[1] This paper provides a review of stratosphere-troposphere exchange (STE), with a focus on processes in the extratropics. It also addresses the relevance of STE for tropospheric chemistry, particularly its influence on the oxidative capacity of the troposphere. After summarizing the current state of knowledge, the objectives of the project Influence of Stratosphere-Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), recently funded by the European Union, are outlined. Several papers in this Journal of Geophysical ResearchAtmospheres special section present the results of this project, of which this paper gives an overview. STACCATO developed a new concept of STE in the extratropics, explored the capacities of different types of methods and models to diagnose STE, and identified their various strengths and shortcomings. Extensive measurements were made in central Europe, including the first monitoring over an extended period of time of beryllium-10 ( 10 Be), to provide a suitable database for case studies of stratospheric intrusions and for model validation. Photochemical models were used to examine the impact of STE on tropospheric ozone and the oxidizing capacity of the troposphere. Studies of the present interannual variability of STE and projections into the future were made using reanalysis data and climate models.

[1] Regional climate-air quality simulations were performed over Europe for two future decades, 2041-2050 and 2091-2100 under IPCC A1B scenario and the control decade 1991-2000. The simulations serve as a theoretical experiment investigating the impact of changing climate on summer surface ozone. Our simulations suggest that changes in summer surface ozone imposed by climate change until the 2040s are below 1 ppbv and more pronounced until the 2090s. The median of summer near surface temperature for whole Europe is 2.7 K higher at the end of the 21st century than to the end of the 20th century with more intense temperature increase simulated for southern Europe. A prominent feature is the decrease of cloudiness mostly over western Europe at the end of the 21st century associated with an anticyclonic anomaly which favors more stagnant conditions and weakening of the westerly winds for the larger part Europe southern of 50°N. Biogenic emissions double in the simulation at the end of the 21st century for latitudes below 50°and together with changes in circulation patterns, temperature, and solar radiation, contribute to the enhanced average ozone concentrations at the end of the 21st century. The change is more intense over southwest Europe, where the median of ozone increases by 6.2 ppbv. Sensitivity simulations suggest that biogenic emissions, temperature and solar radiation have a comparable impact on average surface ozone in the examined range of perturbations. The maximum response of the imposed perturbations was seen over southern Europe.Citation: Katragkou, E., P. Zanis, I. Kioutsioukis, I. Tegoulias, D. Melas, B. C. Krüger, and E. Coppola (2011), Future climate change impacts on summer surface ozone from regional climate-air quality simulations over Europe,

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This paper presents regional scale simulations aiming to assess the sensitivity of future air quality under anticipated climate change, with a focus on near-surface ozone (O 3 ) and particulate matter with a diameter < 10 µm (PM 10 ). Constant anthropogenic emissions and biogenic emissions varying with climate were used. The modelling was carried out with regional climate models coupled to Chemical Transport Models for 3 decadal time slices, under the IPCC A1B scenario, in both coarse (50 km) and high (10 km) resolution for Europe and for targeted domains of Central-Eastern Europe (CEE), respectively. Two modelling systems were applied: the RegCM/ CAMx and ALADINClimate/CMAQ driven by ECHAM5 and ARPEGE global climate models, respectively. A comprehensive 'operational' evaluation of the performance of modelling systems driven by re-analysis of ECMWF ERA-40 fields was carried out for one full year. Our modelling systems fulfilled the fractional bias (FB) and fractional error (FE) skill criteria and the benchmark of index of agreement (IA) for maximum daily running 8 h mean O 3 , with FBs ranging from + 4 to −11%, FEs of 14 to 31% and IAs of 0.63 to 0.87. The models' performance for annual, winter and daily mean PM 10 was weaker, with FBs of −3 to −49% and FEs of 38 to 66%, but skill criteria for PM were met. Those results justified the use of proposed modelling systems for future time projections. The simulated changes in climate has rather weak impacts on the air quality of the mid-century (2041−2050). For the end-century (2091−2100), our study shows an increase in summer mean O 3 and a decrease in annual mean PM 10 in CEE. The main climate factors responsible for projected changes were an increase in summer temperature and a decrease in summer precipitation for O 3 , and an increase in winter precipitation for PM 10 .

Abstract. Regional climate-air quality decadal simulations over Europe were carried out with the RegCM3/CAMx modeling system for the time slice 1991-2000, in order to study the impact of different meteorological forcing on surface ozone. The RegCM3 regional climate model was firstly constrained by the ERA40 reanalysis dataset which is considered as an experiment with perfect meteorological boundary conditions and then it was constrained by the global circulation model ECHAM5. A number of meteorological parameters were examined including the 500 mb geopotential height, solar radiation, temperature, cloud liquid water path, planetary boundary layer height and surface wind. The different RegCM meteorological forcing resulted in changes of near surface ozone over Europe ranging between ± 4 ppb for winter and summer. The area showing the greatest sensitivity in O 3 during winter is central and southern Europe while in summer central north continental Europe. The different meteorological forcing impacts on the atmospheric circulation, which in turn affects cloudiness and solar radiation, temperature, wind patterns and the meteorology depended biogenic emissions. For comparison reasons, the impact of chemical boundary conditions on surface ozone was additionally examined with a series of sensitivity studies, indicating that surface ozone changes are comparable to those caused by the different meteorological forcing. These findings suggest that, Correspondence to: E. Katragkou (katragou@auth.gr) when it comes to regional climate-air quality simulations, the selection of external meteorological forcing can be as important as the selection of adequate chemical lateral boundary conditions.