Nearly every day a stainless-steel flask, about the size of a large soda bottle, arrives at a National Oceanic and Atmospheric Administration (NOAA) laboratory in the American city of Boulder.
The containers, which hail from places as far away as Australia and Antarctica, hold something that isnt typically shipped halfway around the world: air. But this invisible cargo has a higher scientific calling.
Analytical chemist Stephen Montzka and his team at NOAAs 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 whats in them, says Montzka, a senior scientist at the Global Monitoring Lab.
Montzka is on the frontlines of a global effort to track the health of the ozone layer, the invisible shield that protects Earth from the suns ultraviolet radiation.
The work is largely unsung. After reaching a feverish pitch in the 1980s, worries 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 bans, ozone-depleting substances continue to make their way into the atmosphere.
We've noticed the ozone layer hasn't continued to get worse, but things clearly still are not great, says Montzka, a contributing author to a seminal 2022 United Nations-backed report on the state of the ozone layer.
Thats why, he says, checks like those done by his lab are critical. If you don't monitor, you won't be able to understand how things have changed. I have a colleague who asks: If you're on a diet, do you use the scale or not?
The ozone layer hovers 15 to 35 kilometres above the Earths surface in a region known as the stratosphere. The ozone molecules that give the layer its name sop up ultraviolet radiation from the sun, including UV-B. In high doses, this invisible light can damage the DNA of living things, leading to conditions like cataracts and skin cancer. Left completely unchecked, UV-B rays would likely sterilize the surface of the Earth.
So, alarm bells went off in the 1970s whenscientists Sherwood Rowland and Mario Molina released a paper suggesting certain chemicals used in refrigerants, foams and aerosols were damaging the ozone layer. These chlorofluorocarbons (CFCs), it posited, were breaking down when they reached the stratosphere, releasing chlorine gas, which 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 time, scientists discovered that the ozone layer over Antarctica had thinned dramatically, likely because of CFCs. This came to be known as a hole even though, strictly speaking, there was never a spot completely devoid of ozone.
In 1987, the countries of the world adopted the Montreal Protocol, which has since phased out 99 per cent of ozone-depleting substances and helped arrest the ozone layers decline.
Montzka says researchers have in recent years seen a slight hint of ozone increasing around the mid-latitudes a band of the planet, which are between 30 degrees and 60 degrees north and south of the equator. A UN report found that barring a surge in ozone-depleting substances, the 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 best, with countries coming together to solve an environmental crisis, says Megumi Seki, Executive Secretary of the Ozone Secretariat, which provides administrative and organizational services to the Vienna Convention and the Montreal Protocol. But the key thing to remember is this: the ozone layer isnt 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.)
How does the ozone layer work?
Ozone, which is made up of three oxygen atoms, absorbs UV-B radiation from the sun. When an ozone molecule absorbs UV-B, it comes apart into an oxygen molecule (O2) and a separate oxygen atom (O). Later, the two components can come together again to reform the ozone molecule (O3), leaving it ready to soak up other UV-B rays.By absorbing UV-B in the stratosphere, the ozone layer prevents harmful levels of this radiation from reaching Earths surface.
At the NOAA lab in Boulder, the 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 flasks, a process that requires liquid nitrogen.
It smokes and it whistles and it does different things like that, says Montzka, laughing.
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. Usually, the results arent surprising. Ninety-five per cent of the time, the next morning we come in and we say Wow, look 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 Hawaii, weather modelling experts helped researchers track the chemicals to eastern Asia, where 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 important, say experts. Another: once ozone-depleting substances are out of the bag, they cant 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 out, the chemicals continue to affect the atmosphere. The Montreal Protocol allows some CFCs to be used as feedstocks in the production of other chemicals. Meanwhile, ageing appliances, like fridges and air conditioners, still contain CFCs. Some countries and companies are also sitting on stockpiles of old ozone-depleting substances. Without proper management, all of these sources can leak CFCs into the atmosphere.
There are also mounting concerns about nitrous oxide, which is commonly found synthetic fertilizers. The compound is not controlled under Montreal Protocol and its use is rising rapidly, found a 2024 UNEP report.
Still, experts say the ozone layer is in a much better place that it was 40 years ago, largely due to the Montreal Protocol, one 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 problem, it 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 signing, in 1987, of the Montreal Protocol, a landmark agreement to phase out substances that deplete the ozone layer.
