Transport of Regional Pollutants Through a Remote Trans-Himalayan Valley in Nepal
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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. Transport of regional pollutants through a remote trans-Himalayan valley in Nepal Shradda Dhungel1, Bhogendra Kathayat2, Khadak Mahata3, and Arnico Panday1,4 1Department of Environmental Sciences, University of Virginia, Charlottesville, VA22904, 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 Correspondence: Shradda Dhungel ([email protected]) Received: 16 September 2016 – Discussion started: 8 November 2016 Revised: 18 November 2017 – Accepted: 20 December 2017 – Published: 30 January 2018 Abstract. Anthropogenic emissions from the combustion of days to a week during non-monsoon months. Our observa- fossil fuels and biomass in Asia have increased in recent tions of increases in BC concentration and fluxes in the val- years. High concentrations of reactive trace gases and light- ley, particularly during pre-monsoon, provide evidence that absorbing and light-scattering particles from these sources trans-Himalayan valleys are important conduits for transport form persistent haze layers, also known as atmospheric of pollutants from the IGP to the higher Himalaya. brown clouds, over the Indo–Gangetic plains (IGP) from De- cember through early June. Models and satellite imagery suggest that strong wind systems within deep Himalayan valleys are major pathways by which pollutants from the 1 Introduction IGP are transported to the higher Himalaya. However, ob- servational evidence of the transport of polluted air masses Persistent atmospheric haze, often referred to as atmospheric through Himalayan valleys has been lacking to date. To eval- brown cloud (ABC) (Ramanathan and Crutzen, 2003), af- uate this pathway, we measured black carbon (BC), ozone fects broad geographic regions including the Indo–Gangetic (O3), and associated meteorological conditions within the plains (IGP) in southern Asia (Ramanathan and Carmichael, Kali Gandaki Valley (KGV), Nepal, from January 2013 to 2008), eastern China (Ma et al., 2010), southeast Asia (En- July 2015. BC and O3 varied over both diurnal and seasonal gling and Gelencser, 2010), sub-Saharan Africa (Piketh et al., cycles. Relative to nighttime, mean BC and O3 concentra- 1999), Mexico (Vasilyev et al., 1995), and Brazil (Kaufman tions within the valley were higher during daytime when the et al., 1998). In southern Asia, the haze covers extensive ar- up-valley flow (average velocity of 17 m s−1) dominated. BC eas particularly during the period of mid-November to mid- and O3 concentrations also varied seasonally with minima June preceding the summer monsoon season. Major combus- during the monsoon season (July to September). Concentra- tion sources – including the anthropogenic burning of agri- tions of both species subsequently increased post-monsoon cultural waste, garbage, biofuel, and fossil fuels as well as and peaked during March to May. Average concentrations wildfires – emit volatile and particulate-phase compounds to for O3 during the seasonally representative months of April, the atmosphere that contain oxidized and reduced forms of August, and November were 41.7, 24.5, and 29.4 ppbv, re- sulfur, nitrogen, and organic carbon (OC) together with ele- spectively, while the corresponding BC concentrations were mental (black) carbon (BC) and other species. These emis- 1.17, 0.24, and 1.01 µg m−3, respectively. Up-valley fluxes of sions are intermixed and chemically interact with mechani- BC were significantly greater than down-valley fluxes during cally produced aerosols (e.g., sea salt and mineral dust). Im- all seasons. In addition, frequent episodes of BC concentra- portant secondary pollutants such as ozone (O3) also form tions 2–3 times higher than average persisted from several from photochemical reactions involving nitrogen oxides. To- gether, this mixture of atmospheric species constitutes the Published by Copernicus Publications on behalf of the European Geosciences Union. 1204 S. Dhungel et al.: Transport of regional pollutants through a remote trans-Himalayan valley in Nepal ABC or brown haze over southern Asia (Ramanathan et al., tal role in regulating the regional climate due to its effect on 2005; Gustafsson et al., 2009). These optically thick lay- the Asian summer monsoon (ASM) and the hydrologic cycle. ers include high concentrations of light-absorbing and light- In the TP, light-absorbing aerosols may not only serve warm scattering particles (Menon et al., 2002) that modulate ra- the atmosphere but – when deposited onto snow and ice sur- diative transfer. Light-absorbing aerosols (primarily BC and faces – may also decrease albedo and thereby substantially crustal dust) contribute to atmospheric warming, while light- increase the melting rates of snow and glaciers (Kang et al., scattering aerosols (primarily S-, N-, and OC-dominated par- 2010). Indeed, model simulations (Qian et al., 2011) have ticles) drive cooling at the surface. The combined effects shown that these light-absorbing aerosols change the surface of light-absorbing and light-scattering aerosols from anthro- radiative flux in the higher Himalaya and the TP by 5 to pogenic sources serve to reduce UV and visible wavelength 25 Wm−2 during the pre-monsoon months of April and May. radiation at the surface (i.e., surface forcing), increase tro- The interrelated perturbations of the ABC on radiative trans- pospheric warming (i.e., atmospheric forcing), and change fer, air quality, the hydrologic cycle, and crop yields have the net top of the atmosphere solar flux (i.e., top-of-the- important long-term implications for human health, food se- atmosphere forcing) (Andreae and Crutzen, 1997; Kaufman curity, and economic activity over southern Asia. However, et al., 2002; Ramanathan et al., 2005). Light-absorbing an- to date, there has been little observational research to directly thropogenic pollutants like BC also significantly influence demonstrate the role of mountain valleys in the transport of global warming, in terms of direct radiative forcing (Jacob- air pollutants from the IGP and the Himalayan foothills to son, 2001; Bond et al., 2013), and regional influences from higher elevations within and north of the Himalayan range. such pollutants close to sources are greater than those on the In other studied mountain ranges, it is well known that global scale (Ramanathan et al., 2007b). flow patterns carry polluted air masses up valleys to higher The elevated concentrations of aerosols in the anti-cyclone elevations by providing a path of least resistance between have also been shown to weaken circulation patterns and re- tall mountains. In the European Alps, prevailing wind sys- duce total monsoon precipitation over southern India (Ra- tems in the mountain river valleys funnel polluted air from manathan et al., 2005; Fadnavis et al., 2013) while intensi- peripheral source regions to high elevations in a phenomenon fying the monsoon over the foothills of the Himalaya (Lau known as “alpine pumping” (Weissmann et al., 2005). Under et al., 2006). In addition to warming the atmosphere, the ris- fair-weather conditions during daytime, the upslope winds ing concentrations of BC and O3 over southern Asia (e.g., are capable of transporting significant pollutants and mois- Ramanathan and Carmichael, 2008) have detrimental im- ture into the free troposphere (Henne et al., 2004). Relative to pacts on human health. Recent work has shown that elevated air over plains, the air within the valley heats and cools more levels of these two key pollutants can compromise cardiopul- quickly (Steinacker, 1984). The resultant differences in tem- monary and respiratory health (Krupnick et al., 1990; Janssen perature create gradients in pressure and density, which in et al., 2011). In addition, O3 is a leading pollutant contribut- turn drive the transport of air from the plains to higher eleva- ing to biodiversity loss (Royal Society, 2008) as well as de- tions during the daytime (Reiter and Tang, 1984; Whiteman clines in crop yields (Auffhammer et al., 2006). and Bian, 1998; Egger et al., 2000). Such direct evidence of Haze over the IGP often reaches heights of more than a Himalayan mountain valley wind system and its role in 3 kma:s:l: via convection and advection, and the Himalaya pollution transport has been observed at 5079 ma:s:l:, par- range forms a 2500 km long, 8 km high complex topographic ticularly during non-monsoon seasons, in the Khumbu val- barrier along the northern edge of the IGP (Singh et al., 2004; ley in the eastern Himalaya (Bonasoni et al., 2010). How- Dey and Di Girolamo, 2010; Gautam et al., 2011; Lüthi et al., ever, the transport patterns of pollutants at the habitable mid- 2015). Numerous studies have investigated the transport of altitudes within a trans-Himalayan valley have yet to be ob- pollutants from the IGP to the Himalayan foothills, the im- served. Here we present 2.5 years of measurements of BC, mediate source region for potential transport to the Tibetan O3, and associated meteorological data from one of the deep- Plateau (TP) (Pant et al., 2006; Dumka et al., 2008; Komp- est trans-Himalayan valleys, the Kali Gandaki Valley (KGV). pula et al., 2009; Hyvärinen et al., 2009; Ram et al., 2010; We examine seasonal and diurnal patterns of BC and O3, in- Brun et al., 2011; Gautam et al., 2011; Srivastava et al., vestigate potential episodes of enhanced pollution transport 2012). Further, a suite of studies involving satellite imagery up-valley, and make a preliminary estimate of BC mass trans- (Ramanathan et al., 2007a; Brun et al., 2011), back trajec- port. In doing so, we seek to provide the first observational tories (Lu et al., 2011), model calculations (Kopacz et al., evidence of trans-Himalayan valleys acting as conduits for 2011; Zhang et al., 2015), ice core analyses (Lee et al., 2008; pollution transport from the IGP to the higher Himalaya.