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Indicator Specification
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 aluminium 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.
This indicator shows the negative impact of air pollution on ecosystems and vegetation in Europe. In particular, it shows:
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
Ozone
Ozone concentrations: micrograms of ozone per cubic meter (µg/m³) or parts per billion (ppb). Note: 1 ppb ~ 2 µg/m³.
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)
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:
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.'
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.
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.
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.
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).
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).
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 %.
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):
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), 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.
• Ozone
This indicator is rather sensitive to the precision at the reference level (40 ppb or 80 micrograms per m3)
Work specified here requires to be completed within 1 year from now.
Work specified here will require more than 1 year (from now) to be completed.
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/exposure-of-ecosystems-to-acidification-14 or scan the QR code.
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