Fig. 1. Six normal vibration modes of HCHO molecule

Fig. 2. Experimental setup of FC-FTIR spectrometer

Fig. 3. Formaldehyde absorption spectra in range of 2730-2970 cm^-1. Black spectral line is measured formaldehyde spectrum at 296 K and gas pressure of 300 mbar with resolution of 0.08 cm^-1, and point line spectrrum is fitting HITRAN database spectrum

Fig. 4. System sensitivity assessment. (a) Evaluation of system sensitivity using multi-spectral line fitting. Solid line is fitting spectrum and scattered dot is measured spectrum; (b) fitting residual; (c) partial amplification for fitting diagram

Fig. 5. Spectral line fitting to evaluate concentration. (a) ν₁ band of formaldehyde spectrum measured at a resolution of 0.08 cm^-1; (b) spectral line fitting and fitting residuals at 2778.5 cm^-1; (c) spectral line fitting and fitting residuals at 2781.0 cm^-1; (d) comparison of fitting results between original measured spectra of formaldehyde and HITRAN database

Fig. 6. Absorption spectra of formaldehyde under interference of water. (a) Absorption spectra of mixture of formaldehyde and water at 600 mbar; (b) 2790-2870 cm^-1 region local amplification map; (c) local enlarged image; (d) fitting residuals