Effects of Horizontal Transport and Vertical Mixing on Nocturnal Ozone Pollution in the Pearl River Delta

Based on the meteorological, ozone (O-3), and vertical observation data of 2020, this study sought to evaluate the daily variation in O-3, particularly the characteristics of nocturnal ozone pollution. We also discuss the effect of local and mesoscale horizontal transport and vertical mixing on the formation of nocturnal O-3 pollution. Distinct seasonal characteristics of the daily O-3 variation in Shenzhen were identified. In particular, significant nocturnal peaks were found to regularly occur in the winter and spring (November-December and January-April). The monthly average of daily variation had a clear bimodal distribution. During the period, O-3 pollution frequently occurred at night, with the maximum hourly O-3 concentration reaching 203.5 mu g/m(3). Nocturnal O-3 pollution was closely associated with horizontal transport and vertical mixing. During the study period, the O-3 maximum values were recorded on 68 nights, primarily between 23:00 and 03:00, with occasional observation of two peaks. The impact of horizontal transport and vertical mixing on the nocturnal secondary O-3 maximum values was elaborated in two case studies, where vertical mixing was mainly associated with low-level jets, with strong wind shear enhancing turbulent mixing and transporting O-3 from the upper layers to the surface.

Automated summary

Based on the observation data of 2020, this study evaluated the daily variation in O-3 in Shenzhen and focused on the characteristics of nocturnal ozone pollution. The findings revealed frequent nocturnal peaks in winter and spring, and a close association between nocturnal O-3 pollution and horizontal transport and vertical mixing.