The Ozone Layer

The atmosphere that surrounds the Earth consists of multiple layers. The layer closest to the ground is the troposphere. This is where we live out our lives and where nearly all weather occurs. Commercial aircraft typically fly just below the tropopause, the boundary between the troposphere and the layer above: the stratosphere. It is in this layer of the atmosphere that you find the ozone layer, or ozonosphere.

The ozone layer is not a uniform blanket of ozone in the atmosphere, but rather a broad vertical area where ozone is present in relatively high concentrations. It is located about 25 km above the surface of the Earth.  Ozone makes up only 0.00006 percent of the total of the atmosphere, or 0.6 parts per million. In the ozone layer, however, this concentration is much higher at up to 15 parts per million.

Molecules of ozone are composed of three oxygen atoms (O3) and form naturally in the presence of ultra-violet (UV) radiation. In the stratosphere, ozone acts like a screen, filtering out all the UV-C radiation and most of the UV-B. Ninety-nine percent of the ultraviolet light reaching the earth’s surface is UV-A. Without the ozone layer, the entire planet would be bombarded with potentially lethal UV-B radiation. It therefore acts as a critical shield, protecting the biosphere near the surface from solar radiation.


Graph of ozone concentration throughout the atmosphere. Source: NASA.

The hole in the ozone layer

Concentrations of ozone in the atmosphere vary by location, but also change depending on seasons and climate. In general, the least ozone is found in tropical regions where the troposphere is thicker than at the poles. However, there is one place with even less ozone: Antarctica. In 1984, scientists discovered that there was a “hole” in the ozone layer – a region of relatively low ozone concentration – centered above Antarctica. News of the hole spread quickly, and more research was soon conducted to investigate what had caused the dramatic decline in concentration. In 1987, scientists were shocked to learn that the hole in the ozone layer had increased in size and was now as large as the continental US.

Concentration of ozone over Antarctica in 1978 (left) and 2016 (right). Credit: NASA.

There are a few reasons why the hole in the ozone layer was located over Antarctica. A polar vortex isolates the air above Antarctica, so that any chemicals that get caught down there have trouble dispersing. Additionally, during the winter, Antarctica is for a time in perpetual darkness, and the lack of UV light reduces the production of new ozone. Every spring the hole in the ozone layer is at its maximum. In September, after the Antarctic winter, measurements may be as low as 0 Dobson units in some areas.

CFCs, such as those formerly found in aerosol sprays, have been banned in many countries.

Ozone-reactive chemicals called chlorofluorocarbons (CFCs) have been used in aerosol sprays, solvents, and packing materials due to their low cost and durability. Unfortunately, when these molecules are released into the atmosphere they react with ozone in the stratosphere and deplete the ozone layer. CFCs are considered among the most potent ozone-depleting chemicals produced by people.

Chemicals can stay in the air for more than 10 years before they reach the ozone layer. Consequently, there is a significant delay between the release of these chemicals and their effects. It can then take a hundred years for chlorofluorocarbons to be broken down in the atmosphere, and therefore the consequences of using these chemicals may be felt for a long time. Some of the consequences of a reduced ozone layer are:

  • More skin cancer in people and animals and quicker aging of skin
  • Decreased immune system in organisms
  • Damaged crops and a decrease in yield
  • Reduction in growth of phytoplankton, the foundation of the food chain
  • Cooling of the Earth’s stratosphere and climate change

Research and measuring the hole in the ozone layer

To slow the damage we are causing to the ozone layer, a plan was formulated and signed by the international community in 1987. The Montreal Protocol on Substances that Deplete the Ozone Layer would phase out the production and consumption of ozone-depleting chemicals and eliminate the use of chlorofluorocarbons. Since then, more research has been done and more evidence has emerged on the devastating effects of anthropogenic (man-made) chemicals on the ozone layer.

Since the discovery of the hole in the ozone layer, governments around the world have initiated projects to keep better track of the ozone layer. The EASOE experiment in 1991 and 1992 found that the ozone concentration in Europe was at its lowest ever that year, but scientists suggested that it may be due to the eruption of the Pinatubo volcano in the Philippines, which ejected a vast amount of hydrogen chloride into the atmosphere. Typhoon Yunya, however, occurred shortly after and washed most of the chemical out of the atmosphere before it could reach the stratosphere. Nevertheless, the eruption impacted the climate and global temperatures for a few years. From 1993 to 1995, the SESAME project showed that ozone levels in Europe had dropped another 30%. This time scientists concluded that the coldest winter in 30 years had caused the decline in ozone levels.

In 2017, scientists from NASA and the National Oceanic and Atmospheric Administration (NOAA) in the U.S. announced that the hole in the ozone layer above Antarctica was the smallest it has been since 1988. This is still two and a half times the size of the hole in 1987 when scientists were shocked by its size. It was expected as well. Warm stratospheric temperatures in 2016 and 2017 minimized cloud formation in the stratosphere and constrained the growth of the hole. Unfortunately, scientists also pointed out that “the smaller ozone hole extent in 2016 and 2017 is due to natural variability and not a signal of rapid healing.” Real recovery is not expected to happen before 2070.


National Oceanic and Atmospheric Administration (NOAA)
NASA Ozone Watch
Paul Ward, The Antarctic Ozone Hole
Plan of action on the ozone layer: A joint publication of the World Meteorological Organization and the United Nations

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