Four New Ozone Depleting Substances Detected in the Atmosphere

Ozone-depleting substances (ODSs) are synthetic chemicals partially responsible for the degradation of the protective layer of ozone in the stratosphere.¹ As this layer of ozone is instrumental for blocking harmful UV rays emitted from the sun, its depletion due to anthropogenic sources has been a primary environmental concern since the discovery of this problem in the late 1970s.  The synthetic chemicals responsible for ozone depletion (chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, etc.) notably contain carbon-halogen bonds that can undergo photodissociation to extrude atomic halogens, which subsequently catalyze the decomposition of ozone into elemental oxygen.² Thus, the Montreal Protocol on Substances that Deplete the Ozone Layer (MP) was enacted in 1987 through the Vienna Convention for the Protection of the Ozone Layer. The MP has led to the drastic decrease in the emissions of chemicals that cause ozone depletion.¹

In his NBC new article,³ Alister Doyle reports on the discovery of four new industrially-produced gases that are linked to ozone depletion. In his effort to summarize the results originally reported in Nature Geoscience, Doyle neglects to name exactly which new gases (CFCl₂CFCl₂, CF₂ClCCl₃, CF₃CCl₃, CF₃CH₂Cl) have been detected. This is arguably an important detail of the report that has been excluded presumably due to the general audience being targeted through this media source. Despite this lack of specific information, Doyle does provide a reasonable explanation of how the new gases were detected in ice cores harvested in Greenland and Tasmanian air samples. Doyle is also clear that the scientists reporting these data are unsure of how these new emissions are being produced and that these new emissions exist in low enough concentrations in the atmosphere that they are not yet detrimental to the ozone layer. 

J. C. Laube et al. disclose in Nature Geoscience the detection of three new chlorofluorocarbons, (CFCs = CFCl₂CFCl₂ [a], CF₂ClCCl₃ [b], CF₃CCl₃ [c]) and one new hydrofluorocarbon (HCFC = CF₃CH₂Cl [d]).⁴ Laube and co-workers were able to detect these new gases by analyzing air samples collected in Cape Grim, Tasmania (Southern Hemisphere) from 1978 to 2012 as well as deep firn snow samples collected in Greenland (Northern Hemisphere) in 2008. I enjoyed reading about the comparison of these two data collection methods, and was intrigued about  how the levels of the CFCs and the HCFCs varied from one site to another. Interestingly, these four new gases were documented in higher concentrations in the Greenland location than the Tasmania location. The authors claim this is a result of there being more industrialized countries in the northern hemisphere as opposed to the southern hemisphere. Furthermore, they claim that this result suggests entirely anthropogenic sources of these compounds as they are not detectable in air before 1960. Interestingly, compound c has grown in abundance from the 1960s, when it was first detected, to 2012. This is particularly intriguing as its behavior is the opposite of that of its isomer, CF₂ClCFCl_2, which has been decreasing in concentration for decades. The authors nicely highlight the importance of atmospheric lifetime for these CFCs and HCFCs as atmospheric lifetimes are necessary in estimating the beginning of their global emissions. The authors then discuss the stratospheric measurement of the ozone depletion potential, which represents the global ozone loss due to the release of a particular molecule relative to a reference CFC. Importantly, with these measurements they find that the new CFCs are comparatively dangerous to the ozone layer, although they are not yet present in large enough concentrations to begin having measurable detrimental effects. Interestingly, with the lifetime estimation of these CFCs in combination with a two-dimensional chemical transport model, the authors were able to infer global emission of each of the newly reported chemical compound. Currently, it is unknown where these new CFCs and HCFCs are being produced because of caveats with the Montreal Protocol in addition to officially-granted uses of previously-banned CFCs. Specifically, there are no clear regulations on some intermediates generated during the production of certain chlorofluorocarbons, which could lead to the detection of these new gases. The article concludes by highlighting the limitations of the Montreal Protocol and current research focused on determining the source of these new emissions.

While Alister Doyle does fail to identify the four substances that have been measured in the atmosphere, I don't think this is a significant drawback. This is an NBC News article and as such I don't think the general readership is geared toward those with advanced science degrees. With that said, an active link to the original Nature Geoscience article is included in this short publication, which is greatly appreciated.  Overall, I would rate Alister's article as a 7.5/10. I was able to gather all of the general information out of the NBC News article I needed. Further, I feel the many direct quotes from J. C. Laube validate the scientific merit of this popular media article. However, Doyle begins the article by seeming to make the claim that the Montreal Protocol of 1987 is comprehensive and  clear bans are in place for all CFCs and HCFCs. This is a clear overstatement as rightfully pointed out by Laube toward the end of the Nature Geoscience article where it is stated that there are "many caveats in the Montreal Protocol"  as well as "it might be worth reconsidering its reporting regime, including the differentiation of isomeric forms." I feel that certain phrases such as "industrial gases" could be left out by Doyle, as phrases like this tend to carry negative connotations.  Until we know exactly where these gases are coming from we maybe shouldn't be generalizing these gases as purely "industrial."

1. R. W. Portmann et al. Science 2009, 326, 123.

2. E. Wolff et al. Atmos. Chem. Phys. 2007, 7, 4375.

3. A. Doyle,"Four New Ozone-Killing Gases Detected: Scientists Look for Source," NBCNews.com: March 9, 2014. [NBCNews Article]

4. J.C. Laube et al. Nat. Geo. 2014, 7, 266.
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