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Indicator Assessment
In the period 2001-2010, 6-27 % of the urban population in EU-27 was potentially exposed to ambient nitrogen dioxide (NO2) concentrations above the EU limit value set for the protection of human health (40 microgram NO2/m3 annual mean). There was a slight downwards trend over the period (Figure 1)
Ozone (O3)In the period 2001-2010, 15-61 % of the urban population in EU-27 was exposed to ambient ozone concentrations exceeding the EU target value set for the protection of human health (120 microgram O3/m3 daily maximum 8-hourly average, not to be exceeded more than 25 times a calendar year, averaged over three years and to be achieved where possible by 2010). The 61 % of the urban population exposed to ambient ozone concentrations over the EU target value was recorded in 2003, which was the record year. There was no discernible trend over the period (Figure 1).
In the period 2001-2010, the fraction of the urban population in EU-27 that is potentially exposed to ambient concentrations of sulphur dioxide in excess of the EU limit value set for the protection of human health (125 microgram SO2/m3 daily mean not to be exceeded more than three days a year), decreased to less than 1 %, and as such the EU limit value set is close to being met everywhere in the urban background (Figure 1).
Percentage of urban population resident in areas where pollutant concentrations are higher than selected limit/target values, 2001-2010 (EU-27)
Note: The rationale for selection of pollutant and corresponding limit/target values for CSI 004 is given in the justification for indicator selection. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Luxembourg and Malta, are not included due to the geographical coverage of the Urban Audit.
Percentage of urban population resident in areas for days per year with PM10 concentration exceeding daily limit value, 2001-2010 (EU-27)
Note: The limit value is 50 µg PM10/m3 (24 hour average, i.e. daily), not to be exceeded more than 35 times a calendar year and to be met by 2005. Over the years 2001-2010 the total population for which exposure estimates are made, increased from 68 to 107 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Greece and Malta are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
36th highest 24-hour mean PM10 concentration observed at urban background stations, 2001-2010 (EU-27)
Note: Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Greece and Malta are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
Percentage of population exposed to NO2 annual concentrations in urban areas, 2001-2010 (EU-27)
Note: The annual mean limit value is 40 µg NO2/m3 and to be met by 2010. Over the years 2001-2010 the total population, for which exposure estimates are made, increased from 92 to 116 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus and Malta, are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
Annual mean NO2 concentration observed at urban background stations, 2001-2010 (EU-27)
Note: Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus and Malta are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
Percentage of urban population resident in areas for days per year with ozone concentrations over the long-term objective for protection of human health, 2001-2010 (EU-27)
Note: The target value is 120 µg O3/m3 as daily maximum of 8 hour mean, not to be exceeded more than 25 days per calendar year, averaged over three years and to be achieved where possible by 2010. Over the years 2001-2010 the total population for which exposure estimates are made, increased from 87 to 115 million people due to an increasing number of monitoring stations reporting under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus is not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
26th highest maximum daily 8-hour mean ozone concentration observed at urban background stations, 2001-2010 (EU-27)
Note: Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus is not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
Percentage of urban population resident in areas for days per year with SO2 concentration exceeding daily limit value, 2001-2010 (EU-27)
Note: The limit value is 125 µg SO2/m3 as a daily average, not to be exceeded more than three days in a year and to be met by 2005. Over the years 1997-2009 the total population for which exposure estimates are made, increased from 85 to 100 million people due to an increasing number of monitoring stations reporting under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Denmark, Finland and Malta, are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
4th highest 24-hour mean SO2 concentration observed at urban stations, 2001-2010 (EU-27)
Note: Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Denmark, Finland and Malta, are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
Particulate Matter (PM10)
PM10 in the atmosphere results from direct emissions (primary PM10) and from emissions of particulate precursors (nitrogen oxides, sulphur dioxide, ammonia and organic compounds) which are partly transformed into particles by chemical reactions in the atmosphere (secondary PM10).
For the period before 2001, the number of monitoring stations in some areas of EU-27 was relatively small. Since 2001 the coverage improved, firstly in the EU-15 country and since 2005 in the whole EU-27. Notwithstanding these limitations, it is clear that a significant proportion of the urban population (18-41 %) was exposed to concentrations of particulate matter in excess of the EU limit values set for the protection of human health during the period 2001-2010 (Figure 2).
The observed time series is short and the natural meteorological variability is large therefore it is not possible to draw firm conclusions on a possible trend in the data. Preliminary analyses indicate a downward change in the highest daily mean PM10 values although for the majority of stations the observed change is statistically not significant. In Figure 3, the 36th highest daily average is shown; compliance with the short-term limit value is assured when this value is below 50 microgram/m3. The variations in Figures 2 and 3 are partly caused by the variable number of cities or stations. When a consistent set of stations are used in a trend analysis; a small decrease is observed, especially since 2006 ([1]).
Emissions of the gaseous precursors for secondary PM are being reduced by enforcement of EU legislation (e.g. NEC Directive) and UNECE LRTAP Convention protocols (United Nations Economic Commission for Europe, Convention on Long-range Transboundary Air Pollution). Abatement techniques to reduce precursor emissions often also reduce the primary particulate emissions. Other measures (e.g. traffic measures from Auto-Oil-I and II Programme, waste incineration directive, road traffic introduction of EURO standards and the Directive on industrial emissions) should further reduce PM10 emissions.
Despite the likely future reductions in emissions, PM10 concentrations are expected to remain well above the daily limit values in most of the urban areas in the near future.
In total 25 of the 27 EU-27 Member States, which are included in the Urban Audit have submitted information on PM10 concentrations at 'urban background' and 'sub-urban background' stations to the air quality database AirBase. Data for Greece and Malta are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
([1]) ETC/ACM Technical Paper 2011/1. European exchange of monitoring information and state of the air quality in 2009: http://acm.eionet.europa.eu/reports/ETCACM_TP_2011_1_EoI_AQ_meta_info2009
Nitrogen dioxide (NO2)
The main source of nitrogen oxides emissions to the air is the use of fossil fuels; road transport, power plants and industrial boilers account for more than 95% of European emissions.
In the period 2001-2010, the fraction of urban population living in cities with urban background concentrations in excess of the 40 microgram NO2/m3 limit value gradually decreased from 27 to 6 % (Figure 4). However, it is expected that also in cities where the urban background concentration is below the limit value, limit values are exceeded at hot spots, in particular in locations with high density of traffic. The decrease in recent years is mainly caused by the fact that in a few very large cities (London, Manchester, Madrid and Barcelona) the averaged background concentrations dropped below the limit value.
Enforcement of current EU legislation (Directive on industrial emissions, Auto-Oil programmes, the National Emissions Ceilings Directive (NECD)) and LRTAP Convention protocols have resulted in a reduction of nitrogen oxides (NOx) emissions. Until now this reduction has not been fully reflected in the annual averages of NO2 concentrations observed at the urban background stations. Figure 5 shows a prevailingly decreasing trend.
Peak nitrogen dioxide levels occur often in busy streets in cities where road traffic is the main source. Since the introduction of catalytic converters at the end of the 1980s, their growing penetration in the car fleet and other measures have contributed to reducing emissions ([1]). The result has been a downward trend in the number of exceedances of the hourly limit value ([2]). Peak levels depend on meteorological conditions; year-to-year fluctuations are 10 to 20 % or more.
In total 25 of the EU-27 Member States, which are included in the Urban Audit have submitted information on nitrogen dioxide concentrations at 'urban background' and 'sub-urban background' stations to the air quality database 'AirBase'. Data for Cyprus and Malta are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
([1]) The EMEP Centre on Emission Inventories and Projections (CEIP): http://www.ceip.at/
([2]) EEA Report No 2/2007. Air pollution in Europe 1990-2004: http://reports.eea.europa.eu/eea_report_2007_2/en
Ozone (O3)
Although reductions in emissions of ozone precursors appear to have led to lower peak concentrations of ozone in the troposphere, the current target level is frequently exceeded for a substantial part of the urban population of the EU-27 Member States. Figure 6 shows estimates for 2010, indicating that 88 % of the urban population experienced exceedance of the 120 microgram O3/m3 level during at least one day (the long-term objective for protection of human health), while about 17 % of the urban population was exposed to concentrations above the target level of 120 microgram O3/m3 level during more than 25 days. The target level was exceeded over a wide area (and much more than just 25 days).
Figure 7 suggest that the 26th highest daily maximum 8-hour average value (if this parameter drops below 120 microgram O3/m3, there is compliance with the target value) does not show any trend. This statement has to be treated with caution. Firstly because the inter-annual variability of the weather has a large impact on the ozone levels and secondly because the number of stations is not stable over the full period.
The reductions in ozone precursor emissions that should result from enforcement of the NEC Directive and the LRTAP Convention protocols are unlikely to reduce ozone concentrations to below the current target value and long-term objective over the whole of the EU-27 Member State area.
In total 26 of the EU-27 Member States, which are included in the Urban Audit have submitted information on ozone concentrations at 'urban background' and 'sub-urban background' stations to the air quality database 'AirBase'. Data for Cyprus is not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.
Sulphur dioxide (SO2)
Sulphur in coal and oil and in mineral ores is the main source of sulphur dioxide in the atmosphere. Up to 1960s, coal and oil combustion in large and small sources was the typical situation in many European cities, resulting in very high sulphur dioxide and PM concentrations. Since then, the combustion of sulphur-containing fuels have largely been removed from urban and other populated areas, first in western Europe and now also increasingly in most central and eastern European countries. Large point sources (power plants and industries), remain the predominate source of sulphur emissions. These sources, usually with high stacks, are most often located away from population centres.
As a result of the important reductions in sulphur dioxide emissions achieved in the last decades, the fraction of the urban population exposed to concentrations above the EU limit value has been reduced to less than 1 % (Figure 8). The reduction in sulphur dioxide peak concentrations is more clearly seen in the trend of the 4th highest daily sulphur dioxide concentration on urban stations in the period 2001-2010 (Figure 9). Compliance with the limit value for the daily mean is assured when the 4th highest concentration is below 125 microgram SO2/m3. A further decline in concentrations is expected in the coming years. However, peak concentrations above EU limit values still occur, especially close to sources and in cities. Peak levels strongly depend on meteorological conditions; year-to-year fluctuations are 10-20 % or more.
Several factors have contributed to the decrease in sulphur dioxide concentrations. The first (1985) and the second (1994) sulphur protocol under the UNECE LRTAP Convention, together with EC limit values set in the previous Air Quality Directive (89/427/EEC amending 80/779/EEC) have resulted in major European emission reductions and correspondingly decreasing ambient concentrations. Political changes in the beginning of 1990s in the central and eastern European countries connected with economic restructuring, decline of heavy industry and adoption of abatement measures on large point sources has contributed to decreasing winter smog episodes in central and western European countries. Policies and measures such as the Directive on industrial emissions, standards regulating emissions from transport, the National Emission Ceilings Directive, and the reductions agreed under LRTAP Convention are expected to further reduce sulphur dioxide levels. Programmes for the reduction of sulphur emission from ships are also underway.
In total 23 of the EU-27 Member States, which are included in the Urban Audit have submitted information on sulphur dioxide concentrations at 'urban background' and 'sub-urban background' stations and the data is available in the air quality database AirBase. However, the majority of the information on sulphur dioxide concentrations results from stations in EU-15 Member States. The limit values tend to be more widely exceeded in the Central and Eastern European countries ([1]).
([1]) See AirBase ‘Produced maps and graphs’: http://www.eea.europa.eu/data-and-maps/figures/airbase-exchange-of-information-3/so2-2008-concentration/fancybox.html
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:
[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.
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.
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.
Two limit values have been set for the protection of human health. Both limit values had to be met by 1 January 2005.
Both limit values have to be met by 1 January 2010:
Both limit values had to be met by 1 January 2005.
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.
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 within the boundaries of the cities that take part in the Urban Audit. The selected AirBase stations include station types classified as 'urban background' and 'sub-urban background'. It is assumed that the whole city population is exposed to this type of environment. Stations classified as 'traffic' or 'industrial' are influenced either by traffic emissions or other local emissions. Such environments are generally not representative for residential areas. The traffic and industrial stations are therefore not selected for 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.
The geo-political domain for calculating this indicator can be that of EEA member countries, EU Member States or individual states.
For each station, the number of days with a daily averaged concentration in excess of the limit value (125 microgram/m3 as a daily mean) is calculated from the hourly values (if available) or daily values. Only time series with a data coverage of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.
The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.
Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 1-3 days, 3-6 days, >6 days).
The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.
For each station the number of days with a daily mean concentration in excess of the daily limit value of 50 microgram/m3 is calculated from the hourly values (if available) 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, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.
The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.
Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 0-7 days, 7-35 days, >35 days).
The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.
The annual mean concentration in an Urban Audit city is calculated using the values measured at the selected stations (urban and sub-urban background). Only time series with a data capture of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year).
The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.
Depending on the mean concentration, each city (and its population) is then classified uniquely in one of the 4 classes of concentration (0-26 microgram/m3, 26-32 microgram/m3, 32-40 microgram/m3), >40 microgram/m3).
The percentage of urban population allocated to different concentration classes is calculated by using the population in those cities assigned to each individual concentration class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.
For each station, the number of days with a daily maximum 8-hourly mean concentration in excess of the target value (120 microgram/m3) is calculated from the hourly values. Only time series with a data coverage of at least 75 % per calendar year are used (that is, in the case of hourly values, having more than 6576 valid hourly values per calendar year, or 6594 hours if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.
The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.
Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 0-25 days, 25-50 days, >50 days).
The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.
No gap-filling is applied for this indicator.
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.
Sulphur dioxide (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.
Particulate Matter (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.
Nitrogen dioxide (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.
Ozone (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.
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/exceedance-of-air-quality-limit-1/exceedance-of-air-quality-limit-4 or scan the QR code.
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