Paired-in-time differences between observed and modeled MDA8 ozone at AQS stations for each season and analysis region. Model results are for BASE (red), BC H-CMAQ (blue), BC GEOS-Chem (green), and BC AM3 (orange). For each season and region, the observed MDA8 ozone concentrations were rank ordered at each station. Next, differences between CMAQ simulations and observations were computed for each observed percentile by selecting the model value corresponding to the date of the observed percentile. Finally, the median value of these paired-in-time differences across all AQS stations in a given season and region was then computed for each observed percentile and is depicted in this figure.