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. Much of Europe’s lands is exposed to ozone levels above the threshold and long-term objective values set in the EU’s Ambient Air Quality Directive (AAQD) for the protection of vegetation. For instance, after a 6-year period (2009-2014) of relatively low ozone values, the fraction of arable land exposed to levels above the AAQD threshold increased to 30% in 2015, falling to 19% in 2016, before increasing again to 26% in 2017 and 45% in 2018 and decreased only to 37% in 2019.

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 an accumulated ozone exposure threshold (AOT40) value, applicable from 2010, based on the sum of hourly ozone values that exceed 80 μg/m3 (40 ppb), of 18,000 μg/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 and 2006 are disregarded, the data do indicate a general decreasing trend between 2001 and 2017. The peak in 2003 can be explained by meteorological conditions favourable for ozone formation, resulting in exceptionally high concentrations; in June and July 2006, a large number of ozone episodes resulted in the relatively high ozone values in this year.

In 2019, the fraction of land exposed to levels of ozone exceeding the threshold decreased by about 8 percentage points, compared with 2018, to 37%, amounting to a total area of 870 million km2 being exposed to levels above the air quality threshold. Ozone exposure values in 2019 were lower in northern countries and in central Europe than they were in 2018.

The Ambient Air Quality Directive also sets a long-term objective for the protection of vegetation from ground-level ozone: to reduce the exposure of vegetation to 6,000 μg/m3.hour or less. This 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). In 2019, this long-term objective was met for only about 14% of the total agricultural area of the EEA countries, increasing from 4% in 2018.

The UNECE CLRTAP (1979) defines a critical ozone exposure level for the protection of forests, of 10,000 μg/m3.hour. Between 2004 and 2019, large variations in the exposure of forested areas to ozone were observed. In 2004 and 2006, almost all forests were exposed to levels exceeding the critical level, whereas, in 2007, more than 40% of forests were exposed to levels lower than the critical level. The percentage of forests exposed to ozone below the critical level had been more or less stable since 2007 (with the exception of a decrease in 2008, 2018 and 2019), but, in 2018, decreased to the lowest value (13%) since 2007. In fact, in 2018, the proportion of forested areas exposed to levels in excess of 30,000 μg/m3.hour increased to 40% but decreased to 31% in 2019.

As for crop areas, the proportions of forested areas exposed to ozone above the critical threshold value were relatively low in northern Europe and highest in the countries around the Mediterranean. In 2019, the critical level was not exceeded in most of in Scandinavia, the United Kingdom, Ireland and Iceland, whereas, in southern Europe, North Macedonia, Greece and Turkey, levels above 50,000 μg/m3.hour were observed.

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