Fig. 1. The absorption spectrum of volume fraction 1000×10^-6 CO₂

Fig. 2. System structure of the OA-ICOS CO₂ sensor system

Fig. 3. Structure of the absorption cell

Fig. 4. The measured direct absorption spectral signal and data fitting using volume fraction 1 000×10^-6 CO₂ sample

Fig. 5. Relationship between modulation amplitude, second harmonic amplitude and modulation depth

Fig. 6. The measured second harmonic signal using volume fraction 1000×10^-6 CO₂ sample

Fig. 7. Calibration of the sensor system

Fig. 8. System stability measurement results on volume fraction 0×10^-6 CO₂ sample

Fig. 9. Allan deviation curve of the sensor system

Fig. 10. Dynamic response of the sensor system

Fig. 11. Atmospheric CO₂ concentration measured from 21:00 on November 5, 2019 to 13:30 on November 6

Fig. 12. Measured CO₂ concentration results of human breath