June 27, 2017

expert reaction to study looking at dichloromethane and the ozone layer

Publishing in Nature Communications researchers report that levels of dichloromethane – chemicals previously assumed not to be a threat to the ozone layer – have increased in concentration in the atmosphere, and authors suggest they may be contributing to ozone depletion.

Dr Paul Young, Lecturer and Atmospheric Scientist, Lancaster Environment Centre, Lancaster University, said:

“The control of the ozone depleting substances by the Montreal Protocol, and the subsequent healing of the ozone layer is a great environmental science and environmental policy success story.  By preserving the ozone layer, the Montreal Protocol has avoided catastrophic ozone loss and large surface UV increases that would have precipitated human and ecosystem health disasters.

“But there is always devil in the detail.  This timely study points to the ozone-destroying potential of reactive, halogen-containing compounds from industrial sources that weren’t thought to be a threat to the ozone layer, and whose emissions are not controlled under the Montreal Protocol.  While these compounds will not reverse the trend of ozone layer healing, their unchecked growth could result in a substantial delay in its recovery.  Depending on how these emissions grow, recovery could be delayed decades beyond our previous estimates of the mid/end of this century.

“The Montreal Protocol has proven itself to be an effective mechanism for controlling the emissions of ozone depleting substances, and I imagine that the concerns raised by this study will be high on the agenda for the next Meeting of the Parties.  This study is an excellent start in quantifying the risk from these emissions, and the next step will be to quantify their impacts in a range of global models in order to better guide the policy process.”

Dr Grant Allen, Atmospheric Physicist, University of Manchester, said:

“This work has been carried out by an expert atmospheric modelling team and is based on sound global measurement datasets that have robustly detected an increasing trend in atmospheric concentrations of dichloromethane.  This important work shows that globally-averaged concentrations of dichloromethane have almost doubled since 2004 with important consequences for the recovery of the ozone layer, which helps to  protect us from harmful UV radiation.

“The Montreal protocol, which curtailed the global use of a range of CFCs since the 1980s, acted to successfully reduce human-induced damage to the ozone layer.  This new finding shows that this policy success could be undone if sources of this ozone-destroying gas are not identified and stopped immediately.  The work identifies India and Southeast Asia as potential source areas, which should be prioritised for further measurement studies.

“Whatever the source (or sources) of this gas, which have yet to be properly identified, we must act now to stop its release to the atmosphere in order to prevent undoing over 30 years of exemplary science and policy work which has undoubtedly saved many lives.  Scientists must now work to identify the sources of this gas and engage policymakers to mitigate emissions to prevent new damage to the ozone layer.”

Dr David Rowley, Senior Lecturer and Atmospheric Chemist, UCL, said:

“Chlorofluorocarbons, also known as CFCs, have long been known to cause the destruction of the ozone layer which protects the surface of the planet from dangerous ultraviolet (UV-C and UV-B).  They are able to do so precisely because they are exceptionally chemically inert and therefore have very long lifetimes in the atmosphere.  As a result they survive unchanged for years, eventually reaching the stratosphere where they are broken down by ultraviolet light releasing chlorine atoms that catalyse ozone destruction – each chlorine atom is capable of breaking down hundreds of ozone molecules.  This understanding led to the highly effective Montreal Protocol of 1987 that bans the use of CFCs from industrial use.

“However, shorter lived and less exotic chemical compounds have flown under the radar.  Although widely used as industrial solvents and in everyday applications like degreasing and dry-cleaning, other chlorocarbons had been assumed to break down too quickly in the lower atmosphere to reach the stratosphere.  The prevailing view of the atmospheric community has therefore been that substances with short lifetimes did not present an impact to stratospheric ozone.

“This paper by Pyle and coworkers suggests otherwise.  This is superb, well considered research, using the best global stratospheric forecasting model available, SLIMCAT.  The conclusions are well backed up by some observational data, but of course a number of the extrapolations don’t have data – however, a sensible range of scenarios are addressed.  It needs to be noticed that any extrapolations are in their very nature speculative, based upon projected emissions of in this case dichloromethane (DCM), CH₂Cl₂.  Nonetheless, the interpretations of such possible emissions are concrete and the sensitivity of implications to emission scenarios is considered.  It is clear that the potential for DCM to affect the global ozone budget is profound.  It remains to be seen if the emission scenarios considered encompass what reality might deliver.

“The study specifically identifies dichloromethane (DCM), the simplest and most widely used of these short-lived molecules, as a serious potential contributor to ozone destruction.  Dichloromethane is used in things like degreasers and paint strippers – it is a common and cheap solvent.  This study reports that direct measurements of its concentration in the Northern and Southern hemispheres show its concentration to be rising very steeply and for growing quantities to be leaking into the stratosphere.

“The threat from this molecule is manifold.  If dichloromethane reaches the stratosphere, it is exceptionally effective in destroying the ozone.  At the same time, in the lower atmosphere, it also presents a threat to how the atmosphere can clean itself, as it is reduces the so-called ‘oxidising capacity’ which governs the abundance of pollutants, notably greenhouse gases like methane and the HFCs.

“A striking point about the ozone hole has been that in spite of the success of the Montreal protocol, the speed of recovery of the ozone layer has been less than had been anticipated.  This paper is important in identifying much more common chemical compounds as serious global threats to our atmosphere, and calculates their potential consequences.  The results are significant, across a range of scenarios, since they imply a significant reversal of Montreal-led reductions in total stratospheric chlorine, and the consequent effects on ozone.  It also implies that we need to take a closer look at the impact of a much wider range of molecules containing halogens – especially chlorine and bromine – to understand more fully the impact of our activities on our world.  It also suggests that protecting the ozone layer presents a much greater industrial and political challenge than previously thought.  Because of their much wider range of uses, updating the Montreal Protocol to include solvents like DCM can be expected to be far more arduous and controversial than for CFCs.”

* ‘The increasing threat to stratospheric ozone from dichloromethane’ by Ryan Hossaini et al. published in Nature Communications on Tuesday 27 June 2017. 

Declared interests

Dr Paul Young: “Co-author of WMO Scientific Assessments of Ozone Depletion, 2014 and 2018.  The lead author of this study is a colleague at Lancaster, and I am a Co-Investigator on submitted grant of his.”

Dr Grant Allen: “I don’t have any interests to declare in the context of this study.”

None others received.