Pre-monsoon submicron aerosol composition and source contribution in the Kathmandu Valley, Nepal

The Kathmandu Valley in Nepal suffers from unhealthy air quality, with mean pre-monsoon submicron particulate matter concentration (PM1) of 40 mu g m(-3) and daily peaks over 75 mu g m(-3). A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measured ambient non-refractory submicron PM (NR-PM1) in a Kathmandu suburb in April 2015. The mean NR-PM1 composition was 15.8 mu g m(-3) (39.7%) organic aerosol (OA), 10.5 mu g m(-3) (25.0%) BC, 8.4 mu g m(-3) (21.1%) SO42-, 2.96 mu g m(-3) (7.4%) NH4+, 1.22 mu g m(-3) (3.1%) NO3-, and 0.92 mu g m(-3) (2.3%) Cl-. NR-PM1 exhibited a diurnal pattern with concentration maxima at 08:00 local standard time. Afternoon winds and an elevated planetary boundary layer then mitigated NR-PM1. Positive matrix factorization (PMF) of OA mass spectral data identified four primary source components: hydrocarbon-like OA (HOA, 15%), biomass burning OA (BBOA, 15%), trash burning OA (TBOA, 11%), a local sulfur-containing OA (sLOA, 7%), and three oxidized OA (OOA, 50%) factors. sLOA contains the ion C3HSO+, associated with emissions from coal used in brick kilns. Secondary aerosols account for 50% of all OA mass within the valley. Gas-phase carbon monoxide (CO), carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O-3), methane (CH4), and black and brown carbon (BC, BrC) measurements also characterized pollution in the valley. Composition, concentration, and source analysis from AMS-PMF and filter-based chemical mass balance measurements agree well (OC slope = 1.05, R-2 = 0.47). This work identifies and quantifies significant sources of PM1 in the Kathmandu Valley and provides insight into the impact of these sources on overall air quality throughout South Asia.