The healing of the ozone layer could be delayed for 30 years or more by rising emissions of a substance hitherto ignored by environmental regulators. Ironically, its principal use is as a feedstock to make “ozone-friendly” chemicals for air conditioners and refrigerators.
As emissions of CFCs and other ozone-eating chlorine compounds are curbed under the 30-year-old Montreal Protocol, emissions of another chemical called dichloromethane – also known as methylene chloride – have been rising, says Ryan Hossaini of Lancaster University, UK. They now total over a million tonnes a year, and concentrations of dichloromethane in the lower atmosphere have doubled since 2004.
The chemical, a volatile gas, has many uses, including as an industrial solvent and paint remover. The recent growth in its emissions – stemming either from production leaks or deliberate venting – are particularly tied to its increasing role in the manufacture of a hydrofluorocarbon called HFC-32, a widely used replacement for CFCs.
Once in the atmosphere, dichloromethane has an average lifetime of only around five months before it breaks down, releasing chlorine that can destroy ozone if it reaches the stratospheric ozone layer.
Until recently, it was thought that dichloromethane was too short-lived for much of it to reach the stratosphere. So it was not controlled under international treaties such as the Montreal Protocol, introduced after a hole opened up in the ozone layer over Antarctica in the early 1980s.
But that view has been changing. And now Hossaini has modelled the likely impact of a continued rise in dichloromethane emissions. He says that by 2050, it could account for “a quarter of all the chlorine in the lower stratosphere”. The present figure is less than 1 per cent.
Current forecasts are that the Antarctic ozone hole should fill by about 2065, but they ignore dichloromethane. Hossaini says that if those emissions are included, and if they continue to increase at the rate seen since 2004, the hole will not be filled until at least 2095.
Exactly which regions of the world contribute most to the emissions is unclear, says Hossaini. But a major cause of concern is the manufacture of refrigeration systems in Asia.
Analysis of air samples taken by commercial aircraft at the lower edge of the stratosphere show especially high levels of dichloromethane over the Indian sub-continent and South-East Asia during the Asian monsoon, says Emma Leedham Elvidge at the University of East Anglia, UK.
This, she says, is due in part to rapidly rising emissions in the region and in part to the monsoon circulation, which fast-tracks the chemical into the upper atmosphere before it has time to decay. “Our findings show that emissions from the Indian sub-continent have increased two- to fourfold in a decade,” she said.
Hossaini says there are likely to be other human-made sources of chlorine that could delay the ozone layer’s recovery. They include 1,2-dichloroethane, used in the manufacture of PVC. “Long-term atmospheric measurements of this compound are not available at present,” he says. “But sporadic data suggest it is a significant source of chlorine in the atmosphere.”
Journal reference: Nature Communications, DOI: 10.1038/ncomms15962