Responses in Ozone and Its Production Efficiency Attributable to Recent and Future Emissions Changes in the Eastern...

Ozone production efficiency (OPE), a measure of the number of ozone (O₃) molecules produced per emitted NO_X (NO + NO₂) molecule, helps establish the relationship between NO_X emissions and O₃ formation. We estimate long-term OPE variability across the eastern United States using two novel approaches: an observation-based empirical method and a chemical transport model (CTM) method. The CTM approach explicitly controls for differing O₃ and NO_X reaction product (NO_Z) deposition rates and separately estimates OPEs from on-road mobile and electricity generating unit sources across a broad spatial scale. We find lower OPEs in urban areas and that average July OPE increased over the eastern United States domain between 2001 and 2011 from 11 to 14. CTM and empirical approaches agree at low NO_Z concentrations, but CTM OPEs are greater than empirical OPEs at high NO_Z. Our results support that NO_X emissions reductions become more effective at reducing O₃ at lower NO_Z concentrations. Electricity generating unit OPEs are higher than mobile OPEs except near emissions locations, meaning further utility NO_X emissions reductions will have greater per unit impacts on O₃ regionally.