Ozone Transport in the U.S. Mid-Atlantic

The United States Environmental Protection Agency (EPA) revised the primary and secondary national ambient air quality standard (NAAQS) for ozone from 0.075 to 0.070 parts per million (ppm) in 2015 to provide protection of public health and welfare. Observational data in the Mid-Atlantic region show that in hot summer days in which suitable conditions for ozone production exist, ozone concentrations are above the standard and this cause that region to be nonattainment. With the EPA’s new proposed standard for ozone in 2015 (70 ppb), states in the Mid-Atlantic are expected to experience more ozone exceedance than before. Because the U.S. Mid-Atlantic region is affected by transport of ozone and its precursors; anthropogenic nitrogen oxides (NOx) and volatile organic compound (VOC) from upwind states, therefore local emission-reduction strategies are not necessarily the most effective to reach attainment.

Regional air quality and sensitivities of ground-level ozone to emissions from up-wind states in eastern U.S. were simulated for three summer months (June, July, and August) in 2011 using the Comprehensive Air Quality Model with Extensions (CAMx) - inclusive of source apportionment and localized emission reductions, to study the extent of ozone precursor transport in the Mid-Atlantic and the efficacy of emission-reduction strategies for 2011 summer season, with a focus on Delaware. Delaware is a small state in the Mid-Atlantic region and is in nonattainment of the 2015 national 8-hour ozone standard (0.070 ppm) despite its relatively low local emissions, thus it is an ideal test case to study transport. The emission data were taken from the EPA 2011v6 emission-modeling platform based on the 2011 National Emission Inventory (NEI). We performed source apportionment run using CAMx with APCA (Anthropogenic Precursor Culpability Assessment) for summer 2011 on eastern United States to find the upwind states that contribute more to Delaware ozone pollution. In addition, a set of simulations with NOx and VOC emission reductions was performed with CAMx for summer 2011. All the emission reduction cases were run with the same meteorology, initial and boundary conditions as the control case.

Based on source apportionment and emission reduction runs we found that in Delaware; local emission reduction of NOx and VOC by 20% in Delaware has the minimum benefit. The same emission reduction (20% NOx and VOC) in each of five upwind states individually lowers ozone but not sufficiently to reach attainment. With a coordinated effort by the five upwind states together (MD, PA, VA, OH, and WV) Delaware ozone can be lowered significantly. Finally, the emission reduction needed by the five states together is approximately 10%, half that of each individual state. This suggests that coordinated efforts and long-term, multi-state strategies are necessary to protect air quality in the Mid-Atlantic.