This study reviews ozone concentrations from rural monitoring stations across Europe between 1996 and 2010 across a range of statistics with regard to the various objectives, target values and thresholds established by the 2008 Ambient Air Quality Directive 2008/50/EC. The findings reveal that whilst there have been complex and varied changes in patterns of ozone concentrations during the last 15 years, there are still extensive exceedences of the various regulatory standards and that while peak concentrations may have been decreasing, background concentrations are on the rise.
The Air Quality Directive 2008/50/EC established two target values for ozone, one based on the protection of human health (120 µg/m3 as maximum daily 8-hour mean, not to be exceeded more than 25 days per year averaged over three years) and another for protection of vegetation (AOT40 (Accumulated Ozone over a Threshold of 40 ppb) 18,000 µg/m3-h averaged over five years. The directive also established a long-term objective of 120 µg/m3 as maximum daily 8-hour mean (no exceedences) and an AOT40 of 6,000 µg/m3-h (no achievement date defined). The 2008 Directive also set an information threshold of 180 µg/m3 and an alert threshold of 240 µg/m3 (both hourly averages).
Methodology and Results
Data were collected from the European Airbase database (v6) and following screening of the data, 286 rural background stations were selected for analysis over a 10-year period from 2001-2010, and 186 stations for a fifteen year period from 1996-2001. The analysis covered both absolute concentrations, as well as long-term trends in concentrations calculated using Theil Sen slope estimates, following a methodology established by Wilson et al. (2012).
The findings of the analysis are that that there is no clear single trend in ozone concentrations. However, the following points can be noted:
1. Mean concentrations tended to increase over the period 1996-2005, whilst decreasing 2001-2010;
2. During this latter period, mean and 5th percentile (background) concentrations tended to reduce, but this pattern was less obvious for 95th percentile (peak) concentrations;
3. Monitoring stations recording the highest mean ozone concentrations (>60 µg.m-3) tended to show the greatest downward trends over the whole 15 year period;
4. Different spatial regimes exist for mean concentrations and trends, on the one hand and maximum 1-hour or 8-hour mean concentrations, on the other.
5. Any discernible downward trend in some of the exceedence statistics is relatively insignificant in the context of year-to-year variations due to changing meteorological conditions.
6. The extent to which trends are evident is highly dependent on the chosen measure, e.g. average concentrations or exceedences of particular thresholds.
Patterns of ozone pollution over Europe are complex, and the EU monitoring network is only coming to the stage where widespread spatial trends can be evaluated. What becomes clear from the analyses undertaken is that it is unlikely that a ‘one size fits all’ policy will be suitable for all Member States, and therefore better information on the nature of any region’s problem will be vital for effective management strategies.
This work was undertaken as part of a service contract for DG ENV, European Commission. The work as presented is from a report that has not yet been approved or published by the Commission.
Wilson, R. C., et al. (2012) Have primary emission reduction measures reduced ozone across Europe? An analysis of European rural background ozone trends 1996–2005, Atmos. Chem. Phys., 12, 437-454, doi:10.5194/acp-12-437-2012.