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 are 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.

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Figure 1. Exposure of agricultural area to ozone in EEA member countries
Year0-6 000 (μg//m³).h0-6 000 μg/m3.h6 000-12 000 (μg//m³).h6000-1 2000 (μg//m³).h6 000-12 000 μg/m3.h12 000-18 000 (μg//m³).h12 000-18 000 μg/m3.h> 18 000 (μg//m³).hGreater than 18 000 (μg//m³).hGreather than 18 000 μg/m3.h

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 episodesresulted in the relatively high ozone values in this year.

However, following the general decreasing trend since 2001, in 2018, the fraction of land exposed to levels of ozone exceeding the threshold increased, by about 19% compared with 2017, to 45%, amounting to a total area of 1 065 million km2 being exposed to levels above the air quality threshold. Ozone exposure values in 2018 were higher in northern countries and in central and southern Europe than they were in 2017.

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 2018, this long-term objective was met for only about 4% of the total agricultural area of the EEA countries (ETC/ATNI, 2020), decreasing from 24% in 2017. Areas in which the long-term objective had been met were mainly in Scandinavia, the Baltic states, the United Kingdom, Portugal, Ireland and Iceland.

Figure 2. Exposure of forest area to ozone in EEA member countries
YearLower than 10 000 (µg/m³).hLower than 10 000 (mg/m³).hLower than 10 (mg/m³)•hLower than 10 mg/m³.h10 000-20 000 (µg/m³).h10 000-20 000 (mg/m³).h10-20 (mg/m³)•h10-20 mg/m³.h20 000-30 000 (µg/m³).h20 000-30 000 (mg/m³).h20-30 (mg/m³)•h20-30 mg/m³.h30 000-50 000 (µg/m³).h30 000-50 000 (mg/m³).h30-50(mg/m³)•h30-50 mg/m³.hGreater than 50 000 (µg/m³).hGreater than 50 000 (mg/m³).hGreater than 50 (mg/m³)•hGreater than 50 mg/m³.h

The UNECE CLRTAP (1979) defines a critical ozone exposure level for the protection of forests, of 10 mg/m3.hour. Between 2004 and 2018, 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), 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 mg/m3.hour increased to 40%.

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 2018, the critical level was not exceeded in most of in Scandinavia, the Baltic states, the United Kingdom, Ireland and Iceland, whereas, in southern Europe, levels above 50 mg/m3.hour were observed.

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