Composited analyses of the chemical and physical characteristics of co-polluted days by ozone and PM_2.5 over 2013–2020 in the Beijing–Tianjin–Hebei region

Abstract. The co-polluted days by ozone (O3) and PM2.5 (particulate matter with an aerodynamic equivalent diameter of 2.5 µm or less) (O3–PM2.5PDs) were frequently observed in the Beijing–Tianjin–Hebei (BTH) region warm seasons (April–October) 2013–2020. We applied 3-D global chemical transport model (GEOS-Chem) to investigate physical characteristics O3–PM2.5PDs composited analyses such that captured both observations model. Model results showed that, when occurred, concentrations hydroxyl radical total oxidant, sulfur oxidation ratio, nitrogen ratio all high, sulfate at surface highest among pollution types. also found unique features vertical distributions aerosols during O3–PM2.5PDs; decreased altitude near but remained stable 975–819 hPa. Process secondary (nitrate, ammonium, sulfate) had strong productions 913–819 hPa, which then transported downward, resulting quite uniform profiles hPa on O3–PM2.5PDs. weather patterns for characterized anomalous high-pressure system 500 as well southerlies high RH 850 latter resulted around are different from those polluted either O3 alone have important implications air quality management.