What is Ozone?

Ozone is a gas composed of three atoms of oxygen (O3). It is a natural component of the atmosphere. Most Ozone is found high up in the stratosphere, the layer of the atmosphere between 12km and 50km above sea level. Stratospheric ozone is essential to life on earth as it protects us from harmful rays from the sun.

Ozone is also found in the troposphere, the layer of the atmosphere next to the earth. This ground-level Ozone is not emitted directly into the atmosphere and is a secondary pollutant produced by reaction between Nitrogen Dioxide, Hydrocarbons and sunlight. Ozone can be described as good, if found high in the atmosphere, or bad, when found at ground level.

The formation of ground level ozone (O3) is complex. It is formed from reactions between pollutants such as NOX, carbon monoxide and various volatile organic compounds (VOCs) in the presence of sunlight. Ozone is also a transboundary pollutant - it originates in one country but is able to cause damage in another country's environment, by crossing borders. Its impacts mainly affect central and southern Europe during the summer months. Ozone levels over Ireland can be influenced by the transport of pollutants from other European regions and across the Atlantic from North America. High concentrations of ground level ozone can affect the functioning of the respiratory system and damage crops and other vegetation. Exposure to high concentrations of tropospheric ozone causes chest pains, nausea and coughing in humans.

Long term exposure to moderate concentrations causes a reduction in lung capacity and can worsen heart disease, bronchitis, emphysema and asthma. Tropospheric ozone contributes to the greenhouse effect and subsequent global climate change.

Levels of Ozone in Ireland are moderate.

Ozone can be “good” or “bad” for health and the environment depending on where it’s found in the atmosphere. Stratospheric ozone is “good” because it protects living things from ultraviolet radiation from the sun. Ground-level ozone is “bad” because it can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma. Learn more about ground-level ozone.

Ozone levels are higher in Summer than Winter: Ozone formation follows patterns. During the day, ozone formation occurs. However, during the night, when solar radiation and temperatures are low ozone is destroyed. Similar sequences of reactions occur on an annual basis, with ozone levels lower in winter (more destroyed) and highest in summer (more formed). As a result ozone concentrations tend to be higher in June, July and August in the northern hemisphere.

Ozone levels are higher in Rural areas than Urban: In urban areas, where nitrous oxides and VOCs reach high levels (due to greater volumes of traffic and greater levels of fossil-fuel combustion) ozone levels are low in comparison to rural sites. The nitrous oxides are oxidised by ozone gas, which causes a reduction in ozone levels- more ozone is destroyed by forming different pollutants such as NO2. Rural areas, which have a lot less traffic and industrial activity, tend to have higher concentrations of ozone (i.e., less nitrous oxides are present to deplete the ozone concentrations).


NO2 + O2 (+ solar UV-light, + heat) --> NO + O3

In human language: nitrogen dioxide and oxygen react, which results in nitrogen monoxide and ozone. This reaction is speeded up in warmer conditions and with more UV-light.

This is an equilibrium reaction, which means the reaction also works in the other direction (whereby ozone gets degraded again): NO + O3 --> NO2 + O2.

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