Fig. 1. Schematic of the TD-CRDS system

Fig. 2. Cross section of NO₂, O₃, water vapour and diode laser spectrum. (a) Cross section of O₃ and water vapour; (b) cross section of NO₂ and diode laser spectrum

Fig. 3. Fitting results of cavity ring-down signal without NO₂

Fig. 4. Temperature distribution in quartz glass tube at flow rate of 1 L/min

Fig. 5. Temperature fluctuation at 450 ℃

Fig. 6. Comparison of dual-channel consistency. (a) Time series of NO₂ concentration between the two channels; (b) correlation plot between the data from two channels

Fig. 7. Detection limit of CRDS system. (a) Continuous of NO₂ concentration sampled only under zero air; (b) Allan deviation plot for NO₂ concentration in CRDS system, in which the minimum value equals the optimum integration time

Fig. 8. Comparison of CRDS and LP-DOAS. (a) Time series of NO₂ concentration sampled by CRDS and LP-DOAS; (b) correlation plot between the data from CRDS and LP-DOAS

Fig. 9. Time series of ON and NO₂ concentration during October 16 to 23, 2019

Fig. 10. Daily average variation during the observation