Fig. 1. Schematic of continuous-wave NO₂-DIAL system based on high-power multimode laser diode

Fig. 2. NO₂ absorption cross-section (294 K), as well as normalized emission spectra of 450 nm laser diode at λ_on and λ_off wavelengths

Fig. 3. Relationship between aerosol extinction coefficient and wavelength with different Ångström exponents

Fig. 4. Distribution of aerosol extinction coefficient under inhomogeneous atmospheric conditions

Fig. 5. Relationship between simulated atmospheric lidar signals intensity I and distance. (a) Homogeneous distribution of atmospheric aerosol; (b) inhomogeneous distribution of atmospheric aerosol

Fig. 6. Differential absorption curves and segmental fitting. (a) Differential absorption curve and segmental fitting curve when atmospheric aerosol is homogeneously distributed; (b) differential absorption curve and segmental fitting curve when atmospheric aerosol is inhomogeneously distributed; (c) fitting residuals (fitting range is 500 m)

Fig. 7. Relationship between retrieval error of NO₂ mass concentration and fitting range when atmospheric aerosol is homogeneously distributed. (a) Retrieval errors of NO₂ mass_{concentration resulted from aerosol extinction coefficient (ΔNα‑err) and backscattering coefficient (ΔNβ‑err); (b) sum of retrieval errors of NO2 mass concentration resulted from aerosol extinction coefficient and backscattering coefficient (ΔNα‑err+ΔNβ‑err), as well as retrieval error of NO2 mass concentration considering both aerosol extinction and backscattering coefficients (ΔNTot‑err)}

Fig. 8. Relationship between aerosol extinction coefficient induced retrieval error of NO₂ mass concentration and wavelength interval, when atmospheric aerosol is homogeneously distributed

Fig. 9. Relationship among aerosol extinction coefficient induced retrieval error of NO₂ mass concentration, Ångström exponent, and aerosol extinction coefficient of atmospheric aerosol, when atmospheric aerosol is homogeneously distributed

Fig. 10. Relationship between aerosol-induced retrieval error of NO₂ mass concentration and fitting distance on detection path, when atmospheric aerosol is inhomogeneously distributed. (a) Retrieval error of NO₂ mass concentration resulted from aerosol extinction coefficient (ΔNα‑err); (b) retrieval error of NO₂ mass concentration resulted from aerosol backscattering coefficient (ΔNβ‑err); (c) retrieval error of NO₂ mass concentration considering both aerosol extinction and backscattering coefficient (ΔNTot‑err)

Fig. 11. Aerosol-induced retrieval error of NO₂ mass concentration and varies with detecting distance, when atmospheric aerosol is inhomogeneously distributed. (a) Retrieval error of NO₂ mass concentration resulted from aerosol extinction coefficient (ΔNα‑err) and aerosol backscattering coefficient (ΔNβ‑err); (b) sum of retrieval errors of NO₂ mass concentration resulted from extinction coefficient and backscattering coefficient (ΔNα‑err+ΔNβ‑err), as well as retrieval error of NO₂ mass concentration considering both aerosol extinction and backscattering effect (ΔNTot‑err)

Fig. 12. Relationship among aerosol backscattering-induced retrieval error of NO₂ mass concentration, Ångström exponent, and atmospheric aerosol mass concentration, when atmospheric aerosol is inhomogeneously distributed and extinction coefficient of atmospheric aerosol in inhomogeneous range is continuously changed (αa-mean: 0.3 km^-1, αa-peak: 0.3 km^-1→0.9 km^-1)

Fig. 13. Relationship between aerosol-induced retrieval error of NO₂ mass concentration and Ångström exponent, when atmospheric aerosol is homogeneously distributed and value of extinction coefficient is 0.3 km^-1. (a) Comparison of aerosol extinction coefficient-induced errors obtained by simulated calculation method (ΔNα‑err) and approximate model method (εa); (b) comparison of total aerosol-induced error obtained by simulated calculation method (ΔNTot‑err) and approximate model method (εa+εβ)

Fig. 14. Aerosol-induced retrieval error of NO₂ mass concentration varies with detecting distance, when atmospheric aerosol is inhomogeneously distributed. (a) Comparison of aerosol extinction coefficient-induced errors obtained by simulated calculation method (ΔNα‑err) and approximate model method (εa); (b) comparison of aerosol backscattering coefficient-induced errors obtained by simulated calculation method (ΔNβ‑err.) and approximate model method (εβ); (c) comparison of total aerosol-induced errors obtained by simulated calculation method (ΔNα‑err+ΔNβverr) and approximate model method (εa+εβ)

Fig. 15. Comparison of aerosol backscattering coefficient-induced errors of NO₂ mass concentration obtained by simulated calculation method at different fitting distances (ΔNβ‑err) and approximate model method (εβ), when atmospheric aerosol is inhomogeneously distributed

Fig. 16. Aerosol-induced retrieval error of NO₂ mass concentration varies with detecting distance, when atmospheric aerosol is inhomogeneously distributed. (a) Comparison of aerosol extinction coefficient-induced errors obtained by simulated calculation method of differentiation (ΔNα‑err) and approximate model method (εa); (b) comparison of aerosol backscattering coefficient-induced errors obtained by simulated calculation method of differentiation (ΔNβ‑err) and approximate model method (εβ); (c) comparison of total aerosol-induced error sobtained by simulated calculation method of differentiation (ΔNα‑err+ΔNβ‑err) and approximate model method(εa+εβ)

Fig. 17. Aerosol backscattering coefficient-induced retrieval error of NO₂ mass concentrations obtained by approximate model method varies with detecting distance with different Ångström exponents, when atmospheric aerosol is inhomogeneously distributed