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MODIS derived aerosol optical depths (AODs) at 550 nm are compared with sunphotometer CE318 measurements at 7 sites located at Yangtze River Delta (YRD) in China from July to October, 2007.The evaluation result indicates that MODIS AODs (Collection 5, C005) are in good agreement with those from CE318 in dense vegetation regions, but show more differences in those regions with complex underlying surface (such as at lake water and urban surface sites).Reasons for these differences are discussed after removing cases with significant errors caused by validation scheme.The final validation result shows that MODIS AODs are in good agreement with CE318 with a correlation coefficient of 0.85 and RMS of 0.15.90% of MODIS cases fall in the range of △τ=±0.05± 0.20τ, indicating MODIS aerosol retrieval algorithm, aerosol models and surface reflectance estimate are generally suitably reasonable for aerosol retrieval in YRD.However.MODIS AODs show a systemic errors with fitted line of y=0.75x+0.13, indicating underestimation of AOD when aerosol loadings are high.Aerosol models and surface reflectance estimations are dominant sources of MODIS aerosol retrieval errors.