Two winter PM 2.5 pollution types and the causes in the city clusters of Sichuan Basin, Western China.

PM2.5 pollution types were analyzed and the causes were uncovered in Sichuan Basin using PM2.5 data from Chinese Ministry of Environmental Protection (MEP) and multiple meteorology data during January 2015 to February 2017. The events that PM2.5 increased gradually in the periods longer than 10 days and then decreased sharply were defined as "Type I", while the symmetrical variations of PM2.5 during increasing and decreasing periods were defined as "Type II" of PM2.5 pollution. Five cases of Type I and two cases of Type II were identified during the study period inside the basin. The increasing rates were almost comparable between the two PM2.5 pollution types with the range from 4 μg m-3  d-1 to 8 μg m-3  d-1 , while the decreasing rates of Type I were between 25 μg m-3  d-1 and 40 μg m-3  d-1 , which were 3-5 times higher than those of Type II (~8 μg m-3  d-1 ). The rapid reduction of PM2.5 for Type I was mainly related to improvement of vertical and horizontal diffusion conditions induced by invasion of cold air masses, while slowly decreased PM2.5 for Type II was due largely to elevated horizontal wind speeds and shifted wind directions in the city clusters of the basin.