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

摘要

National and European legislation over the past 20 yr, and the modernisationor removal of industrial sources, have significantly reduced European ozoneprecursor emissions. This study quantifies observed and modelled Europeanozone annual and seasonal linear trends from 158 harmonised rural backgroundmonitoring stations over a constant time period of a decade (1996–2005).Mean ozone concentrations are investigated, in addition to the ozone 5thpercentiles as a measure of the baseline or background conditions, and the95th percentiles that are representative of the peak concentration levels.This study aims to characterise and quantify surface European ozoneconcentrations and trends and assess the impact of the changing anthropogenicemission tracers on the observed and modelled trends.Significant (p<0.1) positive annual trends in ozone mean, 5th and 95thpercentiles are observed at 54 %, 52 % and 45 % of sites respectively(85 sites, 82 sites and 71 sites). Spatially, sites in central andnorth-western Europe tend to display positive annual ozone trends in mean,5th and 95th percentiles. Significant negative annual trends in ozone mean5th and 95th percentiles are observed at 11 %, 12 % and 12 % of sitesrespectively (18 sites, 19 sites and 19 sites) which tend to be located inthe eastern and south-western extremities of Europe. European-averaged annualtrends have been calculated from the 158 sites in this study. Overall thereis a net positive annual trend in observed ozone mean(0.16±0.02 ppbv yr?1 (2σ error)), 5th(0.13±0.02 ppbv yr?1) and 95th(0.16±0.03 ppbv yr?1) percentiles, representative of positivetrends in mean, baseline and peak ozone. Assessing the sensitivity of thederived overall trends to the constituent years shows that the Europeanheatwave year of 2003 has significant positive influence and 1998 theconverse effect; demonstrating the masking effect of inter-annual variabilityon decadal based ozone trends.The European scale 3-D CTM CHIMERE was used to simulate hourly O₃concentrations for the period 1996–2005. Comparisons between the 158observed ozone trends to those equivalent sites extracted from regionalsimulations by CHIMERE better match the observed increasing annual ozone(predominantly in central and north-western Europe) for 5th percentiles, thanfor mean or 95th ozone percentiles. The European-averaged annual ozone trendin CHIMERE 5th percentiles (0.13±0.01 ppbv yr?1) matches thecorresponding observed trend extremely well, but displays a negative trendfor the 95th percentile (?0.03±0.02 ppbv yr?1) where apositive ozone trend is observed. Inspection of the EU-averaged monthly meansof ozone shows that the CHIMERE model is overestimating the summer monthO₃ levels.In comparison to trends in EMEP emissions inventories, with the exception ofAustria-Hungary, we do not find that anthropogenic NO_x and VOCreductions have a substantial effect on observed annual mean O₃trends in the rest of Europe. On a ten year time-scale presented in thisstudy, O₃ trends related to anthropogenic NO_x and VOCreductions are being masked as a result of a number of factors includingmeteorological variability, changes in background ozone and shifts in sourcepatterns.