Long-term evolution in the tropospheric concentration of chlorofluorocarbon 12 (CCl₂F₂) derived from high-spectral resolution infrared solar absorption spectra: Retrieval and comparison with in situ surface measurements

The average tropospheric volume mixing ratios of chlorofluorocarbon 12 (CCl₂F₂) have been retrieved from high-spectral resolution ground-based infrared solar-absorption spectra recorded from March 1982 to October 2003 with the McMath Fourier transform spectrometer at the US National Solar Observatory facility on Kitt Peak in southern Arizona (31.9°N, 111.6°W, 2.09 km altitude). The retrievals are based on fits to the unresolved ν 8 band Q-branches near 922 cm^-1 using the SFIT2 retrieval algorithm. The annual increase rate was equal to (16.88 ± 1.37) parts per trillion (10^-12) by volume at the beginning of the time series, March 1982, or (4.77 ± 0.04) % , 1 sigma, declining progressively to (2.49 ± 1.24) parts per trillion, by volume at the end, October 2003, or (0.46 ± 0.24) % , 1 sigma. Average tropospheric mixing ratios from the solar spectra have been compared with average surface flask and in situ sampling measurements from the Climate Monitoring and Diagnostics Laboratory (CMDL) station at Niwot Ridge, CO, (USA) (40.0°N, 105.5°W, 3013 m altitude). The average ratio and standard deviation of the monthly means of the retrieved tropospheric mixing ratios relative to the CMDL surface mixing ratios is (1.01 ± 0.03) for the overlapping time period. Both datasets demonstrate the progressive impact of the Montreal protocol and its strengthening amendments on the trend of CCl₂F₂, though a tropospheric decrease has yet to be observed.