The newly discovered ozone hole is located over the tropics and has gone undetected for decades.

Everyone has heard of ‘the hole in the ozone layer’. During the spring season in the southern hemisphere, the famous ‘ozone hole’ – which is really a continuous thinning of the ozone layer in the stratosphere – forms every year over Antarctica. But apparently this is not the only one. A Canadian researcher has discovered another immense ozone hole over the tropics. A ‘worrying find’, according to discoverer Qing-Bin Lu.

Newly discovered ozone hole

The tropical ozone hole is actually located at an altitude of 10 to 25 kilometers, at latitudes of 30 degrees south to 30 degrees north. And this newly discovered ozone hole is of unprecedented proportion. This turns out to be no less than seven times larger than its well-known Antarctic brother.

The size of the newly discovered hole in the ozone layer over the tropics revealed in part by observations of annual changes in ozone levels. Image: Qing-Bin Lu

According to the researchers, this tropical ozone hole has probably been around since the 1980s. This means that it has gone undetected for decades.

unnoticed

The discovery of the new ozone hole comes as a big surprise. The question is therefore why the tropical ozone hole has not been noticed before. “First, such an ozone hole is not predicted based on current photochemical models,” Lu explains in conversation with Scientias.nl to. “Second, the tropical ozone hole – unlike its Antarctic counterpart – remains the same throughout all seasons, making it difficult to detect.”

Definition

But those aren’t even the only explanations. It is also partly due to how we define an ozone hole. “One speaks of an ozone hole when the total ozone concentration falls below 220 DU (Dobson Unit),” says Lu. “However, this definition does not take into account the normal ozone concentration in a given area. The ozone concentration may have decreased by as much as 60 percent, if it subsequently remains above 220 DU, the ozone loss is not noticeable in the data. Therefore, in our study, we propose a new definition. And that reads: an area in which the depletion of ozone is greater than 25 percent relative to the original ozone concentration. And with this new definition, the tropical ozone hole – along with the Antarctic ozone hole – is clearly visible in the data sets.”

Differences and similarities

Both ozone holes are very similar in many ways. “They both have the same depth,” Lu sums up. “Especially in the centre, 80 percent of the normal ozone concentration has disappeared. In addition, both ozone holes can be associated with the amount of ozone-depleting chemicals – called chlorofluorocarbons or CFCs – in the atmosphere and were similarly conceived.” And even the discovery of both ozone holes is similar. “Both discoveries were unexpected from photochemical theory,” Lu said. “In addition, both were first spotted using ground station measurements.”

However, there are also differences. As mentioned, the newly discovered tropical ozone hole is much larger than its Antarctic brother and is not only present in the spring, but during all seasons. The latter can be easily explained. “Both ozone holes have different stratospheric conditions and climates,” said Lu. “In the case of the tropical ozone hole, this leads to much more ozone being destroyed and thus making appearances all year round.”

New theory

Lu can also explain that the tropical hole is so large – and that its Antarctic brother was exceptionally large and deep in 2020 and 2021 as well – while the use of harmful CFCs is banned. It is generally believed that the depletion of the ozone layer is caused by the release of CFCs, which rise to the stratosphere where they are broken down by the ultraviolet radiation from the sun. This releases chlorine atoms that destroy ozone molecules. But Lu has an alternative theory. According to this theory, harmful cosmic rays also have a finger in the pie. This theory is also called the cosmic-ray-driven electron reaction (or, in Dutch: ‘an electron reaction driven by cosmic rays’).

cosmic rays

According to Lu, the recent upsurge in cosmic rays is why both the Antarctic and the tropical ozone holes are unusually large. “Due to the spike in cosmic rays in the recent solar cycle, maximum ozone holes were observed in 2020 and 2021, while the amount of CFCs in the atmosphere has been reduced thanks to the Montreal Protocol,” said Lu. “In short, without the effect of cosmic rays, both the Antarctic and tropical ozone holes would be much smaller by now, although it will probably take about 10 years longer for the much larger tropical ozone hole to recover.”

Concern

According to the researcher, the discovery of the new ozone hole is quite worrying. The ozone layer protects us against harmful UV radiation and is therefore very important. But with such a large hole, more UV radiation hits the surface. “The tropics make up about 50 percent of the Earth’s surface, where about half of the world’s population lives,” explains Lu. “The hole in the tropical ozone layer is therefore of great concern. For example, when more UV radiation hits the surface, it can increase the risk of skin cancer and cataracts, weaken the human immune system and lower agricultural productivity, in addition to negatively impacting ecosystems.”

What to do

So that we are dealing with not one, but two ozone holes is rather alarming. The question is therefore what we can do to protect ourselves. “Unfortunately, we can’t control the natural cosmic ray effect,” Lu says when asked. “But we can continue to strictly adhere to the Montreal Protocol, which agreed to phase out the production and use of ozone-depleting chemicals.”

At the same time, we can also share many of the new findings published in the trade journal AIP Advances, to learn. “This study will greatly advance our understanding of atmospheric physics and chemistry, ozone depletion and human health,” said Lu. “In addition, the tropical and polar ozone hole play an important role in the regulation of stratospheric temperatures. Therefore, the discovery of the tropical ozone hole could be crucial for a better understanding of global climate change.”