Impacts of Nocturnal Cloud Top Radiative Cooling on Surface O3 in Sichuan Basin, Southwestern China

Nighttime ozone (O3) reserved over the stable boundary layer (SBL) could contribute significantly to surface O3 levels. While many studies have revealed that daytime convective eddies and turbulence induced by nighttime low‐level jets can mix O3‐rich air over the SBL down to the surface, increasing nocturnal surface O3 within the cloud topped boundary layer has rarely been examined. In this study, we used measurements and a single‐column photochemistry model to investigate O3 variations within the nighttime cloud topped boundary layer over the Sichuan Basin, which is the cloudiest region in southwest China. The Santa Barbara Distort Atmospheric Radiative Transfer radiation model was coupled into the single‐column photochemistry model to investigate the nighttime radiative effects of clouds. The results showed that the nocturnal cloud top radiative cooling generated turbulence and enhanced vertical mixing below the cloud top layer. The cloud‐driven turbulent eddies even penetrated through the SBL to the surface. Consequently, it weakened the decoupling of the SBL from the residual layer. O3‐rich air over the SBL was entrained downwards to the surface, decreasing O3 aloft and increasing surface O3 at nighttime. The cloud top radiative cooling rates were enhanced with the increase in the cloud liquid water path. Higher cooling rates produced stronger turbulent transport of O3, leading to higher nocturnal surface O3 levels. The turbulent transport of O3 induced by cloud top radiative cooling is revealed as an important mechanism of vertical transport of O3 within a cloud topped boundary layer, contributing to the maximum nocturnal surface O3, which has great implications for understanding atmospheric O3 variation. Key Points A mechanism of vertical transport of O3 within a cloud topped boundary layer is revealed using measurements and simulations. The contribution from vertical transport to surface O3 increases with cloud liquid water path, with maximum surface O3 enhancing by 30 ppb at nighttime.