in the Kathmandu Valley and Nepal.
The in-situ ambient measurements were made at the Bode site in NAMaSTE 2015. This location is just to the east of Kathmandu. Measurements made in April 2015 were cut short by the 2015 Gorka earthquake so follow-up measurements to complete the dataset were performed in winter of 2017-2018. The winter measurements were made at several other locations in the Kathmandu Valley, and in Lumbini, Nepal in the Indo-Gangetic plain. Differences in aerosol loading and composition are related to the differences in sources and prevailing meteorology at the various sites. Additional gas phase (CO, CO2, CH4, H2O, O3, NOx, SO2,) and particle phase (BC, total PM, filters) measurements were taken in addition to the AMS data and are used to interpret the AMS dataset. Positive Matrix Factorization was used to identify aerosol sources, and chemical signatures were compared to source measurements made during NAMASTE 2015.
The mean concentration of PM2.5 in Bode, April 2015 was 83 ± 45 µg/m3, which is above the 24 hour WHO exposure threshold of 25 µg/m3 and annual continuous exposure limit of 10 µg/m3. AMS species show a clear diurnal pattern, with extremely elevated concentrations in the morning. Daily concentration patterns are driven by westerly winds in the late morning to afternoon. This wind is the main method of ventilation, however, it can also introduce pollutants formed over the IGP. Overnight there is confinement by the boundary layer and stagnation due to temperature inversions. This causes an increase in secondary aerosols from an abundance of ammonium, nitrate, and chlorides.
Supplemental AMS measurements at in the southeastern edge of the Kathmandu Valley (Dhulikhel) suggest a strong adabatic/katabatic flow pattern driven by solar gain. The central Kathmandu site, Ratnapark, is heavily influenced by traffic in the city center, with dramatic increases in aerosol loading observed during peak traffic times. Measurements outside of the valley in the IGP (Lumbini) show heavy loading of organics, NH4, and non-refractory chloride in winter fog episodes.
Localized sources of anthropogenic emissions such as traffic, garbage burning, brick kilns, solid fuel, and dung burning for cooking and agriculture are likely sources of elevated pollutant emissions. Burning of trash and biomass coupled with motor vehicle emissions are a major source of aerosol loading in the measurement region.