A Typical Weather Pattern for the Severe Ozone Pollution Events in North China

Ground-level observations, reanalyzed meteorological fields, and a 3-D global chemical and transport model (GEOS-Chem) are applied in this study to investigate severe ozone (O₃) pollution events (SOPEs) in North China (36.5°N-40.5°N, 114.5°E-119.5°E) during years of 2014-2017. SOPEs are defined as episodes with regionally averaged O₃ maximum daily averaged 8-h (MDA8) concentrations larger than 160μgm^-3 for consecutive 3 days. Observations show that there were 167 O₃ polluted days and 27 SOPEs in North China during 2014-2017, in which 123 days and 21 SOPEs occurred from May to July. We find that a typical weather pattern with high daily maximum temperature, low relative humidity, anomalous northerlies at 850 hPa and anomalous anti-cyclone circulation leads to SOPEs in North China during May to July. A normalized index I_SOPE is defined on the basis of these meteorological parameters to characterize such weather pattern. I_SOPE is able to capture about 81% of observed SOPEs in North China in recent years. The integrated process rate (IPR) analysis is carried out by using the GEOS-Chem model to quantify the impacts of such weather pattern on SOPEs. Strong O₃ chemical production due to the hot and dry weather is found to be the most important process contributing to SOPEs, and the downward transport of O₃ from the lower troposphere to the surface under the high-pressure system is the second important process. I_SOPE defined in this study has implications for the forecast of SOPEs in North China.