The Kathmandu Valley in Nepal suffers from severe wintertime air pollution. Volatile organic compounds (VOCs) are key constituents of air pollution, though their specific role in the valley is poorly understood due to insufficient data. During the SusKat-ABC (Sustainable Atmosphere for the Kathmandu Valley-Atmospheric Brown Clouds) field campaign conducted in Nepal in the winter of 2012-2013, a comprehensive study was carried out to characterise the chemical composition of ambient Kathmandu air, including the determination of speciated VOCs, by deploying a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) -the first such deployment in South Asia. In the study, 71 ion peaks (for which measured ambient concentrations exceeded the 2σ detection limit) were detected in the PTR-TOF-MS mass scan data, highlighting the chemical complexity of ambient air in the valley. Of the 71 species, 37 were found to have campaign average concentrations greater than 200 ppt and were identified based on their spectral characteristics, ambient diel profiles and correlation with specific emission tracers as a result of the high mass resolution (m / m > 4200) and temporal resolution (1 min) of the PTR-TOF-MS. The concentration ranking in the average VOC mixing ratios dur-ing our wintertime deployment was acetaldehyde (8.8 ppb) > methanol (7.4 ppb) > acetone + propanal (4.2 ppb) > benzene (2.7 ppb) > toluene (1.5 ppb) > isoprene (1.1 ppb) > acetonitrile (1.1 ppb) > C8-aromatics (∼1 ppb) > furan (∼0.5 ppb) > C9-aromatics (0.4 ppb). Distinct diel profiles were observed for the nominal isobaric compounds isoprene (m / z = 69.070) and furan (m / z = 69.033). Comparison with wintertime measurements from several locations elsewhere in the world showed mixing ratios of acetaldehyde (∼ 9 ppb), acetonitrile (∼ 1 ppb) and isoprene (∼ 1 ppb) to be among the highest reported to date. Two "new" ambient compounds, namely formamide (m / z = 46.029) and acetamide (m / z = 60.051), which can photochemically produce isocyanic acid in the atmosphere, are reported in this study along with nitromethane (a tracer for diesel exhaust), which has only recently been detected in ambient studies. Two distinct periods were selected during the campaign for detailed analysis: the first was associated with high wintertime emissions of biogenic isoprene and the second with elevated levels of ambient acetonitrile, benzene and isocyanic acid from biomass burning activities. Emissions from biomass burning and biomass co-fired brick kilns were found to be the dominant sources for compounds such Published by Copernicus Publications on behalf of the European Geosciences Union. C. Sarkar et al.: Wintertime high acetaldehyde, isoprene and isocyanic acid in Kathmandu Valleyas propyne, propene, benzene and propanenitrile, which correlated strongly with acetonitrile (r 2 > 0.7), a chemical tracer for biomass burning. The calculated total VOC OH reactivity was dominated by acetaldehyde (24.0 %), isoprene (20.2 %) and propene (18.7 %), while oxygenated VOCs and is...

Kathmandu Valley is one of the largest and most polluted metropolitan regions in the Himalayan foothills. Rapidly expanding urban sprawl and a growing fleet of vehicles, and industrial facilities such as brick factories across the valley have led to conditions where ambient concentrations of key gaseous air pollutants are expected to exceed Nepal's National Ambient Air Quality Standards (NAAQS) and World Health Organization (WHO) guidelines. In order to understand the spatial variation of the trace gases in the Kathmandu Valley, passive samples of SO 2 , NO x , NO 2 , NH 3, and O 3 were collected simultaneously from fifteen locations between March and May 2013. A follow-up study during two separate campaigns in 2014 sampled these gases, except ammonia, one site at a time from thirteen urban, suburban and rural stationary sites. In 2013, urban sites were observed to have higher weekly averaged NO 2 and SO 2 (22.4 ± 8.1 µg m -3 and 14.5 ± 11.1 µg m -3 , respectively) than sub-urban sites (9.2 ± 3.9 µg m -3 and 7.6 ± 2.8 µg m -3 , respectively). Regions located within 3 km of brick factories had higher SO 2 concentrations (22.3 ± 14.7 µg m -3 ) than distant sites (5.8 ± 1.1 µg m -3 ). Higher O 3 (108.5 ± 31.4 µg m -3 ) was observed in rural locations compared to urban sites (87.1 ± 9.2 µg m -3 ), emphasizing the importance of meteorological factors and precursor species for ozone production and titration. Parallel to previous studies, these results suggest that ground-level O 3 , as its levels frequently exceeded guidelines throughout the sampling periods, is an important concern throughout the valley. NH 3 near polluted rivers and SO 2 around brick factories are also important pollutants that need more intensive monitoring, primarily due to their importance in particulate matter formation chemistry.

Abstract. Anthropogenic emissions from the combustion of fossil fuels and biomass in Asia have increased in recent years. High concentrations of reactive trace gases and lightabsorbing and light-scattering particles from these sources form persistent haze layers, also known as atmospheric brown clouds, over the Indo-Gangetic plains (IGP) from December through early June. Models and satellite imagery suggest that strong wind systems within deep Himalayan valleys are major pathways by which pollutants from the IGP are transported to the higher Himalaya. However, observational evidence of the transport of polluted air masses through Himalayan valleys has been lacking to date. To evaluate this pathway, we measured black carbon (BC), ozone (O 3 ), and associated meteorological conditions within the Kali Gandaki Valley (KGV), Nepal, from January 2013 to July 2015. BC and O 3 varied over both diurnal and seasonal cycles. Relative to nighttime, mean BC and O 3 concentrations within the valley were higher during daytime when the up-valley flow (average velocity of 17 m s −1 ) dominated. BC and O 3 concentrations also varied seasonally with minima during the monsoon season (July to September). Concentrations of both species subsequently increased post-monsoon and peaked during March to May. Average concentrations for O 3 during the seasonally representative months of April, August, and November were 41.7, 24.5, and 29.4 ppbv, respectively, while the corresponding BC concentrations were 1.17, 0.24, and 1.01 µg m −3 , respectively. Up-valley fluxes of BC were significantly greater than down-valley fluxes during all seasons. In addition, frequent episodes of BC concentrations 2-3 times higher than average persisted from several days to a week during non-monsoon months. Our observations of increases in BC concentration and fluxes in the valley, particularly during pre-monsoon, provide evidence that trans-Himalayan valleys are important conduits for transport of pollutants from the IGP to the higher Himalaya.

<p><strong>Abstract.</strong> Anthropogenic emissions from the combustion of fossil fuels and biomass in Asia have increased in recent years. High concentrations of reactive trace gases and absorbing and light-scattering particles from these sources over the Indo-Gangetic Plain (IGP) of southern Asia form persistent haze layers, also known as atmospheric brown clouds, from December through early June. Models and satellite imagery suggest that strong wind systems within deep trans-Himalayan valleys are major pathways by which pollutants over the IGP are transported to the high Tibetan Plateau (TP). To evaluate this pathway, we measured black carbon (BC), ozone (O<sub>3</sub>), and associated meteorological conditions within the Kali-Gandaki Valley, Nepal, from January 2013 to August 2015. BC and O<sub>3</sub> varied over both diurnal and seasonal cycles. Relative to nighttime, mean BC and O<sub>3</sub> concentrations within the valley were higher during daytime when the up-valley flow (average velocity of 17 ms<sup>−1</sup>) dominated. Minimal BC and O<sub>3</sub> concentrations occurred during the monsoon season (July to September). Concentrations of both species subsequently increased post monsoon and peaked during March to May. We recorded average concentration for O<sub>3</sub> during April, July, and November were 41.7 ppbv, 24.5 ppbv, and 29.4 ppbv, respectively, while the corresponding BC concentrations were 1.17 µg m<sup>−3</sup>, 0.24 µg m<sup>−3</sup>, and 1.01 µg m<sup>−3</sup>, respectively. Frequent episodes of concentrations two to three fold higher than average persisted from several days to a week during non-monsoon months. Our observations of increases in BC concentration in the valley – especially during pre-monsoon season (April) – support the hypothesis that trans-Himalayan valleys are important conduits for transport of pollutants from the IGP to TP. In addition, the increase in BC concentration in the KGV during high fire activity in Northern India and southern Nepal corroborates the role of trans-Himalayan valleys as vital pollutant transport pathways.</p>

Abstract. The Kathmandu Valley in Nepal suffers from severe wintertime air pollution. Volatile organic compounds (VOCs) are key constituents of air pollution, though their specific role in the Valley is poorly understood due to insufficient data. During the SusKat-ABC (Sustainable Atmosphere for the Kathmandu Valley-Atmospheric Brown Clouds) field campaign conducted in Nepal in the winter of 2012–2013, a comprehensive study was carried out to characterize the chemical composition of ambient Kathmandu air, including the determination of speciated VOCs by deploying a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF-MS)–the first such deployment in South Asia. 71 ion peaks (for which measured ambient concentrations exceeded the 2 σ detection limit) were detected in the PTR-TOF-MS mass scan data, highlighting the chemical complexity of ambient air in the Valley. Of the 71 species, 37 were found to have campaign average concentrations greater than 200 ppt and were identified based on their spectral characteristics, ambient diel profiles and correlation with specific emission tracers as a result of the high mass resolution (m/Δm > 4200) and temporal resolution (1 min) of the PTR-TOF-MS. The highest average VOC mixing ratios during the measurement period were (in rank order): acetaldehyde (8.8 ppb), methanol (7.4 ppb), acetone (4.2 ppb), benzene (2.7 ppb), toluene (1.5 ppb), isoprene (1.1 ppb), acetonitrile (1.1 ppb), C8-aromatics (~ 1 ppb), furan (~ 0.5 ppb), and C9-aromatics (0.4 ppb). Distinct diel profiles were observed for the nominal isobaric compounds isoprene (m/z = 69.070) and furan (m/z = 69.033). Comparison with wintertime measurements from several locations elsewhere in the world showed mixing ratios of acetaldehyde (~ 9 ppb), acetonitrile (~ 1 ppb) and isoprene (~ 1 ppb) to be among the highest reported till date. Two "new" ambient compounds namely, formamide (m/z = 46.029) and acetamide (m/z = 60.051), which can photochemically produce isocyanic acid in the atmosphere, are reported in this study along with nitromethane (a tracer for diesel exhaust) which has only recently been detected in ambient studies. Two distinct periods were selected during the campaign for detailed analysis: the first was associated with high wintertime emissions of biogenic isoprene, and the second with elevated levels of ambient acetonitrile, benzene and isocyanic acid from biomass burning activities. Emissions from biomass burning and biomass co-fired brick kilns were found to be the dominant sources for compounds such as propyne, propene, benzene and propanenitrile which correlated strongly with acetonitrile (r2 > 0.7), a chemical tracer for biomass burning. The calculated total VOC OH reactivity was dominated by acetaldehyde (24.0 %), isoprene (20.2 %) and propene (18.7 %), while oxygenated VOCs and isoprene collectively contributed to more than 68 % of the total ozone production potential. Based on known SOA yields and measured ambient concentrations in the Kathmandu Valley, the relative SOA production potential of VOCs were: benzene > naphthalene > toluene > xylenes > monoterpenes > trimethyl-benzenes > styrene > isoprene. The first ambient measurements from any site in South Asia of compounds with significant health effects such as isocyanic acid, formamide, acetamide, naphthalene and nitromethane have been reported in this study. Our results suggest that mitigation of intense wintertime biomass burning activities, in particular point sources such biomass co-fired brick kilns, would be important to reduce the emission and formation of toxic VOCs (such as benzene and isocyanic acid) in the Kathmandu Valley and improve its air quality.

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Atmospheric visibility, a measure of horizontal distance one can distinctly see with unaided eye, is affected by the scattering and absorption of visible light by the tiny particles (aerosols), or different gases present in the atmosphere. Thus, visibility is commonly considered a proxy for ambient air quality. The long-term trend of visibility may reflect the change in the air quality of a place over the period. In our study, we have analyzed historic climatological data (1977-2020) from the National Oceanic and Atmospheric Administration (NOAA) global hourly archive, for the two cities of west Nepal, namely, Bhairahawa (BWA, 27.506 °N, 83.416 °E) and Surkhet (SKH, 28.6 °N, and 81.617 °E). We have found that both of the synoptic stations exhibited persistent degraded visibility. BWA had poorer visibility conditions (poor air quality) than that of SKH since the beginning of the study period. Since 2014, the ‘annual good day’ (V ≥ 10 km ) at BWA is steadily near 0%, and the ‘annual bad day’ (V < 5 km) is ≈ 60%, suggesting a degraded air quality. Similarly, we observed a notable decline in the 50th percentile of visibility in the mid-1980s at SKH, and a sharp decline of ‘good day’ since 2011. Meteorology modifies the optical properties of aerosol/ gaseous in the atmosphere, thereby, resulting in a change in visibility. In our study, we have investigated the influence of relative humidity (RH) on prevailing visibility. Although the relationship between them exists in both of the stations, it is more distinctly visible at BWA. We observed lower visibility conditions (V ≤ 3 km) occurring at an RH level as low as 50% at BWA. This indicates an abundance of specific hygroscopic aerosols, whose light extinction thresholds are as low. At BWA, the impact of RH is evident during the dry season. In contrast, the threshold value of RH is quite high (80%) at SKH and the relationship is prominent during the wet season. This alarmingly poor air quality at both stations requires a serious concern because of its adverse impact on various sectors like aviation, tourism, and public health.