Baseline continental aerosol over the central Tibetan plateau and a case study of aerosol transport from South Asia

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 (τ) 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 τ = 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 τ of 0.42 on 16 March 2009. The episode was characterized by dominant fine particles with strong absorption. A large τ 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 3–5 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.