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You are here: Home / Data and maps / Indicators / Emissions of ozone precursors

Emissions of ozone precursors

Note: new version is available!

Justification for indicator selection

Emissions of non-methane volatile organic compounds (NMVOCs), nitrogen oxides, carbon monoxide and methane contribute to the formation of ground-level (tropospheric) ozone.

Ozone is a powerful oxidant and tropospheric ozone can have adverse effects on human health and ecosystems. It is a problem mainly during the summer months. High concentrations of ground-level ozone adversely affects the human respiratory system and there is evidence that long-term exposure accelerates the decline in lung function with age and may impair the development of lung function. Some people are more vulnerable to high concentrations than others, with the worst effects generally being seen in children, asthmatics and the elderly. High concentrations in the environment are harmful to crops and forests, decreasing yields, causing leaf damage and reducing disease resistance.

Scientific references:

Indicator definition

This indicator tracks trends since 1990 in anthropogenic emissions of ozone precursor pollutants: nitrogen oxides (NOx), carbon monoxide (CO), methane (CH4) and non methane volatile organic compounds (NMVOCs), each weighted by their respective tropospheric ozone-forming potential.

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).


ktonnes (NMVOC-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 NOx and NMVOCs (the NEC Directive also sets emissions ceilings for ammonia NH3 and sulphur dioxide SO2). There are no specific EU emission targets set for either carbon monoxide (CO) or methane (CH4). However, there are several Directives and Protocols that affect the emissions of CO and CH4. Methane is included in the basket of six greenhouse gases under the Kyoto protocol (see CSI 10: Greenhouse gas emissions and removals).

The European Commission is expected to propose a revised NEC Directive in 2009.


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 NOx and NMVOCs that are either equal to or slightly less ambitious than those in the EU NEC Directive. 



Directive 2001/81/EC, on national emissions ceilings (NECD) for certain atmospheric pollutants.

UNECE (1999). Protocol to the 1979 Convention on Long-Range Transboundary air pollution (LRTAP Convention) to abate acidification, eutrophication and ground-level ozone.


Emissions of NOx and NMVOCs are covered by the EU National Emission Ceilings Directive (NECD) and the Gothenburg Protocol to the UNECE LRTAP Convention (UNECE 1999). 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 ozone precursors NOx and NMVOCs (individual pollutant emission ceilings weighted by ozone formation potential factors prior to aggregation).



1990 - 1990 - 2010 NECD target

1990-2005 Gothenburg











































United Kingdom









Czech Republic
















































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.

Key policy question

What progress is being made in reducing emissions of ozone precursors across Europe?

Specific policy question

How do different sectors and processes contribute to emissions of ozone precursors?


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


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:  

 EEA Code

EEA classification

Non-GHGs (NFR)


National totals

National Total


Energy industries



Industry (Energy)



Fugitive emissions



Road transport



Other transport (non-road mobile machinery)

1A3 (excl 1A3b) + sectors mapped to 8 in table below


Industry (Processes)




4 + 5B





Other (Energy)

1A4a, 1A4b, 1A4b(i), 1A4c(i), 1A5a


Other (non-energy)

3 + 7



Difference between NT and sum of sectors (1-12)


Energy Industries (Power Production 1A1a)


 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 ozone precursor emissions, the emission values of the individual pollutants are multiplied by a troposheric ozone formation potential factor (de Leeuw, 2002) prior to aggregation. The factors are NOx 1.22, NMVOCs: 1, CO: 0.11 and CH4: 0.014. Results are expressed in terms of 'NMVOC 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) (

Methodology references

Data specifications

EEA data references

Data sources in latest figures


Methodology uncertainty

The use of interpolation/extrapolation procedures to gap-fill the underlying emissions dataset and the application of tropospheric ozone formation potential factors both lead to uncertainties. With respect to the tropospheric ozone formation 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 NOx, CO and NMVOC emissions data officially submitted by EU Member States and other EEA member countries follow common calculation (EMEP/EEA 2009) and reporting guidelines (UNECE 2003). CH4 emissions are estimated by countries following IPCC Guidelines (e.g. IPCC 2009).

Nitrogen oxide emission estimates in Europe are thought to have an uncertainty of about +/-20% (EMEP 2009), as the NOx 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.

Uncertainties in emissions of CO are likely to have a similar magnitude of uncertainty as for NOx. NMVOC emissions data have been verified by EMEP and others by means of comparison between modelled and measured concentration throughout Europe (EMEP, 1998). From these studies total uncertainty ranges have been estimated to about +/-50%. Some main source categories are less uncertain.

CH4 estimates are reasonably reliable as they are based on a few well-known emission sources. The IPCC believes that the uncertainty in CH4 emission estimates from all sources, in Europe, is likely to be about +/-20 %. CH4 emissions from some sources, such as rice fields, are much larger (possibly an order of magnitude), but are a minor emission source in Europe. In 2004, EU Member States reported uncertainties in their estimates of CH4 emissions from enteric fermentation as ranging between 0.5 % (UK) and 2.8 % (Ireland) of the total national GHG emissions (EEA 2004).

Incomplete reporting and resulting intra- and extrapolation may obscure some trends.  


  • EEA (2009). Annual European Community greenhouse gas inventory 1990-2007 and inventory report 2009, Technical report No 4/2009. European Environment Agency, Copenhagen.

  • EMEP/EEA 2009. EMEP/CORINAIR 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
  • IPCC (2006). Revised 2006 IPCC Guidelines for National Greenhouse Gas Inventories. JT Houghton, LG Meira Filho, B Lim, K Treanton, I Mamaty, Y Bonduki, DJ Griggs and BA Callender (Eds). IPCC/OECD/IEA. UK Meteorological Office, Bracknell.
  • 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.

Rationale uncertainty

This indicator on emissions of ozone precursors 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.

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.

Work description

Countries should improve the completeness of the time series of their estimates (filling gaps). Further validation and checking is the responsibility of the country and needs especially to lead to improved detailed sectoral time series of emissions. There is also a need for further validation and checking of emission estimates within the framework of LRTAP Convention/EMEP and EEA-ETC/ACC activities.

Resource needs



In progress


2099/01/01 00:00:00 GMT+1

Work description

Improvement in quality of national data delivered to UNFCCC.

Resource needs



In progress


2099/01/01 00:00:00 GMT+1

Work description

Improvement in quality of national data delivered to LRTAP Convention/EMEP.

Resource needs



In progress


2099/01/01 00:00:00 GMT+1

Work description

Improvement of national data delivered under National Emission Ceilings Directive (NECD).

Resource needs



In progress


2099/01/01 00:00:00 GMT+1

General metadata

Responsibility and ownership

EEA Contact Info

Martin Adams


No owners.


Indicator code
CSI 002
Version id: 2
Primary theme: Air pollution Air pollution


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DPSIR: Pressure
Typology: Performance indicator (Type B - Does it matter?)

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