Regional Impacts of Reduced NOx Emissions on Ozone Concentrations in the Eastern USA: Positive Results from the NOx Budget Trading Program (NBP)

Tom Butler 1*,2*, Francois Vermeylen 3, Gene Likens 1, Melissa Rury 4 and Brian Lee 4

We have used statistical random coefficient models to quantify the relation between changing ozone season (May-September) NOx emissions and ozone concentrations at sites in the eastern US for the period 1997 to 2007. The models produced from these analyses show highly significant relationships (p < 0.001) between NOx emissions and warm season max 8-hr ozone concentrations. For the eastern US as a whole, ozone levels have declined by 5 to 6 ppb, a 10% decline in ozone concentrations over the entire region. The Mid-Atlantic and Southeast regions show the greatest declines with a 6 to 7 ppb drop or 12% decline in ozone concentration. The Midwest region has an ozone decline of 5 to 5.5 ppb, a 9% to 10% decline. The Northeast region shows the smallest impact from NBP regulation with a decline of 4.5 ppb, an 8.5% decrease in ozone concentration.

The random coefficient models were extended to separate urban and rural sites. For the Eastern US as a whole, the behavior of rural and urban sites are statistically different from each other (p < 0.05). Greater declines in ozone concentrations have occurred at the rural sites, which show a 7 ppb (12%) decline, while urban sites show a 5 ppb (8%) decline between pre- and post-NBP implementation periods. The greater declines at rural sites, except for the Mid-Atlantic region, may be explained by the fact that stationary source emissions which are dispersed more regionally, are likely to have a larger impact on rural sites because these sites are less impacted by other NOx sources. Urban sites may be more impacted by local transportation sources which have not been regulated to the same extent as stationary source NOx emissions.

The proposed Clean Air Interstate Rule (CAIR) is designed to further reduce power plant pollutant emissions, including a 2 million ton (61%) reduction in NOx stationary source emissions from 2003 levels. The NOx reductions due to CAIR will lead to further ozone declines in all regions, but the future declines will not be as great as those already achieved by the NBP and other stationary source reductions. Over ½ of the 61% decline from 2003 NOx emission levels were already achieved by the period 2004 to 2007. Reduced stationary source emissions have been effective in reducing maximum 8-hr ozone levels, and CAIR implementation will continue that pattern. However significant future reductions in ozone levels should also focus on vehicle emissions as well as stationary sources.

1* The Cary Institute of Ecosystem Studies, Millbrook NY; 211 Rice Hall, Cornell University, Ithaca NY 14853 607 255-3580
2* Cornell University, Ithaca NY;
3 Cornell University Statistical Consulting Unit;
4 EPA Clean Air Markets Division, Office of Air and Radiation