Articles | Volume 16, issue 24
Atmos. Chem. Phys., 16, 15741–15754, 2016
Atmos. Chem. Phys., 16, 15741–15754, 2016

Research article 20 Dec 2016

Research article | 20 Dec 2016

Model sensitivity studies of the decrease in atmospheric carbon tetrachloride

Martyn P. Chipperfield1,2, Qing Liang3,4, Matthew Rigby5, Ryan Hossaini6, Stephen A. Montzka7, Sandip Dhomse1, Wuhu Feng1,8, Ronald G. Prinn9, Ray F. Weiss10, Christina M. Harth10, Peter K. Salameh10, Jens Mühle10, Simon O'Doherty5, Dickon Young5, Peter G. Simmonds5, Paul B. Krummel11, Paul J. Fraser11, L. Paul Steele11, James D. Happell12, Robert C. Rhew13, James Butler7, Shari A. Yvon-Lewis14, Bradley Hall7, David Nance7, Fred Moore7, Ben R. Miller7, James W. Elkins7, Jeremy J. Harrison15,16, Chris D. Boone17, Elliot L. Atlas18, and Emmanuel Mahieu19 Martyn P. Chipperfield et al.
  • 1School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • 2National Centre for Earth Observation, University of Leeds, Leeds, LS2 9JT, UK
  • 3NASA Goddard Space Flight Center, Atmospheric Chemistry and Dynamics, Greenbelt, Maryland 20771, USA
  • 4Universities Space Research Association, GESTAR, Columbia, Maryland 21046, USA
  • 5Atmospheric Chemistry Research Group, School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK
  • 6Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK
  • 7Global Monitoring Division, NOAA Earth System Research Laboratory, Boulder, Colorado 80305, USA
  • 8National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
  • 9Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 USA
  • 10Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0244, USA
  • 11CSIRO Oceans and Atmosphere, Aspendale, Victoria 3195, Australia
  • 12Department of Ocean Sciences, University of Miami, Florida 33149, USA
  • 13Departmet of Geography, University of California, Berkeley, California 94720-4740, USA
  • 14Department of Oceanography, Texas A&M University, College Station, Texas 77840, USA
  • 15Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
  • 16National Centre for Earth Observation, University of Leicester, Leicester, LE1 7RH, UK
  • 17Department of Chemistry, University of Waterloo, Ontario, N2L 3G1, Canada
  • 18Department of Atmospheric Sciences, University of Miami, Miami, Florida 33149, USA
  • 19Institute of Astrophysics and Geophysics, University of Liège, Liège 4000, Belgium

Abstract. Carbon tetrachloride (CCl4) is an ozone-depleting substance, which is controlled by the Montreal Protocol and for which the atmospheric abundance is decreasing. However, the current observed rate of this decrease is known to be slower than expected based on reported CCl4 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 CCl4 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 CCl4 decay. This is partly due to the limiting effect of the rate of transport of CCl4 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 uncertainty in ocean loss (17 % of total) has the largest impact on modelled CCl4 decay due to its sizeable contribution to CCl4 loss and large lifetime uncertainty range (147 to 241 years). With an assumed CCl4 emission rate of 39 Gg year−1, the reference simulation with the best estimate of loss processes still underestimates the observed CCl4 (overestimates the decay) over the past 2 decades but to a smaller extent than previous studies. Changes to the rate of CCl4 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 CCl4 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. These differences imply a difference in emissions which is significant relative to uncertainties in the magnitudes of the CCl4 sinks.

Short summary
Carbon tetrachloride (CCl4) is a compound which, when released into the atmosphere, can cause depletion of the stratospheric ozone layer. Its emissions are controlled under the Montreal Protocol, and its atmospheric abundance is slowly decreasing. However, this decrease is not as fast as expected based on estimates of its emissions and its atmospheric lifetime. We have used an atmospheric model to look at the uncertainties in the CCl4 lifetime and to examine the impact on its atmospheric decay.
Final-revised paper