Ozone response to emission reductions in the southeastern United States

Blanchard, Charles L.; Hidy, George M.

Ozone (inline-formulaO3) formation in the southeastern US is studied in relation to nitrogen oxide (inline-formulaNOx) emissions using long-term (1990s–2015) surface measurements of the Southeastern Aerosol Research and Characterization (SEARCH) network, U.S. Environmental Protection Agency (EPA) inline-formulaO3 measurements, and EPA Clean Air Status and Trends Network (CASTNET) nitrate deposition data. Annual fourth-highest daily peak 8 h inline-formulaO3 mixing ratios at EPA monitoring sites in Georgia, Alabama, and Mississippi exhibit statistically significant (inline-formulapinline-formula< 0.0001) linear correlations with annual inline-formulaNOx emissions in those states between 1996 and 2015. The annual fourth-highest daily peak 8 h inline-formulaO3 mixing ratios declined toward values of inline-formula∼ 45–50 ppbv and monthly inline-formulaO3 maxima decreased at rates averaging inline-formula∼ 1–1.5 ppbv yrinline-formula−1. Mean annual total oxidized nitrogen (inline-formulaNOy) mixing ratios at SEARCH sites declined in proportion to inline-formulaNOx emission reductions. CASTNET data show declining wet and dry nitrate deposition since the late 1990s, with total (wet plus dry) nitrate deposition fluxes decreasing linearly in proportion to reductions of inline-formulaNOx emissions by inline-formula∼ 60 % in Alabama and Georgia. Annual nitrate deposition rates at Georgia and Alabama CASTNET sites correspond to 30 % of Georgia emission rates and 36 % of Alabama emission rates, respectively. The fraction of inline-formulaNOx emissions lost to deposition has not changed. SEARCH and CASTNET sites exhibit downward trends in mean annual nitric acid (HNinline-formulaO3) concentrations. Observed relationships of inline-formulaO3 to inline-formulaNOz (inline-formulaNOyinline-formulaNOx) support past model predictions of increases in cycling of NO and increasing responsiveness of inline-formulaO3 to inline-formulaNOx. The study data provide a long-term record that can be used to examine the accuracy of process relationships embedded in modeling efforts. Quantifying observed inline-formulaO3 trends and relating them to reductions in ambient inline-formulaNOy species concentrations offers key insights into processes of general relevance to air quality management and provides important information supporting strategies for reducing inline-formulaO3 mixing ratios.

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Blanchard, Charles L. / Hidy, George M.: Ozone response to emission reductions in the southeastern United States. 2018. Copernicus Publications.

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