Fig. 1. Schematic of atmospheric CO₂ concentration measurement based on CRDS

Fig. 2. Results of long-time measurement of empty cavity. (a) Ring down measurement result; (b) result of Allen variance

Fig. 3. Spectral fitting of CO₂ with concentration of 99×10^-6 and its residual

Fig. 4. Schematic of high vacuum optical hygrometer based on CRDS^[93]

Fig. 5. Integral absorption coefficients of the four absorption lines selected at dew-point temperature with different vapor pressures^[93]

Fig. 6. Measurement results of trace water vapor absorption spectrum^[93]

Fig. 7. Schematic of atmospheric methane measuring instrument based on CRDS^[96]

Fig. 8. Results of continuous monitoring of atmospheric methane concentration^[96]

Fig. 9. Methane vertical profile measurement system based on balloon-borne CRDS^[89]. (a) System principle diagram; (b) physical diagram of the system

Fig. 10. The site of the second Qinghai-Tibet Plateau comprehensive scientific research test by captive balloon at Lulang^[89]

Fig. 11. Spectral measurement and fitting results^[89]. (a) At 5250 m altitude during the ascent; (b) at 5600 m altitude during the descent

Fig. 12. Measurement results of vertical distribution of methane concentration^[89]. (a) During the ascent; (b) during the descent

Fig. 13. Schematic of in situ analysis system for deep-sea dissolved methane^[88]

Fig. 14. 3D design drawing of optical cavity^[88]

Fig. 15. In the case of water vapor interference, the results of methane spectral fitting and its residual^[88]. (a) Without water vapor spectral fitting; (b) with water vapor spectral fitting

Fig. 16. Compared with the measurement results of atmospheric methane concentration by the AIOFM methanometer and the commercial Piccaro methanometer^[88]