Scientists from the Copernicus atmospheric monitoring service announced on 20 December 2021 that the hole in the ozone layer was one of the longest ever recorded. But what are we talking about? And where does this hole come from?
What is ozone?
Ozone is a colourless gas made up of three oxygen atoms and is chemically referred to as O3. Naturally present in small quantities in the stratosphere (between 12 and 50 km above sea level), this gas is the result of a balance between the sunlight that creates ozone and the chemical reactions that destroy it.
Ozone is created when ultraviolet energy emitted by the sun splits oxygen (O2) into oxygen atoms. Once separated, the oxygen atoms can come together to form O2 or join with O2 molecules to form ozone.
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How was the hole in the ozone layer formed?
The hole in the ozone layer is located over Antarctica. The winter atmosphere in this part of the southern hemisphere is very cold, and these extreme temperatures provide a favourable environment for the formation of polar stratospheric clouds from June onwards, which corresponds to winter at the South Pole.
Composed of droplets of nitric acid solution and water or solid hydrates, the chemicals on the surface of these clouds cause chemical reactions that remove atmospheric chlorine.
When the sun returns to the Antarctic stratosphere in summer (October to February in this part of the world), the sunlight splits chlorine molecules into highly reactive chlorine atoms that rapidly deplete ozone. This depletion is so rapid that it has been called the "ozone hole".
The amount of ozone in the atmosphere is regularly measured from satellites. The ozone hole appears over the Antarctic continent as very low ozone values in the stratosphere. Typically, the Antarctic ozone hole has its largest area in early September and its lowest values between late September and early October.
By 2021, this hole in the ozone layer has been one of the longest ever recorded according to Copernicus scientists.
The Montreal Protocol: an agreement to save the ozone layer
16 September is the International Day for the Preservation of the Ozone Layer. This day celebrates the anniversary, in 1987, of the Montreal Protocol on Substances that Deplete the Ozone Layer. The Montreal Protocol led to a ban on a group of chemicals called halocarbons, which are accused of exacerbating the annual ozone hole.
Although the ozone layer is beginning to recover, it will probably take until the 2060s for the ozone-depleting substances used in refrigerants and aerosol cans to disappear completely from the atmosphere.