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Along with fast economic development, the Yangtze Delta region is experiencing rapid environmental changes. A high concentration of aerosols with diverse properties is emitted in this region, providing a unique opportunity for understanding the impact of environmental change on climate systems, especially in urban regions. Aerosol optical properties including aerosol optical depth (AOD), the Ångström exponent (α), and water vapour and their relationship with aerosol particle concentrations were examined using ground-based measurements at three sites in the Hangzhou district: Hangzhou (120.01° E, 30.26° N), Lin'an (119.73° E, 30.26° N) and Qiandaohu (119.05° E, 29.55° N). The results from these observations show that there exists strong temporal and spatial variation in aerosol optical properties and particle matter (PM) concentrations. Monthly averaged AODs at the Hangzhou and Lin'an sites show a similar trend. AODs in Qiandaohu have the highest values in winter and spring and the lowest values in summer and autumn. In 2008, the minimum monthly mean α for every site was lowest in December due to the weather conditions. From spring to summer, these three locations experienced a drop in AOD and an increase in α due to the humidity-swelling effect. There is a positive correlation between AOD and aerosol particle concentrations; the correlation coefficients were obtained as 0.40, 0.55 and 0.69 in Hangzhou, Lin'an and Qiandaohu, respectively. The association between daily PM10 concentration and AOD is marginally statistically significant at the three sites, which suggests that AOD is a useful tool for mapping PM distribution over large spatial domains. [ABSTRACT FROM PUBLISHER]