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

[摘要] The co-polluted days by ozone (O 3 ) andPM 2.5 (particulate matter with an aerodynamic equivalent diameter of 2.5  µ m or less) (O 3 –PM 2.5 PDs) were frequently observed in the Beijing–Tianjin–Hebei (BTH) region in warm seasons (April–October) of 2013–2020. We applied the 3-D global chemical transport model (GEOS-Chem) toinvestigate the chemical and physical characteristics ofO 3 –PM 2.5 PDs by composited analyses of such days that werecaptured by both the observations and the model. Model results showed that,when O 3 –PM 2.5 PDs occurred, the concentrations of hydroxylradical and total oxidant, sulfur oxidation ratio, and nitrogen oxidationratio were all high, and the concentrations of sulfate at the surface werethe highest among all pollution types. We also found unique features invertical distributions of aerosols during O 3 –PM 2.5 PDs;concentrations of PM 2.5 decreased with altitude near the surface butremained stable at 975–819 hPa. Process analyses showed that secondaryaerosols (nitrate, ammonium, and sulfate) had strong chemical productions at913–819 hPa, which were then transported downward, resulting in the quiteuniform vertical profiles at 975–819 hPa on O 3 –PM 2.5 PDs. Theweather patterns for O 3 –PM 2.5 PDs were characterized by anomalous high-pressure system at 500 hPa as well as strong southerlies and high RH at 850 hPa. The latter resulted in the strong chemical productions around 850 hPa on O 3 –PM 2.5 PDs. The physical and chemical characteristics ofO 3 –PM 2.5 PDs are quite different from those of polluted days byeither O 3 alone or PM 2.5 alone and have important implicationsfor air quality management.

[学科分类] 医学（综合）