| Atmospheric aerosol plays an important role in global and regional climate change. It has a significant impact on the solar radiation budget and the formation and distribution of precipitation. As one of main air pollutants, aerosol has an important effect on the urban and suburban atmospheric environment. The Chinese Sun Hazemeter Network (CSHNET) has been established by us. It is the first standard network to reduce uncertainties in the quantitative assessment of aerosol effects on regional climate and environmental changes in China, which fills the gap of the research field. The synchronous measurements of aerosol optical properties were made with hand-held LED sun hazemeters in the Network, which depends on 19 stations of the Chinese Ecosystem Research Network (CERN), 4 typical cities' sites, and the Xianghe and the Lhasa as calibration sites. The network is conducted by the Atmosphere Sub-Center of Chinese Ecosystem Research Network (SCAS-CERN). A precise calibration and data quality control system was established. The hazemeters were systemically calibrated by Langley plot method and transfer calibration and were uniformly checked by CEMIL once a year. Meanwhile, the operating technology and management system for the ground-base network were well established.In the paper, the data (from August 2004 to June 2006) from CSHNET were collected, controlled, and analyzed. For the first time, the spatial and temporal distribution and the seasonal changing cycles of background aerosol optical properties were found over China. The observational experimental results obtained have come to the following conclusions. In most parts of China, seasonal changing cycles of AODs show a maximum in spring or summer and a minimum in autumn or winter. The minimum values (0.10～0.20) of annual mean AOD (λ= 500 nm) are found in the Qinghai-Tibetan Plateau, the remote northeast comer of China, and Hainan Island. The averages of annual AOD ranges from 0.25 to 0.30 over the oasis, desertification grassland and forest ecosystems in northern China, while annual mean AOD are all moderately high with 0.37 over the desert areas and the Loess Plateau. The maximum values of annual mean AODs with 0.50～0.90 occur in the central-eastern, southern and eastern coastal regions of China where there are the highest density of agricultural and industrial activities and where anthropogenic sulfate aerosol and soil aerosol emissions are constantly high in the whole year. The main types of aerosols (Angstrom exponent,α) over northern China also show a remarkable seasonal changing cycles. Due to biomass and fossil fuel burning from extensive agricultural practices in northern rural areas, concentrations of smoke and soot aerosols rise dramatically during autumn and winter(highα), while the main types of aerosols during spring and summer are dust and soil aerosols (lowα). Over Southeast Asia, the emission of smoke and soot increases by burning a great deal of biomass in spring, which influences the atmospheric environment over there. Over the Tibetan Plateau and Hainan Island where air quality is good, the main types of aerosols show dust aerosol and sea salt respectively.We have evaluated the accuracy of MODIS AOD products over the urban region by using the observational data in typical cities. The comparison of results shows a lack of representative and applicability of AOD retrieved by MODIS at these cities. At Beijing and Shanghai, only 30% of MODIS products fall within the retrieval error standard issued by NASA. At Lanzhou, Taipei and suburban Xianghe, about 50% of MODIS products fall within the retrieval error standard, but there are evident retrieval errors at special period. In addition, MODIS products are few during heavy aerosol pollution, and the spatial differences of them are bigger over urban regions than over other areas.
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