Inside the effort to track the health of the ozone layer

Nearly every day a stainless-steel flaskabout the size of a large soda bottlearrives at a National Oceanic and Atmospheric Administration (NOAA) laboratory in the American city of Boulder. 

The containerswhich hail from places as far away as Australia and Antarcticahold something that isn’t typically shipped halfway around the world: air. But this invisible cargo has a higher scientific calling.  

Analytical chemist Stephen Montzka and his team at NOAA’s Global Monitoring Lab will feed the air into an instrument known as a gas chromatograph and mass spectrometre. The device can detect minute amounts of chemicals capable of chewing away at the ozone layer. It was this monitoring that detected several years ago unexpected releases of trichlorofluoromethane (CFC-11) – a banned ozone-devouring substance. 

“Little parts of the (atmosphere) come to us each and every day so we can characterize what’s in them,” says Montzkaa senior scientist at the Global Monitoring Lab. 

Montzka is on the frontlines of a global effort to track the health of the ozone layerthe invisible shield that protects Earth from the sun’s ultraviolet radiation.  

The work is largely unsung. After reaching a feverish pitch in the 1980sworries about the “hole” in the ozone layer have receded. But experts like Montzka warn the stratospheric barrier remains in a fragile state. And despite global bansozone-depleting substances continue to make their way into the atmosphere.  

“We've noticed the ozone layer hasn't continued to get worsebut things clearly still are not great,” says Montzkaa contributing author to a seminal 2022 United Nations-backed report on the state of the ozone layer.  

That’s whyhe sayschecks like those done by his lab are critical. “If you don't monitoryou won't be able to understand how things have changed. I have a colleague who asks: ‘If you're on a dietdo you use the scale or not?’”  

The ozone layer hovers 15 to 35 kilometres above the Earth’s surface in a region known as the stratosphere. The ozone molecules that give the layer its name sop up ultraviolet radiation from the sunincluding UV-B. In high dosesthis invisible light can damage the DNA of living thingsleading to conditions like cataracts and skin cancer. Left completely uncheckedUV-B rays would likely sterilize the surface of the Earth. 

Soalarm bells went off in the 1970s when scientists Sherwood Rowland and Mario Molina released a paper suggesting certain chemicals used in refrigerantsfoams and aerosols were damaging the ozone layer. These chlorofluorocarbons (CFCs)it positedwere breaking down when they reached the stratospherereleasing chlorine gaswhich was capable of shattering ozone molecules.  

The research in part spurred countries to adopt the Vienna Convention for the Protection of the Ozone Layer in 1985.  

Around the same timescientists discovered that the ozone layer over Antarctica had thinned dramaticallylikely because of CFCs. This came to be known as a “hole” even thoughstrictly speakingthere was never a spot completely devoid of ozone.  

In 1987the countries of the world adopted the Montreal Protocolwhich has since phased out 99 per cent of ozone-depleting substances and helped arrest the ozone layer’s decline.  

Montzka says researchers have in recent years seen “a slight hint” of ozone increasing around the mid-latitudes a band of the planetwhich are between 30 degrees and 60 degrees north and south of the equator. A UN report found that barring a surge in ozone-depleting substancesthe barrier will likely recover to 1980 values by around 2066 over Antarctica and by 2040 for much of the rest of the world. 

“The Montreal Protocol is a sterling example of multilateralism at its bestwith countries coming together to solve an environmental crisis,” says Megumi SekiExecutive Secretary of the Ozone Secretariatwhich provides administrative and organizational services to the Vienna Convention and the Montreal Protocol. “But the key thing to remember is this: the ozone layer isn’t out of the woods yet. We need to remain vigilant to ensure ozone recovery stays on track.” (The UN Environment Programme (UNEP) is the secretariat of the ozone treaties.)  

At the NOAA lab in Boulderthe air sensing instrument – technically known as a gas chromatograph and mass spectrometer – rests on a desk. Montzka had to modify the device so researchers could pump in pressurized air from the metal flasksa process that requires liquid nitrogen. 

“It smokes and it whistles and it does different things like that,” says Montzkalaughing.  

The device is sensitive: it can detect one molecule of an ozone-depleting substance in a hundred trillion molecules of air. 

Montzka and his team process around 10,000 samples a year from two dozen locations around the world. Usuallythe results aren’t surprising. “Ninety-five per cent of the timethe next morning we come in and we say “Wowlook at that. We're great.’” 

But starting in 2013 Montzka and his colleagues began to find elevated levels of as CFC-11 in air samples from the Mauna Loa volcano in the American state of Hawaii. The use of CFC-11 was supposed to have been phased out under the Montreal Protocol.  

From Hawaiiweather modelling experts helped researchers track the chemicals to eastern Asiawhere air monitoring stations identified their origins to be China. The discovery led to what one UNEP report described as a crackdown by China on the illegal uses of CFC-11.  

The discovery of the elevated CFC-11 emissions was a prime reason why atmospheric monitoring is importantsay experts. Another: once ozone-depleting substances are out of the bagthey can’t be put back in. 

“The chemicals causing the problem (are) only removed from the atmosphere by natural processes,” says Montzka. “And that process is very slow. They don't disappear on a time scale of days or weeks or even years. It's decades to centuries.” 

While most ozone-depleting substances have been phased outthe chemicals continue to affect the atmosphere. The Montreal Protocol allows some CFCs to be used as feedstocks in the production of other chemicals. Meanwhileageing applianceslike fridges and air conditionersstill contain CFCs. Some countries and companies are also sitting on stockpiles of old ozone-depleting substances. Without proper managementall of these sources can leak CFCs into the atmosphere. 

There are also mounting concerns about nitrous oxidewhich is commonly found synthetic fertilizers. The compound is not controlled under Montreal Protocol and its use is rising rapidlyfound a 2024 UNEP report. 

Stillexperts say the ozone layer is in a much better place that it was 40 years agolargely due to the Montreal Protocolone of the rare treaties to be ratified by every UN Member State.  

Montzka says the agreement demonstrates the importance of science in addressing global environmental problems and the power of international cooperation. 

“If we as a global community had put our head in the sand back in the 1980s and ignored this problemit would be a much different world today.” 

On 16 September the world marks the International Day of the Preservation of the Ozone Layer. It commemorates the date of the signingin 1987of the Montreal Protocola landmark agreement to phase out substances that deplete the ozone layer.