What Exactly Is The Ozone Layer?

The earth’s ozone layer is part of the atmosphere and contains high concentrations of a substance called ozone (O3). This layer is important to the functioning of the world below, because it absorbs between ninety-three and ninety-nine percent of the high frequency ultraviolet light the earth receives from the sun.  This light is potentially damaging to life.

Over ninety-one percent of the ozone on the entire planet is in this layer, mostly located in the lower part of the stratosphere, or about ten to fifty kilometres from the surface. The thickness of the layer varies by location and by season. We’ve known about the layer since the early part of the twentieth century. It was discovered by Charles Fabry and Henri Buisson in 1913. Later on, G. M. B. Dobson, a British meteorologist explained the properties in greater detail and developed an instrument that could measure the layer from the ground. He established a network of stations around the world to monitor the layer and that network is still in operation today.

The ozone is created when UV light strikes oxygen molecules (02), breaking them up. These broken up molecules attach to unbroken ones to create 03, or ozone. This is an unstable molecule but it can live in the stratosphere for some time. When UV light hits an ozone molecule again, it will split into a stable oxygen molecule and one oxygen atom. Eventually, the atom will join up with another oxygen molecule to create ozone again.

The process is cyclical, which is why a continuous layer of ozone is present. The highest concentrations of ozone (two to eight parts per million) are located between twenty and forty kilometres from the earth’s surface. This isn’t very much, however, compress all that ozone to sea level air pressure and it would be just a few millimetres in thickness. That small amount absorbs a lot of radiation, UV-A, UV-B and UV-C radiation are all screened out, but some UV-B and UV-A still reaches the surface. All UV-C radiation is screened out by ozone at about thirty-five kilometres up.

The thickness of the ozone layer varies across the world. There is generally less near the equator and more near the poles. It’s also thicker in the spring of the northern hemisphere and thinner in the autumn.  This variation is due to solar intensity and atmospheric patterns of circulation. The lowest amounts of ozone anywhere in the world are in the Antarctic, during September and October, due to the hole in the ozone layer.

Free radical catalysts like atomic chlorine and bromine, hydroxyl, and nitric oxide can deplete the ozone layer. These are mostly released by humans, especially in the form of CFCs and bromofluorocarbons.  These compounds are stable and make it to the stratosphere before breaking down. Ozone is not produced as readily because of their presence and more UV light is able to get through to the earth below. This increases the danger of excessive sun exposure. More than one ozone hole is present but the largest is in the southern hemisphere, over the Antarctic.

Since the mid 1990′s, Europe, North America and many other countries have completely banned CFC use, which may be slowing the development of these holes. However, ozone levels over just the northern hemisphere are dropping by four percent per year, and larger declines have been seen around the north and south poles. This is due to existing contaminants, plus the use of ozone depleting chemicals in countries which have not yet banned them.