Earlier this year, the United Nations announced that the ozone layer, which shields the Earth from the sun’s harmful ultraviolet radiation, and which was severely depleted by decades of human-derived, ozone-destroying chemicals, is on the road to recovery.

The dramatic turnaround is a direct result of regulations set by the 1987 Montreal Protocol, a global treaty under which nearly every country in the world, including the United States, successfully acted to ban the production of chlorofluorocarbons (CFCs), the main agents of ozone depletion.

As a result of this sustained international effort, the United Nations projects that the ozone layer is likely to completely heal by around the middle of the century.

But a new study led by the Massachusetts Institute of Technology (MIT), published today in the journal Nature Geoscience, identifies a new threat to the ozone layer’s recovery: chloroform — a colorless, sweet-smelling compound that is primarily used in the manufacturing of products such as Teflon and various refrigerants.

The researchers – which include scientists from the University of Bristol – found that between 2010 and 2015, emissions and concentrations of chloroform in the global atmosphere have increased significantly.

They were able to trace the source of these emissions to East Asia, where it appears that production of products from chloroform is on the rise. If chloroform emissions continue to increase, the researchers predict that the recovery of the ozone layer could be delayed by four to eight years.

Co-author Ronald Prinn, the TEPCO Professor of Atmospheric Science at MIT, said: “Ozone recovery is not as fast as people were hoping, and we show that chloroform is going to slow it down further.”

Chloroform is among a class of compounds called “very short-lived substances” (VSLS), for their relatively brief stay in the atmosphere (about five months for chloroform).

Co-author, Dr Anita Ganesan from the University of Bristol’s School of Geographical Sciences, added: “Chlorinated substances which are very short-lived had previously not been considered for regulation because they were expected to be destroyed before reaching the ozone layer.

“But emissions from certain parts of the world such as East Asia can be lofted to the ozone layer very quickly through monsoon systems, bypassing the normal destruction that occurs in the lower atmosphere of other parts of the world.

“So, regulation may need to be considered in the future for these substances because they do have a significant impact on the recovery of the ozone layer.”

Professor Prinn and his colleagues monitor such compounds, along with other trace gases, with AGAGE, the Advanced Global Atmospheric Gases Experiment — a network of coastal and mountain stations around the world that has been continuously measuring the composition of the global atmosphere since 1978.

When Dr Xuekun Fang, Senior Postdoctoral Associate in Professor Prinn’s group at MIT, and lead author of the paper, began looking through AGAGE data, he noticed an increasing trend in the concentrations of chloroform around the world between 2010 and 2015.

Using an atmospheric model, Bristol researchers quantified global emissions, demonstrating that a rapid rise occurred after 2010. Dr Fang observed that most stations in the AGAGE network did not measure substantial increases in the magnitude of concentration spikes in chloroform, indicating negligible emission rises in their respective regions, including Europe, Australia, and the western United States. However, two stations in East Asia — one in Hateruma, Japan, and the other in Gosan, South Korea — showed dramatic increases in the frequency and magnitude of spikes in the ozone-depleting gas.

To investigate further, the team used two different three-dimensional models that simulate the movement of gases and chemicals in the atmosphere. Bristol researchers estimated China’s contribution to global emissions using a model developed by the UK Met Office, which tracks the origins of air before being measured at the stations. The team fed AGAGE data from 2010 to 2015 into the two models and found that they both agreed on chloroform’s source: East Asia.

Dr Fang added: “We conclude that eastern China can explain almost all the global increase. We also found that the major chloroform production factories and industrialized areas in China are spatially correlated with the emissions hotspots.

“And some industrial reports show that chloroform use has increased, though we are not fully clear about the relationship between chloroform production and use, and the increase in chloroform emissions.”

The fact that the rise in chloroform stems from East Asia, where strong monsoon systems transport very short-lived chlorinated substances directly to the ozone layer, adds further urgency to the situation.

Dr Fang and Professor Prinn say that the study is a “heads-up” to scientists and regulators that the journey toward repairing the ozone layer is not yet over.

This research was supported by NASA, the National Institute of Environmental Studies in Japan, the National Research Foundation of Korea, the Department for Business, Energy & Industrial Strategy and the Natural Environment Research Council in the UK, the Commonwealth Scientific and Industrial Research Organization of Australia, and other organisations.

Adapted from a news story by MIT