Headline: Source Apportionment Of Regional Sources And Spatial Difference Of Aerosol Composition From The Nepal Ambient Monitoring And Source Testing Experiment [NAMaSTE] In The Kathmandu Valley, Nepal

The Kathmandu Valley, an urban basin in Nepal, is subject to extreme air pollution events. Over 10% of Nepali fatalities are from lung disorders, making it the country's most common cause of mortality. The Nepal Ambient Monitoring and Source Testing Experiment [NAMaSTE-1] (2015) campaign establishes aerosol composition backgrounds and diurnal patterns at a suburban site east of Kathmandu. NAMaSTE-2 (2017-2018) builds upon this initial study with high time resolution aerosol and trace gas measurements at four air quality monitoring sites centered on the valley. Spatially resolved mobile AMS measurements taken across Kathmandu exhibit compositional differences due to topography and local sources. Aerosol composition near Kathmandu shows a distinct diurnal pattern, with extremely elevated concentrations overnight and into morning. This pattern is primarily driven by meteorology and strong afternoon westerly winds. Overnight the valley experiences weak easterly wind flows leading to increased concentrations of aerosols through confinement and stagnation from temperature inversions. Brick kilns, trash fires, as well as dung and biomass burning are ubiquitous regional primary emission sources; additional partitioning of secondary species such as ammonium, nitrate, and chlorides increase aerosol concentrations. Traffic remains a major source in Kathmandu; measurements at the Ratnapark urban site indicate, during peak traffic times, a four-fold increase in organic particles, a three-fold increase in chlorides, and large increases in NOx and black carbon. Relative contributions to aerosol composition at the different measurement sites will be discussed and provide integral information for mitigation efforts aimed at reducing exposure to ambient aerosols.