Ozone in the Eastern United States: Production Efficiency Variability Over Time and Between Sources

Abstract

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 NO_X (NO_X = 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 unit and mobile source sensitivities, and controls for differences in O₃ and NO_Z deposition rates. Measurements were taken from the SEARCH network, which reports sub-daily observations of many gaseous and particulate species along with meteorological measurements at eight sites in the southeastern US. Using measurement data, we stratified days based on their emissions-independent photochemical state, and estimated OPE using a spline model to assess the relationship between O₃ and NO_X reaction products (denoted NO_Z). Both approaches yield an increase in OPE with decreasing NO_Z, indicating an increasing effectiveness at lowering O₃ for subsequent NO_X emissions reductions. Electricity generating unit OPEs are low near individual sources, but generally higher than on-road mobile source OPEs throughout the domain, suggesting that further utility NO_X emissions reductions will reduce regional O₃ concentrations more efficiently than mobile source NO_X emissions reductions.