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Exposure of ecosystems to acidification, eutrophication and ozone


Justification for indicator selection

Excess deposition of air pollutants can lead to disturbances in the function and structure [1] of ecosystems. Deposition of sulphur and nitrogen compounds contributes to the acidification of soils and freshwaters. Negative effects are the leaching of plant nutrients from soils and damage to flora and fauna (changes in biodiversity). Deposition of nitrogen compounds can lead to an oversupply of nutrient nitrogen in terrestrial and water ecosystems. Effects can be changes in vegetation abundances or leaching of nitrate to groundwater.

The risk of damage of a sensitive ecosystem receptor at a certain location can be evaluated by comparing the estimated deposition of acidifying and eutrophying air pollutants to the critical load for that location. The critical load is the deposition below which adverse effects on specified sensitive elements of an ecosystem do not occur according to present knowledge. Critical loads for several (semi-)natural areas in Europe are computed under the Convention on Long-range Transboundary Air Pollution by the Coordination Centre for Effcets (CCE; see Hettelingh et al., 2008). Thus, an ecosystem is at risk of acidification or eutrophication when its critical load is exceeded by acidifying and eutrophying air pollutants, respectively. When critical loads are exceeded the actual damage to sensitive elements of an ecosystem may involve a time delay, dependent on soil, water and vegetation properties as well as combined effects due to for example climate change.

Integrated assessment models applied for assessing the effects of air pollutant mitigation measures include information on air emissions, atmospheric dispersion and depositions of air pollutants and critical loads for a range of sensitive European ecosystems (e.g. the RAINS/GAINS model [2]. Such models have been used in support of negotiations of the sulphur protocol (Oslo, 1994) and the multi-pollutant multi-effect Protocol (Gothenburg, 1999) under the 1979 UNECE Convention on Long Range Transboundary Air Pollution (LRTAP Convention). Reductions in critical loads exceedances as short and long term objectives are also addressed in the European Union's (EU) National Emissions Ceiling Directive (NECD; 2001/81/EC) [3]. The NECD, sets slightly stricter emissions ceilings for EU Member States than those negotiated under the LRTAP Convention protocols.

Ground level ozone is one of the most prominent air pollution problems in Europe, mainly due to effects on human health, crops, and natural ecosystems. Threshold levels aiming at the protection of human health and vegetation have been set by the EU in the Air Quality Directive 2008/50/EC. Critical ozone levels for vegetation were further defined under the Convention on Long-range Transboundary Air Pollution (CLRTAP). Those environmental quality standards are exceeded in wide areas of Europe to a high extend. Ozone is a secondary pollutant formed in the atmosphere. Important precursors in Europe are nitrogen oxides and volatile organic compounds, and - to a lesser extent - carbon monoxide and methane. There is a strong chemical interaction between ozone and nitrogen oxides. Close to source, freshly emitted nitrogen monoxide may react very fast with ozone resulting in a depletion of ozone while nitrogen dioxide is formed; at larger distances to the source photochemical ozone formation might occur. High temperatures and sunlight favour ozone formation. Tropospheric ozone is further an important greenhouse gas (positive radiative forcing effect)

[1] Function = for example the ability of forest soils to buffer airborne acidifying pollutants without excessive leaching of plant nutrients to ground and surface waters; structure = e.g. loss of (protected) species
[3] The NEC Directive is currently under revision

Scientific references:

  • No rationale references available

Indicator definition

The indicator shows the ecosystem or crops areas at risk of exposure to harmful effects of ozone as a consequence of air pollution, and shows the state of change in acidification, eutrophication and ozone levels of the European environment. The risk is estimated by reference to the 'critical level' for ozone for each location, this being a quantitative estimate of the exposure to these pollutants below which significant and harmful effects do not occur in the long term at present knowledge.

The fraction of agricultural crops that is potentially exposed to ambient air concentrations of ozone in excess of the EU target value and long-term objective set for the protection of vegetation is also shown.

  • Eutrophication and acidification
    Critical loads of acidity and of nutrient nitrogen are employed to describe exposure to acidification and to eutrophication for forests and semi-natural areas in Europe, including Natura 2000 sites. The area where the deposition of acidifying and eutrophying pollutants is in exceedance of critical loads provides also an indication of the extent of European ecosystem area which is at risk of damage to biodiversity. By analysing the change of exceedances over time (comparative static analysis) an indication of the effects of changing air pollutant emissions over time is obtained. The magnitude of the exceedance (deposition minus critical load) is an important input to the dynamic modelling of time delays in damage. Inversely, once critical loads are no longer exceeded, recovery may take some time as well. By including the risk to be met within a legislative target and year the distance from this target can be evaluated.
  • Ozone
    AOT40 is 'Accumulated ozone exposure over a threshold of 40 ppb'. The indicator shows the ecosystem or crop areas at risk of exposure to harmful levels of ozone as a consequence of air pollution. The risk is estimated by referring to the 'critical level' of ozone for sensitive areas. Thus, the indicator is a quantitative estimate of the exposure to ozone below which significant and harmful effects do not occur in the long term according to present knowledge.
    The fraction of agricultural crops that is potentially exposed to ambient air concentrations of ozone in excess of the EU target value set for the protection of vegetation is also shown.


Eutrophication and acidification

  • Regions at risk: % of total sensitive ecosystem area
  • Critical loads/threshold, depositions, exceedance:
           - Acidifying equivalents (H+) per hectare and year (eq H+.ha-1.a-1)
           - Eutrophication equivalents (N) per hectare and year(eq N.ha-1.a-1)
  • Change over time: % of change compared to base year.


  • AOT40: means 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 8.00 – 20.00 Central European Time. For crops the accumulation is from 1 May to 31 July. For forest the accumulation is over the summer period (1 April – 30 September).  each day ozone concentrations. AOT40 is expressed in (μg/m3).hour
  • Regions at risk: % of total agricultural area
  • Change over time: % of change compared to base year.
  • Percentage of the arable land in Europe potentially exposed to ambient air concentrations of ozone (O3) in excess of the EU target value set for the protection of vegetation.

Policy context and targets

Context description

This indicator is relevant information for the EU's 6th Environmental Action Programme (6EAP) and the Thematic Strategy on Air Pollution. The 6EAP sets the long-term objective of not exceeding critical loads.
A combined ozone, acidification and eutrophication abatement strategy has been developed by the European Commission, resulting in the National Emission Ceiling Directive (2001/81/EC) and the CAFE Thematic Strategy. In this legislation, target values have been set for air pollutant emissions causing acidification and eutrophication, as well as for ozone levels and for ozone precursor emissions. The EU legislation sets for ozone both a target value (to be met in 2010) and a long-term objective. This long-term objective is largely consistent with the long-term critical level of ozone for crops as defined in the UNECE LRTAP Convention protocols to abate acidification, eutrophication and ground level ozone.
Within the LRTAP Convention there is a discussion whether a concentration-base or a flux-based critical level is the best indicator for the impact on ecosystems (see, for example, EMEP,2010). As the target value and long-term objective in air quality directive are concentration-based, the AOT40 has been chosen here as relevant parameter.


  • National Emission Ceilings Directive 2001/81/EC, Article 5
    The aim of the directive is to 'limit emissions of acidifying and eutrophying pollutants and ozone precursors in order to improve the protection in the Community of the environment and human health against risks of adverse effects from acidification, soil eutrophication and ground-level ozone and to move towards the long-term objectives of not exceeding critical levels and loads… by establishing national emission ceilings, taking the years 2010 and 2020 as benchmarks National emission ceilings with interim environmental objectives for the Community as a whole...'.
    The following interim environmental objectives, for the Community as a whole, by 2010 has been set for acidification: The areas where critical loads are exceeded shall be reduced by at least 50 % (in each grid cell) compared with the 1990 situation.
    The interim environmental objective for vegetation-related ground-level ozone exposure is: By 2010 the ground-level ozone load above the critical level for crops and semi-natural vegetation (AOT40 = 3 ppm.hour) shall be reduced by one-third in all grid cells compared with the 1990 situation. In addition, the ground-level ozone accumulated concentration shall not exceed an absolute limit of 10 ppm.hour, expressed as an exceedance of critical accumulated concentration in any grid cell.

  • UNECE CLRTAP Gothenburg Protocol (1999)
    To abate acidification, eutrophication and ground level ozone it sets emission limits with target dates. Whilst environmental quality objectives are not specified, full attainment of emission targets is intended to bring an improvement in the state of the environment estimated at:
    1. Reduction in the European area with excessive levels of acidification from 93 million ha in 1990 to 15 million ha in 2010, and with excessive eutrophication from 165 million ha in 1990 to 108 million ha in 2010. The number of days with excessive ozone levels will be halved. The exposure of vegetation to excessive ozone levels will be 44 % less in 2010 compared to 1990.
    2. Long-term objective, atmospheric depositions will not exceed critical loads of acidity and critical loads of nutrient nitrogen.
  • Air Quality Directive (2008/50/EC)
    For the protection of vegetation to ozone exposure the Air Quality Directive defines:
    1. The target value for the protection of vegetation as AOT40-value (calculated from hourly values from May to July) of 18 (mg/m3).h, averaged over five years. This target value should be met in 2010.
    2. A long-term objective of 6 (mg/m3).h.

Related policy documents


Methodology for indicator calculation

  • Acidification and eutrofication
    Air emission data is reported annually by national authorities to UNECE/EMEP (Convention on Long-range Transboundary Air Pollution) and to the European Community. Reported data includes both newest estimates (two years in arrears) and recalculated emissions from previous years. Emission data is stored and verified at EMEP/CEIP (Centre on Emission Inventories and Projections) [1].
    Using these emissions, EMEP/MSC-W [2] calculates atmospheric transport of sulphur and nitrogen pollutants using the EMEP Unified Model at a spatial resolution of 50 x 50 km2 and according to modelled meteorological conditions adjusted towards observations.
    The Coordination Centre for Effects (CCE) has produced an update of the critical loads database in 2008 (Hettelingh et al., 2008) [3] for use in support of revisions of European air pollution agreements. In 2004 the CCE updated this database with national updates of critical loads (see below section on gap-filling where countries did not provide data). The CCE collaborates with IIASA (CIAM) [4] and EMEP to assess information on ecosystem specific deposition which the CCE then uses to compute and map exceedances in European natural areas including Natura 2000 areas.
    Nitrogen and sulphur deposition in each model grid-cell are used for calculation of the average accumulated exceedances of the critical loads, which is the area-weighted average of exceedances accumulated over all ecosystem points in an EMEP grid cell. The total area of ecosystems exposed to exceedances in a country is expressed as a percentage of the total country area. These areas are summed up to provide two estimates, one for the EU-27 Member States, and for one for a larger region comprising most countries that are Parties to the Convention on Long-range Transboundary Air Pollution (including the non-EU EEA member countries and the EEA cooperating countries).
  • Ozone
    According to the definition in the ozone directive, AOT40 values are calculated from hourly data measured between 08.00 and 20.00 CET at all rural background stations available in AirBase. For crops AOT40 is accumulated during the three month summer period (May-July); for forest accumulation is during the full summer (April-September). Only data series with more than 75 % valid data were considered.

    The AOT40 maps have been created by combining measurements data from the rural background stations combined with the results of the EMEP dispersion model [Fagerli et al 2004] altitude field and surface solar radiation in a linear regression model, followed by the interpolation of its residuals by ordinary kriging [see de Smet et al, 2009 and reference cited therein for more details]. As altitude dataset GTOPO30 (Global Digital Elevation Model) at a resolution of 30 x 30 arcsecond has been used [ESRI, Redlands, California, USA, 2005]. The solar radiation has been obtained from ECMWF [ECMWF: Meteorological Archival and Retrieval System (MARS). It is the main repository of meteorological data at ECMWF].  Kriging is a method of spatial statistics (see e.g. N. Cressie, Statistics for spatial data, New York, 1993) which 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, (ii) on the overall spatial arrangement among the measured points. The type of kriging at its parameters (in particular the parameters describing the semivariogram) are chosen in order to minimize the RMS error.

    The AOT40 maps have been overlayed in a GIS with the land cover CLC2000 map. The resolution was 500 x 500 m2 to generate maps for the agricultural area at risk due to ozone exposure. Exposure of 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 to 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 towards incomplete data capture and/or special meteorological conditions leading to extreme values.(see Gilbert, R.O., 1987. Statistical Methods for Environmental Pollution Monitoring. Van Nostrand Reinhold, New York).


Methodology for gap filling

  • Acidification and eutrophication
    National submissions are used where available. For European countries which have never submitted national totals the CCE uses its European background critical load database (Hettelingh et al., 2004). Turkey has not been included in the analysis due to a not sufficient data basis for calculating critical loads.
  • 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 Switzerland has only been included since 2008 onwards.

Methodology references

No methodology references available.

Data specifications

EEA data references

External data references

Data sources in latest figures


Methodology uncertainty

  • Critical loads
    A comprehensive uncertainty analysis of the integrated assessment approach, including ecosystem effects (critical loads) was compiled by Suutari et al. (2001) [1].
  • Ozone
    The air quality data is officially submitted according to the Exchange of Information decision (Council Decision 97/101/EC). It is assumed that the air quality data has been validated by the national data supplier. Station characteristics and representativeness is often insufficiently documented, which may imply that stations that are not representative for background conditions have been included. Methodology uncertainty is 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 %.

[1] Suutari, R., Amann, M., Cofala, J. Klimont, Z., Schöpp, W. and Posch, M. (2001): From Economic Activities to Ecosystem Protection in Europe – An Uncertainty Analysis of Two Scenarios of the RAINS Integrated Assessment Model:

Data sets uncertainty

  • Ozone
    Most data have been officially submitted to the Commission under the Exchange of Information Decision (and/or to EMEP under the UN ECE Convention). Air quality monitoring station characteristics and representativeness are often not well documented and coverage of territory and in time is incomplete. The different definition of AOT40-values (accumulation during 8.00 to 20.00 CET following the Ozone Directive versus accumulation during daylight hours following the definition in the NEC Directive) is expected to introduce minor inconsistencies in the data sets. The indicator as chosen provides information on the area for which monitoring information is available. Yearly changes in monitoring density will influence the total monitored area. Due to deficiencies in meta-information, the selection of background sites may include some non-background stations, probably leading to a slight underestimation of the indicator.

    The indicator is 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. For instance, the relatively favourable values for 1998 are largely due to unfavourable condition for ozone formation (in other words: '1998 was a bad summer'). 2003 was a hot 'high-ozone' summer in most of Europe. When averaging over Europe this meteorologically induced variation may be less, provided spatial data coverage is sufficient.

    In spite of a generally reasonable level of accuracy and precision of ozone measurements, the indicator is rather sensitive to the precision at the reference level (40 ppb or about 80 micrograms/m3), and to the accuracy of measured ozone levels. Moreover, the number of available data series varies considerably from year to year and for some years it is very low.

Rationale uncertainty

No uncertainty has been specified

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Work description

Introduce a country split, also for the ozone part of the indicator. Follow-up new developments in critical level calculations (using O3 fluxes) and projections based on further developed methodologies. Critical Loads: Add column showing total country area, respectively.

Resource needs

No resource needs have been specified


Not started


2012/12/31 00:00:00 GMT+1

Long term work

Work specified here will require more than 1 year (from now) to be completed.

General metadata

Responsibility and ownership

EEA Contact Info

Michel Houssiau


European Environment Agency (EEA)


Indicator code
CSI 005
Version id: 2
Primary theme: Air pollution Air pollution


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Frequency of updates

Updates are scheduled once per year in October-December (Q4)


DPSIR: State
Typology: Performance indicator (Type B - Does it matter?)
European Environment Agency (EEA)
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