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The effect of aerosols on the global climate change has drawn much attention in recent years due to considerable uncertain climate forcing.Aerosol can influence the Earth’s radiative budget directly by scattering and absorbing solar radiation and indirectly by influencing cloud properties and lifetime.The aerosol optical properties (aerosol optical depth,particle size distribution and absorption properties etc.) are required in evaluating the radiative forcing of aerosols.Rapidly increasing urbanization and population lead to a significant increase in aerosol loading in China.The Yangtze River Delta,is one of the metropolitan areas of China,experience high particulate matter pollution and poor visibility due to human activities.In order to understand seasonal variation of aerosol optical properties in Yangtze River Delta,5-year measurements were conducted during September 2005 and December 2009 at Taihu,China.The monthly averages of aerosol optical depth were commonly larger than 0.6 with the maximum seasonal average 0.93 occurred in summer.The magnitude of ?ngstr?m exponent was found to be high throughout the year with the highest values occurred in autumn (1.33) and the lowest in spring (1.08).Approximately 81% of aerosol optical depth ranged between 0.1 and 1.2,and nearly 80% of ?ngstr?m exponent varied from 1.0 to 1.6 occurred in summer.The highest values of water vapor content occurred in summer (3.36 cm).The aerosol volume size distribution showed a bimodal logarithm normal structure: fine mode (radius < 0.6 μm) and coarse mode (radius > 0.6 μm).The fine modes of volume size distribution showed the maxima peak at radius about 0.15 μm in spring,autumn and winter,and radius about 0.22 μm in summer.The coarse modes showed the maxima peak at radius 2.9 μm in spring,summer and autumn,and 3.8 μm in winter.The volume concentration of fine and coarse modes showed clear positive dependence on aerosol optical depth.