Urban aerosols have a large effect on the deterioration of air quality and the degradation of atmospheric visibility.Characterization of the chemical composition of PM 2.5 and in situ measurements of the optical properties of aerosols were conducted in July 2008 at an urban site in Guangzhou,Southern China.The mean PM 2.5 concentration for the entire period was 53.7±23.2 μg m 3.The mean PM 2.5 concentration (82.7±25.4 μg m 3) on hazy days was roughly two times higher than that on clear days (38.8±8.7 μg m 3).The total water-soluble ion species and the total average carbon accounted for 47.9%±4.3% and 35.2%±4.5%,respectively,of the major components of PM 2.5.The increase of secondary and carbonaceous aerosols,in particular ammonium sulfate,played an important role in the formation of haze pollution.The mean absorption and scattering coefficients and the single scattering albedo over the whole period were 53±20 M m 1,226±111 M m 1,and 0.80±0.04,respectively.PM 2.5 had a high linear correlation with the aerosol extinction coefficient,elemental carbon (EC) was correlated with aerosol absorption,and organic carbon (OC) and SO 4 2 were tightly linked to aerosol scattering.
The semi-diurnal mean aerosol mass concentration, chemical composition, and optical properties of PM2.s were investigated in Shanghai during the spring of 2012. Slight pollution was observed during the study period. The average PM2.s concentration was 64.11± 22.83μg/m3. The mean coefficients of extinction, scattering, and absorption at 532 nm were 125.9 ± 78.5, 91.1 ± 56.3, and 34.9 ±23.6 Mm-1, respectively. A relatively low mean single scattering alhedo at 532 nm （0.73 ±0.04） and low level of elemental carbon （EC, 2.67± 1.96 μg/m3） suggested that the light absorption was enhanced due to the internal mixing of the EC. Sulfate contributed the most to aerosol light scattering in Shanghai. The chemical composition of PM2.5 was dominated by particulate organic matter, sulfate, nitrate, ammonium, and EC. Anthropogenic sources made a significant contribution to the emission and loading of the particulate pollutants. A relatively good correlation between the aerosol chemical composition and the cloud condensation nuclei （CCN） activation indicated that aerosol chemistry is an important factor that influences the saturated hygroscopicity and growth of the aerosol.
