Fig. 1. (a) Observing site (Science Island in the western suburb of Hefei); (b) Observing site (Environmental monitoring center of Shanghai); (c) FTIR spectrometer

Fig. 2. Amplitude modulation efficiency and phrase error of the spectrometer

Fig. 3. Typical spectral fitting and residuals of CO₂ in two spectral windows near (a) 6 220 cm^-1, (b) 6 339.5 cm^-1

Fig. 4. Time series of retrieved Xair

Fig. 5. The time series of XCO₂ observed in Hefei
The light blue circles represent individual measurements; the red circles represent daily average XCO₂; the dark blue lines with blue triangles represent monthly average XCO₂

Fig. 6. XCO₂ observed by portable FTIR and high-resolution FTIR spectrometer in 2017

Fig. 7. Daily average of XCO₂ measured by portable FTIR spectrometer plotted against the corresponding values of the high-resolution FTIR spectrometer
The line shows the best fit to the ratio of the two values

Fig. 8. XCO₂ observed by GOSAT and portable FTIR spectrometer
Blue circles are GOSAT data and orange triangle are FTIR data

Fig. 9. Scatter plot of XCO₂ observed by GOSAT and portable FTIR spectrometer

Fig. 10. The time series of XCO₂ observed in Hefei and Shanghai
The black and red circles indicate the data observed in Hefei and Shanghai, respectively