An underappreciated cyclonic-like circulation drives high summer ozone in North China Plain

Abstract. China continues to experience severe ozone pollution, particularly over the North China Plain (NCP) during summer. Ozone pollution is generally considered to be associated with anticyclonic circulation. However, this study reveals that a previously underappreciated cyclonic-like circulation also plays a substantial role in ozone pollution over the NCP. Based on a systematic analysis of summertime observations from 2017 to 2022, we identify 209 ozone pollution days, 60 of which are associated with cyclonic-like circulation. Under cyclonic-like circulation, northwesterly winds prevail over the NCP. As the airflow crosses the Taihang Mountains, it undergoes adiabatic descent and induce foehn winds, leading to anomalous warming (+1.78 °C) and drying (−15 %) in the western NCP. Foehn-induced warming substantially enhances ozone photochemical production, resulting in severe ozone pollution over the western NCP, with MDA8 ozone concentrations exceeding 102.2 ppb. In addition, subsiding airflow transports ozone-rich air from the residual layer downward, leading to elevated nighttime ozone along the leeward foothills. Consequently, the impact of cyclonic-like circulation on ozone pollution is characterized by pronounced spatial heterogeneity, in contrast to the relatively uniform ozone enhancement over the NCP under anticyclonic circulation. More importantly, the frequency of cyclonic-like circulation exhibits an increasing trend during 1980–2024, suggesting its growing importance in modulating ozone pollution. We further demonstrate that emission control strategies should be tailored to different circulations. Under cyclonic-like circulation, local emission reductions within the NCP are most effective, whereas under anticyclonic circulation, reductions in the adjacent southeastern region yield greater mitigation benefits.