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Ground-level ozone adversely affects human health, vegetation and ecosystems across Europe. This leads to premature deaths, decreased crop yields and forest growth, and loss of biodiversity. In 2023, 12.5% of Europe’s agricultural lands were exposed to ozone levels above the threshold value set for the protection of vegetation in the EU’s Ambient Air Quality Directives. In addition, the long-term objective was not met in 90.8% of agricultural lands.

Figure 1. Exposure of agricultural area to ozone in EEA member countries

Ground-level ozone is a serious air pollution 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.

Two standards related to ozone are set for the protection of vegetation in the EU Ambient Air Quality directives (EU, 2008) (EU, 2024): a target value and a long-term objective. Both are based on the accumulated ozone exposure above a threshold of 40ppb (parts per billion) (AOT40). The 2024 revised Air Quality Directive has kept the same levels, but has set a deadline of 1 January 2050 for attaining the long-term objective.

The target value for protection of vegetation is set at 18,000μg/m3.hour, calculated over five years. However, this indicator examines its value every single year, referred to as the target value threshold. The long-term objective for protection of vegetation is set at 6,000μg/m3.hour. Most agricultural land in the EEA member countries is exposed to ozone levels above this long-term objective, as shown in Figure 1.

Exceedances of the target value threshold have been regularly observed in central, southern and eastern Europe over time. The share of agricultural land exposed to ozone levels exceeding the target value threshold decreased by 20 percentage points in 2023 compared to 2022, dropping from 32.5% to 12.5%. A total agricultural area of 276,485km2 was exposed to levels above the target value threshold in 2023.

According to Copernicus, 2023 was the fifth warmest summer in Europe on record, while the summer of 2022 was the warmest one. This may provide one explanation for the relatively lower values in 2023 compared to 2022. However, more detailed analysis of other influencing factors could present more robust insights.

Eighteen EEA member countries had all of their agricultural land exposed to values below the target value threshold in 2023: five Nordic countries, the three Baltic Republics, Belgium, Bulgaria, Hungary, Ireland, Luxemburg, Netherlands, Poland, Portugal, Romania and Slovakia.

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 2023, this long-term objective was met for only 9.2% of the total agricultural area of EEA member countries. Two EEA member countries (Iceland and Ireland) had all their agricultural land exposed to values below the long-term objective.

Figure 2. Exposure of forest area to ozone in EEA member countries

The UNECE Air Convention defines a critical ozone exposure level for the protection of forests, as AOT40 defined from April to September at 10,000μg/m3.hour. Figure 2 shows that between 2005 and 2023, significant variations were observed in the exposure of forested areas to ozone. In 2023, the share of forests exposed to the highest ozone concentrations (above 30,000μg/m3.hour)  decreased; nonetheless only 28% of forested areas were exposed to ozone levels below the critical level (10% percentage points less than in 2022).

In 2023, the critical level for forests was reached in almost all forested areas of Finland, Iceland and Ireland.

Ozone is a secondary pollutant of photochemical origin, and therefore its concentrations are determined by precursor emissions and meteorology. Meteorological stagnation, high temperatures, high solar radiation, and low precipitation enhance tropospheric ozone formation. Hence ozone concentrations tend to be naturally higher in certain regions, such as southern Europe.