Fig. 2: Diurnal variations of O3 and its precursors.

Mean diurnal variations in the surface O3 concentrations in a Hefei, b Huaibei, and c Tai’an during April–October, 2020–2023. Mean diurnal variations in the d–f NO2 vertical distributions, g–i HCHO vertical distributions, and j–l FNR vertical distributions in Hefei, Huaibei, and Tai’an during April–October, 2020–2023.