Files containing daily maximum 8-hr ozone mixing ratio observations and WRF/CMAQ simulations that were contributed by EPA/ORD/NERL/CED researchers to the manuscript “On the Limit to the Accuracy of Regional Air Quality Models”. This dataset is associated with the following publication: Rao, S.T., H. Luo, M. Astitha, C. Hogrefe, V. Cover, and R. Mathur. On the limit to the accuracy of regional-scale air quality models. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, GERMANY, 20(3): 1627–1639, (2020).