Solving the mystery of Carbon Tetrachloride (CCl4)

Carbon tetrachloride (CCl4 or CTC) is a major ozone depleting substance and greenhouse gas: ozone depletion potential (with respect to CFC-11) of 0.72 (WMO, 2015), and a 100-year global warming potential of 1730 (WMO, 2014). Unfortunately, estimated CTC sources and sinks remain inconsistent with abundance observations. The WMO (2014) total global lifetime estimate (26 years) combined with the observed atmospheric trend implies emissions of 57 (40–74) Gg/yr. This emission level cannot be reconciled with emissions from reported net production. Liang et al. (2014) updated (WMO, 2014) by using surface observations of trends and the inter-hemispheric gradient to estimate a 35 (32-37) year global lifetime and 39 (34-45) Gg/yr. The large discrepancy between the near zero UNEP report emissions and 39 Gr/yr top-down emissions suggest that there is a large unknown source of CTC.

This activity brought together science, industry, and technology experts to solve the CTC mystery. The effort involved observation scientists, together with experts in photochemistry, ocean and soil losses, emissions, modeling, and industrial processes.

As a first step, we brought together experts in all of these areas together for a 2-day workshop. This workshop involved both solicited and submitted presentations related to all aspects of the CTC problem.

Workshop topics included:

  • Observations of CTC from ground stations, aircraft, balloon, satellite, and ships
  • Photochemistry of CTC loss
  • Ocean and soil losses
  • Global and regional emission estimates
  • CTC consumption and production for both historic and current usage
  • CTC feedstock usage and potential CTC fugitive emissions
  • Legacy emissions from brown-field sites
  • Reconciling the emission and loss processes of CTC with observations in a global modeling perspective

The workshop provided information across disciplines and areas that would not necessarily interact with one another. The workshop took place from 5-6 October 2015 in Zurich, Switzerland. For further details please consult:

In 2016, this activity published its final assessment report (SPARC Report No. 7).


Liang, Q. et al. (2014) Constraining the carbon tetrachloride (CCl4) budget using its global trend and inter-hemispheric gradient. Geophys. Res. Lets., 41, 5307-5315.

SPARC (2016) SPARC Report on the Mystery of Carbon tetrachloride. Q. Liang, P.A. Newman, and S. Reimann (Eds.), SPARC Report No. 7, WCRP-13/2016.

WMO (2014) Scientific Assessment of Ozone Depletion: 2010, Global Ozone Research and Monitoring Project. Report No. 52, Geneva, Switzerland, 516 pp.

Activity leaders

Qing Liang, USRA/NASA, USA

Paul Newman, NASA, USA

Stefan Reimann, Empa, Switzerland


Montreal Protocol SAP and TEAP, UNEP, WMO, industry, NGOs, Empa, NASA, NOAA.

Published results

SPARC, 2016: SPARC Report on the Mystery of Carbon tetrachloride. Q. Liang, P.A. Newman, and S. Reimann (Eds.), SPARC Report No. 7, WCRP-13/2016, available at