Atmospheric brown clouds reach the Tibetan Plateau by crossing the Himalayas

Atmos. Chem. Phys., 15, 6007–6021, 2015 www.atmos-chem-phys.net/15/6007/2015/ doi:10.5194/acp-15-6007-2015 © Author(s) 2015. CC Attribution 3.0 License. Atmospheric brown clouds reach the Tibetan Plateau by crossing the Himalayas Z. L. Lüthi1 , B. Škerlak2 , S.-W. Kim3 , A. Lauer4 , A. Mues4 , M. Rupakheti4 , and S. Kang5,1 1 Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research and CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences (CAS), Beijing, China 2 ETH Zurich, Institute for Atmospheric and Climate Science, Zürich, Switzerland 3 School of Earth and Environmental Sciences, Seoul National University, Seoul, Republic of Korea 4 IASS Institute for Advanced Sustainability Studies, Potsdam, Germany 5 State Key Laboratory of Cryospheric Science, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences (CAS), Lanzhou, China Correspondence to: Z. L. Lüthi () and S. Kang () Received: 20 August 2014 – Published in Atmos. Chem. Phys. Discuss.: 13 November 2014 Revised: 9 March 2015 – Accepted: 12 April 2015 – Published: 1 June 2015 Abstract. The Himalayas and the Tibetan Plateau region (HTP), despite being a remote and sparsely populated area, is regularly exposed to polluted air masses with significant amounts of aerosols including black carbon. These dark, light-absorbing particles are known to exert a great melting potential on mountain cryospheric reservoirs through albedo reduction and radiative forcing. This study combines groundbased and satellite remote sensing data to identify a severe aerosol pollution episode observed simultaneously in central Tibet and on the southern side of the Himalayas during 13– 19 March 2009 (pre-monsoon). Trajectory calculations based on the high-resolution numerical weather prediction model COSMO are used to locate the source regions and study the mechanisms of pollution transport in the complex topography of the HTP. We detail how polluted air masses from an atmospheric brown cloud (ABC) over South Asia reach the Tibetan Plateau within a few days. Lifting and advection of polluted air masses over the great mountain range is enabled by a combination of synoptic-scale and local meteorological processes. During the days prior to the event, winds over the Indo-Gangetic Plain (IGP) are generally weak at lower levels, allowing for accumulation of pollutants and thus the formation of ABCs. The subsequent passing of synopticscale troughs leads to southwesterly flow in the middle troposphere over northern and central India, carrying the polluted air masses across the Himalayas. As the IGP is known to be a hotspot of ABCs, the cross-Himalayan transport of polluted air masses may have serious implications for the cryosphere in the HTP and impact climate on regional to global scales. Since the current study focuses on one particularly strong pollution episode, quantifying the frequency and magnitude of similar events in a climatological study is required to assess the total impact. 1 Introduction The Himalayas and Tibetan Plateau region (HTP), sometimes called the “third pole”, contains the largest volume of ice outside the polar regions and impacts radiative budgets and climate (Ye and Wu, 1998; Ma et al., 2009). Recently, a growing body of research has demonstrated that the atmosphere and cryosphere in the HTP are undergoing extraordinary changes, including atmospheric warming (Gautam et al., 2010; Thompson et al., 2000; Kang et al., 2010) and in many parts rapid glacier melting (Bolch et al., 2012; Yao et al., 2012). Consequently, the seasonal water availability of important Asian river systems are very likely to be affected (Immerzeel et al., 2010; Kehrwald et al., 2008). In addition to greenhouse gases, increasing ambient concentrations of black carbon (BC) appear to be an anthropogenic driving force of the observed changes in these remote regions (Lau et al., 2010; Ramanathan and Carmichael, 2008). Lightabsorbing aerosol particles such as mineral dust and BC con- Published by Copernicus Publications on behalf of the European Geosciences Union. 6008 Z. L. Lüthi et al.: Atmospheric brown clouds reach the Tibetan Plateau by crossing the Himalayas tribute to the atmospheric heating and the albedo reduction once deposited on glaciers. Albeit only contributing a few percent to the total aerosol mass, BC exerts major radiative effects (Bond et al., 2013; Jacobson, 2001), especially over the HTP during pre-monsoon seasons when the solar radiative flux at the surface is very high (Flanner et al., 2007). Even though background air pollution levels in the HTP are very low, recurring pre-monsoonal BC peaks have been documented at high altitudes of the south-facing Himalayan slopes (e.g., Marinoni et al., 2010, 2013; Decesari et al., 2010) which are sometimes directly exposed to atmospheric brown clouds (ABC) (Ramanathan et al., 2007b; Bonasoni et al., 2010). Brown clouds have been defined as “huge blankets or layers of haze generally composed of lightabsorbing submicrometer-sized carbonaceous aerosol particles” (Engling and Gelencser, 2010). Areas that are particularly affected by brown clouds, so-called ABC hotspots, are characterized by an anthropogenic aerosol optical depth (AOD) larger than 0.3 and an absorbing aerosol optical depth (AAOD) greater than 0.03 for at least one season (Ramanathan et al., 2007a). Such conditions are frequently observed on the southern side of the Himalayas, especially over the Indo-Gangetic Plain (IGP) during the dry months (November to May). BC transported from this regional hotspot might thus contribute to the retreat of Himalayan glaciers (Engling and Gelencser, 2010). Furthermore, recent studies show BC observations on snow and in ice even further north on the Tibetan Plateau (TP) (e.g., Xu et al., 2009; Qian et al., 2011; Ming et al., 2013; Kaspari et al., 2011) and springtime episodes with large amounts of pollutants observed on the TP have been reported (Engling et al., 2011; Xia et al., 2011). As a consequence of such episodes, BC concentrations on glaciers could significantly increase on the TP and affect the surface albedo (Zhao et al., 2013). Recent GEOS-Chem and HYSPLIT model calculations of BC advection to the TP suggest that the dominant source regions depend on season and receptor location, but South and East Asia show the highest overall contribution (Kopacz et al., 2011; Lu et al., 2012). However, to date, the mechanisms of pollutant transport from the ABC hotspot in the IGP and from the foothills of the Himalayas to the TP have not been investigated in detail. One reason for the limited knowledge about aerosol pollution on the HTP is the small number of long-term in situ observations. In addition, the results from chemistry transport models still have large uncertainties due to the complex terrain and the specific meteorological conditions, as do the emis (...truncated)