Fig. 1. MAX-DOAS measurement site

Fig. 2. A fitting spectrum of a typical measured spectrum

Fig. 3. Correlation of tropospheric NO₂ VCDs observed between MAX-DOAS and TROPOMI

Fig. 4. Daily variations of tropospheric NO₂VCDs

Fig. 5. Diurnal variation characteristics of tropospheric NO₂ VCDs during weekdays and weekends

Fig. 6. Time series of the daily average values of NO₂ VCDs observed by MAX-DOAS from June 21 to September 30, 2019
The shaded area is there periods

Fig. 7. Mean 24 h air mass backward trajectories and each trajectory cluster arriving at Shanghai at 500 m altitudes and the percentage of allocation to each cluster during the three typical periods
(a): Jun. 28—Jul. 15; (b): Jul. 28—Aug. 20; (c): Aug. 26—Sep. 15