Fig. 1. Schematic of WM-OA-ICOS measurement system

Fig. 2. Simulation of CH4 and H2O absorbance near 7185.87 cm-1 using HITRAN 2012 database

Fig. 3. Relationship between modulation amplitude and 2f signal peak height at different pressures

Fig. 4. Variations of 2f signal peak height with (a) modulation phase and (b) modulation frequency

Fig. 5. (a) Relationship between 2f signal peak height and time; (b) Allan variance

Fig. 6. Waveforms of (a) 2f signal and (b) 1f signal at different CH4 concentrations

Fig. 7. Measured (a) 2f signal peak height and (b) 1f signal medium value over a long period of measurement for CH4 of ten different concentrations

Fig. 8. Variations of 2f signal peak height and the ratio of 2f signal peak height to 1f signal medium value (2f/1f) with CH4 concentration

Fig. 9. (a) Variations of 2f signal peak height and 2f/1f value with time; (b) linear relationship between 2f signal peak height or 2f/1f value and CH4 concentration

Fig. 10. (a) Absorption line of CH4 and (b) fitting residual obtained by OA-ICOS technique

Fig. 11. 2f signal obtained by WM-OA-ICOS technique