First ground-based Fourier transform infrared (FTIR) spectrometer observations of HFC-23 at Rikubetsu, Japan, and Syowa Station, Antarctica

Takeda, Masanori; Nakajima, Hideaki; Murata, Isao; Nagahama, Tomoo; Morino, Isamu; Toon, Geoffrey C.; Weiss, Ray F.; Mühle, Jens; Krummel, Paul B.; Fraser, Paul J.; Wang, Hsiang-Jui

page5956We have developed a procedure for retrieving atmospheric abundances of inline-formulaHFC-23 (inline-formulaCHF3) with a ground-based Fourier transform infrared (FTIR) spectrometer and analyzed the spectra observed at Rikubetsu, Japan (43.5inline-formula N, 143.8inline-formula E), and at Syowa Station, Antarctica (69.0inline-formula S, 39.6inline-formula E). The FTIR retrievals were carried out with the SFIT4 retrieval program, and the two spectral windows of 1138.5–1148.0 inline-formulacm−1 and 1154.0–1160.0 inline-formulacm−1 in the overlapping inline-formulaν2 and inline-formulaν5 vibrational–rotational transition bands of inline-formulaHFC-23 were used to avoid strong inline-formulaH2O absorption features. We considered inline-formulaO3, inline-formulaN2O, inline-formulaCH4, inline-formulaH2O, HDO, inline-formulaCFC-12 (inline-formulaCCl2F2), inline-formulaHCFC-22 (inline-formulaCHClF2), peroxyacetyl nitrate (PAN) (inline-formulaCH3C(O)OONO2), inline-formulaHCFC-141b (inline-formulaCH3CCl2F), and inline-formulaHCFC-142b (inline-formulaCH3CClF2) to be interfering species. Vertical profiles of inline-formulaH2O, HDO, and inline-formulaCH4 are preliminarily retrieved with other independent spectral windows because these profiles may induce large uncertainties in the inline-formulaHFC-23 retrieval. Each inline-formulaHFC-23 retrieval has only one piece of vertical information with sensitivity to inline-formulaHFC-23 in the troposphere and the lower stratosphere. Retrieval errors mainly arise from the systematic uncertainties of the spectroscopic parameters used to obtain inline-formulaHFC-23, inline-formulaH2O, HDO, and inline-formulaCH4 abundances. For comparison between FTIR-retrieved inline-formulaHFC-23 total columns and surface dry-air mole fractions provided by AGAGE (Advanced Global Atmospheric Gases Experiment), FTIR-retrieved inline-formulaHFC-23 dry-air column-averaged mole fractions (inline-formulaXHFC-23) were calculated. The FTIR-retrieved inline-formulaXHFC-23 values at Rikubetsu and Syowa Station have negative biases of inline-formula−15 % to inline-formula−20 % and inline-formula−25 % compared to the AGAGE datasets, respectively. These negative biases might mainly come from systematic uncertainties of inline-formulaHFC-23 spectroscopic parameters. The trend of the FTIR-retrieved inline-formulaXHFC-23 data at Rikubetsu was derived for December to February (DJF) observations, which are considered to represent the background values when an air mass reaching Rikubetsu has the least influence by transport of inline-formulaHFC-23 emissions from nearby countries. The DJF trend of Rikubetsu over the 1997–2009 period is 0.810 inline-formula± 0.093 inline-formulapptinline-formulayr−1 (ppt: parts per trillion), 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 inline-formula± 0.013 inline-formulappt yr−1). The DJF trend of Rikubetsu over the 2008–2019 period is 0.928 inline-formula± 0.108 inline-formulappt yr−1, which is consistent with the trend in the AGAGE in situ measurements at Trinidad Head (41.1inline-formula N, 124.2inline-formula W) for the same period (0.994 inline-formula± 0.001 inline-formulappt yr−1). The trend of the FTIR-retrieved inline-formulaXHFC-23 data at Syowa Station over the 2007–2016 period is 0.819 inline-formula± 0.071 inline-formulappt yr−1, which is consistent with that derived from the AGAGE in situ measurements at Cape Grim (40.7inline-formula S, 144.7inline-formula E) for the same period (0.874 inline-formula± 0.002 inline-formulappt yr−1). Although there are systematic biases in the FTIR-retrieved inline-formulaXHFC-23 at both sites, these results indicate that ground-based FTIR observations have the capability to monitor the long-term trend of atmospheric inline-formulaHFC-23. If this FTIR measurement technique were extended to other Network for the Detection of Atmospheric Composition Change (NDACC) ground-based FTIR sites around world, the measurements reported from these sites would complement the global AGAGE observations by filling spatial and temporal gaps and may lead to improved insights about changes in regional and global emissions of inline-formulaHFC-23 and its role in global warming.

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Takeda, Masanori / Nakajima, Hideaki / Murata, Isao / et al: First ground-based Fourier transform infrared (FTIR) spectrometer observations of HFC-23 at Rikubetsu, Japan, and Syowa Station, Antarctica. 2021. Copernicus Publications.

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