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Model sensitivity studies of the decrease in atmospheric carbon tetrachloride

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posted on 14.03.2017 by Martyn P. Chipperfield, Qing Liang, Matthew Rigby, Ryan Hossaini, Stephen A. Montzka, Sandip Dhomse, Wuhu Feng, Ronald G. Prinn, Ray F. Weiss, Christina M. Harth, Peter K. Salameh, Jens Mühle, Simon O'Doherty, Dickon Young, Peter G. Simmonds, Paul B. Krummel, Paul J. Fraser, L. Paul Steele, James D. Happell, Robert C. Rhew, James Butler, Shari A. Yvon-Lewis, Bradley Hall, David Nance, Fred Moore, Ben R. Miller, James W. Elkins, Jeremy J. Harrison, Chris D. Boone, Elliot L. Atlas, Emmanuel Mahieu
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 yearg-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 yearg-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.
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This work was supported by the UK Natural Environment Research Council (NERC) through the TROPHAL project (NE/J02449X/1). The TOMCAT modelling work was supported by the NERC National Centre for Atmospheric Science (NCAS). The ACE-FTS CCl4 work was supported by the NERC National Centre for Earth Observation (NCEO). The ACE mission is funded primarily by the Canadian Space Agency. The University of Liège involvement has primarily been supported by the F.R.S.–FNRS, the Fédération Wallonie-Bruxelles and the GAW-CH programme of Meteoswiss. Emmanuel Mahieu is a research associate with F.R.S.–FNRS. We thank the International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat (HFSJG, Bern) for supporting the facilities needed to perform the FTIR observations and the many colleagues who contributed to FTIR data acquisition. AGAGE is supported principally by NASA (USA) grants to MIT and SIO, as well as by Department of Energy and Climate Change (DECC, UK) and NOAA (USA) grants to Bristol University and by CSIRO and BoM (Australia). The operation of the station at Mace Head was funded by DECC through contract GA01103. Martyn P. Chipperfield is supported by a Royal Society Wolfson Merit award. Qing Liang is supported by the NASA Atmospheric Composition Campaign Data Analysis and Modeling (ACCDAM) programme. NOAA observations were made possible with technical and sampling assistance from station personnel (D. Mondeel, C. Siso, C. Sweeney, S. Wolter, D. Neff, J. Higgs, M. Crotwell, D. Guenther, P. Lang and G. Dutton) and were supported, in part, through the NOAA Atmospheric Chemistry, Carbon Cycle, and Climate (AC4) programme. Elliot L. Atlas acknowledges X. Zhu and L. Pope for technical support and the National Science Foundation AGS Program for support under grants ATM0849086 and AGS0959853.

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Citation

Atmospheric Chemistry and Physics, 2016, 16 (24), pp. 15741-15754

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Atmospheric Chemistry and Physics

Publisher

European Geosciences Union (EGU), Copernicus Publications

issn

1680-7316

eissn

1680-7324

Acceptance date

28/11/2016

Copyright date

2016

Available date

14/03/2017

Publisher version

http://www.atmos-chem-phys.net/16/15741/2016/

Notes

The output from the TOMCAT model experiments can be obtained by emailing Martyn Chipperfield.

Language

en

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