Precursor reductions and ground-level ozone in the Continental United States
Creators: name: Hidy, George M.
name: Blanchard, Charles L.
Publisher: name: Taylor & Francis
Year: 2015
Subjects: subject: Chemistry
subject: Cell Biology
subject: Plant Biology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: Science Policy
subject: Inorganic Chemistry
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences
subject: Biological Sciences
subject: Ecology
subject: Earth and Environmental Sciences
subject: Evolutionary Biology
subject: Biochemistry
subject: Biophysics

description: Numerous papers analyze ground-level ozone (O<sub>3</sub>) trends since the 1980s, but few have linked O<sub>3</sub> trends with observed changes in nitrogen oxide (NO<sub>x</sub>) 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. O<sub>3</sub> concentrations follow the general decreases in both NO<sub>x</sub> and VOC emissions and ambient concentrations of precursors (nitrogen dioxide, NO<sub>2</sub>; nonmethane organic compounds, NMOCs). Annual fourth-highest daily peak 8-hr average ozone and annual average or 98th percentile daily maximum hourly NO<sub>2</sub> concentrations show a statistically significant (<i>p</i> &lt; 0.05) linear fit whose slope is less than 1:1 and intercept is in the 30 to &gt;50 ppbv range. This empirical relationship is consistent with current understanding of O<sub>3</sub> photochemistry. The linear O<sub>3</sub>–NO<sub>2</sub> relationships found from our multispatial scale analysis can be used to extrapolate the rate of change of O<sub>3</sub> with projected NO<sub>x</sub> emission reductions, which suggests that future declines in annual fourth-highest daily average 8-hr maximum O<sub>3</sub> concentrations are unlikely to reach 65 ppbv or lower everywhere in the next decade. Measurements do not indicate increased annual reduction rates in (high) O<sub>3</sub> concentrations beyond the multidecadal precursor proportionality, since aggressive measures for NO<sub>x</sub> and VOC reduction are in place and have not produced an accelerated O<sub>3</sub> reduction rate beyond that prior to the mid-2000s. Empirically estimated changes in O<sub>3</sub> with emissions suggest that O<sub>3</sub> is less sensitive to precursor reductions than is found by the CAMx (v. 6.1) photochemical model. Options for increasing the rate of O<sub>3</sub> change are limited by photochemical factors, including the increase in NO<sub>x</sub> sensitivity with time (NMOC/NO<sub>x</sub> ratio increase), increase in O<sub>3</sub> production efficiency at lower NO<sub>x</sub> concentrations (higher O<sub>3</sub>/NO<sub>y</sub> ratio), and the presence of natural NO<sub>x</sub> and NMOC precursors and background O<sub>3</sub>.Implications: <i>This analysis demonstrates empirical relations between O<sub>3</sub> and precursors based on long term trends in U.S. locations. The results indicate that ground-level O<sub>3</sub> concentrations have responded predictably to reductions in VOC and NO<sub>x</sub> since the 1980s. The analysis reveals linear relations between the highest O<sub>3</sub> and NO<sub>2</sub> 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.</i>