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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-534
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
08 Aug 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Ozone Response to Emission Reductions in the Southeastern United States
Charles L. Blanchard1 and George M. Hidy2 1Envair, Albany, CA, 94706, USA
2Envair/Aerochem, Placitas, NM, 87043, USA
Abstract. Ozone (O3) formation in the southeastern U.S. is studied in relation to nitrogen oxide (NOx) emissions using long-term (1990s–2015) surface measurements of the Southeastern Aerosol Research and Characterization (SEARCH) network, U.S. Environmental Protection Agency (EPA) O3 measurements, and EPA Clean Air Status and Trends Network (CASTNet) nitrate deposition data. 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 NOx emissions in 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 NOx emissions lost to deposition has not changed over time. SEARCH and EPA CASTNet sites exhibit comparable downward trends in mean annual nitric acid (HNO3) concentrations. Mean annual total oxidized nitrogen (NOy) mixing ratios at SEARCH sites declined in proportion to NOx emission reductions. Annual 4th-highest daily peak 8-hour O3 mixing ratios at EPA monitoring sites in Georgia, Alabama, and Mississippi exhibit statistically-significant (p < 0.0001) linear correlations with annual NOx emissions in those states between 1996 and 2015. The annual 4th-highest daily peak 8-hour O3 mixing ratios are declining toward non-zero values of ~ 45–50 ppbv. The O3 declines are less than proportional to the decreases in NOx emissions: emissions decreased by ~ 60 % and O3 maxima declined ~ 30–35 % at rates averaging ~ 1 ppbv y−1. Ozone production efficiency (OPE, molecules of O3 produced per molecule of NOx oxidized) increased between 1999 and 2014, which affected the magnitude of the O3 response to NOx emission reductions by partially offsetting precursor decreases and contributing to the nonlinear O3 response. The results suggest increasing responsiveness of O3 to NOx, but the effectiveness of ongoing NOx emission reductions will depend on the balance between changes in observed OPE and ambient NOx in the context of changes in anthropogenic emissions of volatile organic compounds (VOC).

Citation: Blanchard, C. L. and Hidy, G. M.: Ozone Response to Emission Reductions in the Southeastern United States, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-534, in review, 2017.
Charles L. Blanchard and George M. Hidy
Charles L. Blanchard and George M. Hidy
Charles L. Blanchard and George M. Hidy

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Short summary
Ozone (O3) formation in the southeastern U.S. was studied in relation to nitrogen oxide (NOx) emissions using long-term (1990s–2015) surface measurements of the Southeastern Aerosol Research and Characterization (SEARCH) network and U.S. Environmental Protection Agency (EPA) data. The annual 4th-highest daily peak 8-hour O3 mixing ratios declined toward ~ 45–50 ppbv at ~ 1 ppbv y−1. NOx emissions decreased by ~ 60 % and O3 maxima declined ~ 30–35 % with increasing responsiveness of O3 to NOx.
Ozone (O3) formation in the southeastern U.S. was studied in relation to nitrogen oxide (NOx)...
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