Baseline continental aerosol over the central Tibetan plateau and a case study of aerosol transport from South Asia
Highlights
► The background level of aerosol at a Tibetan site is revealed. ► A spring atmospheric pollution episode over the central TP is shown. ► Aerosol transportation from South Asia to the central TP is revealed.
Introduction
The Tibetan Plateau (TP), the third pole, is a vast and elevated plateau in Asia that extends over the area of 27–45°N, 70–105°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 warming 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 anthropogenic 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 anthropogenic 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) heat the middle troposphere by absorbing solar radiation. Heating produces an atmospheric dynamical feedback, i.e., the so-called elevated-heat-pump (EHP) effect, resulting in increased precipitation over much of India (Lau and Kim, 2006). The theory of an EHP is based on the assumption that the direct aerosol effect is reinforced in high altitudes and above bright surfaces. Therefore whether anthropogenic aerosol can be transported to the Tibetan Plateau is crucial. These bright surfaces, however, exist mainly in high altitudes, e.g., on snow-covered Himalayan mountains. For aerosols to interact with these surfaces, they have to ascend to ≥5 km a.s.l (Kuhlmann and Quaas, 2010). Natural aerosols in the region originate mainly from local and remote deserts. Elemental compositions of aerosol at sites such as, for example, in Udaoliang, Lhasa, and Gongga, showed that the average dust concentration at Tibetan stations was 82 μg m−3 (Zhang et al., 2001). Transport of dust aerosols from the Taklimakan Desert to the northern slope of the TP during summer was evidenced by CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) and by Multi-angle Imaging SpectroRadiometer (MISR) aerosol measurements (Huang et al., 2007, Xia et al., 2009). Anthropogenic aerosols around the TP have their sources both in the Indian and Chinese urban areas but also in the rural areas (Cong et al., 2007). Atmospheric aerosols build up over the southern slope of the TP during the premonsoon season, and they are lifted up by the Himalayan orography. Significant build-up of aerosol and black carbon concentrations was observed during a dry period, accompanied by the occurrence of fires and by the transport of pollution from the nearby regions of South Asia and the northern part of the Indian Peninsula (Engling et al., 2011). Observations at the Nepal Climate Observatory-pyramid (NCO-P) provide clear evidence that, especially during the premonsoon period, the southern side of the high Himalayan valleys serve as a “direct channel” able to transport brown cloud pollutants up to 5 km a.s.l., where the pristine atmospheric composition can be strongly influenced (Bonasoni et al., 2010). Transport of optically active material to the very sensitive regions of the Himalayas is therefore a key issue to improve the description of the absorbing aerosol effect on Indian Summer monsoon and, in particular, its impact on precipitation in the HKH regions, and therefore on frozen-water storage (Lau et al., 2010). However, the effects of the Tibetan aerosol on regional climate variability remain largely unknown, due partly to an insufficient number of observations over the plateau.
The objective of this study was first to present baseline continental aerosol loading using 22 months of sunphotometer observations at Nam Co, an Aerosol Robotic NETwork (AERONET) site located in the central TP. More importantly, an extraordinary aerosol pollution case with aerosol optical depth (τ) an order of magnitude higher than the baseline was studied using ground and satellite remote-sensing data. The motivation of this case study is to present how anthropogenic activities dramatically perturb the background aerosol levels and discuss how aerosol optical properties change in such case. The research results are significant for studying climate change in TP and how anthropogenic activities may contribute to climate change in TP.
Section snippets
Site description
Ground-based remote-sensing aerosol data were acquired using the CIMEL sunphotometer at Nam Co Station for Multisphere Observation and Research (Briefly Nam Co Station, 30.773°N, 90.962°E, 4730 m a.s.l.). The site is occasionally impacted by dust episodes during spring. Under the influence of the Indian summer monsoon, summer is characterized by higher temperature and humid weather. Cloudless skies are prevalent in autumn, and winter is cold and the surface is generally snow covered, resulting
Seasonal variation and baseline continental aerosol
Monthly statistics of aerosol optical depth at 500 nm (τ) and Angstrom exponent (α) are presented in Fig. 1 and Table 1. Monthly mean and median τ from October to February are <0.03, indicating pristine condition during these months. These values are close to τ (0.02) at Mauna Loa (3.4 km a.s.l.) in the mid-Pacific (Holben et al., 2001) and τ (0.02 at 440 nm) at Dome C in Antarctica (Delphine et al., 2005). The maximum seasonal mean of ∼0.07 occurs during spring. This is mostly due to presence
Discussion and conclusions
The data used in this study provide important preliminary insights into the background condition of aerosol optical properties and the sources of aerosols in this remote part of the TP. A case study unveiled a substantial regional buildup of atmospheric brown clouds in the southern TP slope due to fire activities and transport of pollution from the nearby regions of South Asia and the northern part of the Indian Peninsula to the central TP. Aerosol optical depth and aerosol absorption during
Acknowledgments
We thank B. Holben, S. Tripathi, S. Janjai and staff for their effort in maintaining AERONET sites (Kanpur, Nainital, Chiang_Mai_Met_Sta and Nam Co). We would like to thank the NOAA Air Resources Laboratory team for providing the HYSPLIT_4 trajectory model. The CALIPSO and OMI data were available from the Atmospheric Science Data Center. The research was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant no. KZCX2-YW-QN201), the National Basic Research
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