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 2021, 18% 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 19.5% of agricultural lands.

Year	0-6,000 μg/m3·h	6,000-12,000 μg/m3·h	12,000-18,000 μg/m3·h	Greater than 18,000 μg/m3·h
2000	10.7	25.4	21.2	42.6
2001	8.2	20.7	22	49.1
2002	4.2	25.1	23.6	47.1
2003	5.7	10.5	13.2	70.5
2004	15	24.6	23.2	37.3
2005	7.7	16	26.8	49.5
2006	1.9	4.9	20.3	73
2007	17.8	26.9	19.9	35.5
2008	4.3	23.3	33.5	38.8
2009	15	34.5	25.7	24.7
2010	9.9	29.6	37.2	23.3
2011	6.5	28.5	45.4	19.5
2012	8.2	34.6	28.2	29.1
2013	14.7	38.6	24.5	22.2
2014	9.2	38.5	33	19.3
2015	15.6	26.6	25.4	32.4
2016	18.9	34.3	26.3	20.5
2017	19.5	32.7	20	27.8
2018	4	16.8	31.1	48.2
2019	9.3	25	26.4	39.3
2020	23.3	48.3	23	5.5
2021	19.5	37	25.4	18.1

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 period is from May to July for the protection of vegetation and crops. The target value for protection of vegetation is set at 18,000μg/m³.hour, calculated over five years, although this indicator examines its value in every single year (what we call the target value threshold). The long-term objective for protection of vegetation 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 over the years since 2001. 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 a 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%. This increased again in 2021 to 18%, amounting to a total area of 401 000 km²being exposed to levels above the target value threshold. All these differences indicate the interannual variation of the ozone concentrations, partially due to different meteorological conditions, and, therefore, exposure of vegetation.

Fourteen EEA member countries had all their agricultural land exposed to values below the target value threshold in 2021: the five Nordic countries, the three Baltic Republics, the Benelux, Czechia, Ireland and Romania; while in Germany and Portugal only one km² has been estimated to be exposed above the target value threshold.

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 2021, this long-term objective was met for about 19.5% of the total agricultural area of the EEA countries. Iceland and Ireland were the only two EEA countries that had all their agricultural land exposed to values below the long-term objective.

Year	Lower than 10,000 µg/m3·h	10,000-20,000 µg/m3·h	20,000-30,000 µg/m3·h	30,000-50,000 µg/m3·h	Greater than 50,000 µg/m3·h
2005	20.6	20.6	19.1	30.1	9.7
2006	0.1	30.5	18	40.8	10.6
2007	38.9	14.6	13.6	25.5	7.4
2008	21.5	30.1	21.5	26.4	0.5
2009	33.5	19.2	18.4	25.6	3.3
2010	37.7	14.9	25.6	19.8	2
2011	32.1	16.4	26.2	22.2	3
2012	35.8	18.9	18.4	23.2	3.6
2013	33.6	24.4	19.6	18.6	3.8
2014	32.4	32.1	25.5	9.9	0.1
2015	41.4	7.8	17.5	29.2	4.1
2016	38	19.1	24.3	17.5	1
2017	42.9	17	18.5	18.6	3
2018	12.5	30.1	16.3	36.3	4.8
2019	14.8	32.2	21.2	29.3	2.4
2020	40.9	22.1	28.7	7.9	0.4
2021	38.8	29.2	20.2	10.8	1

The UNECE Air Convention defines a critical ozone exposure level for the protection of forests, as AOT40 defined from April to September, of 10,000 μg/m³.hour. Between 2005 and 2021, 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 87.5% of the forested area; on the contrary, in 2015, 2017 and 2020, more than 40% of forests were exposed to levels lower than the critical level, being close to this share (39%) in 2021.

In 2021, the critical level was not exceeded in Finland, and it was exceeded in less than 2% of forested areas of Estonia, Iceland and Ireland.

Supporting information

Definition

This indicator shows the exposure of areas covered with specific vegetation (crops and forests) to ground-level ozone.

Ground-level ozone is one of the most prominent air pollution problems in Europe, mainly due to its effects on human health, crops and natural ecosystems. When absorbed by plants, it damages plant cells, impairing their ability to grow and reproduce, and leading to reduced agricultural crop yields, decreased forest growth and reduced biodiversity.

The risk is estimated by reference to either a target value and a long-term objective, or to the 'critical level' for ozone for each location. The target value and the long-term objective are levels fixed with the aim of avoiding, preventing or reducing harmful effects on the environment. For ozone, the critical level is a concentration in the atmosphere, above which direct adverse effects on receptors, such as human beings, plants, ecosystems or materials, may occur according to present knowledge .

Methodology

Methodology for indicator calculation

AOT40 estimated values are calculated from hourly data at all rural background stations available in the Air Quality e-reporting database (former AirBase). Only data series with more than 75% valid data are considered.

The AOT40 maps have been created by combining measurement data from the rural background stations combined with the results of the EMEP dispersion model, altitude field and surface solar radiation in a linear regression model, followed by the interpolation of its residuals by ordinary kriging. For altitude, dataset GTOPO30 (Global Digital Elevation Model) at a resolution of 30x30 arcseconds has been used. The solar radiation has been obtained from ECMWF's Meteorological Archival and Retrieval System (MARS). Kriging is a method of spatial statistics that makes use of spatial autocorrelation (the statistical relationship between the monitoring points expressed in the form of variograms). Kriging weights the surrounding measured values to derive an interpolation for each location. The weights are based (i) on the distance between the measured points and the interpolated point, and (ii) on the overall spatial arrangement among the measured points. The type of kriging at its parameters (in particular the parameters describing the semivariogram) is chosen in order to minimise the RMS error.

The AOT40 maps have been overlayed in a geographical information system with the land cover CLC2018 map. The resolution was 2 x 2km² to generate maps for the agricultural area at risk due to ozone exposure. Exposure of the agricultural area (defined as the CORINE land cover level-1 class 2 Agricultural areas, encompassing the level-2 classes 2.1 Arable land, 2.2 Permanent crops, 2.3 Pastures and 2.4 Heterogeneous agricultural areas) and forest areas (defined as the CORINE land cover level-2 class 3.1. Forests) have been calculated at the country-level.

AOT40 is used to be in line with the Air Quality Directive. However, in considering the latest scientific knowledge concerning vegetation ozone exposure, it should be noted that, at present, ozone impacts on vegetation are better modelled by fluxes of ozone into stomatal openings of vegetation .

Methodology for gap filling

In the AOT40-mapping, Türkiye has been excluded up to 2015 (the 2016 map was the first time it was included) due to the lack of reported measurements at rural background stations. In the exposure estimates, the number of countries has been growing since 2004. Figures 1 and 2 show the situation in individual years and not a trend.

Policy/environmental relevance

This indicator provides relevant information for the EU's Eighth Environmental Action Programme (8th EAP). The 8th EAP supports the objectives of the European Green Deal in line with the long-term objective to live well, within the planetary boundaries by 2050 at the latest, as already established in the 7th EAP.

The 8th EAP aims to accelerate the green transition to a climate-neutral, sustainable, non-toxic, resource-efficient, renewable energy-based, resilient and competitive circular economy in a just, equitable and inclusive way, and to protect, restore and improve the state of the environment by, inter alia, halting and reversing biodiversity loss. It supports and strengthens an integrated policy and implementation approach, building upon the European Green Deal.

Internationally, a first step to address air-pollution related impacts on health and the environment was the 1979 United Nations Economic Commission for Europe (UNECE) Geneva Convention on Long-range Transboundary Air Pollution (LRTAP or Air Convention).

A centrepiece of the convention is the 1999 Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone, subsequently amended in 2012. Critical ozone levels for vegetation were also defined under the Air Convention.

The Air Quality Directive sets both a target value (to be met in 2010) and a long-term objective for ozone for the protection of vegetation. The long-term objective is largely consistent with the long-term critical level of ozone for crops, as defined in the UNECE LRTAP Convention.

Targets

UNECE CLRTAP Gothenburg Protocol (1999; amended in 2012)

Using a stepwise approach and taking into account advances in scientific knowledge, the long-term target under the amended protocol is that atmospheric depositions or concentrations do not exceed for parties within the geographical scope of EMEP, the critical levels of ozone, as given in Annex I.

Annex I of the amended protocol includes a short definition of critical levels for ozone.

Critical levels for the protection of crops and forests (AOT40f) have also been defined under the Air Convention. The critical level for crops is consistent with the EU long-term objective for vegetation. The critical level for forests relates to the accumulated sum during the growing season (considered as April to September) and is set at 10,000μg/m³·h.

Air Quality Directive (2008/50/EC)

For the protection of vegetation from ozone exposure, the Air Quality Directive defines:

a) the target value for the protection of vegetation as AOT40-value (calculated from hourly values from May to July, considering the growing season) of 18,000 (μg/m³)·h, averaged over five years. This target value should be met in 2010 (2010 being the first year from which data will be used in the calculation over the following five years).

b) a long-term objective as AOT40-value (calculated from hourly values from May to July) of 6,000(μg/m³)·h, with no defined date of attainment.

In the assessment part of the indicator, the target value threshold is also considered. This is the target value considered only for one year and not for the averaged period of five years.

The European Green Deal

The European Green Deal aims, among other objectives, to protect, conserve and enhance the EU's natural capital. To achieve this aim, it is essential to increase the value given to protecting and restoring natural ecosystems and biodiversity. In this context, both the Zero pollution action plan and the EU Biodiversity Strategy for 2030 want to reduce the impact of air pollution in the loss of biodiversity and in the ability of ecosystems to provide natural services.

The 8th Environment Action Plan

The 8th EAP has the long-term priority objective that by 2050 at the latest, people live well, within the planetary boundaries in a well-being economy where nothing is wasted, growth is regenerative, climate neutrality in the Union has been achieved and inequalities have been significantly reduced. A healthy environment is an environment in which biodiversity is conserved, ecosystems thrive, and nature is protected and restored, leading to increased resilience to climate change, weather- and climate-related disasters and other environmental risks.

Furthermore, it has two thematic priority objectives related to ecosystems:

1. pursuing zero pollution, and protecting the health and well-being of ecosystems from environment-related risks and negative impacts.

2. protecting, preserving and restoring marine and terrestrial biodiversity by, inter alia, halting and reversing biodiversity loss and improving the state of ecosystems and their functions and the services they provide.

Methodology uncertainty

This indicator provides information on the area for which monitoring data is available. In previous years, yearly changes in monitoring density influenced the total monitored area. By using interpolated maps, this problem is largely solved; maps are less sensitive for changes in the central part of the network (though more sensitive for changes in the number of stations in the outskirts).

The indicator is also subject to year-to-year fluctuations as it is mainly sensitive to episodic conditions, and these depend on particular meteorological situations, the occurrence of which varies from year to year. When averaging over Europe, this meteorologically induced variation may be less, provided spatial data coverage is sufficient. Methodology uncertainty is also given by uncertainty in mapping AOT40 based on the interpolation of point measurements at background stations. The mean interpolation uncertainty of the map of AOT40 for crops is estimated to be about 35%.

Data sets uncertainty

Most data have been officially submitted to the Commission under the Exchange of Information Decision, the Implementing Decision on exchange of information and reporting and/or to EMEP under the Air Convention. Air quality monitoring station characteristics and representativeness may not be well documented, which may imply that stations that are not representative for background conditions have been included, probably leading to a slight underestimation of the indicator. Coverage of territory and time may be incomplete.

Rationale uncertainty

This indicator is rather sensitive to the precision at the reference level (40 ppb or 80 micrograms per m³).

Data sources and providers

Metadata

DPSIRTopicsTagsTemporal coverageGeographic coverage

TypologyUN SDGsUnit of measureFrequency of disseminationContact

References and footnotes

EU, 2008, Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe

^a^b^c^d
EEA, 2007, Air pollution by ozone in Europe in summer 2006, Technical report, 5/2007, European Environment Agency.

^↵
UNECE, 1979, Convention on Long-range Transboundary Air Pollution, United Nations Economic Commission for Europe.

^a^b^c
Loran, C., 2019, 'Manual for Modelling and Mapping Critical Loads & Levels', Umweltbundesamt (https://www.umweltbundesamt.de/en/manual-for-modelling-mapping-critical-loads-levels) accessed December 15, 2021.

^a^b^c^d
EMEP_Status_Report_1_2021.pdf, (https://emep.int/publ/reports/2021/EMEP_Status_Report_1_2021.pdf) accessed September 29, 2022.

^↵
Cressie, N. A. C., 1993, Statistics for spatial data, John Wiley & Sons, Inc., New York, NY.

^↵
EU, 1997, Council Decision of 27 January 1997 establishing a reciprocal exchange of information and data from networks and individual stations measuring ambient air pollution within the Member States (97/101/EC), OJ L 35, 5.2.1997, p. 14-22.

^↵
EU, 2011, Commission Implementing Decision of 12 December 2011 laying down rules for Directives 2004/107/EC and 2008/50/EC of the European Parliament and of the Council as regards the reciprocal exchange of information and reporting on ambient air quality (notified under document C(2011) 9068) (2011/850/EU), OJ L 335, 17.12.2011, p. 86-106.

^↵

Exposure of Europe's ecosystems to ozone

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

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

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Supporting information

Methodology for indicator calculation

Methodology for gap filling

Targets

Related policy documents

Methodology uncertainty

Data sets uncertainty

Rationale uncertainty

Metadata

References and footnotes