Submicron Aerosol Composition and Source Contribution across the Kathmandu Valley, Nepal, in Winter.

The Kathmandu valley experiences an average wintertime PM 1 concentration of ∼100 μg m -3 and daily peaks over 200 μg m -3 . We present ambient nonrefractory PM 1 chemical composition, and concentration measured by a mini aerosol mass spectrometer (mAMS) sequentially at Dhulikhel (on the valley exterior), then urban Ratnapark, and finally suburban Lalitpur in winter 2018. At all sites, organic aerosol (OA) was the largest contributor to combined PM 1 (C-PM 1 ) (49%) and black carbon (BC) was the second largest contributor (21%). The average background C-PM 1 at Dhulikhel was 48 μg m -3 ; the urban enhancement was 120% (58 μg m -3 ). BC had an average of 6.1 μg m -3 at Dhulikhel, an urban enhancement of 17.4 μg m -3 . Sulfate (SO 4 ) was 3.6 μg m -3 at Dhulikhel, then 7.5 μg m -3 at Ratnapark, and 12.0 μg m -3 at Lalitpur in the brick kiln region. Chloride (Chl) increased by 330 and 250% from Dhulikhel to Ratnapark and Lalitpur on average. Positive matrix factorization (PMF) identified seven OA sources, four primary OA sources, hydrocarbon-like (HOA), biomass burning (BBOA), trash burning (TBOA), a sulfate-containing local OA source (sLOA), and three secondary oxygenated organic aerosols (OOA). OOA was the largest fraction of OA, over 50% outside the valley and 36% within. HOA (traffic) was the most prominent primary source, contributing 21% of all OA and 44% of BC. Brick kilns were the second largest contributor to C-PM 1 , 12% of OA, 33% of BC, and a primary emitter of aerosol sulfate. These results, though successive, indicate the importance of multisite measurements to understand ambient particulate matter concentration heterogeneity across urban regions.