Twelve-Year Trends of PM₁₀and Visibility in the Hefei Metropolitan Area of China

China has been experiencing severe air pollution and previous studies have mostly focused on megacities and a few hot spot regions. Hefei, the provincial capital city of Anhui province, has a population of near 5 million in its metropolitan area, but its air quality has not been reported in literature. In this study, daily PM₁₀and visibility data in 2001–2012 were analyzed to investigate the air quality status as well as the twelve-year pollution trends in Hefei. The results reveal that Hefei has been suffering high PM₁₀pollution and low visibility during the study period. The annual average PM₁₀concentrations are 2~3 times of the Chinese Ambient Air Quality Standard. PM₁₀shows fluctuating variation in 2001–2007 and has a slightly decreasing trend after 2008. The annual average visibility range is generally lower than 7 km and shows a worsening trend from 2001 to 2006 followed by an improving trend from 2007 to 2012. Wind speed, precipitation, and relative humidity have negative effects on PM₁₀concentrations in Hefei, while temperature could positively or negatively affect PM₁₀. The results provide a general understanding of the status and long-term trends of PM₁₀pollution and visibility in a typical second-tier city in China.