Modeling assessment of point source NOx emission reductions on ozone air quality in the eastern United States

This study investigates the effects of reductions in nitrogen oxide (NOx) emissions from major point sources on daily maximum 8-h ozone concentrations in the eastern United States. The Community Multiscale Air Quality (CMAQ) model was utilized in photochemical simulations on a matrix of modeling scenarios permitting an examination of the separate effects of emission changes and meteorological influences on maximum ozone levels over a 3-month period during the summers of 2002 and 2004. Two modeling scenarios involved base case 2002 emissions and post-control emissions, reflecting the point source NOx emission reductions implemented before the ozone season of 2004, using summer 2002 meteorological conditions. Results revealed that point source NOx emission reductions caused decreases in daily maximum 8-h ozone concentrations over the eastern United States. At the 50th and 95th percentiles of the cumulative frequency distribution, daily maximum 8-h ozone values in the emission reduction scenario were lower than corresponding base case values over 70% and 90% of the modeling domain, respectively. During southwesterly wind flows across the Ohio River Valley, morning ozone concentrations aloft were lower over northeastern states downwind of the emissions-rich region in the NOx reduction scenario results. Another notable feature of the NOx emission reduction scenario results is that greater decreases in daily maximum 8-h ozone occurred at higher concentrations. Results from other modeling scenarios revealed strong differences in meteorological conditions between these two summer periods greatly impacted the daily 8-h maximum ozone concentrations with the meteorological effects on ozone being greater than those from emission changes over the northern part of the modeling domain. Using backtrajectory analysis, greater percentage decreases in daily maximum 8-h ozone occurred at monitoring sites when they were downwind of the Ohio River Valley, which is a notable emission source region, as compared to cases when the sites were not downwind of it.