| 19 Jun 2026
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.
This is a quite complete study about the ozone concentrations in the North China Plain under cyclonic and anticyclonic circulations and orographic effects play a main role. Observations were obtained from a network that includes 39 urban stations. Summertime observations (June-August) were considered during 2017-2022. Simulations with the GEOS-Chem model were conducted during June of 2017-2022. Moreover, 5 June 2022 was selected by way of example. Since the paper is quite elaborated, only some minor changes are required for establishing the paper restrictions.
Firstly, the authors should indicate if cyclonic circulation is thermally induced. A comparison with similar situations around the world would increase the potential readers
Ozone concentrations are different in urban and rural environments. In consequence, maps showing the ozone spatial distribution present this contrast where cities highlight as ozone sinks due to the precursor levels, whereas ozone concentrations are higher at rural sites. This contrast is not observed in the presented maps. Perhaps the model resolution does not allow the definition of cities. In addition, only cities are used in this study, where precursors play a main role. A comment about the ozone concentrations at rural sites would increase the paper value.
Varied periods are employed. For instance, observations cover the summertime, whereas simulations are restricted to June. The authors should comment possible implications of this time disagreement on the obtained results. Moreover, Figure 7 extends for 45 years, whereas 5 June 2022 is highlighted for specific results. The convenience of all these time intervals should be justified.
At the end of the paper, the authors present the model response under three types of precursor reductions, such as 10, 30 and 50%. Since such reductions should have a noticeable impact on the human activity due to the affected sources, perhaps potential readers wonder if such reductions are realistic and if expected results agree with those from the model, i.e., if the model was tested under such conditions.