Air pollution by ozone

Modelled change in tropospheric ozone concentrations over Europe

Note: The modelled changes shown are only due to climate variability and climate change

Data source:

Andersson, C.; Langner, J. and Bergström, R., 2007. Interannual variationand trends in air pollution over Europe due to Climate variability during 1958-2001 simulated with a regional CTM coupled to the ERA40 reanalysis, Tellus 59B: 77-98.

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Change in number of ozone exceedance days between 1993-1996 and 2000-2004

Note: The map shows the change in the average number of ozone exceedence days from April to September observed at regional background stations (EMEP monitoring stations) from the period 1990-1994 to the period 1999-2004, excluding the abnormally warm summer of 2003. A day is counted as an exceedence day when the daily maximal 8-hr-average ozone concentration exceeds 60 ppbV.

Data source:

Van Dingenen, R.; Raes, F.; Dentener, F.; Putaud, J. P. and Micale, F., 2008. The relation between temperature anomaly and ozone exceedance as an indication for the role of climate change on ozone exceedance in Europe (manuscript in preparation).

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Past trends

A modelling study from 1958 to 2001 (Andersson et al., 2007) shows that climate variability and change contributed to increased ozone concentrations during the period 1979-2001 over south-central and south-western Europe, and a decrease in north-eastern Europe (Figure 1). The reason for this is a combination of changes in temperature, wind patterns, cloud cover and stability. Further, temperature plays a role in various processes which directly affect the formation of ozone, like the emission of biogenic organic compounds (e.g. isoprene), and the photo-dissociation of NO₂.
A link between temperature and ozone concentration is also evident from observations. A statistical analysis of ozone and temperature measurements in Europe for 1993-2004 shows that in central-western Europe and the Mediterranean area, a change the increase in the daily maximum temperature in 2000-2004 compared with 1993-1996 contributed to extra ozone exceedences (Figures 2 and 3). In south and central Europe, the temperature trend was responsible for an average of 8 extra annual exceedence days of 120 micrograms/m³, i.e. 17 % of the total number of exceedences observed in that region. An analysis of trends over the past twelve years indicates that in the EU the average number of hours when ozone concentration exceeded the information threshold of 180 micrograms/m³ was higher in summer 2003 than in all previous years (Fiala et al., 2003).

Projections

The projected trends for ozone and other air pollutants are closely linked to projections for radiation, temperature, cloudiness, and precipitation. On a global scale, the effect of climate change alone on tropospheric ozone concentrations is expected to be small, because of a reduction in ozone lifetime as a consequence of higher humidity (Stevenson et al., 2006). However, regional differences can be large. Regions where climate change is expected to result in an increased frequency of stable anticyclonic conditions with associated high temperatures, large solar inputs and little boundary layer ventilation may experience a deterioration of air quality (Hogrefe et al., 2004; Sousounis et al., 2002). A 30-year model study for the period 2071-2100, based on the IPCC A2 and B2 scenarios for CO₂ emissions (but with otherwise constant emissions of pollutants) shows that daily peak ozone amounts as well as average ozone concentrations will increase substantially during the summer in future climate conditions (Meleux et al., 2007), in particular in central and western Europe, in line with observed trends from the past. The study also finds that summer ozone levels in future climate conditions are similar to those found during the exceptionally hot summer of 2003. The expected impact on human health may be exacerbated by the aging of the population, the elderly being more susceptible to air pollution than the average population (OECD, 2008).