What is Stratospheric Ozone?

Ozone is comparatively rare in the atmosphere - there are only 3 molecules of ozone for every ten million air molecules. 90% of the planet's ozone is in the "ozone layer" which exists in the lower level (20-25 kilometres above sea level) of the stratosphere.

The stratosphere is the region of the atmosphere which exists between 10 and 50 kilometres above the surface of the earth. Solar wavelengths in the ultraviolet range (180-240 nanometers) are absorbed by and break apart oxygen molecules (which are made of two oxygen atoms). Some of the resulting unattached pairs of oxygen atoms then recombine into triplets to form ozone. A different range of wavelengths of ultraviolet (290-300 nm) are strongly absorbed by ozone, which breaks down as a result and reforms into molecular oxygen again.

The higher up in the atmosphere you go, the thinner the air is, and thus the less oxygen there is to absorb the 180-240 nm ultraviolet to form ozone. This means that ozone amounts tend to decrease as you go higher. The lower down in the atmosphere you go, the more oxygen the ultraviolet has to pass through to get there, and the greater the chances are that it has already been absorbed to create ozone somewhere higher up. This means that very low down, the ozone concentrations tend to be lower. From roughly 12 to 30 km, the two tendencies balance out, and the highest ozone concentrations are found there, in what is called the "ozone layer". This is why the ozone layer exists in the lower part of the stratosphere.

The lower layer of the atmosphere that immediately surrounds the Earth is called the troposphere. Stratospheric ozone is a naturally-occurring gas that filters the sun's ultraviolet (UV) radiation. This is typically regarded as 'good' ozone since it reduces the harmful effects of ultraviolet (UV-B) radiation. A diminished ozone layer allows more radiation to reach the Earth's surface. Excessive exposure to UV-B at the surface of the earth has been shown to cause harmful effects in plants and animals. Absorption of UV-B by ozone in the stratosphere reduces the amount of UV-B reaching the earth's surface and also generates heat that plays a role in maintaining the temperature structure of the atmosphere.

Ozone that occurs in the troposphere is a much smaller proportion of the total planetary ozone and is regarded as 'bad' ozone since it reacts easily with other Molecules making it highly toxic to living organisms. Tropospheric ozone is known to have negative impacts on such things as crop production, forest growth and human health. Tropospheric ozone is a key component of photochemical smogs which are observed in many cities.

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