Fig. 1. Correlation demodulation schematic of photoacaustic signals

Fig. 2. Spectral absorption of 400×10^-6 CO₂ and 5 000×10^-6 H₂O based on HITRAN database

Fig. 3. Structure diagram of CO₂ detection system based on FPGA

Fig. 4. Schematic diagram of FPGA internal program

Fig. 5. Photoacoustic signal amplitudes of CO₂ with a concentration of 500×10^-6 under different pressures

Fig. 6. 2f signal amplitude at different modulation amplitudes of 500×10^-6 CO₂

Fig. 7. Amplitude of 2f signal at different modulation frequencies

Fig. 8. 2f signals and its peak-to-peak value at different CO₂ concentrations

Fig. 9. Raw data from 920×10^-6 CO₂ concentration measurements and Allan variance curve of CO₂ photoacoustic sensing system

Fig. 10. Seed activity was measured by standard germination test and respiration rate measurement

Fig. 11. Relationship between CO₂ respiration rate and seed activity