Analysis on the Causes of the Heaviest Pollution Episode of Nanjing in 2007

With API (Air Pollution Index) value reaching 234 and air quality graded as 4A, June 5 was one of the dirtiest days of 2007 in Nanjing. At that time, Nanjingpsilas air quality was moderately polluted with the minimum visibility less than 500 meters. Surrounding cities such as Yangzhou, Zhenjiang, Chuzhou and Mapsilaanshan suffered the same problem. The API values were 171, 196, 177, 124, and air quality grades were 3B, 3B, 3B, and 3A for Yangzhou, Zhenjiang, Chuzhou and Mapsilaanshan, respectively. The main reason of the worst air quality lied in burning of crop straw coupled with temperature inversion, which was an adverse meteorological condition particularly in high air humidity and low wind speed. Temperature inversion promoted gas-to-particle transformations and hindered the vertical mixing of air, which aggravated the air quality and led directly to the worst air pollution episode of Nanjing.