Meteorological influences on co-occurrence of O_(3) and PM_(2.5)pollution and implication for emission reductions in Beijing-Tianjin-Hebei

Co-occurrence of surface ozone(O_(3))and fine particulate matter(PM_(2.5))pollution(CP)was frequently observed in Beijing-Tianjin-Hebei(BTH).More than 50%of CP days occurred during April-May in BTH,and the CP days reached up to 11 in two months of 2018.The PM_(2.5)or O_(3) concentration associated with CP was lower than but close to that in O_(3) and PM_(2.5)pollution,indicating compound harms during CP days with double-high concentrations of PM_(2.5)and O_(3).CP days were significantly facilitated by joint effects of the Rossby wave train that consisted of two centers associated with the Scandinavia pattern and one center over North China as well as a hot,wet,and stagnant environmental condition in BTH.After 2018,the number of CP days decreased sharply while the meteorological conditions did not change significantly.Therefore,changes in meteorological conditions did not really contribute to the decline of CP days in 2019 and 2020.This implies that the reduction of PM_(2.5)emission has resulted in a reduction of CP days(about 11 days in 2019 and 2020).The differences in atmospheric conditions revealed here were helpful to forecast the types of air pollution on a daily to weekly time scale.The reduction in PM_(2.5)emission was the main driving factor behind the absence of CP days in 2020,but the control of surface O_(3) must be stricter and deeper.