Spatial patterns of conditions leading to peak O 3 concentrations revealed by clustering analysis of modeled data Abstract Air quality models are currently the best available tool to estimate ozone (O3) concentrations in the Metropolitan Area of Buenos Aires (MABA). While the DAUMOD-GRS has been satisfactorily evaluated against observations in the urban area, a Monte Carlo (MC) analysis showed that it is the region around the MABA, where the lack of observations impedes model testing, that concentrates not only the greatest estimated O3 peak levels but also the largest model uncertainty. In this work, we apply clustering analysis to these MC outcomes in order to study the spatial patterns of conditions leading to peak ozone hourly concentrations. Results show that families of conditions distribute, as emissions, radially around the city. A cluster exhibiting an O3 morning peak dominates in low-emission areas, a behavior that can be explained both from theory and from the few monitoring campaigns carried out in the city. Its distinct dynamics compared with the typical O3 diurnal profile occurring in the urban area suggests the need of new ozone measurements in the surroundings of the MABA which could contribute to improve our understanding of O3 formation drivers in this region. The results illustrate the potential of applying clustering analysis on large ensembles of modeled data to better understand the variability in model solutions.