Abstract
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.
Highlights
Persistent atmospheric haze, often referred to as atmospheric brown cloud (ABC) (Ramanathan and Crutzen, 2003), affects broad geographic regions including the Indo–Gangetic plains (IGP) in southern Asia (Ramanathan and Carmichael, 2008), eastern China (Ma et al, 2010), southeast Asia (Engling and Gelencser, 2010), sub-Saharan Africa (Piketh et al, 1999), Mexico (Vasilyev et al, 1995), and Brazil (Kaufman et al, 1998)
This study provides new in situ observational evidence of the role of a major Himalayan valley as an important pathway for transporting air pollutants from the IGP to the higher Himalaya
We found that concentrations of black carbon (BC) and O3 in the Kali Gandaki Valley (KGV) exhibited systematic diurnal and seasonal variabilwww.atmos-chem-phys.net/18/1203/2018/
Summary
Persistent atmospheric haze, often referred to as atmospheric brown cloud (ABC) (Ramanathan and Crutzen, 2003), affects broad geographic regions including the Indo–Gangetic plains (IGP) in southern Asia (Ramanathan and Carmichael, 2008), eastern China (Ma et al, 2010), southeast Asia (Engling and Gelencser, 2010), sub-Saharan Africa (Piketh et al, 1999), Mexico (Vasilyev et al, 1995), and Brazil (Kaufman et al, 1998). Model simulations (Qian et al, 2011) have shown that these light-absorbing aerosols change the surface radiative flux in the higher Himalaya and the TP by 5 to 25 W m−2 during the pre-monsoon months of April and May. The interrelated perturbations of the ABC on radiative transfer, air quality, the hydrologic cycle, and crop yields have important long-term implications for human health, food security, and economic activity over southern Asia. The resultant differences in temperature create gradients in pressure and density, which in turn drive the transport of air from the plains to higher elevations during the daytime (Reiter and Tang, 1984; Whiteman and Bian, 1998; Egger et al, 2000) Such direct evidence of a Himalayan mountain valley wind system and its role in pollution transport has been observed at 5079 m a.s.l., during non-monsoon seasons, in the Khumbu valley in the eastern Himalaya (Bonasoni et al, 2010).
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