Variation trends of cloud and aerosol optical thicknesses over four sites in China during 1960–2009

Cloud and aerosol optical thicknesses (COT and AOT) are retrieved from pyrheliometer and pyranometer measurements over Beijing, Zhengzhou, Lhasa and Golmud meteorological observatories in China during 1960-2009. As shown in the retrieval results, there are COT-decreasing trends over Lhasa and Golmud with yearly decreasing rate of 0.029 and 0.026 but COT-increasing trends over Beijing and Zhengzhou with yearly increasing rate of 0.159 and 0.121. There are AOT-increasing trends with yearly increasing rate of 0.0007, 0.0002, 0.0016 and 0.0075 during 1960-2009 for Lhasa, Golmud, Beijing, and Zhengzhou, respectively. The AOT variations are very smooth over Lhasa and Golmud locating on the Tibetan plateau. The AOT increases over Zhengzhou site are particularly strong, changing from about 0.2 to 0.6 from 1960s to 2000s. There is considerably increasing trend of AOT over Beijing site during 1960-1999 but decreasing trend from 2000 to 2009.