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To evaluate the impact of PM size distribution on aerosol properties,PM1 and PM2.5 samples were collected and aerosol scattering coefficients (bsp) were measured synchronously at urban site in Guangzhou,during the typical summer and winter periods.The excellent,good,lightly polluted,moderately polluted and heavily polluted days based on PM2.5 mass were categorized,according to Chinas Air Quality Index (AQI) (HJ633-2012).Bsp at 550nm were 88.5±13.0 Mm-1 (excellent),186.4±60.4 Mm-1 (good),384.1±83.8 Mm-1 (lightly polluted),588.8±84.0 Mm-1 (moderately polluted) and 757.6±97.4 (heavy polluted),which were 1.3-1.9 times of those in summer period.PM1 and PM1-2.5 accounted for 58.7% and 41.3%,respectively,of the bsp during the summer period.However,the contribution of PM1 to bsp was 2 times of PM1-2.5 in winter (65.2% and 34.8%).Among the different types of pollution,the contribution of chemical composition to aerosol properties reflected differences in different size ranges.In summer,as pollution aggravated,the contribution of OM to bsp in PM1 down from 34.2% to 26.4%,and (NH4)2SO4 became the predominant factor (accounted for 35.5%).The contribution of (NH4)2SO4 in PM1-2.5 was slightly larger than that of OM during slightly polluted,while high polluted in opposite.