Exposure of ecosystems to acidification, eutrophication and ozone
Published (reviewed and quality assured)
Justification for indicator selection
Excess deposition of air pollutants can lead to disturbances in the function and structure of ecosystems (EEA, 2014a). Atmospheric deposition of sulphur and nitrogen compounds contributes to a depletion of the buffering capacity or an excessive demand on buffering rates in soils and waters, and thus to a reduction in pH levels. The consequences of this are the release of toxic metals, such as aluminum, and the leaching of nutrients from soils, causing damage to flora and fauna. Additionally, the atmospheric deposition of nitrogen compounds can lead to an oversupply of nutrient nitrogen in terrestrial and water ecosystems. Effects can be the leaching of nitrate to groundwater and changes in species richness, i.e. changes in biodiversity. The latter occurs through the excessive growth of a few species, which thrive in the presence of the added nutrients, to the detriment of a larger number of species, which have long been part of the ecosystems but are accustomed to a lower-nutrient environment.
Ground level ozone is one of the most prominent air pollution problems in Europe, mainly due to its effects on human health, crops and natural ecosystems. When absorbed by plants, it damages plant cells, impairing their ability to grow and reproduce, and leading to reduced agricultural crop yields, decreased forest growth and reduced biodiversity.
- EEA, 2007 Air pollution by ozone in Europe in summer 2006. EEA Technical report 5/2007, European Environment Agency.
- EEA, 2014a Air Quality in Europe – 2014 Report, EEA Report 5/2014.
- EEA, 2014b NEC Directive status report 2013, Reporting by the Member States under Directive 2001/81/EC of the European Parliament and of the Council of 23 October 2001 on national emission ceilings for certain atmospheric pollutants, EEA Technical report No 10/2014, European Environment Agency.
- EU, 1997 Council Decision 97/101/EC on the exchange of information and data on ambient air quality, replaced by (EU, 2011).
- EU, 2001 Directive 2001/81/EC of the European Parliament and of the Council of 23 October 2001 on national emission ceilings for certain atmospheric pollutants.
- EU, 2008 Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe.
- EU, 2011 Commission Implementing Decision 2011/850/EU laying down rules for Directive 2004/107/EC and 2008/50/EC of the European Parliament and of the Council as regards the reciprocal exchange of information and reporting on ambient air.
- Hettelingh, J.-P., Posch, M., and Slootweg, J. (eds.), EC4MACS Uncertainty Treatment, The CCE-EIA Ecosystem Impact Model, European Consortium for Modelling of Air Pollution and Climate Strategies – EC4MACS (Life project). 2012.
- ICP on Modelling and Mapping, 2015 ICP on Modelling and Mapping critical loads and levels approach.
- Skeffington, R.A., 2006 Quantifying uncertainty in critical loads: (A) Literature review, Water, Air, and Soil Pollution 169: 3–24.
- Suutari, R. Amann, M., Cofala, J., Klimont, Z., Schöpp, W. and Posch, M., From Economic Activities to Ecosystem Protection in Europe – An Uncertainty Analysis of Two Scenarios of the RAINS Integrated Assessment Model. EMEP CIAM/CCE Report 1/2001, International Institute for Applied Systems Analysis, Laxenburg, Austria. 2001.
- UNECE, 2015. Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends Mapping Manual, UNECE Convention on Long-range Transboundary Air Pollution, ICP Modelling and Mapping.
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, last year's rural concentrations of ozone, and the annual variation at the European level of rural concentrations of ozone.
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 the 'critical level' for ozone for each location. This is a concentration of ozone 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). 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 AOT40 definition, (i.e. 08:00 to 20:00 h 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
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 and NOx. The directive requires 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 is part of the Clean Air Programme for Europe proposed by the European Commission at the end of 2013. It aims at compliance with the amended Gothenburg Protocol by 2020, followed by more ambitious reductions from 2030 onwards.
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 computations 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) 2001/81/EC
The NECD (EU, 2001) sets pollutant-specific and legally binding emissions ceilings for nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOC), sulphur dioxide (SO2) and ammonia (NH3) for each EU Member State. The directive sets specific environmental objectives that address acidification and eutrophication impacts on ecosystems and the harmful effects of ozone on vegetation and human health. The directive requires the Member States to have met the ceilings and interim environmental objectives by 2010 and in subsequent years.
The NEC Directive's 2010 interim objective for the protection of sensitive ecosystems from acidification aims to reduce areas where critical loads of acid deposition are exceeded by at least 50 % (in each grid cell) compared with the 1990 situation. The eutrophication objective under the NEC Directive aims to reduce areas with depositions of nutrient nitrogen in excess of the critical loads by about 30 % in 2010 compared with 1990.
The interim environmental objective for vegetation-related ground-level ozone exposure is to reduce the ground-level ozone load above the critical level for crops and semi-natural vegetation (AOT40 = 3 ppm/hour) by one-third in all grid cells by 2010, compared with the 1990 situation. In addition, the ground-level ozone accumulated concentration shall not exceed an absolute limit of 10 ppm per hour, expressed as an exceedance of critical accumulated concentration in any grid cell.
- 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 to 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 five years. This target value should be met in 2010 (2010 being the first year from which data will be used in the calculation over the following five years).
b) a long-term objective as AOT40-value (calculated from hourly values from May to July) of 6 000 (μg/m3)·h, with no defined date of attainment.
In the assessment part of the indicator, the target value threshold is also considered. This is the target value considered only for one year and not for the averaged period of five years.
- The 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 Environmental 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’
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 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.
Key policy question
What progress is being made towards targets for the reduction of ecosystem exposure to acidification, eutrophication and ozone?
Specific policy question
Which areas in Europe remain most affected by eutrophication, acidification and ground-level ozone?
Methodology for indicator calculation
- Acidification and eutrofication (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 NEC Directive 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); Hettelinghet 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 EEA33 member countries).
AOT40 estimated values are calculated from hourly data (EU, 2008) at all rural background stations available in 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 Qaulity 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.
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.
- 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.
EEA data references
- No datasets have been specified here.
Data sources in latest figures
- 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).
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):
- Expert judgement by single expert – no supporting data;
- Consensus judgement by group of experts;
- Uncertainty estimated from observations, but not at this particular site;
- Uncertainty estimated from model calculations based on general knowledge or theory;
- 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”.
Most data have been officially submitted to the Commission under the Exchange of Information Decision (EU, 1997) 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. The different definition of AOT40-values (accumulation during 08.00 to 20.00 CET following the Air Quality Directive (EU, 2008) versus accumulation during daylight hours following the definition in the NEC Directive (EU, 2001)) is expected to introduce minor inconsistencies in the data sets.
This indicator is rather sensitive to the precision at the reference level (40 ppb or 80 micrograms per m3)
Short term work
Work specified here requires to be completed within 1 year from now.
Long term work
Work specified here will require more than 1 year (from now) to be completed.
Work descriptionConsider to introduce a country split, also for the ozone part of the indicator. Follow-up new developments in critical level calculations (using O 3 fluxes) and projections based on further developed methodologies.
No resource needs have been specified
Deadline2018/12/31 00:00:00 GMT+1
Responsibility and ownership
EEA Contact InfoAlberto Gonzalez Ortiz
Frequency of updates
Typology: Performance indicator (Type B - Does it matter?)
For references, please go to www.eea.europa.eu/soer or scan the QR code.
This briefing is part of the EEA's report The European Environment - State and Outlook 2015. The EEA is an official agency of the EU, tasked with providing information on Europe's environment.
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