Fig. 1. Absorption line of CO₂ from HITRAN database

Fig. 2. DFB laser emission wavelength as the function of diode temperature and injection current

Fig. 3. The absorption line of CO₂ and H₂O within the tuning range of the used DFB laser

Fig. 4. Simulation and physical implementation of Herriott multi-pass cell

Fig. 5. Experimental setup diagram

Fig. 6. 2f signal amplitude as the function of CO₂ concentration

Fig. 7. Relationship between TDLAS signal amplitude and CO₂ concentration

Fig. 8. Raw and real-time Kalman processed signals and noise

Fig. 9. 8 h continuous monitoring of CO₂ concentration over time in the laboratory

Fig. 10. 24 h continuous monitoring of CO₂ on the roof of the Jinan University Science and Engineering Building