Atmospheric Carbon Tetrachloride -- A Conundrum?

Carbon tetrachloride (CCl₄) is a significant ozone-depleting gas in the atmosphere that has been decreasing in concentration at just under 1 ppt y^-1 (< 1% y^-1) over the past decade. This turnover and decrease in reported mixing ratio can be attributed to lowered production as agreed upon by the parties signing the Montreal Protocol on Substances that Deplete Stratospheric Ozone. The rate of decline, however, is inconsistent with available information regarding the production and atmospheric lifetime of this gas. Other useful information for CCl₄ is in the interhemispheric difference, which has changed little since the initial turnover in mixing ratio. This is unlike that of the CFCs and methyl chloroform (CH₃CCl₃), for which the interhemispheric difference in mixing ratio has become smaller, in some cases rapidly, with the decline in emissions in the mid-1990s. This suggests that emissions of CCl₄ continue to be significant, particularly in light of the lowered estimates for the atmospheric lifetime of CCl₄. This seeming paradox raises a number of questions, not just regarding production and emission, but also about the calculated lifetimes of atmospheric trace gases. The lifetime of CCl₄ from losses solely within the atmosphere currently is calculated at 35(21-43 ) y. Recently reported losses to the ocean have reduced the lifetime estimate to 26(17-36) y and an even more recent estimate of losses to soils reduces the lifetime further to 20(13-32) y. These newly reported losses of CCl₄ force a re-examination of the magnitudes and distributions of the sources and sinks of this gas. This poster presents some new data and an evaluation of what is known and unknown with regard to the budget of atmospheric CCl4, highlighting uncertainties and the need for additional research to reduce them. Montzka, S. A., et al., Controlled substances and other source gases, Chapter 1, in Scientific Assessment of Ozone Depletion: 2002, Global Ozone Res. and Monitor. Proj., Vol. No. 47, World Meteorol. Organ., Geneva, Switzerland, 2003. Yvon-Lewis, S. A., and J. H. Butler, Effect of oceanic uptake on atmospheric lifetimes of selected trace gases, J. Geophys. Res., 107(D20), 4414, doi:10.1029/2001JD001267, 2002. Happell, J. D., and M. P. Roche, Soils: A global sink of atmospheric carbon tetrachloride, Geophys. Res. Lett., 30(2), 1088, doi:10.1029/2002GL015957, 2003.