Indicator Assessment

Exposure of ecosystems to acidification, eutrophication and ozone (version 1)

Indicator Assessment
Prod-ID: IND-30-en
  Also known as: CSI 005
Published 14 Nov 2005 Last modified 11 May 2021
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  • Eutrophication has fallen slightly since 1980. However, only limited further improvement is expected by 2010 with current plans.
  • There have been clear reductions in acidification of Europe's environment since 1980, but with some tailing off in that improvement after 2000.
  • Most agricultural crops are exposed to ozone levels exceeding the EU long term objective and a significant fraction is exposed to levels above the target value. AOT40 values showed an increasing tendency over the last years.

EU-25 and European-wide ecosystem damage area (average accumulated exceedance of critical loads), 1980-2020

Note: Data source of deposition-data used to calculate exceedances: EMEP/MSC-W

Data source:

UNECE - Coordination Center for Effects

Exposure of crops to ozone (exposure expressed as AOT40 in (mg/m3).h) in EEA member countries, 1996-2002

Note: The target value for protection of vegetation is 18 (mg/m3).h while the long-term objective is set to 6 (mg/m3).h

Data source:

Air quality database Airbase (ETC/ACC)

Exposure above AOT40 target values for vegetation around rural ozone stations (EEA member countries), 2002

Note: Reference period: May - July 2002

Data source:



There have been substantial reductions in areas subjected to deposition of excess acidity since 1980 (Figure 1). Current data, however, makes it difficult to assess the quantitative improvements since 1990 (these being the standards established in the National Emissions Ceilings Directive, NECD, 2001/81/EC) as acidification status in this base year (1990) remains to be estimated using the latest critical loads and deposition calculation methodology.

Progress on a country basis indicates that by 2000 all except six countries had less than 50% of their ecosystem areas in exceedance of acidity critical loads. Further substantial progress is anticipated for virtually all countries in the period 2000-2010. The current status in the countries of the EU-25 remains poorer than across the broader European continent.

Evaluating progress in achieving the NECD acidification 50% reduction target is hampered not only by recently revised modelled deposition estimates and the lack of updated 1990 data, but by the changes in critical load assessments completed by countries themselves since negotiation of the Directive. Difficulties in managing 'double counting' (areas containing critical loads for more than one ecosystem type) exist for more than one country.


Eutrophication shows less progress (Figure 1). There have been limited improvements at the European level since 1980, and very little further improvement is expected in some countries between 2000 and 2010. The broader European continent continues to have a lesser problem than the countries of the EU-25.


There is a substantial fraction of the arable area in EEA-31+ countries (1) (in 2002, about 38% of a total area of 133 million ha) where the target value is exceeded (Figure 2 and 3). The long-term objective is met in less than 9% of the total arable area, mainly in the UK, Ireland and the northern part of Scandinavia.

(1) Due to missing information Bulgaria, Cyprus and Turkey are not included in the analyses; Switzerland is included.

Exceedance of the critical loads for eutrophication in Europe (as average accumulated exceedances), 2000

Note: Distribution of the magnitude of average accumulated exceedance of the critical loads for eutrophication in 2000.

Data source:

UNECE - Coordination Center for Effects; Deposition-data: EMEP/MSC-W

Exceedance of the critical loads for acidity in Europe (as average accumulated exceedances), 2000

Note: Distribution of the magnitude of average accumulated exceedance of the critical loads for eutrophication in 2000.

Data source:

UNECE - Coordination Center for Effects; Deposition-data: EMEP/MSC-W

Country-wise ecosystem damage area for eutrophication in Europe, 2000-2010

Note: N/A

Data source:

UNECE - Coordination Center for Effects

Country-wise ecosystem damage for acidity in Europe, 2000-2010

Note: N/A

Data source:

UNECE - Coordination Center for Effects

The maps (Figure 4 and 5) show the spatial distribution over Europe - as additional illustration to the key message - on acidification and eutrophication.

The country-wise barcharts (Figure 6 and 7) show - as additional illustration to the key message - the progress made and to be expected in each European country.

Annual variation in the ozone AOT40 value (May-July), (EEA member countries), 1995-2003

Note: Average values over all rural stations which reported data over at least six years in the period 1996-2002

Data source:

Airbase (ETC/ACC)

Strong year-to-year fluctuations both in meteorological conditions as well as in the number and location of monitoring stations and lack of data prevent general conclusions on time trends of the exposed area. However, using the observed concentration data, an analysis of recent trends (1996-2002) can be made for about 200 rural stations which have provided AOT40 values for at least six annual periods. Although the variation in measured AOT40 values is large, the overall averaged value shows a increasing tendency over the last years (Figure 8 and 9).

A data summary for EEA countries(2) of crops (in 106 ha) for each exposure class;
no data refers to the area for which data is lacking (see fotenote (2)); total refers to the total area of crops in the EEA-31 countries and Switzerland:

Crops areas (106 ha) exposed to AOT40 levels

AOT40 values
0-6 mg/m3.h
6-12 mg/m3.h
12-18 mg/m3.h
> 18 mg/m3.h
covered area
no data
no coverage

(2) The fraction labelled with "no information" refers to areas in Greece, Iceland, Norway, Sweden, Estonia, Lithuania, Latvia, Malta, Romania, and Slovenia for which either no ozone data from rural background stations or no detailed land cover data is available. The countries Bulgaria, Cyprus, and Turkey are not included.

Supporting information

Indicator definition

The indicator shows the ecosystem or crops areas at risk of exposure to harmful effects of acidification, eutrophication and 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 load' for acidification and eutrophication and 'critical level' for ozone for each location, this being a quantitative estimate of the exposure to these pollutants below which significant and harmful such effects do not occur in the long term at present knowledge.

Two critical loads, for acidity and for nutrient nitrogen, are employed to describe exposure to acidification and to eutrophication respectively. The area over which the deposition of acidifying and eutrophying pollutants is in exceedance of critical loads provides an indication of the ecosystem area in which such damage could occur. The magnitude of the potential risk is displayed as the percentage of total ecosystem areas exposed to exceedence of these critical loads.  By showing the change in risk over time, the state of change in acidification and eutrophication is displayed. By including the risk to be met within a legislative target and year the distance from this target is displayed.

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.


  • Regions at risk: % of total ecosystem area
  • Critical loads/threshold, depositions, exceedance: acidifying equivalents (H+) per hectare per year (eq H+.ha-1.a-1).
  • 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

No context has been specified


No targets have been specified

Related policy documents

No related policy documents have been specified



Methodology for indicator calculation

Acidification and eutrofication

Air emission data is reported annually by national authorities to UNECE/EMEP and to EU. Reported data includes both newest estimates (two years in arrears) and updates of emissions from previous years. Emission data is stored and verified at EMEP/MSC-W.

Using these emissions, EMEP/MSC-W calculates atmospheric transport of sulphur and nitrogen pollutants using the EMEP Unified Model at a spatial resolution of 50km and according to modelled meteorological conditions adjusted towards observations.

The Co-ordination Centre for Effects uses the resulting deposition estimates to calculate exceedances over reported critical loads for acidity and nutrient nitrogen. In 2004 the CCE updated this database with national updates of critical loads (see section on gap filling where countries did not provide data). These updated estimates have been used for the calculations for 1980, 2000, 2010, and 2020.

Nitrogen and sulphur deposition in each model grid-cell are used for calculation of the average accumulated exceedances of the critical loads, that is the area-weighted average of exceedances accumulated over all ecosystem points in an EMEP gridcell. 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 to provide two estimates, one for the EU25 States, and for one for a larger region comprising most countries Party to the Convention on Long-range transboundary air pollution.


According to the definition in the ozone directive, AOT40 values are calculated from hourly data measured between 08.00 and 20.00 CET at Airbase rural background stations. For crops AOT40 is accumulated during the three month summer period (May - July). Only data series with more than 75% valid data were considered. The AOT40 value measured at a background station is assumed to be representative for an area within 100 km from the station. Interpolation is done with the use of kriging. 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). In the case of AOT40 ordinary co-kriging is performed where the altitude is included as additional variable because there is a statistical dependency between AOT40 and altitude.

Kriging results have been overlayed in a GIS with the HBEU_LC land cover database, which was delivered with CLC90. It includes the UK data and some troubles at the time of the GIS processing with the spatial reference used for CLC90 was avoided that way. The raster resolution of HBEU_LC is 1000 x 1000 meters. The class no. 2 "Strongly artificial vegetated areas" was used as the the land cover type to estimate the crops exposed to ozone.

The temporal trends have been estimated using a Mann-Kendal test.

Methodology for gap filling

Acidification and eutrophication

Older national submissions are used where available, and for European countries which have never submitted national totals the CCE uses its European background critical load database (Hettelingh et al, 2004).

Methodology references



Methodology uncertainty

Acidification and eutrophication

The estimate of the deposition of acidifying and eutrophying pollutants is a calculation directly dependent on reported emissions. Monitored depositions are not employed for any other reasons than comparison against the EMEP model. Thus, the exceedance of deposition over critical loads presented in this indicator is itself a calculation derived from reported air emissions. As negotiation of emission reduction agreements has been based on similar model calculations, reporting of emission reductions in accordance with those agreements would be expected to indicate the improvement in environmental quality required by policy objectives. Model estimates of pollutant depositions are used rather than observed depositions on account of their higher spatial resolution.


The air quality data is officially submitted. It is assumed that the AQ data has been validated by the national data supplier. Methodology uncertainty is given by uncertainty in mapping AOT40 based on interpolation of point measurements at background stations. Station characteristics and representativeness is often insufficiently documented which may implies that stations have been included which are not representative for background conditions.

Data sets uncertainty

Acidification and eutrophication

There are uncertainties behind the modelled estimates of air pollutant supply and the estimated critical loads of ecosystems across Europe.

Computer modelling uses officially reported national pollutant emission totals and their geographical distributions using documented procedures. Temporal and spatial coverage is imperfect, however, as a number of annual national totals and geographical distributions are not reported according to time schedules. These are estimated as necessary using expert opinion by the Meteorological Synthesising Centre - West of EMEP (MSCW) and by the International Institute for Applied Systems Analysis (IIASA). The resolution of the computer estimates has improved recently to 50km grid averages. Local pollutant sources or geographical features below this scale will not be well resolved. The meteorological parameters used for modelling pollutant supply are largely computations, with some adjustment towards observed conditions.

The critical load estimates are reported by official national sources, but face difficulties of geographical coverage and comparability. The latest reporting round in 2004 supplied estimates for 16 of the 38 European Environment Agency countries. For a further nine countries earlier submissions were reported as still valid. Those reporting did so for a variety of ecosystem classes, although reported ecosystems typically covered less than 50% of their total country area. For other countries the most recently submitted critical loads data is used. Variations in approach between the 25 reporting countries can lead to double counting of land area. Not all countries apply the steady state mass balance method to compute critical loads, and not all countries map the same ecosystems.  Norway mapped both forest soils and catchment areas which lead to a 119% coverage of its country surface. In the assessment of different endpoints it is possible that ecosystem areas are counted more than once. The ecosystem types considered is a national decision, and hence there is some variation country to country in the detail of subdivision used from the European Nature Information System (EUNIS) categorisation. All reporting countries include forest, but not all include waters and other vegetation types. For the countries which have never submitted data, critical loads were derived using a European background database recognized by the LRTAP Convention (CCE-Progress report 2003; Posch M, Reinds GJ, Slootweg J, The European Background database, in: Posch et al. 2003, pp.37-44)


Geographical coverage: EEA31 countries although not the whole geographical are is covered: No data available (either because no ozone data from rural background stations is available or the country is not included in the land cover map)  for Greece, Iceland, Norway, Sweden, Bulgaria, Cyprus, Estonia, Lithuania, Latvia, Malta, Romania, Slovenia; Switzerland is included.

Strength and weakness (at data level): Most data have been officially submitted to the Commission under the Exchange of Information Decision and/or to EMEP under the UN-ECE CLRTAP. Station characteristics and representativeness is 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 NECD) is expected to introduce minor inconsistencies in the data set. 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. 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.

Reliability, accuracy, robustness, uncertainty (at data level): 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 80 microgram/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 is very low. The indicator is subject to large year-to-year variations due to meteorological variability. 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"). When averaging over Europe this meteorologically induced variation may be less provided spatial data coverage is sufficient.

Rationale uncertainty

No uncertainty has been specified

Data sources

Other info

DPSIR: State
Typology: Performance indicator (Type B - Does it matter?)
Indicator codes
  • CSI 005
EEA Contact Info


Geographic coverage





Filed under:
Filed under: ozone, pollution, csi, air
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