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To obtain a thorough knowledge of PM2. 5 chemical compositionand its impact on aerosol optical properties across China, existing fieldstudies conducted after the year 2000 are reviewed and summarized in terms ofgeographical, interannual and seasonal distributions. Annual PM2. 5 wasup to 6 times the National Ambient Air Quality Standards (NAAQS) in somemegacities in northern China. Annual PM2. 5 was higher in northern thansouthern cities, and higher in inland than coastal cities. In a few citieswith data longer than a decade, PM2. 5 showed a slight decrease only inthe 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 ofPM2. 5 and its major chemical components were typically observed in thecold seasons. Annual average contributions of secondary inorganic aerosols toPM2. 5 ranged from 25 to 48 %, and those of carbonaceous aerosolsranged from 23 to 47 %, both with higher contributions in southernregions due to the frequent dust events in northern China. Sourceapportionment analysis identified secondary inorganic aerosols, coalcombustion and traffic emission as the top three source factors contributingto PM2. 5 mass in most Chinese cities, and the sum of these three sourcefactors explained 44 to 82 % of PM2. 5 mass on annual average acrossChina. Biomass emission in most cities, industrial emission in industrialcities, dust emission in northern cities and ship emission in coastal citiesare other major source factors, each of which contributed 7–27 % toPM2. 5 mass in applicable cities. The geographical pattern of scattering coefficient (bsp) wassimilar to that of PM2. 5, and that of aerosol absorption coefficient(bap) was determined by elemental carbon (EC) mass concentrationand its coating. bsp in ambient condition of relative humidity (RH)  =  80 % canbe amplified by about 1.8 times that under dry conditions. Secondaryinorganic aerosols accounted for about 60 % of aerosol extinctioncoefficient (bext) at RH greater than 70 %. Themass scattering efficiency (MSE) of PM2. 5 ranged from 3.0 to5.0 m2 g−1 for aerosols produced from anthropogenic emissions andfrom 0.7 to 1.0 m2 g−1 for natural dust aerosols. The massabsorption efficiency (MAE) of EC ranged from 6.5 to 12.4 m2 g−1in urban environments, but the MAE of water-soluble organic carbon was only0.05 to 0.11 m2 g−1. Historical emission control policies inChina and their effectiveness were discussed based on available chemicallyresolved PM2. 5 data, which provides the much needed knowledge forguiding future studies and emissions policies

atmospheric chemistry and physics

A review of current knowledge concerning PM&amp;lt;sub&amp;gt;2. 5&amp;lt;/sub&amp;gt; chemical composition, aerosol optical properties and their relationships across China