1. Long-Term Trend of Carbon Tetrachloride (CCl 4 ) from Ground-based High Resolution Infrared Solar Spectra Recorded at the Jungfraujoch Curtis P. Rinsland, 1 Emmanuel Mahieu, 2 Philippe Demoulin, 2 Linda Chiou, 3 Rodolphe Zander, 2 and Jean-Michel Hartmann 4 1 NASA Langley Research Center, Atmospheric Sciences Competency, Hampton, U.S.A 2 Institute of Astrophysics and Geophysics, University of Liege, Liege, Belgium 3 Science Systems and Applications, Inc., 10210 Greenbelt Road, Suite 600, Latham, MD 20706 U.S.A. 4 Laboratoire Inter-universitaire des Systèmes Atmosphériques, (LISA), UMR CNRS/INSU 7583, Universités Paris VII et Paris XII, 94010 Créteil Cédex, France

2. Atmospheric Importance of Carbon Tetrachloride (CCl 4 ) Carbon tetrachloride (CCl 4 ) is a key component of the chlorine budget [Zander et al., 1994; 2008, Nassar et al., 1996; Allen et al., 2009], and a potent greenhouse gas with a global warming potential relative to CO 2 of 1400 on a 100 year horizon [Solomon et al. 2007] CCl 4 also has the longest emissions history beginning in 1908, but experimental uncertainties from surface station measurements are high (3-5%) with a significant spreading due to differences in the adjusted surface and adjusted chemical model records [WMO, 2007, Chapter 1; Martinerie et al., 2009] Firn-air results also indicate that nearly all of the CFCs, halons, methyl chloroform and CCl 4 present in today’s atmosphere are the result of anthropogenic production, though the existence of small natural sources or some anthropogenic production and use in years predating the oldest firn air samples cannot be ruled out A recent study of Atmospheric Chemistry Experiment (ACE) solar occultation spectra between February 2004 and August 2007 produced CCl 4 profile measurements between 70°S and 80°N latitude [Allen et al., 2009], but they did not report a trend measurement

3. Jungfraujoch FTS Measurements and Retrieval Approach The long-term trend and seasonal cycle of the CCl 4 total column are deduced from high spectral resolution infrared solar absorption spectra recorded with a Fourier transform spectrometer (FTS) at the Jungfraujoch station (46.5°N latitude, 8.0°E longitude, 3580 m altitude) The measurements cover the 1999 to 2010 time period and have typical signal-to-noise ratio of 1000 The analysis shows good sensitivity is achieved for zenith angles between 75° to 85° and that range was adopted Our results provide an updated assessment by remote sensing of the CCl 4 trend, and we compared to those reported previously [WMO, 2007, Chapter 1, Table 1-2] A factor of two reduction in the standard deviation of the residuals was obtained by taking into account line mixing in a nearby CO 2 Q branch at 791 cm -1, a procedure not implemented in previous remote sensing CCl 4 retrievals, though its importance has been noted in several papers

4. ISSJ Time Series, Retrieval Procedures, and Assumed Spectroscopic Parameters The number of initial days with clear sky measurements per year was initially about 60 per year until 1990 and has increased to about 110 thereafter Retrievals are based on the spectroscopic parameters measured by Nemtchinov and Varanasi [2003] covering the strong 12.7 µm 3 and weaker 1 + 4 combination band at temperatures between 208 and 296 K and pressures. Those spectra were recorded with an FTS at an unapodized resolution of 0.03 cm -1 between 10 and 1013 hPa The measured laboratory spectra were converted to “pseudolines” by G. C. Toon (Jet Propulsion Laboratory, private communication, 2008) with a procedure that has an estimated uncertainty of 4%

5. Retrieval Procedure Retrievals were performed with SFIT2 version 3.81 The approach is based on the semi-empirical implementation of the Rodgers optimal estimation method The covariance matrix is specified for each layer as a percentage of the a priori profile and a correlation length, which is interpreted as an exponentially decaying correlation between layers Multiple windows may be fitted simultaneously allowing retrieval of volume mixing ratio profiles or columns characterized with volume mixing ratio or column averaging kernels The analysis procedure includes first-order line mixing approximation [Rosenkranz,1985] and an algorithm developed by one of us and also including an empirical solar linelist [Hase et al. 2005]

6. ISSJ Sample Simulation and Measured Spectra

7. Total Column Averaging Kernel

8. Fit to ISSJ Example Spectrum with and without Line Mixing

9. ISSJ CCl 4 Daily and Monthly Mean Total Column Time Series

11. Summary and Conclusions Measurements of the long-term trend and seasonal cycle of the total column of carbon tetrachloride (CCl 4 ) have been retrieved from high-spectral resolution solar spectra recorded at the Jungfraujoch for the 1999-2009 time period with SFIT2 version 3.81 taking into account all interferences (and solar lines) and line mixing in the nearby CO 2 Q branch The time series shows a statistically-significant long-term decrease in the CCl 4 total atmospheric burden of (-1.18±0.10%)/yr -1, at the 95% confidence level, using 2005 as reference The fit to the total column data set also reveals a seasonal cycle with a peak-to-peak amplitude of 10.2%, with minimum and maximum values found in mid-February and early August, respectively This seasonal modulation can however be attributed to mostly tropopause height changes throughout the season The results quantify the continued impact of the regulations implemented by the Montreal Protocol and its strengthening amendments and adjustments for a molecule with high global warming potential Although a statistically significant decrease in the total column is inferred, the CCl 4 molecule, it remains an important contributor to the stratospheric chlorine budget and burden