Indicator Specification

Energy-related emissions of acidifying substances

Indicator Specification
  Indicator codes: ENER 006
Published 12 Jan 2011 Last modified 04 Sep 2015
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Emissions of combined SO2 and NOx (also NH3 where applicable) in 1000 tonnes acid equivalents. Gaps filled by EEA/ETC-ACC where necessary using simple interpolation techniques

Assessment versions

Published (reviewed and quality assured)
  • No published assessments


Justification for indicator selection

Energy production and use accounts for the majority of nitrogen oxides (NOx) and sulphur dioxide (SO2) emissions, but only a small fraction of ammonia (NH3) emissions. These pollutants all contribute to acid deposition. Acidification is caused by emissions of sulphur dioxide, nitrogen dioxide and ammonia into the atmosphere, and their subsequent chemical reactions and deposition on ecosystems and materials (see also EEA 2008). Deposition of acidifying substances causes damage to ecosystems, buildings and materials (corrosion). The adverse effect associated with each individual pollutant depends on its potential to acidify and the individual properties of the ecosystems and materials. The deposition of acidifying substance still often exceeds the critical loads1 of the ecosystems across Europe2. Efforts to reduce the effects of acidification are therefore focused on reducing the emissions of acidifying substances. NOx and SO2 can react in the atmosphere and transform into small-diameter particulate matter which when inhaled, can have direct or indirect impacts on human health causing harmful effects such as respiratory problems. See EN07 for more information about energy-related particulate emissions. NOx is also a tropospheric ozone precursor that reacts in the atmosphere in the presence of sunlight to form ozone which, in high concentrations, can lead to significant health impacts and damage to crops and other vegetation (see also EN05). Furthermore, an excessive input of nitrogen nutrients from atmospheric deposition or via run-off following atmospheric deposition can lead to eutrophication of waters.

Scientific references

  • No rationale references available

Indicator definition

Emissions of combined SO2 and NOx (also NH3 where applicable) in 1000 tonnes acid equivalents. Gaps filled by EEA/ETC-ACC where necessary using simple interpolation techniques


Emissions kt acid equivalents


Policy context and targets

Context description

Several EU-wide emissions limits and targets exist for the reduction of total SO2, NOx and NH3 emissions, including the National Emissions Ceiling Directive (NECD; 2001/81/EC) and the UNECE LRTAP Convention Gothenburg Protocol under UNECE LRTAP Convention (UNECE 1999). This indicator provides relevant information for assessing the achievement of these targets and also for analyses performed within the European Commission’s Clean Air for Europe programme (CAFE). This thematic strategy on air quality was released in September 2005 (The CAFE Programme/implementation of the Thematic Strategy on Air Pollution,
The NEC Directive includes emission reduction targets that are slightly stricter than the targets set in the Gothenburg Protocol and requires the introduction of national emission ceilings for emissions of SO2, NOx and NH3 (and also for NMVOCs) in each Member State, as well as setting interim environmental objectives for reducing the exposure of ecosystems and human populations to damaging levels of the acid pollutants. Targets for the new Member States are temporary and are without prejudice to the on-going review of the NECD. A proposal for a revised NEC Directive (which will set 2020 emission ceiling targets for these acidifying pollutants), is expected in spring 2008. Targets for Bulgaria and Romania are provisional and not binding. Hence, the existing EU25 NECD Target has been used in the following analysis.
In terms of the energy sector, the most relevant NEC Directive targets for the EU-25 (exclude Romania and Bulgaria) as a whole are:

  • SO2: emissions reduction of 74 % by 2010 from 1990 levels;
  • NOx: emissions reduction of 53 % by 2010 from 1990 levels.

NH3 emissions are also an important source of acid deposition and have an emissions target under NEC (emissions reduction target of 15 % by 2010 from 1990 levels), but energy-related emissions of ammonia are insignificant, accounting for only 2.5 % of total EU-27 ammonia emissions in 2005. Agriculture is by far the largest contributing sector to EU ammonia emissions.

Other key policies that have contributed to the reduction of acidifying emissions across Europe include:

  • The Integrated Pollution Prevention and Control (IPPC) Directive (96/61/EC), which entered into force in 1999. It aims to prevent or minimise pollution of water, air and soil by industrial effluent and other waste from industrial installations, including energy industries, by defining basic obligations for operating licences or permits and by introducing targets, or benchmarks, for energy efficiency. It requires the application of Best Available Techniques in new installations (and for existing plants over 10 years, according to national legislation).
  • The Large Combustion Plant Directive (2001/80/EC) is important in reducing emissions of SO2, NOx and dust from combustion plants with a thermal capacity greater than 50 MW. The Directive sets emission limits for licensing of new plants and requires Member States to establish programmes for reducing total emissions. Emissions limits for all plants are also set under the IPPC Directive.


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. See CSI001

Related policy documents

  • Council Directive 96/61/EC (IPPC)
    Council Directive 96/61/EC of 24 September 1996 concerning Integrated Pollution Prevention and Control (IPPC). Official Journal L 257.
  • Directive 2001/80/EC, large combustion plants
    Directive 2001/80/EC of the European Parliament and of the Council of 23 October 2001 on the limitation of emissions of certain pollutants into the air from large combustion plants
  • 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.
  • UNECE Convention on Long-range Transboundary Air Pollution
    UNECE Convention on Long-range Transboundary Air Pollution.


Methodology for indicator calculation

Combination of emission measurements and emission estimates based on volume of activities and emission factors. Recommended methodologies for emission data collection are compiled in the Joint EMEP/CORINAIR Atmospheric Emission Inventory Guidebook 3rd edition, EEA, Copenhagen EEA.

Acid Equivalents: Weighting factors (w) are used for SO2, NOx and NH3, which are multiplied with the emissions (Em, Gg) and the resulting acid equiv. emissions are added (de Leeuw 2002). Thus, total acid equivalent emission = w(SO2)*Em(SO2) + w(NOx)*Em(NOx) + w(NH3)*Em(NH3) where weight factors are given by: w(SO2) = 2/64 acid eq/g = 31.25 acid eq/kg w(NOx) = 1/46 acid eq/g = 21.74 acid eq/kg w(NH3) = 1/17 acid eq/g = 58.82 acid eq/kg These factors are assumed to be representative for Europe as a whole; on the (very) local scale different factors might be estimated; see de Leeuw (2002) for a more extensive discussion on the uncertainties in these factors. Due to the variation in potential TOFP factors that might be determined on a local scale, the use of such factors does not always have wide support or recognition in EU Member States. The energy supply sector includes public electricity and heat production, oil refining, production of solid fuels and fugitive emissions from fuels. The transport sector includes emissions from road and off-road sources (e.g. railways and vehicles used for agriculture and forestry). Industry (Energy) relates to emissions from combustion processes used in the manufacturing industry including boilers, gas turbines and stationary engines. ‘Other (energy-related)’ covers energy use principally in the services and household sectors.

Base data, reported in SNAP, draft NFR or NFR are converted into EEA sector codes to obtain a common reporting format across all countries and pollutants:
- Energy industry: Emissions from public heat and electricity generation - Fugitive emissions: Emissions from extraction and distribution of solid fossil fuels and geothermal energy
- Industry (Energy): relates to emissions from combustion processes used in the manufacturing industry including boilers, gas turbines and stationary engines
- Industry (Processes): Emissions from production processes
- Road transport: light and heavy duty vehicles, passenger cars and motorcycles;
- Off-road transport: railways, domestic shipping, certain aircraft movements, and non-road mobile machinery used in agriculture, forestry;
- Agriculture: manure management, fertiliser application, field-burning of agricultural wastes
- Waste: incineration, waste-water management.
- Other (energy-related) covers energy use principally in the services and household sectors
- Other (Non Energy): Emissions from solvent and other product use.
The following table shows the conversion of NFR sector codes into EEA sector codes (EEA, 2006):


0 National totals National Total National totals without LUCF (sector
1 Energy industries 1A1 1A1
3 Industry (Energy) 1A2 1A2
2 Fugitive emissions 1B 1B
11 Transport n.a. n.a. n.a.
7 Road transport 1A3b 1A3b
8 transport (non-road mobile machinery) 1A3 (excl 1A3b) + sector mapped to 8 in table below 1A3a, 1A3c, 1A3d, 1A3e
9 Industry (Processes) 2 2
4 Agriculture 4 + 5B 4
5 Waste 6 6
6 Other (Energy) 1A4a, 1A4b, 1A4b(i), 1A4c(i), 1A5a 1A4+1A5
10 Other (non-energy) 3+7 3+7
14 Unallocated difference between NT and sum of sectors (1-12)  
12 Energy Industries (Power Production 1A1a) 1A1a 1A1a
15 residential n.a. 1A4 b
16 services, agriculture and other sectors n.a. 1A4a + 1A4c + 1A5

Methodology for gap filling

EEA/ETC-ACC gap-filling methodology. To allow trend analysis where countries have not reported data for one or several years, data has been interpolated to derive annual emissions. If the reported data is missing either at the beginning or at the end of the time series period, the emission value has been considered to equal the first (or last) reported emission value. It is recognised that the use of gap-filling can potentially lead to artificial trends, but it is considered unavoidable if a comprehensive and comparable set of emissions data for European countries is required for policy analysis purposes. The gap-filled air emissions spreadsheet is available on

Methodology references

No methodology references available.


Data specifications

EEA data references

Data sources in latest figures



Methodology uncertainty

Officially reported data following agreed procedures and Emission Inventory Guidebook (EEA 2009), e.g. regarding source sector split.  The incomplete reporting and resultant extrapolation may obscure some trends.

Data sets uncertainty

The uncertainties of total sulphur dioxide emission estimates in Europe are relatively low, as the sulphur emitted mainly comes from the fuel burnt and therefore can be 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 (which include emission data as one of the model parameters) and measured concentrations throughout Europe (EMEP 2005). From these studies the uncertainties associated with the modelled annual averages for a specific point in time have been estimated in the order of ± 30 %. This is consistent with an inventory uncertainty of ±10 % (with additional uncertainties arising from the other model parameters, modelling methodologies, and the air quality measurement data etc). In contrast, NOx emission estimates in Europe are thought to have higher uncertainty, as the NOx emitted comes both from the fuel burnt and the combustion air and so cannot be estimated accurately from fuel nitrogen alone. EMEP has compared modelled and measured concentrations throughout Europe (EMEP 2005). From these studies differences for individual monitoring stations of more than a factor of two have been found. This is consistent with an inventory of national annual emissions having an uncertainty of ±30% or greater (there are also uncertainties in the air quality measurements and especially the modelling). For some countries, reported time-series emissions data may be inconsistent. This may occur where for example different inventory reporting definitions have been used in different years and/or where changes made to estimation methodologies have not been applied back to 1990. For all emissions the trend is likely to be much more accurate than individual absolute annual values - the annual values are not independent of each other.

Rationale uncertainty

No uncertainty has been specified

Further work

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.

General metadata

Responsibility and ownership

EEA Contact Info

Cinzia Pastorello


European Environment Agency (EEA)


Indicator code
ENER 006
Version id: 2
Primary theme: Energy Energy


DPSIR: Pressure
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)




Filed under:
Filed under: energy
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