Emissions of short-lived chlorine-based chemicals that deplete ozone are increasing worldwide. But over some regions of Asia, these chemicals may be on a fast track to the ozone layer
October 27, 2017 — A rise in the emission of short-lived chlorine-based chemicals over the past decade has created a possible new threat to the health of Earth’s protective, yet fragile, ozone layer.
A recent study shows that human-made emissions of short-lived chlorocarbons, chemicals not regulated by the internationally ratified Montreal Protocol, have increased in a region of the atmosphere where air from the surface is thought to get rapidly pumped into the upper troposphere. These chlorocarbons, once further lofted into the stratosphere, may eat away at stratospheric ozone.
Just where are these chlorocarbons coming from? From industrial and agricultural processes in Southeast Asia, particularly in China, explained David Oram, a research fellow at the National Centre for Atmospheric Science at the University of East Anglia in Norwich, U.K. Oram and his team published these results in Atmospheric Chemistry and Physics on 12 October.
Scientists have noticed that atmospheric concentrations of short-lived chlorocarbons, particularly dichloromethane and 1,2-dichloroethane, have been on the rise worldwide since 2003. This increase was a surprise to the scientific community, as it came after nearly a decade of slowly declining concentrations. Oram’s team suspected that the sharp rise in pollution could be coming from nations with rapidly developing industries, and they sought to test that.
The researchers collected air samples at surface-level research stations in Malaysia and Taiwan. Chemical analysis of those samples revealed dichloromethane and 1,2-dichloroethane concentrations more than 20 times higher than expected from previous reports.
These levels were concerning, Oram explained. Short-lived chlorocarbons were recently recognized as some of the most effective ozone depleters.
Oram’s team next sought to pinpoint just where these pollutants were coming from and where they might be going. To do so, they collected additional air samples at altitudes of 10–12 kilometers across Southeast Asia using Europe’s In-service Aircraft for a Global Observing System’s Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC). The aircraft is specially designed to take high-altitude atmospheric samples. The team’s upper troposphere samples came from several CARIBIC flights spanning Southeast Asia from Kuala Lumpur, Malaysia, to as far west as Karachi, Pakistan.
Within those high-altitude air samples, the team found tropospheric concentrations of dichloromethane and 1,2-dichloroethane up to 3 times higher than expected and “in a region where air is known to be transferred into the stratosphere,” Oram said.
For the last piece of the puzzle—where the pollutants originated—the researchers used computer simulations to backtrack where air over Malaysia is pulled from. The simulations indicated that the pollutants were likely emitted 1–2 weeks earlier in continental Southeast Asia.
The team then compared the predicted emission dates and locations to publicly reported industrial emissions data from the region. These data allowed them to quantitatively link the significantly increased chlorine concentrations to industrial growth in China from 2000 onward.
Dichloromethane and 1,2-dichloroethane are frequently used in human-controlled processes like paint stripping, agricultural foam blowing, solvent and degreasing applications, and polyvinyl chloride (PVC) manufacturing. The researchers estimated that China may be responsible for around 50%–60% of current global emissions of these two chlorocarbons.
Holes in the Montreal Protocol
The Montreal Protocol on Substances That Deplete the Ozone Layer, which took effect on 1 January 1989, has been successful in making large reductions in the emission of chemicals that damage stratospheric ozone. Research conducted in the past decade has shown that the ozone layer has begun to heal in the time since the Montreal Protocol began regulating emissions of anthropogenic halogen-based chemicals containing fluorine, chlorine, or bromine, which are the most damaging to ozone.
However, the Montreal Protocol does not regulate the emission of short-lived halocarbons because in small doses they have a negligible effect on the health of the ozone layer compared to longer-lived species. At the time the Montreal Protocol took effect, emissions of the chlorocarbons examined by Oram’s team were low enough that scientists did not think that they caused much damage in their less than 6-month life span.
But now, this research calls into question past assumptions. Could high emissions of short-lived chlorocarbons over Asia have lasting consequences? At the very least, “these chemicals are slowing down the decline in atmospheric chlorine abundance,” said Oram. From 2008 to 2012, chlorine concentrations declined by an average of 0.4% per year, slower than the 0.6% decline from 2002 to 2004.
The researchers also found that the short-lived chlorocarbons’ potential effect on ozone largely depends on where those compounds originate geographically. Put a different way, Asia’s chlorocarbon emissions occurred in just the right spot for a quick trip to the stratosphere.
Oram explained that in areas of the globe that do not have quick atmospheric pathways from the surface to the stratosphere, short-lived chlorocarbons can do little damage. But scientists have long suspected that atmospheric patterns created fast tracks to the stratosphere above the Indian subcontinent and western Pacific Ocean, tropical regions with strong atmospheric convection and vertical uplift.
Chlorocarbons that find their way to these fast tracks, Oram added, could reach the upper troposphere in approximately 10 days, well within the chemicals’ life spans. By contrast, “emissions of short-lived compounds in, for example, North America or Europe are potentially less harmful as it typically takes air much longer to find its way to the tropics,” he explained.
A Short-Lived Threat or Nonexistent?
Because the chemicals are mostly anthropogenic, the researchers argue that humans can and should begin to control production of short-lived chlorocarbons and their emission into the atmosphere, especially in areas like Southeast Asia that have a mechanism for rapid stratospheric transport.
“Short-lived chlorocarbons have been generally overlooked in terms of ozone loss in recent years,” said University College London atmospheric chemist David Rowley, who was not involved in the study. “This was wrong.”
But although the research shows that short-lived chlorocarbons make it to an upper tropospheric region that pulls air into the stratosphere, the fate of these chemicals in the stratosphere is unknown. As a result, some scientists are not convinced that these chlorocarbons pose much of a threat once the chemicals actually reach the stratosphere.
“The measurements report dichloromethane at an altitude of 10–12 kilometers—this is still the troposphere,” said Susan Strahan, who researches atmospheric transport processes at NASA Goddard Space Flight Center in Greenbelt, Md., and also did not participate in this research. “In the additional weeks required to travel to the lower stratosphere, which is above 16 kilometers, even more of the compound will be destroyed.”
Still, Oram believes that their results “are highlighting a gap in the Montreal Protocol that may need to be addressed in the future, particularly if atmospheric concentrations continue to rise.” He believes that “these chemicals should potentially be considered for inclusion in the list of ozone-depleting substances controlled by Montreal Protocol.”