Fig. 1. Schematic of passive infrared remote sensing of polluted gases

Fig. 2. Schematic of experimental instrument

Fig. 3. Simulated spectra under different CPL values. (a) Spectra of methane; (b) spectra of water vapor

Fig. 4. Measured and simulated spectra. (a) Brightness temperature spectra of brick wall and low elevation sky backgrounds; (b) simulated transmittance spectra by MODTRAN

Fig. 5. Fitting results by proposed method and Harig's method. (a) Brick wall background; (b) low elevation sky background

Fig. 6. Reconstructed background spectra by proposed method and Harig's method. (a) Brick wall background; (b) low elevation sky background

Fig. 7. Extracted features and correlation coefficients by proposed method and Harig's method. (a) Brick wall background; (b) low elevation sky background

Fig. 8. Measured spectra and simulated spectra under different relative humidity conditions. (a) Measured spectra; (b) simulated transmittance spectra by MODTRAN

Fig. 9. Comparation between reconstructed backgrounds and extracted features by two methods under different relative humidities. (a) Measured spectrum and reconstructed backgrounds by two methods under relative humidity of 67%; (b) reference spectrum and extracted features by two methods corresponding to (a); (c) measured spectrum and reconstructed backgrounds by two methods under relative humidity of 56%; (d) reference spectrum and extracted features by two methods corresponding to (c); (e) measured sp

Fig. 10. Comparison of measured spectra and simulated spectra of ethylene. (a) Brightness temperature spectra of ethylene at different stages of release; (b) simulated transmittance spectra of ethylene under different CPL values

Fig. 11. Spectra of ethylene at different stages of release. (a) Spectra of ethylene at early stage of release and reconstructed backgrounds by two methods; (b) reference spectrum and extracted features by two methods corresponding to (a); (c) spectra of ethylene at middle stage of release and reconstructed backgrounds by two methods; (d) reference spectrum and extracted features by two methods corresponding to (c); (e) spectra of ethylene at end stage of release and reconstructed backgrounds by two meth