Seasonal changes in the recent decline of combined high PM and O pollution and associated chemical and meteorological drivers in the Beijing-Tianjin-Hebei region, China.

China suffers from combined air pollution (CAP) comprising dual high O and PM, particularly in the Beijing-Tianjin-Hebei (BTH) region, which is an urban agglomeration in the North China Plain. To characterize the seasonal changes in regional CAP, 82 CAP days were identified during the study period from 2015 to 2019 with the co-occurring pollution of O and PM in the BTH. It is found that CAP seasonality has undergone distinct changes with a declining trend in the interannual variations in CAP over recent years.