Precursor reductions and ground-level ozone in the Continental United States

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Numerous papers analyze ground-level ozone (O3) trends since the 1980s, but few have linked O3 trends with observed changes in nitrogen oxide (NOx) and volatile organic compound (VOC) emissions and ambient concentrations. This analysis of emissions and ambient measurements examines this linkage across the United States on multiple spatial scales from continental to urban. O3 concentrations follow the general decreases in both NOx and VOC emissions and ambient concentrations of precursors (nitrogen dioxide, NO2; nonmethane organic compounds, NMOCs). Annual fourth-highest daily peak 8-hr average ozone and annual average or 98th percentile daily maximum hourly NO2 concentrations show a statistically significant (p < 0.05) linear fit whose slope is less than 1:1 and intercept is in the 30 to >50 ppbv range. This empirical relationship is consistent with current understanding of O3 photochemistry. The linear O3–NO2 relationships found from our multispatial scale analysis can be used to extrapolate the rate of change of O3 with projected NOx emission reductions, which suggests that future declines in annual fourth-highest daily average 8-hr maximum O3 concentrations are unlikely to reach 65 ppbv or lower everywhere in the next decade. Measurements do not indicate increased annual reduction rates in (high) O3 concentrations beyond the multidecadal precursor proportionality, since aggressive measures for NOx and VOC reduction are in place and have not produced an accelerated O3 reduction rate beyond that prior to the mid-2000s. Empirically estimated changes in O3 with emissions suggest that O3 is less sensitive to precursor reductions than is found by the CAMx (v. 6.1) photochemical model. Options for increasing the rate of O3 change are limited by photochemical factors, including the increase in NOx sensitivity with time (NMOC/NOx ratio increase), increase in O3 production efficiency at lower NOx concentrations (higher O3/NOy ratio), and the presence of natural NOx and NMOC precursors and background O3.Implications: This analysis demonstrates empirical relations between O3 and precursors based on long term trends in U.S. locations. The results indicate that ground-level O3 concentrations have responded predictably to reductions in VOC and NOx since the 1980s. The analysis reveals linear relations between the highest O3 and NO2 concentrations. Extrapolation of the historic trends to the future with expected continued precursor reductions suggest that achieving the 2014 proposed reduction in the U.S. National Ambient Air Quality Standard to a level between 65 and 70 ppbv is unlikely within the next decade. Comparison of measurements with national results from a regulatory photochemical model, CAMx, v. 6.1, suggests that model predictions are more sensitive to emissions changes than the observations would support.