Abstract
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 8 h 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 8 h O3 mixing ratios declined toward values of ∼ 45–50 ppbv and monthly O3 maxima decreased at rates averaging ∼ 1–1.5 ppbv yr−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.
Highlights
Ozone (O3) is a well known and important product of photochemical processes in the troposphere involving nitric oxide (NO), nitrogen dioxide (NO2), and volatile organic compounds (VOCs)
The largest nitrogen oxide (NOx) emission changes in the southeast occurred between 2007 and 2009 due to reductions of emissions from electric generating units (EGUs) and from diesel engine vehicles, and they were accompanied by more gradual year-to-year reductions of gasoline-engine mobile-source NOx emissions
Linear regression slopes indicate that the annual nitrate deposition fluxes at the Georgia and Alabama Clean Air Status and Trends Network (CASTNET) sites correspond to 30 % of Georgia emissions and 36 % of Alabama emissions on an annual and statewide basis (Fig. 1)
Summary
Ozone (O3) is a well known and important product of photochemical processes in the troposphere involving nitric oxide (NO), nitrogen dioxide (NO2), and volatile organic compounds (VOCs). Regulatory actions address extreme O3 mixing ratios: the US National Ambient Air Quality Standard (NAAQS), currently 70 ppbv, is applicable to the annual fourth-highest daily 8 h maxima averaged over 3-year periods (U.S EPA, 2015b, c). O3 management has generally relied on precursor reduction requirements estimated from models that integrate descriptions of nonlinear chemical and atmospheric processes (e.g., Seigneur and Dennis, 2011), and guidance has derived from so-called “observationbased” models linking O3 and its precursors based on chemical reactions that are believed to drive ambient mixing ratios (e.g., NARSTO, 2000; Schere and Hidy, 2000). Short-term data are available from aircraft flights, for example, or summer field
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