Fig. 1. The absorption spectrum of CH₄ and H₂Oin the near-infrared

Fig. 2. The absorption spectrum of CH₄， C₂H₆， C₃H₈， C₂H₂ and C₂H₄ within 6 030~6 055 cm^-1

Fig. 3. Gas-liquid separation system

Fig. 4. Structure of the near-infrared methane sensor system based on OA-ICOS

Fig. 5. Curve of incident light intensity I₀， absorbed light intensity I and （I₀/I-1）for the 10×10^-6 CH₄ sample

Fig. 6. Differential direct absorption amplitude at different concentration of CH₄ gas

Fig. 7. Relationship between amplitude of differential direct absorption signal and measurement time under different CH₄ concentration levels

Fig. 8. Fitting curve of CH₄ gas concentration and the amplitude of the differential direct absorption signal

Fig. 9. Measurement results of CH₄ concentration under N₂ atmosphere

Fig. 10. Allan deviation curve

Fig. 11. The measured concentration results of the dissolved CH₄ in tap water， rainwater and lake water using the developed CH₄ sensor system