Indicator Assessment

Exposure of Europe's ecosystems to acidification, eutrophication and ozone

Indicator Assessment
Prod-ID: IND-30-en
  Also known as: CSI 005 , AIR 004
Published 02 Oct 2018 Last modified 11 May 2021
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  • In the EU-28, critical loads for acidification were exceeded in 7 % of the ecosystem area in 2010, down from 43 % in 1980. The figure also decreased to 7 % of the ecosystem area across all EEA member countries. There are still some areas where the interim objective for reducing acidification, as defined in the EU's National Emission Ceilings Directive, has not been met. 
  • The EU-28 ecosystem area in which the critical loads for eutrophication were exceeded peaked at 84 % in 1990 and decreased to 63 % in 2010 (55 % in the EEA member countries). The area in exceedance is projected to further decrease to 54 % in 2020 for the EU-28 (48 % in the EEA member countries), assuming current legislation is implemented. The magnitude of the exceedances is also projected to decline considerably in most areas, except for a few 'hot spot' areas in western France and the border areas between Belgium, Germany and the Netherlands, as well as in northern Italy.
  • Looking ahead, only 4 % of the EU-28 ecosystem area (3 % in EEA member countries) is projected to exceed acidification critical loads in 2020 if current legislation is fully implemented. The eutrophication reduction target set in the updated EU air pollution strategy proposed by the European Commission in late 2013, will be met by 2030 if it is assumed that all maximum technically feasible reduction measures are implemented, but it will not be met by current legislation.
  • For ozone, most of Europe's vegetation and agricultural crops are exposed to ozone levels that exceed the long term objective specified in the EU's Air Quality Directive. A significant fraction is also exposed to levels above the target value threshold defined in the directive. The effect-related concentrations show large year-to-year variations. Over the period 1996-2015, the concentrations observed at rural background stations increased until 2006, after which it decreased. After a six-year period (2009-2014) of relatively low values, the fraction of agricultural crops exposed to levels above the target value increased again to 30 % in 2015. However, at the low end of the exposure spectrum there was an increase in the area with levels below the long-term objective from 15 % (2014) to 21 % (2015).
  • During the past 5 years, around 58-62 % of the forest area was exposed to ozone concentrations above the critical level set by the United Nations Economic Commission for Europe (UNECE) for the protection of forests. 

Exposure of ecosystems to acidification

Note: The maps show the average accumulated exceedance of critical loads for acidification in 1980 (top left), 1990 (top right), 2000 (middle left), 2010 (middle right), 2020 under the revised Gothenburg Protocol scenario emission reduction agreements (bottom left) and 2030 assuming maximum technically feasible reduction (bottom right).

Exposure of ecosystems to eutrophication

Note: The maps show areas where critical loads for eutrophication of freshwater and terrestrial habitats are exceeded (CSI 005) by nitrogen depositions caused by emissions between 1980 (top left) and 2030 (bottom right)

Exposure of agricultural area to ozone in EEA member countries

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Agricultural area in EEA member countries for each ozone exposure class

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Rural concentration of the ozone indicator AOT40 for crops, 2015

Note: Accumulated ozone exposure values for crops — over a threshold of 40 parts per billion (AOT40c) — for 2015, as calculated for the fusion maps and as measured at rural background stations.

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Rural concentration of the ozone indicator AOT40 for forest, 2016

Note: Accumulated ozone exposure values for forest — over a threshold of 40 parts per billion (AOT40f) — for 2016, as calculated for the fusion maps and as measured at rural background stations.

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Exposure of forest area to ozone in EEA member countries

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Acidification and eutrophication

Exceedances of critical loads for acidificationAn assessment made by applying the scientific methods used when policy objectives were set shows that the 2010 interim objective for acidification set in the National Emission Ceilings Directive 2001/81/EC (NEC Directive) was achieved in the vast majority of grid cells across the EU. This means that the area at risk was reduced by more than 50 % in comparison with 1990 in all Member States, except for one grid cell located in northern Germany (EEA, 2012; Hettelingh et al., 2013).

Figure 1 shows the exposure and area at risk for European countries between 1980 and 2030, using updated scientific knowledge developed since 2000 (see 'policy context' in the Indicator Specification). The reduction of anthropogenic acidifying emissions since 1980 (see CSI040) is evident. For example, the areas shaded red on the maps (exceedances of more than 1 200 equivalents per hectare and year) show a clear reduction between 1980 and 2010. In the EU-28, the ecosystem area in which acidification critical loads were exceeded decreased from peak values of 43 % in 1980 to 7 % in 2010 (7 % across all EEA member countries). However, in 2010, several areas in the EU-28 did not meet the National Emission Ceilings Directive's (2001/81/EC) interim objective for acidification. When implementing current legislation, only 4 % of the EU-28 ecosystem area was projected to be in exceedance in 2020 (3 % in EEA member countries; see CCE 2014). A few hot spots can still be found particularly in the Czech Republic, the Netherlands and Poland. When considering uncertainties in the calculations, the projected area at risk in 2030 is not significantly further reduced if a 'maximum technically feasible reduction' (MTFR) scenario is assumed.

Exceedances of critical loads for eutrophication. An assessment performed on the basis of 2000 knowledge indicated that the area at risk of eutrophication declined by 34 % in the EU-27 as a whole, thus meeting the NEC Directive's objective at the EU level (EEA, 2012).

However, this is only the case when the original average nitrogen deposition modelling approach is employed to assess attainment. Figure 2 shows the development of eutrophication critical load exceedances in Europe starting in 1980, if present scientific knowledge is applied (see 'policy context' in the Indicator Specification). It also shows the projection of the exceedances in 2020 (current legislation scenario) and the projection of exceedances in 2030 according to an MTFR scenario. The EU-28 ecosystem area in which the critical loads for eutrophication were exceeded peaked at 84 % in 1990 and fell to 63 % in 2010 (55 % in EEA member countries). Most central European areas in which there were very high exceedances in 1980 (red shading: larger than 1 200 equivalents of nitrogen per hectare per year) are on track to be markedly reduced by 2020. However, modelling results predict that there will still be hot spots with very high exceedances in western France and the border areas between Belgium, Germany and the Netherlands, as well as in northern Italy.

According to the scenario analyses, the EU-28 area at risk due to atmospheric nutrient nitrogen deposition will decrease from 63 % in 2010 to 54 % in 2020 under the 2012 amended Gothenburg Protocol (the figures are 55 % and 48 % respectively for the EEA member countries). Thus, in 2020, more than 50 % of the EU ecosystem areas classified according to the European Nature Information System (EUNIS) are still expected to be at risk of excessive atmospheric nitrogen (N) deposition. However, the magnitude of exceedance is predicted to fall considerably in these areas (see the equivalent N per hectare and year ranges in Figure 2). In the EU-28, the area at risk of eutrophication is projected to decrease only slightly by 2030. The updated air pollution strategy proposed by the European Commission in late 2013 aims to achieve a situation in which the EU-28 ecosystem area exceeding critical loads for eutrophication is reduced by 35 % by 2030, relative to 2005. This target would be met in 2030 under an MTFR scenario, but would not be met if only current legislation were fully implemented.

Ground-level ozone pollution

The target value threshold for the protection of vegetation is exceeded in a substantial fraction of the agricultural area of the EEA-33 member countries. In 2016, this was the case in about 20 % of a total area of 2.444 million km2 (Figures 3 and 4), an increase of about 38 % compared with the previous year. Exceedances of the target value threshold have been observed in central, southern and eastern Europe (see red and dark red areas in Figure 5). The long-term objective has been met in about 23 % of the total agricultural area (green areas in Figure 5), mainly in Scandinavia, the Baltic States, the United Kingdom, Ireland and Iceland.

In 2003, the meteorological conditions were very favourable for ozone formation resulting in exceptionally high concentrations. Also, compared with other years, in June and July 2006, there were a large number of ozone episodes (EEA, 2007) resulting in a much higher accumulated ozone exposure value over a threshold of 40 parts per billion (AOT40). Compared with 2015, the AOT40 values in 2016 were higher in the northern countries and lower in central and southern Europe.

Figure 6 shows a map with the AOT40 results for forests (AOT40f). The gradients of the AOT40f values are similar to those of the AOT40 for crops/protection of vegetation: relatively low in northern Europe, with the highest values observed in the countries around the Mediterranean. The critical level was met (green areas) in 2015 in most of Estonia, Finland, Iceland, Ireland and the United Kingdom, Denmark, Latvia, Norway, and Sweden (the total forested area with concentrations below the critical level is 39 % of a total area of 1.69 million km2). In southern Europe the relatively high observed levels in 2015, decreased again and the mapping results show levels below 50 000 (µg/m3).h.

Figure 7 shows the exposure of forested areas between 2004 and 2016, when large variations were observed. While in 2004 and 2006, almost all forests were exposed to levels exceeding the critical level, in 2007 and 2016, 40 % of forests were exposed to levels lower than the critical level. This percentage has been more or less stable since then (with a decrease in 2008). However, in 2016, the area where levels in excess of 30 000 (µg/m3).h were decreased compared to 2015.



The ecosystem area at risk of acidification and the magnitude of exceedance in each country

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The ecosystem area at risk of eutrophication and the magnitude of exceedance in each country

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Annual variation in the ozone AOT40 value for crops (May-July)

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Acidification and eutrophication

In most European countries, the area at risk of acidification was below 10 % in 2010 and is projected to decrease further by 2020, when current legislation is fully implemented (Figure 8). In addition, the magnitude of exceedance was below 50 eq H+ per hectare and year in most countries in 2010, and will reach values close to zero in 2020. However, there are a few exceptions, particularly in the Czech Republic, Lithuania, the Netherlands and Poland where exceedances were still high in 2010, and the acidification problem is not predicted to be solved by 2020.

As SO2 emissions have fallen, the relative contribution made by ammonia (NH3) emitted from agricultural activities and nitrogen oxides (NOX) emitted from combustion processes to surface water and soil acidification has increased or become predominant in some regions in Europe.

The ecosystem area where critical loads for nutrient nitrogen are exceeded was still close to 100 % in many European countries in 2010 (Figure 9). Although a decrease is predicted by 2020 if current legislation is implemented, the exceedance area will be above 50 % in most countries. Extremely high magnitudes of exceedances can be found in countries exposed to high nitrogen deposition, both in 2010 and 2020, particularly in Denmark, Luxembourg, Hungary, the Netherlands and Switzerland. This is due to high nitrogen deposition rates and/or ecosystems, such as nutrient poor grasslands, which are very sensitive to an excess supply of nitrogen via the atmosphere (see for example EEA, 2014c).


Observed AOT40 concentrations for crops show large variation. Disregarding the extreme peaks in 2003 and 2006, the data show some decreasing tendency after 2001. Between 1996 and 2015, there were 271 rural background stations in 21 countries (with missing data from eastern Europe, for instance from Bulgaria, Greece or Romania) providing valid data during at least 15 of the 20 years (Figure 10). A total of 78 stations show a significant downward trend; and 34 stations show increasing trends though they are statistically significant at only three stations.



Supporting information

Indicator definition

This indicator shows the negative impact of air pollution on ecosystems and vegetation in Europe. In particular, it shows:

  • ecosystem areas with exceedances of the critical loads for acidification and eutrophication; and
  • exposure of areas covered with vegetation (crops and forests) to ground-level ozone, the latest available rural concentrations of ozone and the annual variation of rural concentrations of ozone at the European level.

In the case of acidification and eutrophication, the area as well as the magnitude of critical load exceedances in ecosystems are shown. A critical load is a quantitative estimate of an exposure to one or more pollutants, below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge (ICP on Modelling and Mapping, 2015; UNECE, 2015). It represents the upper limit of one or more pollutants deposited on the Earth's surface that an ecosystem, such as a lake or a forest, can tolerate without its function (e.g. the nutrient nitrogen cycle) or its structure (e.g. with respect to plant species' richness) being damaged.

A positive difference between the deposition loads of acidifying and/or eutrophying airborne pollutants and the critical loads is termed an 'exceedance'.

In the case of ozone, 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 (ICP on Modelling and Mapping, 2015; UNECE, 2015).  


Acidification and eutrophication 

  • The magnitude of the critical load exceedance is expressed in (acid or nitrogen) equivalents per hectare per year, i.e. as equivalents (H+ or N) per hectare per year (eq. ha–1 a–1)
  • For the ecosystems area, a percentage (%) of the total ecosystem area in each grid cell is calculated (for details see under ‘Methodology’).
  • Average accumulated exceedance (AAE) is the area-weighted average of exceedances, accumulated over all sensitive habitats (or ecosystem points) defined in a grid cell.



 Ozone concentrations: micrograms of ozone per cubic meter (µg/m³) or parts per billion (ppb). Note: 1 ppb ~ 2 µg/m³. 

    • AOT40 (Accumulated ozone exposure over a threshold of 40 parts per billion): the sum of the differences between hourly concentrations greater than 80 µg/m3 (= 40 parts per billion) and 80 µg/m3 accumulated over all hourly values measured between 08:00 and 20:00 Central European Time. For crops, the accumulation period is defined as 1 May to 31 July (growth period until harvest). For forest, the accumulation is defined as 1 April to 30 September (vegetation/growth period). AOT40 is expressed in (μg/m3)·hours.
    • AOT40 estimate: in cases where, due to missing values, all possible measured data are not available, the AOT40 values are calculated according to the following formula:

AOT40estimate = AOT40measured x [(total possible number of hours)/(number of measured hourly values)]

Where 'total possible number of hours' is the number of hours within the time period of the AOT40 definition, (i.e. 08:00 to 20:00 CET from 1 May to 31 July each year, for vegetation protection and from 1 April to 30 September each year for forest protection)

    • For the risk of ozone damage due to ozone exposure, a percentage (%) of the total agricultural/forest area in each country in excess of the reference level is calculated.



Policy context and targets

Context description

This indicator provides relevant information for the EU's Seventh Environmental Action Programme (7th EAP) and the new Clean Air Programme for Europe proposed by the European Commission at the end of 2013. The long-term strategic objective and core of the new air package is to attain 'air quality levels that do not give rise to significant negative impacts on, or risks for, human health and the environment'. The 7th EAP kept the intermediate objectives already set in the 6th EAP and the 2005 Thematic Strategy on Air Pollution to further reduce air pollution and its impacts on ecosystems and biodiversity by 2020. This will be accomplished by achieving full compliance with existing legislation. Furthermore, the long-term objective to not exceed critical loads and levels remains in place.

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 Convention).

A centrepiece of the convention is the 1999 Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone, subsequently amended in 2012. This protocol set national ceilings (limits) for the main air pollutants and established the critical loads concept as a tool to inform political discussions concerning damage to sensitive ecosystems. Critical ozone levels for vegetation were also defined under the LRTAP Convention.

The Gothenburg Protocol was followed in 2001 by the EU's National Emission Ceilings (NEC) Directive (EU, 2001), which set specific environmental objectives addressing acidification and eutrophication impacts on ecosystems, to be met by 2010. The directive introduced legally binding national emissions limits for four main air pollutants, including three important pollutants that contribute to acidification and eutrophication: sulphur dioxide (SO2), nitrogen oxides (NOX) and ammonia (NH3). Furthermore, the directive defines national emissions ceilings for ozone precursors, non-methane volatile organic compounds (NMVOCs) and NOx. The directive required EU Member States to have met emissions ceilings by 2010 and in subsequent years, although in reality around half of all Member States had missed at least one of their ceilings by 2010 (EEA, 2014b). A revision of the NEC Directive was part of the Clean Air Programme for Europe proposed by the European Commission at the end of 2013. The revised NEC Directive, which entered into force at the end of 2016 (EU, 2016), aims at compliance with the 2012 amended Gothenburg Protocol. In July 2017, the EU ratified the 2012 amendments to the 1999 protocol.

The Air Quality Directive (EU, 2008) 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 (UNECE, 2015), as defined in the UNECE LRTAP Convention.

When performing an assessment of progress made in reducing harm caused by air pollution, advances in scientific knowledge since the approval of the NEC Directive should clearly not be disregarded. These allow a more accurate picture to be obtained than by using the original techniques alone. Such developments include the facts that:

  • more complete and improved emissions inventories for the base year (1990) are now available;
  • the scientific knowledge on environmental impacts has improved, including the development of ecosystem-specific critical loads;
  • the air pollution dispersion modelling approach used by the LRTAP Convention's European Monitoring and Evaluation Programme (EMEP) to calculate the impacts of air pollution has changed; and
  • the level of detail of air pollution modelling computation has improved, with results now available on a 50 x 50 km2 and 28 x 28 km2 grid cell basis, as compared to the older 150 x 150 km2 grid used at the time of determining the NEC Directive ceilings and its interim objectives.


  • National Emission Ceilings Directive (NECD) (EU) 2016/2284

Under the revised NEC Directive, the 2010 emission ceilings remain applicable until 2019 (EU, 2016). The new NEC Directive also sets emission reduction commitments for SO2, NOx, NH3, NMVOCs and primary fine particulate matter (PM2.5), relative to the emissions in 2005, for the years 2020 to 2029. Greater reduction commitments will take effect from 2030.

Concerning effects of air pollution on human health and ecosystems, the Directive states: 'Member States should implement this Directive in a way that contributes effectively to achieving the Union's long-term objective on air quality, as supported by the guidelines of the World Health Organization, and the Union's biodiversity and ecosystem protection objectives by reducing the levels and deposition of acidifying, eutrophying and ozone air pollution below critical loads and levels as set out by the LRTAP Convention.'


  • 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:

a) for parties within the geographical scope of the European Monitoring and Evaluation Programme (EMEP) and Canada, the critical loads of acidity, as described in annex I, that allow ecosystem recovery;

b) for parties within the geographical scope of EMEP, the critical loads of nutrient nitrogen, as described in annex I, that allow ecosystem recovery; and

c) 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 loads for acidification/eutrophication and critical levels for ozone.

Critical levels for the protection of crops and forests (AOT40f) have also been defined under the LRTAP Convention (UNECE, 2015). 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 10 000 μg/m3·h.


  • Air Quality Directive (2008/50/EC)

For the protection of vegetation from ozone exposure, the Air Quality Directive (EU, 2008) 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 5 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 5 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 1 year and not for the averaged period of 5 years.


  • The Clean Air Programme for Europe

New air policy objectives for 2030 are specified in the Clean Air Programme for Europe proposed by the European Commission in 2013, in line with the long term objective of reaching no exceedance of the critical loads and levels: in 2030, the ecosystem area exceeding eutrophication limits will be 35 % of the area in 2005.

Related policy documents

  • 7th Environment Action Programme
    DECISION No 1386/2013/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 20 November 2013 on a General Union Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. In November 2013, the European Parliament and the European Council adopted the 7 th EU Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. This programme is intended to help guide EU action on the environment and climate change up to and beyond 2020 based on the following vision: ‘In 2050, we live well, within the planet’s ecological limits. Our prosperity and healthy environment stem from an innovative, circular economy where nothing is wasted and where natural resources are managed sustainably, and biodiversity is protected, valued and restored in ways that enhance our society’s resilience. Our low-carbon growth has long been decoupled from resource use, setting the pace for a safe and sustainable global society.’
  • 1999 Protocol to Abate Acidification, Eutrophication and Ground-level Ozone
    Convention on Long-range Transboundary Air Pollution 1999 Protocol to Abate Acidification, Eutrophication and Ground-level Ozone, amended on 4 May 2012.
  • A Clean Air Programme for Europe
    Communication from the Commission to the Council, the European Parliament, the European Economic and Social Committee and the Committee of the Regions - "A Clean Air Programme for Europe", COM(2013) 918 final
  • Directive (EU) 2016/2284, reduction of national emissions of certain atmospheric pollutants
    The directive is amending Directive 2003/35/EC (providing for public participation in respect of the drawing up of certain plans and programmes relating to the environment) and repealing Directive 2001/81/EC. It entered into force at the end of 2016 and aims at compliance with the 2012 amended Gothenburg Protocol. In July 2017, the EU ratified the 2012 amendments to the 1999 protocol.
  • Directive 2001/81/EC, national emission ceilings
    Directive 2001/81/EC, on nation al emissions ceilings (NECD) for certain atmospheric pollutants. Emission reduction targets for the new EU10 Member States have been specified in the Treaty of Accession to the European Union 2003  [The Treaty of Accession 2003 of the Czech Republic, Estonia, Cyprus, Latvia, Lithuania, Hungary, Malta, Poland, Slovenia and Slovakia. AA2003/ACT/Annex II/en 2072] in order that they can comply with the NECD.
  • Directive 2008/50/EC, air quality
    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.
  • Thematic Strategy on Air Pollution
    Communication from the Commission to the Council and the European Parliament - Thematic Strategy on air pollution (COM(2005) 0446 final)
  • UNECE Convention on Long-range Transboundary Air Pollution
    UNECE Convention on Long-range Transboundary Air Pollution.


Methodology for indicator calculation


  • Acidification and eutrophication (critical loads)

For the area where critical loads for acidification or eutrophication are exceeded, a percentage of the total ecosystem area in each grid cell can be calculated. 'Total ecosystem area' is defined as the area of ecosystem-types classified according to the EUNIS Habitats classification (EEA, 2014d). The European background information used around 2001, when the old NEC Directive (EU, 2001) was adopted, covered only forest ecosystems (EUNIS class G) and freshwaters. Now semi-natural vegetation (EUNIS classes D, E and F) is also included. For details on changes in the scientific knowledge base since 2001, please see EEA (2012 and 2014c); Hettelingh et al. (2013) and CCE (2014).

An AAE can be computed for an ecosystem, a grid cell and any region or country for which multiple critical loads and deposition values are available.

The European database of critical loads for acidification and eutrophication used in this indicator is compiled by the Coordination Centre for Effects (CCE) under the Convention on Long-range Transboundary Air Pollution. CCE applies methods that are described in detail in CCE Status Reports and adopted by the Task Force of the International Cooperative Programme on Modelling and Mapping (ICP M&M) in the so-called Mapping Manual (UNECE, 2015), for use by National Focal Centres (NFCs) under the ICP M&M. NFCs compute critical loads and submit data to the CCE at regular intervals following consensus of Parties to the LRTAP Convention (including most EEA-33 member countries). 

  • Ozone

AOT40 estimated values are calculated from hourly data (EU, 2008) 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 (EMEP, 2014), altitude field and surface solar radiation in a linear regression model, followed by the interpolation of its residuals by ordinary Kriging (Horalek et al, 2013). For altitude, dataset GTOPO30 (Global Digital Elevation Model) at a resolution of 30 x 30 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 (N. Cressie, 1993) 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 CLC2006 map. The resolution was 500 x 500 m2 to generate maps for the agricultural area at risk due to ozone exposure. Exposure of the agricultural area (defined as the 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 land cover level-2 class 3.1. Forests) have been calculated at the country-level.

The temporal trends have been estimated using a Mann-Kendal statistical test. This test is particularly useful since missing values are allowed and the data need not conform to any particular distribution. Moreover, as only the relative magnitudes of the data rather than their actual measured values are used, this test is less sensitive to incomplete data capture and/or special meteorological conditions leading to extreme values (Gilbert, 1987).

AOT40 is used to be in line with the Air Quality Directives. 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 (Mills et al., 2011a and 2011b).

Methodology for gap filling

  • Acidification and eutrophication
    The Coordination Centre for Effects under the LRTAP Convention uses a European background database to compute critical loads for ecosystems in countries that do not submit data.
  • Ozone
    In the AOT40-mapping, Turkey has to be excluded 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 show the situation in individual years and not a trend.

Methodology references

  • CCE, 2014 Modelling and Mapping the impacts of atmospheric deposition on plant species diversity in Europe, CCE Status Report 2014, RIVM report no. 2014-0075. 
  • N. Cressie, Statistics for spatial data. Wiley, New York, 1993.
  • EEA, 2012 Evaluation of progress under the EU National Emission Ceilings Directive. Progress towards EU air quality objectives, EEA Technical report No 14/2012, European Environment Agency
  • EEA, 2014c Effects of air pollution on European ecosystems, Past and future exposure of European freshwater and terrestrial habits to acidifying and eutrophying air pollutants, EEA Technical report No 11/2014. 
  • EEA, 2014d 'About the European Nature Information System, EUNIS', European Environment Agency.
  • EMEP, 2014 Transboundary particulate matter, photo-oxidants, acidifying and eutrophying components, EMEP Status Report 2014.
  • Gilbert, R. O., 1987 Statistical Methods for Environmental Pollution Monitoring. Van Nostrand Reinhold, New York
  • Hettelingh, J.-P., Posch, M., Velders, J.M., Ruyssenaars, P., Adams, M., de Leeuw, F., Lükewille, A., Maas, R., Sliggers, J., Slootweg, J.  Assessing interim objectives for acidification, eutrophication and ground-level ozone of the EU National Emissions Ceilings Directive with 2001 and 2012 knowledge, Atmospheric Environment 75: 129-140.  2013.
  • Jan Horálek, Peter de Smet, Pavel Kurfürst, Frank de Leeuw, Nina Benešová, European air quality maps of PM and ozone for 2011 and their uncertainty. ETC/ACC Technical Paper 2013/13.  2013.
  • Mills, G., dHayes, F., Simpson, D., Emberson, L., Norris, D., Harmens, H., and Buker, P.: Evidence of widespread effects of ozone on crops and (semi-)natural vegetation in Europe (1990–2006) in relation to AOT40-and flux-based risk maps, Global Change Biol., 17, 592–613, doi:10.1111/j.1365-2486.2010.02217.x, 2011a.
  • Mills, G., Pleijel, H., Braun, S., Buker, P., Bermejo, V., Calvo, E., Danielsson, H., Emberson, L., Fernandez, I., Grunhage, L., Harmens, H., Hayes, F., Karlsson, P., and Simpson, D.: New stomatal flux-based critical levels for ozone effects on vegetation, Atmos. Environ., 45, 5064–5068, doi:10.1016/j.atmosenv.2011.06.009, 2011b.
  • ETC/ACM 2015 European air quality maps for 2015, ETC/ACM provided by European Topic Centre on Air pollution and Climate change mitigation (ETC/ACM)  


Methodology uncertainty

  • Acidification and eutrophication

Since the critical loads exceedance approach is a tool used in policy-related analysis, the assessment of biases and robustness of approach is a major focus when addressing uncertainties. This assessment is also determined by the uncertainties in the emissions and atmospheric dispersion modelling estimates, and the scenarios used. Sensitivity analyses using, for example, different emissions estimates for future years or excluding the correlation in the dispersion of pollutants give information on the uncertainty range (Hettelingh et al., 2012).

A comprehensive uncertainty analysis of the integrated assessment approach, including ecosystem effects (critical loads) was compiled by Suutari et al. (2001).


  • Ozone

    This indicator provides information on the area for which monitoring information 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

  • Acidification and eutrophication

National Focal Centres compute critical loads and submit data to the Coordination Centre for Effects (CCE) at regular intervals following the consensus of Parties to the LRTAP Convention (including most EEA-33 member countries). The CCE uses a European background database to compute critical loads for ecosystems in countries that do not submit data.

Five main categories for uncertainties in data inputs have been suggested for the calculation of the critical loads themselves, with category 1 being the least certain (Skeffington, 2006):

  1. Expert judgement by single expert – no supporting data;
  2. Consensus judgement by group of experts;
  3. Uncertainty estimated from observations, but not at this particular site;
  4. Uncertainty estimated from model calculations based on general knowledge or theory;
  5. Uncertainty estimated from site-specific observations. 

Skeffington (2006) concludes in his literature review analysis that 'although the values for the input parameters are often poorly known, the resulting critical loads are not so uncertain as to make them unusable for environmental policy development'.

  • Ozone

    Most data have been officially submitted to the Commission under the Exchange of Information Decision (EU, 1997), the Implementing Decision on exchange of information and reporting (EU, 2011) and/or to EMEP under the LRTAP 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

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

Data sources

Other info

DPSIR: State
Typology: Performance indicator (Type B - Does it matter?)
Indicator codes
  • CSI 005
  • AIR 004
Frequency of updates
Updates are scheduled once per year
EEA Contact Info


Geographic coverage

Temporal coverage