Still high ozone levels despite less air pollution

Ozone levels remain largely unchanged in many European countries, even though emissions of the air pollutants involved in ozone formation have been reduced significantly.

Efforts to combat European ozone levels have so far achieved only limited success, according to a recent report from the European Environment Agency (EEA). Although overall European emissions of the air pollutants that lead to ozone formation have been gradually lowered since 1990, ozone levels remain largely unchanged in many countries.

The EEA study explores the reasons for this apparent contradiction, using data from the European air quality database, AirBase, and computer models to investigate ground-level ozone formation in Europe.

Ground-level ozone is among the most harmful air pollutants in Europe today. Elevated ozone levels cause health problems, premature deaths, reduced agricultural crop yields, changes in ecosystem species composition and damage to physical infrastructure and cultural heritage. Ozone is also an important greenhouse gas, ranked third behind carbon dioxide and methane.

Ozone (O₃) is not directly emitted into the atmosphere – it is formed in complex photochemical reactions from ozone precursor gases, namely nitrogen oxides (NOx), volatile organic compounds (VOCs) and carbon moNOxide (CO). Moreover, ozone formation depends strongly on meteorological conditions, such as solar intensity and temperature.

Based on emission statistics, anthropogenic emissions of ozone precursor gases were reduced by 37 per cent across the EEA-32 countries between 1990 and 2006. However, during the same time period, population exposure to ozone levels above the target value set in the EU legislation for protection of human health did not decrease. Short-term peaks in ozone concentrations reportedly dropped in the first part of the 1990s, while between 1997 and 2006 there was a year-on-year increase in daily 8-hour maximum concentration at most monitoring stations.

While emission statistics show a steady decline in overall European anthropogenic emissions of NOx and VOCs, the reductions were largest during the early years both in absolute and relative terms. From 1990 to 1995 the total emissions of NOx and VOCs dropped 17 and 19 per cent, respectively, whereas during 2000–2005 the cuts were 11 and 12 per cent.

There are also large differences between the individual countries. Some countries, such as Spain, Greece, Portugal and Austria, even increased their NOx emissions, whereas others, such as Germany, Italy, Sweden, and the United Kingdom, cut their emissions of both NOx and VOCs by about 40-60 per cent.

Key findings of the study:

• Variations in weather conditions have a significant impact on ozone levels. Discerning the effect of reduced ozone precursor emissions therefore requires data from stable monitoring networks covering a long period of time. Unfortunately, extended time series data are generally unavailable, particularly in southern Europe where ozone pollution is a major problem.
• Significant uncertainties exist regarding the magnitude and distribution of intercontinental inflows of ozone and its precursors, and the size and distribution of isoprene emissions from plants
• The importance of meteorological conditions in ozone formation suggests that predicted changes in climate could also lead to increased ground-level ozone in many regions of Europe.
• Ground-level ozone has become a hemispheric or even global air pollution and climate change problem. Ozone abatement should be integrated into local, regional and global strategies and measures that simultaneously address emissions of air pollutants and greenhouse gases.

Christer Ågren

The EEA report “Assessment of ground-level ozone in EEA member countries, with a focus on long-term trends”, EEA Technical report No 7/2009, can be downloaded from: www.eea.europa.eu/publications