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The Kathmandu Valley in Nepal suffers from severe wintertime air pollution.\udVolatile organic compounds (VOCs) are key constituents of air pollution,\udthough their specific role in the valley is poorly understood due to\udinsufficient data. During the SusKat-ABC (Sustainable Atmosphere for the\udKathmandu Valley–Atmospheric Brown Clouds) field campaign conducted in Nepal\udin the winter of 2012–2013, a comprehensive study was carried out to\udcharacterise the chemical composition of ambient Kathmandu air, including the\uddetermination of speciated VOCs, by deploying a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) – the first such deployment in South\udAsia. In the study, 71 ion peaks (for which measured ambient concentrations exceeded the\ud2σ detection limit) were detected in the PTR-TOF-MS mass scan\uddata, highlighting the chemical complexity of ambient air in the valley. Of\udthe 71 species, 37 were found to have campaign average concentrations greater\udthan 200 ppt and were identified based on their spectral\udcharacteristics, ambient diel profiles and correlation with specific emission\udtracers as a result of the high mass resolution (m ∕ Δm  >  4200) and temporal resolution (1 min) of the PTR-TOF-MS. The\udconcentration ranking in the average VOC mixing ratios during our wintertime\uddeployment was acetaldehyde (8.8 ppb)  >  methanol (7.4 ppb)\ud >  acetone + propanal (4.2 ppb)  >  benzene (2.7 ppb)  > \udtoluene (1.5 ppb)  >  isoprene (1.1 ppb)  >  acetonitrile\ud(1.1 ppb)  >  C8-aromatics ( ∼ 1 ppb)  >  furan\ud( ∼ 0.5 ppb)  >  C9-aromatics (0.4 ppb). Distinct diel\udprofiles were observed for the nominal isobaric compounds isoprene\ud(m ∕ z  =  69.070) and furan (m ∕ z  =  69.033).\udComparison with wintertime measurements from several locations elsewhere in\udthe world showed mixing ratios of acetaldehyde ( ∼  9 ppb),\udacetonitrile ( ∼  1 ppb) and isoprene ( ∼  1 ppb) to\udbe among the highest reported to date. Two "new" ambient compounds,\udnamely formamide (m ∕ z  =  46.029) and acetamide\ud(m ∕ z  =  60.051), which can photochemically produce isocyanic\udacid in the atmosphere, are reported in this study along with nitromethane (a\udtracer for diesel exhaust), which has only recently been detected in ambient\udstudies. Two distinct periods were selected during the campaign for detailed\udanalysis: the first was associated with high wintertime emissions of biogenic\udisoprene and the second with elevated levels of ambient acetonitrile,\udbenzene and isocyanic acid from biomass burning activities. Emissions from\udbiomass burning and biomass co-fired brick kilns were found to be the\uddominant sources for compounds such as propyne, propene, benzene and\udpropanenitrile, which correlated strongly with acetonitrile (r2 > 0.7), a\udchemical tracer for biomass burning. The calculated total VOC OH reactivity\udwas dominated by acetaldehyde (24.0 %), isoprene (20.2 %)\udand propene (18.7 %), while oxygenated VOCs and isoprene\udcollectively contributed to more than 68 % of the total ozone\udproduction potential. Based on known secondary organic aerosol (SOA) yields and measured ambient concentrations in the Kathmandu Valley, the relative SOA production potential of VOCs were benzene  >  naphthalene  >  toluene  >  xylenes  >  monoterpenes  >  trimethylbenzenes  >  styrene  >  isoprene.\udThe first ambient measurements from any site in South Asia of compounds with\udsignificant health effects such as isocyanic acid, formamide, acetamide,\udnaphthalene and nitromethane have been reported in this study. Our results\udsuggest that mitigation of intense wintertime biomass burning activities, in\udparticular point sources such biomass co-fired brick kilns, would be\udimportant to reduce the emission and formation of toxic VOCs (such as benzene\udand isocyanic acid) in the Kathmandu Valley

atmospheric chemistry and physics

Overview of VOC emissions and chemistry from PTR-TOF-MS measurements during the SusKat-ABC campaign: high acetaldehyde, isoprene and isocyanic acid in wintertime air of the Kathmandu Valley