The eastern United States has seen dramatic air pollution emissions reductions since the turn of the century. These emissions reductions have in turn been linked to widespread reductions in ozone (O3)—between 2000 and 2016, the US EPA reports a reduction in 4th highest mean daily annual 8-hr O3 of 15% (from 82.3 to 69.6 ppb) across 206 sites nationwide. Reductions, however, have not been spatially uniform or linear with emissions reductions, and therefore motivate an investigation into spatial and source-specific O3 production efficiency (OPE). OPE is a measure of the number of O3 molecules produced per emitted NOX (NOX = NO + NO2) molecule. We assess OPE using both model-based and empirical approaches. We modelled July OPE in 2001 and 2011 using CMAQ-DDM version 5.0 with a 12 km resolution over the eastern US. CMAQ-modelled OPE is taken as a ratio of electricity generating …