Fig. 1. Schematic diagram of a nighttime atmospheric NO₂ detection device based on target reflected light

Fig. 2. Selection of NO₂ differential wavelengths

Fig. 3. Collimation scheme of semiconductor laser beam

Fig. 4. Structure diagram of Newton telescope

Fig. 5. Schematic diagram of NO₂ column concentration detection based on target reflected light

Fig. 6. Semiconductor laser spectra at different temperatures

Fig. 7. Variation of the central wavelength (a) and intensity (b) of semiconductor laser with temperature

Fig. 8. Variation of semiconductor laser light intensity (a) and central wavelength (b) with current

Fig. 9. Intensity absorption of λ_on (a) and λ_off (b) at different NO₂ concentrations

Fig. 10. Fitting of NO₂ differential absorption cross section

Fig. 11. λ_on and λ_off spectra of NO₂ sample gas at the concentration of 1620 mg/m³

Fig. 12. Laser reflection spectra of reflective targets with different materials

Fig. 13. Photograph of field experiment

Fig. 14. Laser differential echo signal from 21:00 to 22:15 on August 15, 2022. (a) 21:00; (b) 21:15; (c) 21:30;(d) 21:45; (e) 22:00; (f) 22:15

Fig. 15. The intensity values of echo signals of λ_on andλ_off at different times

Fig. 16. Time distribution of NO₂ concentration from 21:00 on August 15 to 05:00 on August 16, 2022

Fig. 17. Comparison of the results measured (a) and hourly averages (b) of data measured by the instrument with the data of environmental monitoring station on August 15, 2022

Fig. 18. Correlation between the results measured by this instrument and the data given by environmental monitoring station

Fig. 19. Comparison of NO₂ detection values between the developed system and LP-DOAS on the evenings of October 11 (a) and October 12 (b), 2022

Fig. 20. Correlation between the results measured by this instrument and the data acquired by LP-DOAS

Fig. 21. Variation of NO₂ concentration at different altitudes with time. (a) November 5-6, 2022;(b) November 6-7, 2022; (c) November 7-8, 2022