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Volume 18, issue 2
Atmos. Chem. Phys., 18, 1203–1216, 2018
https://doi.org/10.5194/acp-18-1203-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Atmospheric pollution in the Himalayan foothills: The SusKat-ABC...

Atmos. Chem. Phys., 18, 1203–1216, 2018
https://doi.org/10.5194/acp-18-1203-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 30 Jan 2018

Research article | 30 Jan 2018

Transport of regional pollutants through a remote trans-Himalayan valley in Nepal

Shradda Dhungel1, Bhogendra Kathayat2, Khadak Mahata3, and Arnico Panday1,4 Shradda Dhungel et al.
  • 1Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA
  • 2Nepal Wireless, Shanti Marg, Pokhara, 33700, Nepal
  • 3Institute for Advanced Sustainability Studies, 14467 Potsdam, Germany
  • 4International Center for Integrated Mountain Development, Khulmaltar, Kathmandu, 44700, Nepal

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 light-absorbing 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 (O3), and associated meteorological conditions within the Kali Gandaki Valley (KGV), Nepal, from January 2013 to July 2015. BC and O3 varied over both diurnal and seasonal cycles. Relative to nighttime, mean BC and O3 concentrations within the valley were higher during daytime when the up-valley flow (average velocity of 17 m s−1) dominated. BC and O3 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 O3 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.

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We analyze seasonal and diurnal concentrations of black carbon (BC), ozone, and associated meteorological conditions within a remote trans-Himalayan valley in western Nepal. We observe elevated BC concentrations during non-monsoon seasons, frequent and persistent episodes of higher-than-average concentrations, and net up-valley fluxes throughout the year. The findings provide direct observational evidence of trans-Himalayan valleys serving as vital pollutant transport pathways.
We analyze seasonal and diurnal concentrations of black carbon (BC), ozone, and associated...
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