The Southeastern Aerosol Research and Characterization (SEARCH) study: Temporal trends in gas and PM concentrations and composition, 1999-2010

The SEARCH study began in mid 1998 with a focus on particulate matter and gases in the southeastern United States. Eight monitoring sites, comprising four urban/nonurban pairs, are located inland and along the coast of the Gulf of Mexico. Downward trends in ambient carbon monoxide (CO), sulfur dioxide (SO2), and oxidized nitrogen species (NOy) concentrations averaged 1.2 +/- 0.4 to 9.7 +/- 1.8% per year from 1999 to 2010, qualitatively proportional to decreases of 4.7 to 7.9% per year in anthropogenic emissions of CO, SO2, and oxides of nitrogen (NOx) in the SEARCH region. Downward trends in mean annual sulfate (SO4) concentrations ranged from 3.7 +/- 1.1 to 6.2 +/- 1.1% per year, approximately linear with, but not 1:1 proportional to, the 7.9 +/- 1.1% per year reduction in SO2 emissions from 1999 to 2010. The 95th percentile of the March-October peak daily 8-hr ozone (O-3) concentrations decreased by 1.1 +/- 0.4 to 2.4 +/- 0.6 ppbv per year (1.5 +/- 0.6 to 3.1 +/- 0.8% per year); O-3 precursor emissions of NOx and volatile organic compounds (VOC) decreased at rates of 4.7 and 3.3% per year, respectively. Ambient particulate nitrate (NO3) concentrations decreased by 0.6 +/- 1.2 to 5.8 +/- 0.9% per year, modulated in comparison with mean annual ambient NOy concentration decreases ranging from 6.0 +/- 0.9 to 9.0 +/- 1.3% per year. Mean annual organic matter (OM) and elemental carbon (EC) concentrations declined by 3.3 +/- 0.8 to 6.5 +/- 0.3 and 3.2 +/- 1.4 to 7.8 +/- 0.7% per year. The analysis demonstrates major improvements in air quality in the Southeast from 1999 to 2010. Meteorological variations and incompletely quantified uncertainties for emission changes create difficulty in establishing unambiguous quantitative relationships between emission reductions and ambient air quality.