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Stratospheric threat: future-proofing the climate

Dichloromethane is the latest industrial chemical to be seen as a major risk to the health of the ozone layer Dichloromethane is the latest industrial chemical to be seen as a major risk to the health of the ozone layer
25 Nov
The discovery of increasing levels of ozone-depleting compounds being emitted into the atmosphere puts pressure on the need to expand the Montreal Protocol

Despite the high-profile banning of chlorofluorocarbons (CFCs) by the Montreal Protocol 30 years ago, the conveyer-belt of ozone-depleting chlorine compounds being emitted into the atmosphere continues to whirr. The latest to be added to the list include dichloromethane, used in paint stripping, agricultural fumigation and, in the manufacturing process of various pharmaceuticals and 1,2-dichloroethan, used in the production of PVC.

Emissions of these chemicals steadily decreased during the 1990s and early 2000s, as they gradually faded from manufacturing in Europe and North America. The past decade, however has seen a gradual and significant rise in their atmospheric presence. Given the nature of 21st century manufacturing, China has now become the largest emitter of these compounds, alongside other Asian nations with growing, manufacturing-heavy economies.

Unfortunately, it’s also a particularly damaging part of the world for such compounds to be released. Due to a geographical quirk, a pattern of cold surges capable of quickly transporting industrial pollutants south from Asia has resulted in large quantities of these compounds being found at very high altitudes in the tropics, enabling them to be potentially transferred into the stratosphere, where ozone depletion can occur.

‘These chemicals have lifetimes of three or four months in the atmosphere,’ explains David Oram, from the University of East Anglia. ‘If you release something with an atmospheric lifetime of, say, four months at 50 or 60-degrees north, by the time it gets down to the tropics – particularly to these regions of the tropics where the majority of stratospheric exchange takes place – because of their short lifetimes, a lot of it will have reacted away, broken down lower in the atmosphere before they get the chance to get into the stratosphere.’ Therefore, despite general awareness of these compounds for decades, it has always been anticipated that they would contribute only a small amount of chlorine to the stratosphere.

This new rise of emissions, however, is seeing a far faster process at work, making the short lifespan less significant. ‘They’re transported very quickly into the tropics as part of the winter monsoon mechanism,’ says Oram. ‘Once they’re in that Western Pacific-South China Sea region, that’s where you get some very strong convection. It’s one of the regions on Earth where you get material going into the stratosphere regularly.’

The Montreal Protocol was updated in Kigali, Rwanda, last year to add climate change-causing hydrofluorocarbons (HFCs) to its remit, and Oram expects compounds such as dichloromethane to now be up for inclusion in a similar fashion. ‘The issue of chlorinated short-lived compounds is certainly in that domain now,’ he argues. ‘The scientists are very aware of it. The next international Scientific Assessment of Ozone Depletion is currently being written and is due to be published next year. So it could well be that following the success of HFCs, they will now move on to consider short-lived compounds like dichloromethane.’

This was published in the December 2017 edition of Geographical magazine.

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