Statistical models to evaluate the relationships between large-scale meteorological conditions, prevailing air pollution levels and combined ozone and temperature events, were developed during the 1993–2012 period with Central Europe as regional focus. Combined ozone and temperature events were defined based on the high frequency of coinciding, health-relevant elevated levels of daily maximum tropospheric ozone concentrations (based on running 8-h means) and daily maximum temperature values in the peak ozone and temperature season from April to September. By applying two different modeling approaches based on lasso, logistic regression, and multiple linear regression mean air temperatures at 850 hPa, ozone persistence, surface thermal radiation, geopotential heights at 850 hPa, meridional winds at 500 hPa, and relative humidity at 500 hPa were identified as main drivers of combined ozone and temperature events. Statistical downscaling projections until the end of the twenty-first century were assessed by using the output of seven models of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Potential frequency shifts were evaluated by comparing the mid- (2031–2050) and late-century (2081–2100) time windows to the base period (1993–2012). A sharp increase of ozone-temperature events was projected under RCP4.5 and RCP8.5 scenario assumptions with respective multi-model mean changes of 8.94% and 16.84% as well as 13.33% and 37.52% for mid- and late-century European climate.

High ground‐level ozone concentrations and high air temperatures present two health‐relevant natural hazards. The most severe health outcomes are generally associated with concurrent elevated levels of both variables, representing so‐called compound ozone and temperature (o‐t‐) events. These o‐t‐events, their relationship with identified main meteorological and synoptic drivers, as well as ozone and temperature levels themselves and the linkage between both variables, vary temporally and with the location of sites. Due to the serious health burden and its spatiotemporal variations, the analysis of o‐t‐events across the European domain represents the focus of the current work. The main objective is to model and project present and future o‐t‐events, taking region‐specific differences into account. Thus, a division of the European domain into six o‐t‐regions with homogeneous, similar ground‐level ozone and temperature characteristics and patterns built the basis of the study. In order to assess region‐specific main meteorological and synoptic drivers of o‐t‐events, statistical downscaling models were developed for selected representative stations per o‐t‐region. Statistical climate change projections for all central European o‐t‐regions were generated to assess potential frequency shifts of o‐t‐events until the end of the 21st century. The output of eight Earth System Models from the sixth phase of the Coupled Model Intercomparison Project considering SSP245 and SSP370 scenario assumptions was applied. By comparing midcentury (2041–2060) and late century (2081–2100) time slice differences with respect to a historical base period (1995–2014), substantial increases of the health‐relevant compound o‐t‐events were projected across all central European regions.

<p>Air pollution as well as high air temperature both pose a large risk to human health in Europe. High temperature levels are associated with an exceptionally high mortality rate, only representing the extreme end of a wide range of possible health effects. Tropospheric ozone, a secondary air pollutant, is primarily built by photochemical reactions under solar radiation with the involvement of precursor gases including nitrogen oxides, carbon monoxide, methane, and non-methane volatile organic compounds. Due to the specific characteristics of ozone formation, high levels of ozone and temperature often coincide, posing an even intensified threat to human health.</p><p>The current scientific work focuses on the co-occurrence of these two health stressors as well as their underlying meteorological conditions. A subset of European ozone (AirBase_v8, EEA) and temperature (ECA&D) stations is selected for analysis based on individual station locations and data coverage. Taking into account different settings of air substances concentrations (urban, outer conurbation area, rural regions), these stations are classified and grouped by station type and area type resulting in five distinct station classes: urban traffic, urban background, suburban background, rural background and rural industrial.</p><p>Maximum daily 8-hour average ozone values (MDA8O3, EEA), observed daily maximum air temperatures (TX, ECA&D) and meteorological variables (from ERA5, ECMWF) form the data basis for model building. Current thresholds and extreme definitions e.g. based on WHO air quality guidelines or high percentiles (75<sup>th</sup> and 90<sup>th</sup>) are examined and discussed to describe elevated levels of these variables and to finally define combined ozone-temperature events.</p><p>Possible regional patterns as well as disparities between urban and rural areas regarding the specific settings for ozone formation as well as varying meteorological mechanisms for the occurrence of combined ozone-temperature events are closely examined. The methodological focus is primary on statistical modelling, the application and comparison of varying multivariate statistical approaches and different machine learning methods, e.g. various regression analyses using shrinkage methods or random forests. Consequently, statistical models are generated to analyse the influence of meteorological conditions on the occurrence of combined ground-level ozone and temperature events along with the identification of primary key factors (e.g. ozone persistence or larger-scale air temperature and wind conditions) at each specific location.</p><p>Furthermore, frequency and intensity changes of combined ozone-temperate events in the scope of global warming are assessed. Thus, projections of these co-occurring events under the constraints of ongoing climate change until the end of the 21st century are analysed by integrating projections of general circulation models into the statistical modelling process.</p>

Ground‐level ozone is a major air pollutant harmful to human health. In the scope of climate change, it is essential to provide high‐quality local‐scale assessments of the anticipated changes for public health and policy interventions. Assessments and projections of ground‐level ozone usually rely on numerical modeling, but statistical approaches are also available. The present study enhances the validity of statistical downscaling by taking climate change as well as air pollution changes into account. Besides considering meteorological predictors such as air temperature, short‐wave radiation, humidity, and wind, ozone trends from changes in precursor emissions were included in the statistical models. Meteorological and ozone predictor information extracted from reanalysis data for the observational period and output of seven Earth System Models (ESMs) for the projection periods were used, with three of them having interactive chemical modeling, while the other four used prescribed ozone changes. Ground‐level ozone, more precisely daily maximum 8‐hr running means (MDA8) as well as daily maximum 1‐hr values (MDA1), at 798 measurement stations across the European area in the “ozone season” from April to September were assessed. Results depended strongly on whether only meteorological information or additional information about emission changes were considered. As a general picture under the consideration of climate and emission changes, decreasing ground‐level ozone concentrations were projected under the moderate SSP2‐4.5 scenario, while for the more pessimistic scenario SSP3‐7.0 increasing ozone concentrations over Europe, especially at the end of the 21st century, were assessed.

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