Exposure of Europe's ecosystems to ozone

Ground-level ozone adversely affects not only human health but also vegetation and ecosystems across Europe, leading to decreased crop yields and forest growth, and loss of biodiversity. In 2020, a historical minimum of 5.5% of Europe’s agricultural lands were exposed to ozone levels above the threshold value set in the EU’s Ambient Air Quality Directive (AAQD) for the protection of vegetation; the long-term objective was met in a historical maximum of 23% of agricultural lands.

Published: ‒ 25min read

The pollution of air with ground-level ozone is a serious cause for concern in Europe, not only because of its harmful effects on human health but also because of its damaging effects on vegetation, leading to reduced crop yields and forest growth, and loss of biodiversity. The EU Ambient Air Quality Directive aims to protect vegetation from ozone and sets two standards: a target value and a long-term objective. Both are based on the accumulated ozone exposure above a threshold of 40ppb (AOT40). AOT40 is the sum of the difference between hourly concentrations greater than 80µg/m3 (40ppb) and 80µg/m3 over a given period using only the one-hour values measured between 08:00 and 20:00 Central European Time (CET) each day. The periods are from May to July for the protection of vegetation and crops, and April to September for the protection of forests. The target value is set at 18,000μg/m3.hour, calculated over 5 years, although in this indicator we only consider its value in one year, 2020. The long-term objective is set at 6,000μg/m3.hour.

The fraction of agricultural land in the EEA member countries exposed to ozone levels above the threshold is substantial and exceedances have been observed in central, southern and eastern Europe. Considerable variation from year to year makes trends difficult to identify but, if extremes in 2003 (with meteorological conditions favourable for ozone formation) and 2006 (with a large number of ozone episodes) are disregarded, the data do indicate a general decreasing trend between 2001 and 2017.

After the secondary maximum in 2018 (48%, due to very favourable conditions for O3 formation and high O3 concentrations particularly in northern and central Europe) and the decrease in 2019 (39%), the fraction of land exposed to levels of ozone exceeding the target value threshold reached an absolute minimum in 2020 of 5.5%, amounting to a total area of 121 million km2 being exposed to levels above the target value threshold. Only nine EEA member countries had some of their agricultural land exposed to values above the target value threshold in 2020: Cyprus, Italy, Türkiye, Greece, Spain, Switzerland, Croatia, France and Slovenia. Ozone exposure values in 2020 were lower than they were in 2019 in all the considered countries.

The long-term objective is in line with the critical level of ozone for the protection of crops defined by the United Nations Economic Commission for Europe (UNECE) Convention on Long-range Transboundary Air Pollution (CLRTAP or Air Convention). In 2020, this long-term objective was met for about 23% of the total agricultural area of the EEA countries, reaching a new maximum. Estonia, Finland, Iceland, Ireland, Latvia, Lithuania, and Sweden had all their agricultural land exposed to values below the long-term objective and in Norway and Denmark, only 1 and 3 km2, respectively, have been estimated to be exposed above the long-term objective.

The UNECE Air Convention defines a critical ozone exposure level for the protection of forests, as AOT40, of 10,000 μg/m3.hour. Between 2005 and 2020, large variations in the exposure of forested areas to ozone were observed. In 2006, almost all forests were exposed to levels exceeding the critical level and in 2018 this was the case for 87% of the forested area; on the contrary, in 2015 and 2017, and again in 2020, more than 40% of forests were exposed to levels lower than the critical level.

In 2020, the critical level was not exceeded in Estonia, Finland, Iceland and Latvia, and it was exceeded only in less than 1% of Lithuania.

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