Have primary emission reduction measures reduced ozone across Europe? An analysis of European rural background ozone trends 1996-2005

National and European legislation over the past 20 yr, and the modernisation or removal of industrial sources, have significantly reduced European ozone precursor emissions. This study quantifies observed and modelled European ozone annual and seasonal linear trends from 158 harmonised rural background monitoring stations over a constant time period of a decade (1996-2005). Mean ozone concentrations are investigated, in addition to the ozone 5th percentiles as a measure of the baseline or background conditions, and the 95th percentiles that are representative of the peak concentration levels. This study aims to characterise and quantify surface European ozone concentrations and trends and assess the impact of the changing anthropogenic emission tracers on the observed and modelled trends.

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The European scale 3-D CTM CHIMERE was used to simulate hourly O-3 concentrations for the period 1996-2005. Comparisons between the 158 observed ozone trends to those equivalent sites extracted from regional simulations by CHIMERE better match the observed increasing annual ozone (predominantly in central and north-western Europe) for 5th percentiles, than for mean or 95th ozone percentiles. The European-averaged annual ozone trend in CHIMERE 5th percentiles (0.13 +/- 0.01 ppbv yr(-1)) matches the corresponding observed trend extremely well, but displays a negative trend for the 95th percentile (-0.03 +/- 0.02 ppbv yr(-1)) where a positive ozone trend is observed. Inspection of the EU-averaged monthly means of ozone shows that the CHIMERE model is overestimating the summer month O-3 levels.

In comparison to trends in EMEP emissions inventories, with the exception of Austria-Hungary, we do not find that anthropogenic NOx and VOC reductions have a substantial effect on observed annual mean O-3 trends in the rest of Europe. On a ten year time-scale presented in this study, O-3 trends related to anthropogenic NOx and VOC reductions are being masked as a result of a number of factors including meteorological variability, changes in background ozone and shifts in source patterns.