Exceedance of air quality limit values in urban areas (CSI 004) - Assessment published Nov 2012

Indicator definition

The indicator shows the fraction of the urban population that is potentially exposed to ambient air [1] concentrations of pollutants [2] in excess of the EU limit value set for the protection of human health.

The urban population considered is the total number of people living in cities with at least one monitoring station at a background location. The population data applied for the indicator derives from the Urban Audit, which is conducted at the initiative of the Directorate-General for Regional Policy at the European Commission, in cooperation with Eurostat and the national statistical offices of the 27 current Member States. Currently, the Urban Audit involve more than 620 European cities in 30 EEA member countries. The Urban Audit contains data for over 250 indicators across nine domains (e.g. demography, social aspects, environment, travel and transport).

The Urban Audit aims at a balanced and representative sample of cities in Europe. To obtain such a selection, a few simple rules are applied:

1. Approximately 20% of the national population should be covered by the Urban Audit.

2. All capital cities were included.

3. Where possible, regional capitals were included.

4. Both large (more than 250 000 inhabitants) and medium-sized cities (minimum 50 000 and maximum 250 000 inhabitants) were included.

5. The selected cities should be geographically dispersed within each Member State.

The selection of cities was prepared in close collaboration between the Directorate-General for Regional Policy, Eurostat and the national statistical institutes. To ensure that large and medium-sized cities are equally represented in the Urban Audit, in some of the larger Member States not all large cities could be included.

The Urban Audit works with three different spatial levels: the core city, the larger urban zone (LUZ) and the sub-city district (SCD). For CSI 004 only the the core city level is considered, which is the most important level. To ensure that this level is directly relevant to policy makers and politicians, political boundaries were used to define the city level. In many countries these boundaries are clearly established and well-known. As a result, for most cities the boundary used in the Urban Audit corresponds to the general perception of that city. Due to the highly diverse nature of political boundaries in the European Union, for some cities the political boundary does not correspond to the general perception of that city. In a few cities, Dublin for example, the political boundary of the city is narrower than the general perception of that city.

Exceedance of air quality limit values occurs when the concentration of air pollutants exceeds the limit values specified in the first Daughter Directive of the Air Quality Framework Directive for SO₂, PM₁₀ [3], NO₂ and the target values for O₃ as specified in the third Daughter Directive. Where there are multiple limit values (see section on Policy Targets), the indicator uses the most stringent case:

• Sulphur dioxide (SO₂): the daily limit value;
• Nitrogen dioxide (NO₂): the annual limit value;
• Particulate matter (PM₁₀): the daily limit value;
• Ozone (O₃): the target value.

[1] 'Ambient air' shall mean outdoor air in the troposphere, excluding work places.

[2] 'pollutant' shall mean any substance introduced directly or indirectly by man into the ambient air and likely to have harmful effects on human health and/or the environment as a whole.

[3] 'PM₁₀' shall mean particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 10 microgram aerodynamic diameter.

Units

Percentage of the urban population in Europe potentially exposed to ambient air concentrations (in microgram/m³) of sulphur dioxide (SO₂), particulate matter (PM₁₀), nitrogen dioxide (NO₂) and ozone (O₃) in excess of the EU limit value set for the protection of human health.

Policy context and targets

Context description

This indicator is relevant information for the current European air quality legislation related to the protection of human health in the adopted Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC).

A combined ozone and acidification abatement strategy has been developed by the Commission, resulting in the Ozone Daughter Directive (2002/3/EC) and the National Emission Ceiling Directive (2001/81/EC). In this legislation, target values for ozone levels and for precursor emissions have been set.

Targets

EU limit values on concentrations of sulphur dioxide in ambient air

Two limit values have been set for the protection of human health. Both limit values had to be met by 1 January 2005.

• a limit value of 125 microgram SO₂/m³ as an daily average, not to be exceeded more than three times a calendar year, has been set for the protection of human health in the adopted Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC, Section I of Annex I).
• an hourly limit value for the protection of human health has been set at 350 microgram SO₂/m³; this level may not be exceeded more than 24 times a calendar year.

EU limit values on concentrations of nitrogen dioxide in ambient air

Both limit values have to be met by 1 January 2010:

• In the first Daughter Directive (Council Directive 1999/30/EC, section 1 of Annex II) an annual mean limit value for nitrogen dioxide of 40 microgram NO₂/m³ has been set for the protection of human health.
• In addition, an hourly limit value of 200 microgram NO₂/m³ not to be exceeded more than 18 times a calendar year has been set.

EU limit values on concentrations of PM₁₀ in ambient air

Both  limit values had to be met by 1 January 2005.

• a limit value for PM₁₀ of 50 microgram/m³ (24 hour average, i.e. daily), not to be exceeded more than 35 times a calendar year is set for the protection of human health by the first Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC, Annex III).
• an additional limit value of 40 microgram/m³ as annual average has also been set.

EU target values on concentrations of ozone in ambient air

A combined ozone and acidification abatement strategy has been developed by the European Commission, resulting in a new Ozone Daughter Directive (2002/3/EC) and a National Emission Ceiling Directive (2001/81/EC). In this legislation, target values for ozone levels and for precursor emissions have been set.

• The Ozone Daughter Directive sets a target value for the protection of human health of 120 microgram O₃/m³ as maximum daily 8 hour mean, not to be exceeded more than 25 days per calendar year, averaged over three years. This target should be met in 2010.
• The Ozone Daughter Directive has also set a long-term objective of 120 microgram O₃/m³ as a maximum daily 8 hour average within a calendar year (not to be exceeded any day).

Related policy documents

• Council Directive 96/62/EC of 27 September 1996
  Council Directive 96/62/EC of 27 September 1996 on ambient air quality assessment and management.
• Council Directive 1999/30/EC of 22 April 1999
  Council Directive 1999/30/EC of 22 April 1999 Relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air
• 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 2002/3/ EC...ozone in ambient air
  Directive 2002/3/ EC of the European Parliament and of the Council of 12 February 2002 relating to ozone in ambient air
• 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

Methodology

Methodology for indicator calculation

Initially, AirBase stations are spatially joined with Urban Audit core cities in a Geographical Information System (GIS) in order to select AirBase stations, which fall into the Urban Audit geometry. The selected AirBase stations include station types classified as 'urban background' and 'sub-urban background' since stations classified as 'traffic' or 'industrial' are influenced by local (traffic) emissions and might not be representative for the concentrations in more residential areas and are therefore excluded from the indicator calculations.

Based on the selection of AirBase stations air quality statistics are extracted from the database via Structured Query Language (SQL) server queries in order to update the pollutant specific indicators specified below.

Sulphur dioxide (SO₂)

For each urban station, the number of days with a daily averaged concentration in excess of the limit value (125 microgram SO₂/m³ as a daily mean) is calculated from the available hourly or daily values. Only time series with a data capture of at least 75 % per calendar year are used (that is with more than 274 valid daily values per calendar year). The selected urban stations include station types 'urban background' and 'sub-urban background'. The number of exceedance days per city is obtained by averaging the results of all 'urban' and 'sub-urban background' stations.

Particulate matter (PM₁₀)

For each urban station the number of days with a daily mean concentration in excess of the limit value of 50 microgram/m³ is calculated from the available hourly or daily values. The selected urban stations include station types 'urban background' and 'sub-urban background'. Only time series with a data capture of at least 75% per calendar year are used (that is with more than 274 valid daily values per calendar year). The number of exceedance days per city, is obtained by averaging the results of all 'urban' and 'sub-urban background' stations.

Nitrogen dioxide (NO₂)

The annual mean concentration in a city is calculated as the average of the annual mean value measured at all 'urban background' and 'sub-urban background' stations. Only time series with a data capture of at least 75% are used (that is with more than 274 valid daily values per calendar year).

Ozone (O₃)

The number of exceedance days is calculated for each 'urban background' and 'sub-urban background' stations. A city average number of exceedance days is obtained by averaging over all available 'urban background' and 'sub-urban background' stations. Only time series with a data capture of at least 75% are used (that is with more than 274 valid daily values per calendar year).

Methodology for gap filling

No gap-filling is applied for this indicator.

Methodology references

• European exchange of air quality monitoring meta information in 2002 Buijsman E., van Hooydonk P.R., Mol W.J.A., Cernikovsky L. (2004). ETC/ACC Technical Paper 2004/1.

Uncertainties

Methodology uncertainty

The air quality data is officially submitted. Following  the Exchange of Information Decision it is expected that data has been validated by the national data supplier (see Mol, W.J.A. (2008). Quality checks on air quality data in AirBase and the EoI data in 2008. ETC/ACC Working Paper December 2008. Available at http://air-climate.eionet.europa.eu/databases/airbase/ETCACC_WP_2008_Quality_checks_EoI2008_Airbase.pdf). Station characteristics and representativeness is in some cases insufficiently documented. The data is thought to be representative for the urban population in Europe as a whole. Locally, the indicator is subject to year-to-year variations due to meteorological variability.

Data sets uncertainty

SO₂

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the air quality data has been validated by the national data supplier. Station characteristics and representativeness is often insufficiently documented. Data coverage in non EEA-32 member countries needs improvement; data availability over the period 1980-1995 needs improvement.

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year to year and is for the first part of the 1990s insufficient. The data is generally not representative for the total urban population in a country. Availability of data before 1990 is too low to include in the indicator; data for non EU Member States is largely missing before 1995. Locally, the indicator is subject to large year-to-year variations due to meteorological variability. When averaging over EEA member countries this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban sites (i.e. of the type 'urban background' and 'sub-urban background') might not always result in a representative selection of polluted zones. As a consequence, the indicator may be biased . The representativeness of the selection is different for different cities which reduces the comparability between cities. It is not possible at this stage to select a sufficiently large set of stations covering the entire time period since the stations with available data change from year to year.

PM₁₀

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the national data supplier has validated the air quality data. Station characteristics and representativeness is often insufficiently documented. Geographical coverage and data availability still needs improvement. Data has been considered both from monitoring with the reference method (gravimetry) and with other methods. It is not documented whether countries have applied correction factors for non-reference methods, and if so, which factors have been applied. Uncertainties associated with this lack of knowledge may be several tens of percents.

Reliability, accuracy, robustness, and uncertainty (at data level): In some Member States the density of PM₁₀ monitoring stations are not sufficient to give a representative picture. The number of available data series varies considerably from year-to-year and is insufficient for the period before 1997. The data is generally not representative for the total urban population in a country. Locally, the indicator is subject to year-to-year variations due to meteorological variability. When averaging over EEA-32 this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban (type 'urban background' and 'sub-urban background') sites might not always result in a representative selection of polluted areas. The indicator may be biased due to insufficient representative coverage of the pollution situation. The representativeness of the selection is likely to be different for different cities which reduces comparability.

NO₂

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the air quality data has been validated by the national data supplier. Station characteristics and representativeness is often insufficiently documented. Data availability over the period 1980-1995 needs improvement.

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year-to-year and is for the first part of the 1990s insufficient. The data is generally not representative for the total urban population in a country. Availability of data before 1990 is too low to include in the indicator; data for non EU Member States is largely missing before 1995. When averaging over EEA-32 this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban sites might not always result in a representative selection of polluted zones. As a consequence, the indicator may be biased. The representativeness of the selection is different for different cities which reduces the comparability between cities. It is not possible in this stage to select a sufficiently large set of stations covering the entire time period since the stations with available data change from year to year.

O₃

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the air quality data has been validated by the national data supplier. Station characteristics and representativeness is often insufficiently documented. Data coverage in non EEA-32 member countries needs improvement; data availability over the period 1980-1995 needs improvement.

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year to year and is for the first part of the 1990s insufficient. Yearly changes in indicator value may result from changes in monitoring density and/or selected cities which will influence the total monitored population. The indicator is subject to year-to-year fluctuations as it represents episodic conditions, and these depend on particular meteorological situations, the occurrence of which varies from year to year.

Data availability over the period 1980-1995 needs improvement.

Rationale uncertainty