Twenty years of policies in Europe to reduce emissions that contribute to the formation of ozone have successfully reduced short-term peak ozone levels, according to a new study conducted in central England. However, the study warns that tougher targets are needed to further reduce average annual levels. Ozone can cause health problems for humans, other animals and plants, and is also a greenhouse gas. It is a pollutant gas formed in the lower atmosphere in the presence of sunlight and other harmful precursor emissions, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). Vehicle and industry emissions are the major sources of NOx, and emissions of VOCs predominantly come from vehicles, industry, solvents and paints. Previous policy actions to curb the ozone precursor emissions NOx and VOCs have been unified as part of the EU's Clean Air for Europe (CAF�) Thematic Strategy on Air Pollution. As part of this strategy, the EU National Emissions Ceilings (NEC) Directive sets the national emissions ceilings for NOx and VOCs among other pollutants. In this study, the researchers modelled the impact of the main influences on the formation of ground-level ozone for 20 years (from 1990) in central England. The focus was on four issues, namely, the actual reductions of NOx and VOC precursor emissions achieved; the complex relationship between NOx and VOCs emissions and ozone formation; the long range transport of ozone in the atmosphere across the North Atlantic ocean; and the target levels of precursor emissions set in international policy negotiations. As ozone formation depends on sunlight, ozone concentrations vary, with higher levels found during the summer and late afternoon. Exposure to short-term peak ozone levels and exposure to annual average levels, ie chronic (long-term) exposure, were determined. Modelled results were compared with actual observations. The results suggest that, over central England, short-term episodes of peak ozone levels between 1990 and 2007 decreased significantly, but annual ozone levels rose slightly during the same period. Changes in the intercontinental trans-Atlantic transport of ozone did not appear to affect episodes of peak ozone but had a substantial effect on the annual levels, by offsetting the declining trend caused by the reduction of precursor emissions. By 2007, precursor NOx and VOC emissions had declined by about 50-60%, but peak ozone levels fell by only 30%. The study suggests reductions in VOC emissions have led mainly to declining summertime ozone levels, whereas NOx reductions have led to decreasing summertime levels, but also to increased wintertime levels. Therefore average annual ozone levels have not fallen as much as would be expected. The World Health Organization (WHO) suggests there is evidence that long-term exposure to ozone can cause health problems, but not enough quantitative information on dose response to recommend an annual guideline value for ozone exposure for the general population. The researchers suggest reasonable policy targets to reduce ozone levels should be reductions of NOx by 60% and VOCs by about 30%, or about 30% for VOC and NOx combined, beyond 2010.


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