Emissions of acidifying substances (version 2) (CSI 001) - Assessment published Dec 2008

Indicator definition

The indicator tracks trends since 1990 in anthropogenic emissions of the acidifying pollutants nitrogen oxides (NO_x) , ammonia (NH₃), and sulphur dioxide (SO₂), each weighted by their respective acidifying potential factor.

The indicator also provides information on the sources of emissions from a number of sectors: Energy industries; road and other transport; industry (processes and energy); other (energy); fugitive emissions; waste; agriculture and other (non energy).

Units

ktonnes (acidifying equivalent)

Policy context and targets

Context description

Within the European Union, the National Emission Ceilings Directive (NEC Directive) imposes emission ceilings (or limits) for emissions of the acidifying pollutants nitrogen oxides, sulphur dioxide and ammonia that harm human health and the environment (the NEC Directive also sets emissions ceilings for a fourth pollutant - non-methane volatile organic compounds). The European Commission is expected to propose a revised NEC Directive in 2010.

Other key EU legislation is targeted at reducing emissions of air pollutants from specific sources, for example:

• transport;
• industrial facilities and other stationary sources.

Internationally, the issue of air pollution emissions is also being addressed by the UNECE Convention on Long-range Transboundary Air Pollution (the LRTAP Convention) and its protocols. The Gothenburg 'multi-pollutant' protocol under the LRTAP Convention also contains national emission ceilings for the acidifying pollutants that are either equal to or slightly less ambitious than those in the EU NEC Directive. 

Targets

Emissions of NOx, SOx and NH3 are covered by the NECD and the Gothenburg Protocol to the UNECE LRTAP Convention. Both instruments contain emission ceilings (limits) that countries must meet by 2010.

Table 1. Percentage reduction required by 2010 compared to 1990 levels by country, for aggregated emissions of the acidifying pollutants NOx, SOx and NH3 (individual pollutant emission ceilings weighted by acidifying potential factors prior to aggregation).

Table 1. Percentage reduction required by 2010 from 1990 levels by country, for emissions of ozone precursors NOx and NMVOCs (emission targets weighted by ozone formation potential).

Related policy documents

• 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.
• 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.

Methodology

Methodology for indicator calculation

This indicator factsheet uses emissions data from the EEA dataservice dataset 'EEA aggregated and gap-filled air emission data'. The 2010 projection estimates reported by the EU-27 Member States under the requirements of the NEC Directive are also included in the analysis http://www.eea.europa.eu/themes/air/datasets).

The dataset 'EEA aggregated and gap-filled air emission data' is consistent with the annual 'European Community LRTAP Convention emission inventory' compiled by EEA. This inventory is based on the officially reported emissions data from countries submitted to the UNECE LRTAP Convention and supplemented with additional data reported under the NEC Directive and the EU GHG Monitoring Mechanism/UNFCCC.   

Air pollutant emissions data are reported by countries using the Nomenclature For Reporting (NFR) sectroal classification system developed by UNECE/EMEP.  For the purposes of the 'EEA aggregated and gap-filled air emission dataset', the numerous NFR sectors reported by countries are combined into the following EEA aggregated sectors to allow a simpler analysis: 

• 'Energy industries': emissions from public heat and electricity generation
• 'Fugitive emissions': emissions from extraction and distribution of solid fossil fuels and geothermal energy
• 'Industry (Energy)': emissions from combustion processes used in the manufacturing industry including boilers, gas turbines and stationary engines
• 'Industry (Processes)': emissions from production processes
• 'Road transport': emissions from light and heavy duty vehicles, passenger cars and motorcycles;
• 'Off-road transport': emissions from railways, domestic shipping, certain aircraft movements, and non-road mobile machinery used in agriculture, forestry;
• 'Agriculture': emissions from manure management, fertiliser application, field-burning of agricultural wastes
• 'Waste': emissions from incineration of waste, waste-water management.
• 'Other (energy-related)': emissions from energy use principally in the services and household sectors
• 'Other (Non Energy)': emissions from solvent and other product use.

The following table shows how the NFR categories used by countries to report their emissions are aggregated into the EEA aggregated sectors listed above:

The 'unallocated' sector (14) corresponds to the difference between the reported national total and the sum of the reported sectors for a given pollutant/country/year combination. It can be either negative or positive. Inclusion of this additional sector means that the officially-reported national totals do not require adjustment to ensure they are consistent with the sum of the individual sectors reported by countries.

Where reported data from countries is incomplete, simple gap-filling techniques are used in the 'EEA aggregated and gap-filled air emission dataset' in order to obtain a consistent time-series (see following section).

To obtain an aggregated estimate of the total acidifying substances emissions, the emission values of the individual acidifying pollutants are multiplied by an acidifying potential factor (de Leeuw, 2002) prior to aggregation. The factors are NO_x 0.02174, SO₂ 0.03125 and NH₃ 0.05882. Results are expressed in terms of 'acidification equivalents' (ktonnes).

In addition to historic emissions, Figure 1 of the indicator factsheet also shows the latest 2010 projection estimates reported by the EU-27 Member States under the requirements of the NEC Directive. The "with measures" (WM) projections reported by Member States take into account currently implemented and adopted policies and measures. Where countries have instead reported "business as usual" or "current legislation" projections, it is assumed for comparison purposes that these are equivalent to a WM projection. The "with additional measures" projections reported by Member States take into account additional future planned policies and measures but which are not yet implemented.

Methodology for gap filling

To allow trend analysis, where countries have not reported data for one or more years, data in the 'EEA aggregated and gap-filled air emission dataset' has been interpolated to derive the emissions for the missing year or years. If the reported data is missing either at the beginning or at the end of the period, the emission value is assumed to equal the first or last reported value. The use of gap-filling may lead to artificial trends, but it is considered necessary if a comprehensive and comparable set of emissions data for European countries is to be obtained. A spreadsheet containing a record of the gap-filled data is available from EEA's European Topic Centre on Air and Climate Change (ETC/ACC) (http://air-climate.eionet.europa.eu/)

Methodology references

• EEA/ETC-ACC technical report describing the gap-filling methodologies used for the 2004 EEA/ETC-ACC LRTAP Convention and GHG (CRF) air emissions spreadsheet
• de Leeuw , F. (2002). A set of emission indicators for long-range transboundary air pollution. Environmental Science and Policy, Volume 5, Issue 2, p. 135-145.  

Uncertainties

Methodology uncertainty

The use of interpolation/extrapolation procedures to gap-fill the underlying emissions dataset and the application of acidifying potential factors both lead to uncertainties. With respect to the acidifying potential factors, these are assumed to be representative for Europe as a whole; on the local scale different factors might be estimated. An extensive discussion on the uncertainties in these factors is available in de Leeuw (2002).

Data sets uncertainty

The NO_x, SO₂ and NH₃ emissions data officially submitted by EU Member States and other EEA member countries follow common calculation (EMEP/EEA 2009) and reporting guidelines (UNECE 2003).

Sulphur dioxide emission estimates in Europe are thought to have an uncertainty of about 10% as the sulphur emitted comes from the fuel burnt and therefore can be more accurately estimated.  However, because of the need for interpolation to account for missing data the complete dataset used here will have higher uncertainty. EMEP has compared modelled (using emission inventory data) and measured concentrations throughout Europe (EMEP, 1998). From these studies differences in the annual averages have been estimated in the order of 30% consistent with an inventory uncertainty of 10% (there are also uncertainties in the measurements and especially the modelling).

Nitrogen oxide emission estimates in Europe are thought to have an uncertainty of about ±20% (EMEP, 2009), as the NO_x emitted comes both from the fuel burnt and the combustion air and so cannot be estimated accurately from fuel nitrogen alone.  However, because of the need for interpolation to account for missing data, the complete dataset used will have higher uncertainty. The trend is likely to be more accurate than the individual absolute annual values - the annual values are not independent of each other.

Ammonia emissions are also relatively uncertain. NH₃ emission estimates in Europe are more uncertain than those for NO_x or SO₂ due largely to the diverse nature of agricultural sources - which account for the vast majority of NH₃ emissions. It is estimated that they are around ±30% (EMEP, 2007). The trend is likely to be more accurate than the individual absolute annual values - the annual values are not independent of each other.

References

• EMEP/EEA 2009. EMEP/CORINAIR Emission Inventory Guidebook - 2009 (http://www.eea.europa.eu/publications/emep-eea-emission-inventory-guidebook-2009)
• EMEP (1998). Transboundary Acidifying Air Pollution in Europe, Part 1: Estimated dispersion of acidifying and eutrophying compounds and comparison with observations. EMEP/MSC-W Report 1/98, July 1998.
• EMEP (2009). "Transboundary acidification, eutrophication and ground level ozone in Europe in 2007". EMEP August 2009 http://www.emep.int/publ/reports/2009/status_report_1_2009.pdf
• UNECE (2003). Emission Reporting Guidelines: 'Guidelines for Estimating and Reporting Emission Data under the Convention on Long-range Transboundary Air Pollution', ECE/EB.AIR/80. Air Pollution studies No. 15, United Nations, New York and Geneva. http://www.unece.org/env/documents/2003/eb/air/ece.eb.air.80.E.pdf 

Rationale uncertainty

This indicator on emissions of acidifying pollutants is updated annually by EEA and is used regularly in our reports on the state of the environment. It is therefore important to note the uncertainties related to methodology and data sets.