Fig. 1. Absorption line of CO₂ in range of 2-5 μm

Fig. 2. Absorbance curves of CO₂， CH₄ and H₂O

Fig. 3. Schematic diagram of atmospheric CO₂ detection devices

Fig. 4. Atmospheric CO₂ detection devices

Fig. 5. Data processing flowchart

Fig. 6. Raw data and its fitting curve

Fig. 7. Voigt fitting and absorbance curves and their residual plots

Fig. 8. measured concentration and calibrated concentration results

Fig. 9. Linear relationship between measured concentration and calibrated concentration and residual results

Fig. 10. Allan variance method

Fig. 11. Comparison of TDLAS-CO₂ self-developed instrument and commercial instrument

Fig. 12. Comparison data of CO₂ instrument concentration

Fig. 13. Atmospheric CO₂ concentration over eight-day period