Transport of regional pollutants through a remote trans-Himalayan valley in Nepal
Shradda Dhungel1, Bhogendra Kathayat2, Khadak Mahata3, and Arnico Panday1,41Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA 2Nepal Wireless, Shanti Marg , Pokhara, 33700, Nepal 3Institute for Advanced Sustainability Studies, Potsdam, 14467, Germany 4International Center for Integrated Mountain Development, Khulmaltar, Kathmandu, 44700, Nepal
Received: 16 Sep 2016 – Accepted for review: 27 Oct 2016 – Discussion started: 08 Nov 2016
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 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 (O3), and associated meteorological conditions within the Kali-Gandaki Valley, Nepal, from January 2013 to August 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 ms−1) dominated. Minimal BC and O3 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 O3 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−3, 0.24 µg m−3, and 1.01 µg m−3, 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.
Dhungel, S., Kathayat, B., Mahata, K., and Panday, A.: Transport of regional pollutants through a remote trans-Himalayan valley in Nepal, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-824, in review, 2016.