Decrease of the carbon tetrachloride (CCl ₄) loading above Jungfraujoch, based on high resolution infrared solar spectra recorded between 1999 and 2011

The long-term trend of the atmospheric carbon tetrachloride (CCl ₄) burden has been retrieved from high spectral resolution infrared solar absorption spectra recorded between January 1999 and June 2011. The observations were made with a Fourier transform spectrometer at the northern mid-latitude, high altitude Jungfraujoch station in Switzerland (46.5°N latitude, 8.0°E longitude, 3580m altitude). Total columns were derived from spectrometric analysis of the strong CCl ₄ ν ₃ band at 794cm ^-1, accounting for all interfering molecules (e.g., H ₂O, CO ₂, O ₃, and a dozen weakly absorbing gases). A significant improvement in the fitting residuals and in the retrieved CCl ₄ columns was obtained by taking into account line mixing in a strong interfering CO ₂ Q branch. This procedure had never been implemented in remote sensing CCl ₄ retrievals though its importance was noted in earlier studies. A fit to the CCl ₄ daily mean total column time series returns a statistically-significant long-term trend of (-1.49±0.08×10 ¹³mol/cm ²)/yr, 2-σ. This corresponds to an annual decrease of (-1.31±0.07)pptv for the mean free tropospheric volume mixing ratio. Furthermore, the total column data set reveals a weak seasonal cycle with a peak-to-peak amplitude of 4.5%, with minimum and maximum values occurring in mid-February and mid-September, respectively. This small seasonal modulation is attributed primarily to the residual influence of tropopause height changes throughout the year. The negative trend of the CCl ₄ loading reflects the continued impact of the regulations implemented by the Montreal Protocol and its strengthening amendments and adjustments. Despite this statistically significant decrease, the CCl ₄ molecule currently remains an important contributor to the atmospheric chlorine budget, and thus deserves further monitoring, to ensure continued compliance with these strengthenings, globally. Our present findings are briefly discussed with respect to recent relevant CCl ₄ investigations at the ground and from space.