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

Abstract

The Kathmandu Valley in Nepal suffers from unhealthy air quality, with mean pre-monsoon submicron particulate matter concentration (PM₁) of 40 μg m⁻³ and daily peaks over 75 μg 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 μg m⁻³ (39.7%) organic aerosol (OA), 10.5 μg m⁻³ (25.0%) BC, 8.4 μg m⁻³ (21.1%) SO₄²⁻, 2.96 μg m⁻³ (7.4%) NH₄⁺, 1.22 μg m⁻³ (3.1%) NO₃⁻, and 0.92 μg 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 (O₃), 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, R² = 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.