Tracking an ozone killer

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35,000 tons of undeclared carbon tetrachloride (CCl 4 ) are released into our atmosphere every year - although applications in which this substance is released into the environment have been officially banned by the Montreal Protocol since 2010. So where does this environmental pollutant come from? Empa researchers tracked down carbon tetrachloride and found the possible sources.

«It is detective work,» says Stefan Reimann, a researcher at Empa’s «Air Pollution / Environmental Technology» department and one of the authors of the studies on carbon tetrachloride. In fact, his work is comparable to that of an investigator. CCl 4 is one of the main substances responsible for the depletion of the ozone layer and was officially banned in the Montreal Protocol in 2010. The colorless gas is, however, still approved as an intermediate product for chemical syntheses, but it may no longer be released into the atmosphere.

Therefore, according to the United Nations Environment Programme (UNEP), all nations are obliged to ban emissions of CCl 4 and to report and quantify any emissions that may nevertheless occur. With these quantities reported to UNEP, emissions of just 3,000 tons can be calculated. Nevertheless, around 35,000 tons of undeclared carbon tetrachloride is still released into the atmosphere every year, as an international study has already found out in 2016. The study assumed that chemical emissions were mainly due to factories producing chlorinated solvents, which are still permitted. In particular, the production of dichloromethane (CH 2 Cl 2 ), chloroform (CHCl 3 ) and tetrachloroethene (C 2 Cl 4 ) produces CCl 4 as a by-product and releases it into the atmosphere. Other possible sources are emissions from the production of chlorine gas or from old landfills. These sources have now actually been confirmed by two new studies by international research teams with measurements in South Korea.

The 35,000 tons of CCl 4 do most probably not originate from illegal factories - at least not entirely. As measurements in South Korea show, around 20’000 tons of undeclared CCl 4 emissions originate from China. A large proportion of the gas can actually be traced back to the production of chlorinated solvents, which is still allowed as long as CCl 4 is not emitted into the atmosphere. However, Chinese emissions have not declined after 2010, which indicates that emissions from this source are on-going. The remaining emissions arise from other countries in Asia, but also from Europe and the USA. According to Empa researcher Reimann, it is therefore crucial to implement technical improvements and better regulatory strategies in order to reduce CCl 4 emissions at factory and process level. In addition, continuous global measurements of ozone-depleting substances would need to be continued to identify sources of substances that threaten the recovery of the ozone layer.

A study in «Nature Geoscience» has revealed that the world-wide emissions of ethane and propane have so far been massively underestimated in global models. Calculations deviate by 50% from the values actually measured in the atmosphere, as researchers have found out. Among other data, measurements from the research station on the Jungfraujoch in combination with analyses from Empa researchers contributed to these findings. There are some theories as to where this previously unknown emission of the two gases comes from: They may leak from gas pipelines or be released when natural gas is extracted, for example through fracking. Even the melting of permafrost and the associated release of gases from formerly frozen soils is a possible source.

SB Dalsøren, G Myhre, Ø Hodnebrog, C Lund Myhre, A Stohl, I Pisso, S Schwietzke, L Höglund-Isaksson, D Helmig, S Reimann, S Sauvage, N Schmidbauer, KA Read, LJ Carpenter, AC Lewis, S Punjabi, M Wallasch; Discrepancy between simulated and observed ethane and propane levels explained by underestimated fossil emissions; Nature Geoscience (2018); doi: 10.1038/s41561-018-0073-02018

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