CFC-11 is a chlorofluorocarbon (CFC). It used to be used in refrigerators and insulating foam for houses. It’s like CO₂ a greenhouse gas. In the course of the twentieth century, it was also found to be harmful to the ozone layer. In the stratosphere, CFCs disintegrate under the influence of UV light, after which the remnants of ozone gas (O₃) breaking down.

International treaty

Because CFCs deplete the ozone layer, the United Nations drafted the Montreal Protocol in 1987, after which global production was curtailed. The ozone layer is located at an altitude of between 10 and 40 kilometers and blocks the most harmful part of UV radiation from the sun. This radiation is harmful to plants and animals and can cause skin cancer. Above the South Pole in particular, an ‘ozone hole’ develops every year between September and December, because low temperatures and the formation of clouds high in the atmosphere stimulate the reaction with CFCs.

After the legally binding treaty, emissions of harmful gases decreased, allowing the ozone layer to slowly recover. That takes a long time, because the gases remain in the atmosphere for decades and CFCs from old materials still ‘leak’ into the atmosphere. But when after 2012 measurements in the atmosphere showed that the concentration of CFC-11 decreased less rapidly than expected, there had to be a new – illegal – production source. Researchers pointed to China, based on measurements of the atmosphere in remote places outside of China. If those new releases had continued for longer, the ozone layer’s recovery would slow down.

Air measurements

That now seems to have happened. In 2019, CFC-11 emissions were back to their previous levels, according to new measurements. 60% of this is due to reduced emissions in China. Researchers measured the concentration of the gas in the air. They did this in remote places around China to measure regional emissions, but also further afield – for example, on Hawaii’s Mauna Loa volcano, at an altitude of 3,400 meters.

“The international community has acted quickly and measures have been taken in China to stop these emissions,” says Guus Velders. He is professor of air quality and climate interactions in Utrecht and also affiliated with RIVM. “The main message is that measurements in the atmosphere are essential to verify environmental treaties. This is a lesson for the Paris Agreement. Emission reporting is important, but without independent measurements, we just have to believe in the blue eyes of the countries.”