Determination of Total Column of Trichlorofluoromethane in the Atmosphere Taking Account the Effect of Amorphous Water Ice Sediment on the Spectrometer Detector

Abstract

A ground-based spectroscopic method for determining CCl₃F content from measurements of IR-spectra of solar radiation using an IFS-125HR Fourier Spectrometer (FTIR method) is considered. The detector based on mercury-cadmium-tellurium (HgCaTe), which is used for measurements in the CCl₃F absorption spectral region, is cooled by liquid nitrogen. When the vacuum gradually deteriorates in the metal Dewar vessel of the receiver during cooling, the amorphous ice film grows on the detector. The spectral absorption band of amorphous ice at liquid nitrogen temperatures overlaps the CCl₃F absorption band, and the ice film thickness variability adds additional uncertainty to the estimates of the CCl3F atmospheric content. To eliminate this uncertainty, a technique has been developed to estimate the thickness of the ice film and to account for its spectral absorption in the algorithm of solving the reverse problem. The technique has been applied when measuring the atmospheric concentration of CCl3F in 2017—2019 over the NDACC station, St.Petersburg. The results have been compared with ones obtained earlier using a technique in which the thickness of the ice film was adopted by an unknown parameter specified in the process of solving the inverse problem. The previously obtained CCl₃F atmospheric content has been refined using the proposed technique; the difference reaches 10%.