Classification and Characteristics of PM_2.5 Pollution Days along the Yangtze River in Anhui Province

To explore the causes of PM_2.5 pollution in the areas along the Yangtze River in Anhui Province, this study utilized PM_2.5 concentration, ground meteorological element observation data, ERA5 reanalysis, and GDAS1 data from 2015 to 2023. Based on the PM_2.5 pollution status and synoptic situations in the traversed areas of backward trajectories, pollution days were classified and their characteristics analyzed. The results showed that the average annual number of pollution days in this region was 46.2 days, with the total number of pollution days and the number of light and moderate pollution days both showing a significant decreasing trend. Pollution days can be divided into two categories: external input type and static and stable accumulation type. The former, though less frequent, accounted for a larger proportion of severe and above pollution days, and was closely related to the southward movement of cold air; the input of PM_2.5 it brought was a major cause of heavy pollution in this region. The latter was more numerous and dominated light and moderate pollution days, often associated with synoptic situations such as uniform pressure fields, convergent wind fields, and anticyclones. With no cold air activity, it was primarily characterized by local PM_2.5 accumulation, resulting in relatively light pollution.