To obtain a thorough knowledge of PM2.?5 chemical composition and its impact on aerosol optical properties across China, existing field studies conducted after the year 2000 are reviewed and summarized in terms of geographical, interannual and seasonal distributions. Annual PM2.?5 was up to 6 times the National Ambient Air Quality Standards (NAAQS) in some megacities in northern China. Annual PM2.?5 was higher in northern than southern cities, and higher in inland than coastal cities. In a few cities with data longer than a decade, PM2.?5 showed a slight decrease only in the second half of the past decade, while carbonaceous aerosols decreased, sulfate (SO42?) and ammonium (NH4+) remained at high levels, and nitrate (NO3?) increased. The highest seasonal averages of PM2.?5 and its major chemical components were typically observed in the cold seasons. Annual average contributions of secondary inorganic aerosols to PM2.?5 ranged from 25 to 48?%, and those of carbonaceous aerosols ranged from 23 to 47?%, both with higher contributions in southern regions due to the frequent dust events in northern China. Source apportionment analysis identified secondary inorganic aerosols, coal combustion and traffic emission as the top three source factors contributing to PM2.?5 mass in most Chinese cities, and the sum of these three source factors explained 44 to 82?% of PM2.?5 mass on annual average across China. Biomass emission in most cities, industrial emission in industrial cities, dust emission in northern cities and ship emission in coastal cities are other major source factors, each of which contributed 7–27?% to PM2.?5 mass in applicable cities. The geographical pattern of scattering coefficient (bsp) was similar to that of PM2.?5, and that of aerosol absorption coefficient (bap) was determined by elemental carbon (EC) mass concentration and its coating. bsp in ambient condition of relative humidity (RH)??=??80?% can be amplified by about 1.8 times that under dry conditions. Secondary inorganic aerosols accounted for about 60?% of aerosol extinction coefficient (bext) at RH greater than 70?%. The mass scattering efficiency (MSE) of PM2.?5 ranged from 3.0 to 5.0?m2?g?1 for aerosols produced from anthropogenic emissions and from 0.7 to 1.0?m2?g?1 for natural dust aerosols. The mass absorption efficiency (MAE) of EC ranged from 6.5 to 12.4?m2?g?1 in urban environments, but the MAE of water-soluble organic carbon was only 0.05 to 0.11?m2?g?1. Historical emission control policies in China and their effectiveness were discussed based on available chemically resolved PM2.?5 data, which provides the much needed knowledge for guiding future studies and emissions policies.
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