The ozone layer is a layer of ozone gas around 20-30 km up in the stratosphere, and is of crucial importance in protecting life on Earth from harmful UV-B radiation from the sun. Every southern spring a large hole develops in this layer over Antarctica, however this year’s hole is the largest since 2008. How does the hole form and how important is this year’s growth?
Ozone is a gas consisting of three oxygen atoms in a molecule instead of the usual two. It is formed in the stratosphere when incoming ultraviolet radiation from the sun breaks up normal oxygen molecules (O2) into two single oxygen atoms. These atoms are very reactive and quickly react with another oxygen molecule to form an ozone molecule (O3).
This molecule is unstable, however, and can easily lose the extra oxygen to revert back to an O2 molecule, but the released oxygen atom just reacts again with another O2 molecule, repeating the process. This chemical reaction releases energy, which is why the temperature stops falling or even rises with height through the stratosphere.
In winter, the stratosphere above the poles cools dramatically as the sun’s energy fails to reach it during the Polar Night. This forms a strong circulation around the pole (Polar Vortex), which helps contain the coldest air over the pole. Due to the geography of Antarctica, the vortex is stronger over the South Pole, allowing temperatures to fall much lower than over the North Pole. This colder stratosphere allows Polar Stratospheric Clouds (PSC) to form, and these play an important role in destroying ozone when the sun finally makes an appearance again in the spring (August-October).
As the sun’s radiation increases, the ice crystals in the PSC act as sites for the chemical release of chlorine from the normally-stable hydrogen chloride, chlorine nitrate or CFC molecules. This free chlorine destroys the ozone by stealing an oxygen atom to form Chlorine oxide and an oxygen (O2) molecule. The ClO molecule breaks down again and releases the chlorine atom to go off and attack another ozone molecule, and so on. This drastically reduces the concentration of ozone in the stratosphere, therefore forming the famous Ozone Hole. So one chlorine atom punches well above its weight in terms of its damaging effect on ozone.
Without these Polar Stratospheric Clouds, the reaction above would not take place. As the sun gets stronger as the weeks pass, the temperature rises and the clouds dissipate. The chlorine goes back into its box (reforms the original stable molecules) for another year. The polar vortex weakens, allowing in ozone from lower latitudes . The ozone is also free to renew itself through the original reaction with UV and oxygen, and hence the hole in the layer fills in.
It is therefore clear that temperature plays a key role in the whole process, as the lower the temperature the more PSCs will form, and vice versa. A look at this year’s temperatures shows well below -90 °C through July and August, colder than the same period last year. 2015 is not unique, however, as there were other years with similarly cold temperatures (2008, 2003, 1999, 1993). This would indicate no clear trend in cooling, and hence no indication that the hole will increase in size in the coming years. The Montreal Protocol put an end to the release of CFCs into the atmosphere, halting the worrying trend that had been identified during the 1980s.
As we in the northern hemisphere head towards our winter the polar vortex will start to form, however the many mountain ranges prevent it from becoming as strong and cold as the southern vortex. Sudden Stratospheric Warming events are more common in the north, which break up the vortex and can allow cold air to flood south. We’ll be watching for these events over the next few months and you will read about them here, should one occur.