Rebecca Smith

What is ozone?

Ozone (O3) is a molecule made up of three oxygen atoms1. The atmosphere contains around 3 billion metric tonnes of ozone, equating to 0.00006% of the total atmospheric mass and it is formed as a result of O2, the molecular type of oxygen we breath, being split into individual atoms – O1 – by ultraviolet (UV) radiation; these atoms re-join to produce either O2 (Oxygen), or O3 (ozone). While the total percentage of ozone seems minute, it plays a hugely important role: it absorbs a large proportion of the Sun’s UV rays, and without it, the Sun’s UV rays would sterilize Earth’s surface and would alter the environment.

The Earth has natural ozone and this exists in the stratosphere (10-50 km altitude) which is the second layer of the atmosphere, sitting immediately above the troposphere – the lowermost layer in which our weather occurs. For perspective, commercial airliners sometimes fly in the stratosphere in order to avoid the turbulence of the troposphere.

Simplified visual description of atmospheric stratification2

Ozone can be destroyed when it reacts with molecules containing nitrogen, hydrogen, chlorine or bromine – some of these molecules are naturally produced, whilst others are anthropogenic. Likewise, ozone can also be produced anthropogenically. This type of anthropogenic ozone is present in the troposphere, and can be caused by emissions from cars, gas vapours, and aerosols.

How did the ozone layer deteriorate?

In relatively recent years, anthropogenic emissions which contribute to the destruction of ozone have been widely discussed. This is in part because of Joe Farman’s discovery of an ozone hole above Antarctica in 19855. The hole is essentially a reduction in ozone concentration in the stratosphere as defined by Dobson units6. The hole means that more radiation is able to penetrate to the Earth’s surface, and this is thought to cause an increased risk of cancer, and a changing climate and environment.

Chemicals such as chlorofluorocarbons (CFCs), first created in 1928, and produced commercially in 1930, have contributed to the deterioration of the ozone layer. CFCs were previously released by fridges, freezers, aerosols, and cleaning agents, for example, but it is now acknowledged that CFCs can destroy ozone, since when UV reacts with it, it will produce a chlorine atom, which causes ozone to break apart4. As such, CFC’s have been attributed to widening the ozone hole above Antarctica during winter months3,4.

What’s happening now?

Since the early 1990s, the ozone hole has remained larger than 8 million square miles; equivalent to 8 Kazakhstans, or 1 Russia and 1 India, with a little bit left over for fun. This is despite the fact that in 1987, the Montreal Protocol agreement was signed – an international agreement ratified by 197 parties, tasked with phasing out substances responsible for depleting the ozone layer. As a result of the Protocol, CFCs were phased out in developed countries in 1996, and developing countries by 2010, and as a result there has been a significant downward trend in emissions3. The Montreal Protocol was most recently revised in 2016, and by the year 2030 and 2040, the Protocol promises that we will also see the end of alternatively produced hydrochlorofluorocarbons (HCFCs) in developed countries, and developing countries, respectively3,4. This is significant since HCFCs also break apart to produce chlorine, which causes ozone to break down.

The quantity of chemicals in the atmosphere is currently monitored by NASA’s AURA satellite, which enables scientists to establish forecasts of the size of the ozone hole in the future. Fortunately, scientists now believe that the ozone hole will be permanently smaller than 8 million square miles by the year 20407. Satellites will be instrumental to assess the ozone layer’s path to recovery (i.e., full closure), which is predicted to be at some point before 21007. This is good news for our crops, the climate of our planet, and the health of humans.

The Ozone hole. Green refers to thick ozone, red refers to thinning ozone, blue/purple refers to the thin/depleted ozone 8


1) An atom is the smallest particle that makes up an element, and is composed of neutrons, protons and electrons. See here for more information on atoms.
2) NC State University. 2014. Ozone Layer. Last visited 14/11/2016
3) NOAA. 2010. Has the Montreal Protocol been successful in reducing Q16 ozone-depleting substances in the atmosphere? Last visited 14/11/2016
4) CFC’s are a compound which contain chlorine, fluorine and carbon. Why do CFCs break down ozone? When light, or UV rays, reach CFC (represented chemically as CFCl3), a carbon-chlorine bond breaks apart, which produces a loose chlorine atom (represented as Cl) in the atmosphere. The Cl atom reacts with the O3 in the atmosphere, which breaks it apart (remember the molecules which can destroy O3: nitrogen, hydrogen, chlorine or bromine), and the O3 is no more. HCFCs are essentially the same as CFC’s but include one or more hydrogen atom.
5) Farman et al. 1985 Nature 315:207-210
6) Welch 2016 The Ozone Hole. Last visited 14/11/2016
7) Strahan et al. 2014 J. Geophys. Res. Atmos., 119, 14,098–14,109
8) NASA, Image of the ozone layer. Last visited 14/11/2016

For more information on ozone, see here: and here:

For more information on the recovery of our ozone layer, see here: