An observed nocturnal ozone transport event in the Sichuan Basin, Southwestern China

The ozone (O₃) pollution in China drew lots of attention in recent years, and the Sichuan Basin (SCB) was one of the regions confronting worsening O₃ pollution problem. Many previous studies have shown that regional transport is an important contributor to O₃ pollution. However, very few features of the O₃ profile during transport have been reported, especially in the border regions between different administrative divisions. In this study, we conducted tethered balloon soundings in SCB during the summer of 2020 and captured a nocturnal O₃ transport event during the campaign. Vertically, the O₃ transport occurred in the bottom of the residual layer, between 200 and 500 m above ground level. Horizontally, the transport pathway was directed from southeast to northwest based on the analysis of the wind field and air mass trajectories. The effect of transport in the residual layer on the surface O₃ concentration was related to the spatial distribution of O₃. For cities with high O₃ concentrations in the upwind region, the transport process would bring clean air masses and abate pollution. For downwind lightly polluted cities, the transport process would slow down the decreasing or even increase the surface O₃ concentration during the night. We provided observational facts on the profile features of a transboundary O₃ transport event between two provincial administrative divisions, which implicated the importance of joint prevention and control measures. However, the sounding parameters were limited and the quantitative analysis was preliminary, more integrated, and thorough studies of this topic were called for in the future.