Chemical source apportionment of ambient particulate matter in the Kathmandu Valley, Nepal

Anthropogenic emissions of submicron particulate matter (PM₁) reduce the quality of life and life expectancy globally. The Kathmandu Valley in Nepal endures unmitigated aerosol pollution. The air quality index in this remote urban basin can be as hazardous as megacities', like Delhi, which is eight-fold more populated. Ambient measurements in April 2015 and winter 2018 and mobile measurements in April 2018 characterized PM₁ for the Nepal ambient monitoring and source testing experiment (NAMaSTE). Measurements of non-refractory PM₁ (NR-PM₁) size, concentration, and chemical composition by aerosol mass spectrometers (AMS) characterize the regional aerosols. Understanding ambient PM₁ concentration, chemical composition, organic aerosol sources, and spatial variation is essential for pollution mitigation. The enhancement of PM₁ from background to urban regions was 120%. Organic aerosol (OA, 50%) and black carbon (BC, 20%) dominate stationary PM₁ mass. On-road PM₁ had an elevated BC component of 60%. Carbonaceous aerosols were typically 70% or more of PM₁ mass. Mobile measurements characterized an enhancement of sulfate aerosol (SO₄²⁻ ) by 150% from background to industrial brick kilns regions. Source apportionment of OA by positive matrix factorization (PMF) identifies traffic, biomass, trash, and coal combustion, and a dominant fraction of secondary organic aerosols (57%). Traffic, the prominent primary source of PM₁, contributed 18% of ambient OA and 36% of BC. In the city center the contribution of primary emissions was up to 60%. Brick kilns were a primary source of organic and inorganic sulfate emissions, and trash burning plumes a source of aerosol hydrogen chloride (HCl). The impact of distinct source factors varied by hour of day and measurement location. This study characterized significant ambient PM₁ sources and the effect of measurement location on concentration and composition. Characterizing the impact from each emission source provides critical insight for local air quality management. The fast time response of ground-based mobile measurements and in-situ instrumentations allowed for the determination of distinct plumes and local sources, in addition to regional variation. This distinction is impossible from stationary or satellite measurement platforms. The combination of stationary and mobile measurements across the Kathmandu Valley characterizes the spatial and temporal impacts on PM₁ concentration and chemical composition.