Effects of different types of heat wave days on ozone pollution over Beijing-Tianjin-Hebei and its future projection.

There are concerns that in the 21st century, global warming will lead to more frequent heat wave days (HWDs), which could amplify ozone pollution (OP). However, a recent study projected that future atmospheric circulation variations may benefit OP control in Beijing-Tianjin-Hebei (BTH). To investigate the possible reasons for this contradiction, this paper discussed the ozone amplification capacity between different HWD types and their future projections based on observations and Community Earth System Model Large Ensemble Simulations (CESM-LENS). Composite analysis shows that not all HWDs amplify the OP in BTH. The main factor determining whether HWDs aggravate OP is the accompanying circulation anomalies rather than the intensity of the HWDs. The HWDs that aggravate ozone pollution are usually accompanied by stable saddle-like circulation anomalies and atmospheric blocking (blocking HWDs), which weaken the meridional temperature gradient and strengthen atmospheric stability. In contrast, HWDs with wave-train circulation anomalies have a limited ability to exacerbate OP in BTH due to their weak atmospheric stability. We introduce the Ozone Weather Index (OWI) to discern the influence of meteorological conditions on OP and overcome the lack of ozone concentration data in CESM-LENS under RCP 8.5 scenario. The OWI shows a significant downward trend in the 21st century, which indicates that the summer mean atmospheric circulation variations are beneficial for OP control in BTH. However, the frequency of blocking HWDs, which could amplify OP, will increase significantly in the 21st century. By the end of the 21st century, the frequency of blocking HWDs will be three times more than that of the end of 20th century. These findings inform policymakers that it is imperative to consider the mean climate state and the risks associated with extreme events when formulating future ozone pollution control policies.