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Volume 18, issue 11 | Copyright
Atmos. Chem. Phys., 18, 8183-8202, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Jun 2018

Research article | 13 Jun 2018

Ozone response to emission reductions in the southeastern United States

Charles L. Blanchard1 and George M. Hidy2 Charles L. Blanchard and George M. Hidy
  • 1Envair, Albany, CA 94706, USA
  • 2Envair/Aerochem, Placitas, NM 87043, USA

Abstract. Ozone (O3) formation in the southeastern US 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. Annual fourth-highest daily peak 8h 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 fourth-highest daily peak 8h O3 mixing ratios declined toward values of ∼ 45–50ppbv and monthly O3 maxima decreased at rates averaging ∼1–1.5ppbvyr−1. Mean annual total oxidized nitrogen (NOy) mixing ratios at SEARCH sites declined in proportion to NOx 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 NOx emissions by ∼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 NOx emissions lost to deposition has not changed. SEARCH and CASTNET sites exhibit downward trends in mean annual nitric acid (HNO3) concentrations. Observed relationships of O3 to NOz (NOy–NOx) support past model predictions of increases in cycling of NO and increasing responsiveness of O3 to NOx. 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 O3 trends and relating them to reductions in ambient NOy species concentrations offers key insights into processes of general relevance to air quality management and provides important information supporting strategies for reducing O3 mixing ratios.

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Ozone (O3) formation in the southeastern US was studied in relation to nitrogen oxide (NOx) emissions using long-term (1990s–2015) measurements of the SEARCH network and U.S. EPA data. NOx emissions decreased by ~ 60 %. Annual fourth-highest daily peak 8 h O3 mixing ratios declined toward ~ 45–50 ppbv at ~1 ppbv yr−1 and O3 exhibited increasing sensitivity to NOx. This study illustrates the value of consistent, long-term measurements of O3 and reactive nitrogen made at both urban and rural sites.
Ozone (O3) formation in the southeastern US was studied in relation to nitrogen oxide (NOx)...