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Title: Model sensitivity studies of the decrease in atmospheric carbon tetrachloride

Carbon tetrachloride (CCl 4) is an ozone-depleting substance, which is controlled by the Montreal Protocol and for which the atmospheric abundance is decreasing. But, the current observed rate of this decrease is known to be slower than expected based on reported CCl 4 emissions and its estimated overall atmospheric lifetime. Here we use a three-dimensional (3-D) chemical transport model to investigate the impact on its predicted decay of uncertainties in the rates at which CCl 4 is removed from the atmosphere by photolysis, by ocean uptake and by degradation in soils. The largest sink is atmospheric photolysis (74 % of total), but a reported 10 % uncertainty in its combined photolysis cross section and quantum yield has only a modest impact on the modelled rate of CCl 4 decay. This is partly due to the limiting effect of the rate of transport of CCl 4 from the main tropospheric reservoir to the stratosphere, where photolytic loss occurs. The model suggests large interannual variability in the magnitude of this stratospheric photolysis sink caused by variations in transport. The impact of uncertainty in the minor soil sink (9 % of total) is also relatively small. In contrast, the model shows that uncertaintymore » in ocean loss (17 % of total) has the largest impact on modelled CCl 4 decay due to its sizeable contribution to CCl 4 loss and large lifetime uncertainty range (147 to 241 years). Furthermore, with an assumed CCl 4 emission rate of 39 Gg year -1, the reference simulation with the best estimate of loss processes still underestimates the observed CCl 4 (overestimates the decay) over the past 2 decades but to a smaller extent than previous studies. Changes to the rate of CCl 4 loss processes, in line with known uncertainties, could bring the model into agreement with in situ surface and remote-sensing measurements, as could an increase in emissions to around 47 Gg year -1. Further progress in constraining the CCl 4 budget is partly limited by systematic biases between observational datasets. For example, surface observations from the National Oceanic and Atmospheric Administration (NOAA) network are larger than from the Advanced Global Atmospheric Gases Experiment (AGAGE) network but have shown a steeper decreasing trend over the past 2 decades. The observed differences imply a difference in emissions which is significant relative to uncertainties in the magnitudes of the CCl 4 sinks.« less
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  1. Univ. of Leeds (United Kingdom). School of Earth and Environment and National Centre for Earth Observation
  2. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States); Universities Space Research Association (GESTAR), Columbia, MD (United States)
  3. Univ. of Bristol (United Kingdom). Atmospheric Chemistry Research Group
  4. Lancaster Univ. (United Kingdom). Lancaster Environment Centre
  5. National Oceanic and Atmospheric Administration (NOAA), Boulder, CO (United States). Earth System Research Lab.
  6. Univ. of Leeds (United Kingdom). School of Earth and Environment
  7. Univ. of Leeds (United Kingdom). School of Earth and Environment and National Centre for Atmospheric Science
  8. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
  9. Univ. of California, San Diego, CA (United States). Scripps Institution of Oceanography
  10. Commonwealth Scientific and Industrial Research Organization (CSIRO) Oceans and Atmosphere, Aspendale VIC (Australia)
  11. Univ. of Miami, FL (United States). Dept. of Ocean Sciences
  12. Univ. of California, Berkeley, CA (United States). Dept. of Geography
  13. Texas A & M Univ., College Station, TX (United States). Dept. of Oceanography
  14. Univ. of Leicester (United Kingdom). Dept. of Physics and Astronomy; National Centre for Earth Observation
  15. Univ. of Waterloo, ON (Canada). Dept. of Chemistry
  16. Univ. of Miami, FL (United States). Dept. of Atmospheric Sciences
  17. Univ. of Liege (Belgium). Inst. of Astrophysics and Geophysics
Publication Date:
Grant/Contract Number:
NE/J02449X/1; GA01103; ATM0849086; AGS0959853
Accepted Manuscript
Journal Name:
Atmospheric Chemistry and Physics (Online)
Additional Journal Information:
Journal Name: Atmospheric Chemistry and Physics (Online); Journal Volume: 16; Journal Issue: 24; Journal ID: ISSN 1680-7324
European Geosciences Union
Research Org:
Univ. of Leeds (United Kingdom). School of Earth and Environment
Sponsoring Org:
USDOE; National Aeronautics and Space Administration (NASA)
Country of Publication:
United States
OSTI Identifier: