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

Indicator Specificationexpired Created 07 Oct 2004 Published 07 Jun 2005 Last modified 11 Sep 2015, 12:38 PM
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This content has been archived on 02 Mar 2015, reason: Other (New version data-and-maps/indicators/exposure-of-ecosystems-to-acidification-3 was published)
Indicator codes: CSI 005

Assessment versions

Published (reviewed and quality assured)
  • No published assessments


Justification for indicator selection

Deposition of sulphur and nitrogen compounds contribute to acidification of soils and surface waters, leaching of plant nutrients and damage to flora and fauna. Deposition of nitrogen compounds can lead to eutrophication, disturbance of natural ecosystems, excessive algal blooms in coastal waters and increased concentrations of nitrate in ground water.

The risk of damage is evaluated by comparing the estimated deposition of acidifying and eutrophying air pollutants with the estimated capacity of each location to receive such pollutants without harm. This capacity, or 'critical load' may be thought of as the threshold of air polluting compounds which should not be exceeded if ecosystems are to be protected from risk of damage, according to present knowledge. Thus, risk occurs where the deposit of pollutants exceeds the estimated critical load. In reality exceedence of critical loads is a complex function of the deposition of various pollutants, and of ecosystem, soil and water properties.

Critical load estimates and the modelled  estimates of pollutant deposition have been used to support the negotiation of the multi-pollutant multi-effect Gothenburg Protocol (1999) to the 1979 Geneva Convention on Long Range Transboundary Air Pollution. Account has also taken of these estimates in the development of the slightly stricter National Emissions Ceilings Directive 2001/81/EC. Previously, negotiation of the 1994 sulphur reduction Oslo Protocol to the Geneva convention utilised critical load exceedances. Hence, the indicator is closely comparable with the approach to state of the environment understanding during the development of transboundary air pollution policy in Europe.

Ground level ozone is seen as one of the most prominent air pollution problems in Europe, mainly due to effects on human health, natural ecosystems and crops. Threshold levels set by the European Union for the protection of human health, and vegetation and critical levels agreed under the Convention LRTAP for the same purpose are exceeded widely and largely. Ozone is a secondary pollutant formed in the atmosphere. Important precursors are nitrogen oxides and volatile organic compounds, and - to a lesser extent - CO and methane. Ozone plays also an important role in air pollution by nitrogen dioxide (see CSI 004). Tropospheric ozone is an important greenhouse gas.

Scientific references

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

Key policy question

What progress is being made towards the targets for reducing the exposure of ecosystems to acidification, eutrophication and ozone?

Specific policy question

Which areas of Europe and which country-wise area totals remain most affected by acidification and eutrophication?

Specific policy question

Which areas of Europe remain most affected by ozone?


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

Data specifications

EEA data references

Data sources in latest figures


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

Further work

Short term work

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

Work description

There is a strong need for calculation of estimated deposition of acidifying and eutrophying components in 1990, using the EMEP Unified Model, and for calculation of exceedance status across Europe using these deposition estimates and the recently revised critical load data. This is needed to permit evaluation of progress towards the targets set by the NECD (200/81/EC), art.5 (a). Produce a distance-to-target diagram (as % change over time). This type of figure would have direct relevance to policy questions and accords with EEA practice. Definition of the target should take close account of the European Union legal objective for acidification (NECD, Art.5 (a)) of a 50% reduction in exceeding area in each 150km grid square; aggregation to a 50% reduction in total 1990 area exceeding, whilst not the strict true target, provides a close approximation for the European scale.

Resource needs

No resource needs have been specified


Not started


2005/12/31 00:00:00 GMT+1

Work description

Movement towards independent evaluation of actual on-the-ground achievements will be a significant enhancement of the indicator. The current methodology necessitates employment by the indicator of anticipated current environmental state and change. Furthermore, the anticipated change is driven by expected emission change, much as negotiation of emission changes were related to anticipated environmental change. As under current approaches the attainment of emission targets can be expected to be interpreted essentially as attainment of acidification targets, the current acidification/eutrophication indicator is open to interpretation as largely an alternative expression of the emission indicator. EMEP production of deposition maps derived from a combination of observed and modelled status is currently irregular and has found limited application to date. Development of such or similar approaches in the medium-term will provide a move towards independent verification of the attainment of EU environmental objectives for acidification, and provide greater transparency to evaluating achievement of emission targets. A layout to consider for the graphs which result is to follow that of the emission distance-to-target graphs. On ozone there is a need to improve the coverage in space and time. Further work is needed on interpolation methodology interpretation and combination/assimilation monitoring data with modelling rersults.

Resource needs

No resource needs have been specified


Not started


2008/12/31 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

Peder Gabrielsen


No owners.


Indicator code
CSI 005
Version id: 1
Primary theme: Air pollution Air pollution


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