Baseline continental aerosol over the central Tibetan plateau and a case study of aerosol transport from South Asia Xiangao Xia a, * , Xumei Zong a , Zhiyuan Cong b , Hongbin Chen a , Shichang Kang b, c , Pucai Wang a a Key Laboratory of Middle Atmosphere and Global Environment Observation LAGEO, Institute of Atmospheric Physics, Chinese Academy of Sciences (CAS), P.O. Box 9804, Beijing 100029, China b Key Laboratory of Tibetan Environmental Changes and Land Surface Processes, Institute of Tibetan Plateau Research, CAS, Beijing, China c State Key Laboratory of Cryospheric Sciences, CAS, Beijing, China article info Article history: Received 22 April 2011 Received in revised form 15 July 2011 Accepted 18 July 2011 Keywords: Aerosol Tibetan plateau Baseline Transportation abstract Using 22 months of sunphotometer observations at Nam Co, an Aerosol Robotic Network (AERONET) site located in the central Tibetan Plateau, the background level of aerosol in this remote continental site was analyzed for the first time. An extraordinary pollution episode with an aerosol optical depth at 500 nm (s) of an order of magnitude higher than the baseline was further studied using ground-based and satellite remote-sensing data. The annual aerosol baseline at Nam Co is s ¼ 0.029, which is about half of that over the Pacific Ocean and the Atlantic Ocean. Sunphotometer observations at Nam Co recorded an intense spring atmospheric pollution episode, with the maximum s of 0.42 on 16 March 2009. The episode was characterized by dominant fine particles with strong absorption. A large s value with similar size and absorption also occurred in the Indo-Gangetic plains and South Asia, suggesting the transportation of aerosols to the Tibetan Plateau from surrounding regions. The passive satellite data showed dense plumes associated with active fires in the Himalayan foothills, the Indo-Gangetic plains, and South Asia and a thick, widespread haze piling up against the Himalayan ridges and impinging upon adjacent valleys. A pollution pool with a thick layer rich with aerosol over the Ganges Plain in Northern India, as revealed by the active satellite data, extended up to 3e5 km during this period, making it possible for the transportation of aerosols to the central Tibetan Plateau by the southwesterly wind prevailing at that level. Further studies are urgently required to identify the transport mechanism and to reveal the potential climatic impacts of aerosol transportation to the Tibetan Plateau. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. 1. Introduction The Tibetan Plateau (TP), the third pole, is a vast and elevated plateau in Asia that extends over the area of 27e45  N, 70e105  E, with a mean elevation >4 km above sea level (a.s.l). Not only the Asian monsoon circulation but also the water cycle of the entire Asian continent are influenced by the TP through its dynamical and thermal forcing as well as snow/glacier melt (Lau et al., 2010; Wu et al., 2007; Yeh et al., 1957). There has been growing evidence of climatic changes in the TP regions such as increased warming, early snowmelt, and retreat of high mountain glaciers. These outstanding features have been attributed to greenhouse warming (Duan and Wu, 2006). However, it has been argued that greenhouse warm- ing is not necessarily the sole agent of change in these regions, and that local forcing and feedback processes could play an important role in causing the faster warming rate and the accelerated retreat of the mountain glaciers (Lau et al., 2010; Kang et al., 2010). It was estimated that atmospheric heating by Asian brown clouds doubled the greenhouse warming over South Asia, and that this heating may contribute substantially to the loss of glacier mass in the Himalayas (Ramanathan et al., 2007). The snow-darkening effect by black carbon and dust may also contribute substantially to the early snowmelt and the retreat of the mountain glaciers (Xu et al., 2009; Qian et al., 2011). The TP is surrounded by many important natural and anthro- pogenic aerosol source areas, including the Taklimakan Desert to the north, the Gobi Desert to the northeast, the deserts in Southwest Asia and Middle East to the west and southwest, and by anthro- pogenic emissions over the Indo-Gangetic Plain and biomass burning in South Asia. Due to general circulation patterns, the TP is a strong receptor of these source areas. Studies of the effects of transport and local emission of aerosols on Tibetan’s environment and climate are significant. It has been suggested that absorbing aerosols in the elevated regions of Hindu-Kush-Himalaya (HKH) * Corresponding author. Tel.: þ86 10 82995071; fax: þ86 10 82995073. E-mail address: xxa@mail.iap.ac.cn (X. Xia). Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.07.067 Atmospheric Environment 45 (2011) 7370e7378