Fig. 1. Flow chart of photoacoustic detection

Fig. 2. Structural diagram of integrating sphere chamber

Fig. 3. 3D CAD diagram of integrating sphere

Fig. 4. Frequency response curves of integrating sphere coupling system

Fig. 5. Simulated distributions of sound pressure levels at resonant frequencies. (a) 956 Hz; (b) 2868 Hz; (c) 4780 Hz

Fig. 6. Schematic diagram of general design scheme

Fig. 7. Emission spectrum of light source and NO₂ absorption spectrum

Fig. 8. Functional block diagram of STM32 data processing system

Fig. 9. Frequency response curves

Fig. 10. Flow chart of concentration calibration

Fig. 11. Fitted results of concentration