Observed diel variation of O_x (=O₃+NO₂), and relationship of the ΔO_x as well as daily averaged data for O_x, NO₂, VOCs and solar radiation with temperature during high O_x (maximum hourly averaged O_x >75 ppbv) days in Shanghai, YRD, and NCP regions. The sunny day (maximum of solar radiation >550 W m ⁻²) observations were analyzed based on measurements from 2017 to 2019 of the national monitoring network. Results of Shanghai include 9 sites, that of YRD region include 180 sites in 41 major cities and that of NCP include 140 national monitoring sites in 30 major cities (Figure S1B). Values are averages over 2.5 ppbv NO₂ bins for the daily average (at least 10/50 data points required in city/regional scale). The daily average data of VOCs during the same period were from an urban supersite in Shanghai. The meteorological variables were from the Surface Meteorological Stations in China. (A), (D), (G) Diel variation of O_x during photochemical smog days in Shanghai, YRD and NCP, respectively. The red, black and blue lines represent three temperature bins: <18°C, 18°C– 25°C and >25°C, which indicate cool seasons, warm seasons and mid-summer. (B), (E), (H) Dependences of the daily averaged O_x and daily ΔO_x on averaged temperature. (C), (F), (I) Relationships of daily averaged NO₂, VOCs and maximum solar radiation (SR) with the average temperature in Shanghai, YRD and NCP, respectively. The error bars indicate the standard variations.