A mysterious rise in the emissions of an ozone layer-destroying chemical, trichlorofluoromethane (CFC-11), has been traced back to eastern mainland China, according to a paper published in the journal Nature.

Atmospheric CFC levels have been declining since the mid-1990s, when they were phased out as part of the Montreal Protocol Agreement. Most emissions since then have been due to CFCs leaking from “banks” – buildings and products that were produced with CFCs before the ban.

However, this decline slowed in 2013 and international researchers detected a mysterious rise in emissions of CFC-11, which was used in “foam blowing” – a process used to insulate buildings and other products such as refrigerators.

To trace the origins of this rise, they gathered emissions data from monitoring stations across the globe, including Australia, the US, Europe, Japan and South Korea, and found the emissions came from around the Shandong and Hebei provinces in China. {%recommended 574%}

Study author Paul Fraser, from Australia’s CSIRO Climate Science Centre, says the emissions are “likely due to the new production of insulating foams used in buildings, which is not permitted under the Montreal Protocol”.

The emissions from the Chinese provinces account for at least 40 to 60% of the increase, according to the authors, and were 10 times higher than CFCs released from all fridges disposed of in China between 2014 and 2017. They were equal to emissions predicted for building demolitions for the entire world over 20 years.

This indicates the emissions are due to new, unrecorded production and use of CFCs, the authors say, and could not simply be due to CFCs leaking from existing sources.

CFC-11 is very stable in the lower atmosphere but rises to the stratosphere, 30 kilometres or so above Earth’s surface, “where it is smashed up by radiation,” says Ian Rae from Australia’s University of Melbourne.

“The resulting chlorine atoms cause destruction of ozone molecules in the ozone layer that stops dangerous ultraviolet radiation reaching Earth’s surface.”

Rae says the problem with CFCs is that they’re effective at what they do, so rogue users of old supplies occasionally try to get away with using them again.

“Regulated production and uses of ozone-depleting substances in China, for example, have been phased out with assistance from the multinational fund that developed countries have provided,” he says.

“However, regulators like China’s EPA and some in other countries have found it difficult to deal with the rogues.”