Complex Ozone-Temperature Relationship in the North China Plain Under Heat Extremes

The North China Plain is one of the most heavily polluted regions in the world in terms of surface ozone pollution, and experiences frequent extreme summer heat waves. Concurrently, extreme heat events—capable of triggering severe ozone episodes—are becoming increasingly frequent, suggesting a growing challenge for ozone air quality control in a warming climate. In this study, we utilize the long-term surface measurements to show that, in the North China Plain, ozone-temperature relationship has two distinct regimes of ozone suppression (OS) vs. non-OS in June months, with a regime shift occurring around 2020. The observed OS can be well captured by the GEOS-Chem model and a machine learning model based on meteorological data only; our analysis indicates that OS is primarily driven by different circulation patterns rather than by the previously identified chemical and emissions processes. Furthermore, although the GEOS-Chem successfully captures the shift from OS to no-OS around 2020, the observed non-OS in NCP was strongly underestimated by the model. Using an improved version of GEOS-Chem, we quantify the potential importance of meteorological factors, changes in anthropogenic emissions, and chemical drivers. The results show that at extremely high temperatures, temperature-dependent emission processes such as soil NO_x and anthropogenic VOCs contribute significantly to the ozone temperature slope during no-OS. Whereas the reduction of anthropogenic emissions is unfavorable for the occurrence of no-OS, the contribution of NO_x-producing processes (e.g., PAN decomposition) is amplified at high temperatures due to a shift in ozone production control from NOx-limited to VOC-limited conditions.