Persisting Day and Night High Surface Ozone Pollution Driven by Compound Heatwaves

Abstract While daytime heatwaves are well‐known to exacerbate surface ozone (O 3 ) pollution, the role of compound heatwaves with persistent day‐night hot in O 3 pollution remains unclear. Taking China's Sichuan Basin (SCB) as an example, we find compound heatwaves persistently intensify surface O 3 concentration day and night, with significant positive anomalies of 29.3 μg/m 3 and 12.1 μg/m 3 during daytime and nighttime, respectively. In details, daytime hot triggers high daytime O 3 concentration by enhancing O 3 ‐forming photochemistry, thereby providing a nocturnal residual layer (RL) with O 3 ‐rich air and substantial heat storage. Nighttime, the persistent day‐night hot combined with easterly flow jointly drive a plateau‐basin secondary circulation across RL and stable boundary layer over the SCB, facilitating RL's O 3 ‐rich air mixing to the surface and increasing nighttime O 3 concentration. These findings advance understanding of how extreme heat and large‐scale topographic forcing jointly modulate atmospheric environmental changes in plateau‐basin regions.