Our Changing World: Why we are still monitoring the ozone hole

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From the bright red bottles stacked in the corner against a cement wall, hydrogen gas is flowing into a giant white balloon. It's cheaper and lighter than helium, so as the balloon fills it begins to float off the ground, tethered by a weight to prevent an early escape.

The two scientists handling it are wearing orange suits, masks and eyeglasses - precautionary protective equipment when handling this large amount of inflammable gas.

It's an odd sight against the rural setting of Central Otago. Cows moo in the background, a pair of paradise ducks argue as they waddle by, and birds sing loudly in the sun.

But it's the perfect place for investigating the ozone layer.

Sending a balloon to the stratosphere

When released, the balloon will float upwards, through the troposphere - the name for the part of the atmosphere closest to Earth - and up to the stratosphere, a layer 15-50 kilometres above us.

At about 35 kilometres, the reduced pressure on the balloon will force it to expand and pop. A bright orange parachute guides its package back down - though only about 20 percent are recovered, as most fall into the ocean.

The package contains an ozonesonde - an instrument that can sample air and detect the presence of ozone. As it drifts upwards under the balloon, it sends data back to NIWA's Atmospheric Research Station just outside the small village of Lauder - where we began filling the balloon with hydrogen.

This isn't the only way that ozone is measured here. "We're known as a bit of an ozone super site," says Dr Richard Querel.

Lauder is part of the Detection of Atmospheric Composition Change (NDAC). Within that network there are five recognised techniques for measuring ozone profiles, and Lauder has all five of them. The data collected here feeds back into that network, for scientists around the world to access and use.

Ozone assessment also takes place via satellite - but ozone measurements such as the ones made here help ground-truth that data.

How ozone helps

The stratosphere, the balloon's ultimate destination, is where you find the ozone layer. This is not the only place where you find ozone. It can also be found in the troposphere, especially near dense cities, where it is a pollutant.

But the bulk of it is in the stratosphere, where it plays an important role for us by absorbing high-energy ultraviolet (UV) rays.

Ozone is a molecule of oxygen gas (O2) with an extra oxygen atom added - so three oxygen atoms bound together (O3). Ozone forms when a type of UV light hits oxygen in the stratosphere. Then, if ozone absorbs a different kind of UV light, it can break apart. This natural ozone-oxygen cycle is what protects us from all UV-C light, and most UV-B.

However, when we started adding certain chemical compounds to the mix, it upset this cyclical balance.

In the 1920s a mechanical engineer working at General Motors called Thomas Midgley Jr created a new chemical compound - a chloroflurocarbon (CFC) he named Freon. It was non-toxic, non-flammable and seemed stable - perfect for use as a refrigerant.

The production and use of different CFCs spread rapidly - not just in fridges, but also air conditioners, some types of foam, and aerosols.

Unfortunately, these CFC gases are just the right weight to float up to, and linger in, the stratosphere where they act as an ozone destroyer.

What's going on with the ozone hole?

It's because of these long-lasting CFCs that an ozone hole opens up each year above Antarctica. The "hole" is not actually completely ozone-free. It's an area of very little ozone below a certain threshold.

This thinning happens at around the same time every year. It needs three factors to get going, explains Dr Olaf Morgenstern.

First, you need polar stratospheric clouds - which you only get if you have really cold temperatures (-80 °C). You get these cold temperatures with a strong polar vortex (a strong wind that circles around the pole).

Second, you need CFCs (or other ozone-depleting chemicals). And third, you need sunlight. This means the ozone hole only begins to form after the sun rises in Antarctica in spring.

In terms of the ozone hole's size, there are two things to think about: how long it is open for, and how big it is.

This year's ozone hole formed a little later than usual, due to a weaker than normal polar vortex. But size wise it's about 'normal'.

In 2023, we saw a large and long-lasting ozone hole.

In 2020 we experienced a large one too, and, unusually, a second hole opened up above the Arctic as well.

The annual variations between the size and duration of the ozone hole are one reason why it's tricky to track the general trend.

And the presence of CFCs, or other ozone-depleting chemicals, isn't the only thing that impacts ozone either. The 2019-2020 Australian bushfires launched particles into the stratosphere and appeared to have a depleting effect. Climate change gases that we are emitting also alter the oxygen-ozone cycle in different ways.

Our increased activity in space might even have an impact, with both rocket launches and debris from satellite re-entry of concern.

In general, the ozone trend appears to be good. The CFCs do hang around a long time, but there are suggestions that the ozone hole might be a thing of the past by the late 2060s.

However, some analysis of satellite measurements carried out by researchers at the University of Otago has highlighted the need for a watchful eye. Their results suggest that in some parts of the stratosphere, the hole may be getting worse - potentially due to the climate changes gases.

Meanwhile at the NIWA Atmospheric Research Station in Lauder, now in its 63rd year, the weekly work of ozone measurement continues.

Thanks to Ngā Taonga Sound & Vision for use of recordings from the 1980s and 1990s used in this episode.