Spatiotemporal variations of wintertime secondary PM2.5 and meteorological drivers in a basin region over Central China for 2015-2020

It is important to comprehend the contribution of secondary PM2.5 (particulate matter with aerodynamic diameter <= 2.5 mu m) to ambient PM2.5 for the long-term change of air quality. This study used the approximate envelope method to identify the variations of primary and secondary PM2.5 concentrations based on routine observations of environment and meteorology in the Twain-Hu Basin (THB, located in Central China), during the winters of 2015-2020. It is found that the contribution of secondary PM2.5 to ambient PM2.5 was enhanced with accompanying the descending trend in PM2.5 concentrations over the recent years. The regional average for secondary PM2.5 contributions was 62.26% in the THB during winters of 2015-2020, and the high secondary contributions were centered in the THB urban areas and major transport routes in the northwestern region. Secondary PM2.5 contributions averaged over the THB increased from 50.41% in clean air quality to 77.09% in heavy PM2.5 pollution with a significant contribution of secondary aerosols to heavy pollution. The secondary PM2.5 contributions presented two peaks at 3:00-7:00 a.m. and 13:00-17:00 p.m., which could be attributed respectively to the nighttime high relative humidity and daytime high air temperature. Regional transport of gaseous precursors of aerosols could lead to the increasing secondary PM2.5 levels in the strong oxidizing atmosphere.