Detection of stable isotopic ratio of atmospheric CO2 based on Fourier transform infrared spectroscopy

Long-term measurement of CO2 and its stable isotopes not only obtain the CO2 sources and sink information, but also determine the contributions of different emission sources to atmospheric CO2. Fourier transform infrared spectroscopy (FTIR) is an important technique which can provide highly precise remote sensing of column abundances of atmospheric trace gases. In the study, the stable isotopes of atmospheric CO2, (CO2)-C-13 and (CO2)-C-12, are retrieved from the near-infrared solar absorption spectra collected by a ground-based high-resolution Fourier transform spectrometer. Three spectral windows of (CO2)-C-13 and two spectral windows of (CO2)-C-12 are chosen to retrieve the two species. The root mean square spectral fitting residuals are about 1.2%, 2.3% and 1.2% for the three spectral windows of (CO2)-C-13, and about 0.64% and 0.60% for the two spectral windows of (CO2)-C-12, respectively. The small spectral fitting residuals indicate the high-quality spectral fitting. The mean retrieval errors are (1.18 +/- 0.27)% and (0.89 +/- 0.25)% for (CO2)-C-13 and (CO2)-C-12 during the experiment, respectively. The measurement precision of carbon isotopic ratio delta C-13 for the observation system is estimated to be about 0.041% based on the Allan variance method, comparable to the precision of in situ FTIR measurement. Moreover, long time series of atmospheric delta C-13 in one year from September 18, 2015 to September 24, 2016 is obtained. The results show that atmospheric delta C-13 varies from -7.58 parts per thousand to -11.66 parts per thousand, and the mean value is about (-9.5 +/- 0.57)parts per thousand over the duration of the experiment. Also, time series of carbon isotopic signature delta C-13 has an obvious seasonal trend, with a minimum of (-9.35 +/- 0.47)parts per thousand in winter and a maximum of (-8.73 +/- 0.39)parts per thousand in summer. The further analysis suggests that the increase of emission from the fossil fuel burning due to heating may explain the depletion of heavy isotope (CO2)-C-13 in winter. Additionally, it is revealed that the variation range of atmospheric C-13 observed in Hefei area is consistent with the reported values in Nanjing area based on in situ measurement, while delta C-13 values in summer and winter are higher than the corresponding values detected in Beijing area as indicated in recent publications, which may result from the fact that the CO2 emissions from the fossil fuel combustion in Beijing are more than those in Hefei. The experimental results demonstrate the ability of the ground-based high-resolution FTIR to detect the stable isotopes of atmospheric CO2, (CO2)-C-13 and (CO2)-C-12, and carbon isotopic ratio delta C-13 with a high precision and accuracy.