Ground-level observations, reanalyzed meteorological fields and a 3-D global chemical and transport model (GEOS-Chem) were applied in this study to investigate ozone (O3) pollution events (OPEs) in North China (36.5–40.5° N, 114.5–119.5° E) during 2014–2017. Ozone pollution days (OPDs) were defined as days with maximum daily averaged 8 h (MDA8) concentrations over North China larger than 160 μg m?3, and OPEs were defined as periods with 3 or more consecutive OPDs. Observations showed that there were 167?OPDs and 27?OPEs in North China during 2014–2017, in which 123?OPDs and 21?OPEs occurred from May to July. We found that OPEs in North China occurred under a typical weather pattern with high daily maximum temperature (Tmax), low relative humidity (RH), anomalous southerlies and divergence in the lower troposphere, an anomalous high-pressure system at 500 hPa, and an anomalous downward air flow from 500 hPa to the surface. Under such a weather pattern, chemical production of O3 was high between 800 and 900 hPa, which was then transported downward to enhance O3 pollution at the surface. A standardized index I_OPE was defined by applying four key meteorological parameters, including Tmax, RH, meridional winds at 850 hPa (V850) and zonal winds at 500 hPa (U500). I_OPE can capture approximately 80 % of the observed OPDs and OPEs, which has implications for forecasting OPEs in North China.
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