A3.2.4 Ozone


Ozone (O3) is a gas composed of three oxygen atoms. It is a strong oxidising agent, and therefore highly reactive. It is not usually emitted directly into the air.

According to http://www.airquality.co.uk, at ground-level it is primarily formed by a complicated series of chemical reactions initiated by sunlight. These reactions can be summarised as the sunlight-initiated oxidation of Volatile Organic Compounds (VOCs) in the presence of nitrogen oxides (NOx). The sources of VOCs are similar to those for NOx, but also include other activities such as solvent use, and petrol distribution and handling.


The chemical reactions do not take place instantaneously, but can take hours or days. Therefore Ozone measured at a particular location may have arisen from VOC and NOx emissions many hundreds or even thousands of miles away. Maximum concentrations, therefore, generally occur downwind of the source areas of the precursor pollutant emissions. Ozone irritates the airways of the lungs, increasing the symptoms of those suffering from asthma and lung diseases.

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Ozone has the same chemical structure (O3) whether it occurs miles above the earth or at ground-level and can be “good” or “bad,” depending on its location in the atmosphere.

In the earth’s lower atmosphere, ground-level ozone is considered “bad.” Motor vehicle exhaust and industrial emissions, petrol vapours, and chemical solvents as well as natural sources emit NOx and VOC that help form ozone. Ground-level ozone is the primary constituent of smog. Sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. As a result, it is known as a summertime air pollutant. Many urban areas tend to have high levels of “bad” ozone, but even rural areas are also subject to increased ozone levels because wind carries ozone and pollutants that form it hundreds of miles away from their original sources.

In the higher atmosphere, Ozone is considered “good” because it forms at protective barrier around the earth. It is the hole in the Ozone Layer that are causing problems regarding global temperatures.