Rising exposure to concurrent extreme heat and ozone (O₃) pollution have become a critical constraint on urban habitability in China. As demonstrated in Fig. 1, we have observed that both the frequency and intensity of heatwaves have almost tripled in China since the beginning of this century. Moreover, persistent heatwaves and elevated ozone frequently co-occur in these highly-populated city clusters of China, increasingly endangering city dwellers and urban environment (Ref. 1-3). However, limited understanding of the comprehensive mechanism hinders our ability to mitigate such compound events. In recent years our interest therefore has been focused on the urban meteorology-chemistry coupling in compound heat-ozone extremes over China’s megacities.
Elucidating the governing mechanisms of these compound events demands disentangling the tightly coupled human and natural systems. The years of 2013 and 2022 marked exceptionally hot summers falling outside the ranges of any extreme episodes recorded in China. Leveraging comprehensive field observations at the Station for Observing Regional Processes of the Earth System (SORPES) in Nanjing (Ref. 4) and statistical emission data, we show that persistent heatwaves accelerate photochemical ozone production by boosting anthropogenic and biogenic emissions (Fig. 2; Ref. 1). Furthermore, using meteorology-chemistry coupled modelling, we examined the roles of atmospheric physical and chemical processes in heat-stressed O3 pollution, and highlighted that heat-boosted emissions and suppressed dry deposition due to water-stressed vegetation led to a more than 30% increase in surface ozone pollution in China’s urban areas.
Beyond the interconnections near the surface, long-lasting and widespread heatwave-associated O3 pollution usually extends into the deeper troposphere. Yet vertical insights into the photochemical stratification and intricate connection between heatwaves and O3 are limited due to sparse vertical observations. To close this observational gap, we conducted an intensive airship field campaign near the SORPES in the early summer of 2023, and obtained hundreds of vertical profiles from the surface to a maximum altitude of 1.2 km, which thus shed more lights into the photochemical and boundary layer processes involved in urban O3 pollution. Leveraging airship vertical measurements and meteorology-chemistry coupled modeling, we revealed that heatwave-reinforced turbulence redistributes precursors vertically, which features a notable increase of nitrogen oxides (NOx) aloft while a distinct decline near the surface (Fig. 3; Ref. 5). By reallocating precursors vertically, heatwaves shift the stratification of the photochemical regime and accelerate O3 formation both aloft and at ground level across China’s megacities.
On the basis of meteorology-chemistry modeling, it is estimated that stringent emission control targeting nitrogen oxides could mitigate the heatwave-exacerbated O3 extremes by narrowing the vertical disparity of photochemical sensitivity. Although heatwaves are projected to intensify, urban emission reductions due to China’s carbon neutrality pledge could alleviate O3 pollution by 41–47% in four city clusters of China during heatwaves. Stringent emission controls help tackle the dual challenges of air pollution and global warming, as well as strengthen the climate resilience of cities and underscore the critical leverage of urban emission control in compound heat–ozone extremes.
Our study disentangles the urban meteorology-chemistry coupling in compound heat-ozone extremes, and also indicates the importance of urban emission control in mitigating compound heat-air pollution extreme. For more details about these studies, please read the paper, “Urban meteorology-chemistry coupling in compound heat-ozone extremes”, in Nature Cities (Ref. 5), as well as those listed in the references below.
References
[1] Li, M., Huang, X.* et al., Sci. Bull., 69, 2938-2947 (2024).
[2] Yan, D., Li, M.* et al., Geophys. Res. Lett., 52 (2025).
[3] Wang, W., Zhou, Y., Li, M.* et al., Sci. China Earth Sci., 68, 1448–1457 (2025).
[4] Ding, A.*, Huang, X. et al., Atmos. Chem. Phys., 19, 11791-11801 (2019).
[5] Zhou, X.#, Li, M.#, Huang, X.* et al. Nat. Cities (2025).