Mass extinction efficiency and extinction hygroscopicity of ambient PM_2.5 in urban China

The ambient PM_2.5 pollution problem in China has drawn substantial international attentions. The mass extinction efficiency (MEE) and hygroscopicity factor (f(RH)) of PM_2.5 can be readily applied to study the impacts on atmospheric visibility and climate. The few previous investigations in China only reported results from pilot studies and are lack of spatial representativeness. In this study, hourly average ambient PM_2.5 mass concentration, relative humidity, and atmospheric visibility data from China national air quality and meteorological monitoring networks were retrieved and analyzed. It includes 24 major Chinese cities from nine city-clusters with the period of October 2013 to September 2014. Annual average extinction coefficient in urban China was 759.3±258.3Mm^-1, mainly caused by dry PM_2.5 (305.8.2±131.0Mm^-1) and its hygroscopicity (414.6±188.1Mm^-1). High extinction coefficient values were resulted from both high ambient PM_2.5 concentration (68.5±21.7µg/m³) and high relative humidity (69.7±8.6%). The PM_2.5 mass extinction efficiency varied from 2.87 to 6.64m²/g with an average of 4.40±0.84m²/g. The average extinction hygroscopic factor f(RH=80%) was 2.63±0.45. The levels of PM_2.5 mass extinction efficiency and hygroscopic factor in China were in comparable range with those found in developed countries in spite of the significant diversities among all 24 cities. Our findings help to establish quantitative relationship between ambient extinction coefficient (visual range) and PM_2.5 & relative humidity. It will reduce the uncertainty of extinction coefficient estimation of ambient PM_2.5 in urban China which is essential for the research of haze pollution and climate radiative forcing.