Ozone

The Atmosphere

The Earth is surrounded by a thin envelope of gases called our atmosphere. This life giving bubble is consistes of numerous layers and is composed of a mixture of gases. Four of these gases are carbon dioxide, nitrogen, argon and water vapour.

The Atmosphere

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Most of the ozone found on earth is high up in the sky in the stratospheric ozone layer. There it performs the very important function of protecting us from the sun’s ultraviolet radiation. A small portion of the earth’s ozone is, however, found closer to the ground where it is formed by the action of sunlight on other air pollutants.

Ozone laden air can be transported over large distances and so some of the ozone measured in Ireland originates outside of the country and falls into the category of transboundary pollution.

High concentrations of ozone can cause various health problems, such as reduced lung capacity, bronchitis, asthma, headaches, and irritation of the eyes and mucous membranes. It is a very reactive chemical and can cause damage to crops, to buildings and to materials such as rubber and plastic.

Ozone Concentration
Courtesy EPA Š

Ozone levels in Ireland have remained low in comparison to mainland Europe and were well below the thresholds for effects on human health and vegetation set down in regulations until the recent spike in 2018.This is due to a pollution episode that occurred during the warm weather spell Ireland experienced in June 2018.

The limit is 120 micrograms per cubic metre but the regulations allow for this limit to be breached 25 days in any year. In the long-term the regulations aim to reduce that allowance to zero.

The graph shows the AOT40 for five rural background stations for the years 2007-2018. The AOT40 is an indicator of exposure of vegetation to ozone during the growing season. AOT40 is the sum of the difference between hourly concentrations greater than 80 μg/m3 (40 ppb) and 80 μg/m3 over a given period using only the one-hour values measured between 8:00 and 20:00 Central European Time each day (EPA, 2013).The annual threshold limit is 6,000 μg/m3, and it can be seen that in 2008 it was exceeded although it had since reduced quite significantly, only to be exceeded again in 2016 and singificantly in 2018. Trends show that ozone levels rose significantly for all zones in 2018 apart from Laois.

If levels for 1 hour exceed 180 micrograms per cubic metre then warnings are given in weather reports on radio and television to advise people with respiratory illnesses to stay indoors and to avoid strenuous work. This threshold is breached occasionally in Ireland when there is a combination of transboundary pollution and hot, sunny weather and is referred to as ground level ozone. Due to a ‘photochemical smog’ episode across urban areas in Ireland with heavy traffic, a large amount of ground level ozone was produced leaving the country at 20 ľg/m3 away from declaring a Public Information Alert (recorded at 160 ľg/m3) If weather patterns continue in this trend ground level pollution could become a potential problem.

In 2018, warm sunny weather from 22 to 30 June led to an increase in ozone concentrations across the country (EPA, 2020). During this time, the concentrations reached ≈170 ľg/m3 . Had they reached 180 ľg/m3 , a public information alert would have been triggered to inform the public of the health impacts of elevated (raised) ozone concentrations. Ground-level ozone could be a potential problem pollutant in Ireland if similar conditions are experienced again in the future.

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