First ground-based FTIR observations of HFC-23 at Rikubetsu, Japan, and Syowa Station, Antarctica

Abstract. We have developed a procedure for retrieving atmospheric abundances of HFC-23 (CHF3) with a ground-based Fourier transform infrared spectrometer (FTIR) and analysed the spectra observed at Rikubetsu, Japan (43.5° N, 143.8° E), and at Syowa Station, Antarctica (69.0° S, 39.6° E). The FTIR retrievals were carried out with the SFIT4 retrieval program, and the two spectral windows of 1138.5–1148.0 cm−1 and 1154.0–1160.0 cm−1 in the overlapping ν2 and ν5 vibrational-rotational transition bands of HFC-23 were used to avoid strong H2O absorption features. We considered O3, N2O, CH4, H2O, HDO, CFC-12 (CCl2F2), HCFC-22 (CHClF2), PAN (CH3C(O)OONO2), HCFC-141b (CH3CCl2F), and HCFC-142b (CH3CClF2) as interfering species. Vertical profiles of H2O, HDO, and CH4 are preliminarily retrieved with other independent spectral windows because these profiles may induce large uncertainties in the HFC-23 retrieval. Each HFC-23 retrieval has only one piece of vertical information with sensitivity to HFC-23 in the troposphere and the lower stratosphere. The retrieval errors mainly arise from the systematic uncertainties of the spectroscopic parameters used to obtain the HFC-23, H2O, HDO, and CH4 abundances. For comparison between FTIR-retrieved HFC-23 total columns and surface dry-air mole fractions provided by AGAGE (Advanced Global Atmospheric Gases Experiment), the FTIR-retrieved HFC-23 dry-air column-averaged mole fractions (XHFC-23) were calculated. The FTIR-retrieved XHFC-23 at Rikubetsu and Syowa Station have negative biases compared to AGAGE datasets. The trend derived from the FTIR-retrieved XHFC-23 data at Rikubetsu for December to February (DJF) data over the 1997–2010 period is 0.817 ± 0.087 ppt (parts per trillion) year−1, which is in good agreement with the trend derived from the annual global mean datasets of the AGAGE 12-box model for the same period (0.820 ± 0.011 ppt year−1). The trend of the FTIR-retrieved XHFC-23 data at Rikubetsu for DJF data over the 2007–2020 period is 0.894 ± 0.099 ppt year−1, which is smaller than the trend in the AGAGE in-situ measurements at Trinidad Head (41.1° N, 124.2° W) for the 2007–2019 period (0.984 ± 0.002 ppt year−1). The trend computed from the XHFC-23 datasets at Syowa Station over the 2007–2016 period is 0.823 ± 0.075 ppt year−1, which is consistent with that derived from the AGAGE in-situ measurements at Cape Grim (40.7° S, 144.7° E) for the same period (0.874 ± 0.002 ppt year−1). Although there are systematic biases on the FTIR-retrieved XHFC-23 at both sites, these results indicate that ground-based FTIR observations have the capability to monitor the trend of atmospheric HFC-23.