Changes in Summertime Surface Ozone during the Last Decade over Europe

Long-term trends in summertime surface ozone over Europe are examined using observations from several ground-based European sites (WDCGG and EMEP) and a global chemistry transport model (GEOS-Chem). A simulation that includes interannually varying meteorology and emission inventories from various geopolitical regions as well as several other potentially important factors was conducted from 1991 to 2005. Cumulative probability distributions of hourly daytime surface ozone concentrations in summer are compared between two periods, 1991-1994 vs. 2001-2005 (excluding the anomalously hot year 2003). The observed concentrations have decreased at the high end of the probability distribution and have increased at the low end. The observed crossover is captured by the model at an equivalent ozone concentration (around 40 ppb) but at a lower percentile range (around the 32th percentile) in comparison with the observations (around the 42th percentile). The reasons of this crossover is examined using multiple sensitivity simulations that investigate the influence of emissions and meteorology.