Fig. 1. Measurement route of mobile MAX-DOAS (the thirdring road of Wuhan, map data copyright© Google Earth)

Fig. 2. Mobile MAX-DOAS system

Fig. 3. Fitting example for NO₂ DSCD and the fitting error (RMSE). (a) Fitting example; (b) fitting error

Fig. 4. Schematic of portable ultraviolet DOAS system

Fig. 5. NO₂ VCD spatial distribution measured by mobile MAX-DOAS and the average wind speed (u) and wind direction (the arrows indicate the average wind direction)

Fig. 6. Monthly mean NO₂ mass concentration of Wuhan from September, 2019 to March, 2020

Fig. 7. NO₂ VCD measured by Mobile MAX-DOAS and NO₂ VCD in troposphere measured by Tropomi on Feb. 29^th. (a) Comparison of NO₂ VCDs; (b) correlation coefficient of NO₂ VCDs

Fig. 8. Spatial distribution of R(NO_x) obtained by portable ultraviolet DOAS

Fig. 9. NO_x emission flux of the third ring road of Wuhan and its error

Fig. 10. Proportion (R²) of NO_x emission flux error of the third ring road of Wuhan caused by different error sources (using real-time [NO_x]/[NO₂] and its errors for NO_x emission flux and flux errors calculation)

Fig. 11. Proportion (R²) of NO_x emission flux error of the third ring road of Wuhan caused by different error sources (using average [NO_x]/[NO₂] and its errors for NO_x emission flux and flux errors calculation)