Response of ozone and nitrate to stationary source NOx emission reductions in the eastern USA

This study is an assessment of the impact of reduced stationary source NOx emissions on ground-level concentrations of ozone (O3) and dry-NO3 (HNO3(g) + NO3−(p)) in the eastern United States (EUS). Total anthropogenic NOx and VOC emissions have declined 32% and 20%, respectively, from 1997 to 2005 in the 20 eastern states participating in a NOx Budget Trading Program (NBP). Annual and ozone season (OS) NOx emissions from electric generating units (EGUs) have declined 48% and 66%, respectively, in the EUS.From 1997–1999 to 2006–2008 measured decline in meteorologically adjusted (met-adj) daily maximum 8-h (dm8h) O3, has been 8 ppb or 14% for the EUS during the May–September OS, with the largest declines occurring in the highest concentration category (99th percentile). Random coefficient models, with NBP and Acid Rain Program (ARP) regulated NOx emissions as the independent variable, show highly significant relations (p ≤ 0.0001) to dm8h O3 for 1997–2008 during the OS. Regional declines within the EUS range from 11% to 16%. The patterns of the O3 trends are in general agreement with other recent studies. Rural and urban sites both show similar levels of decline from pre- to post-NBP periods.Dry-NO3 during the OS shows a substantial decline from 3.1 to 1.8 μg NO3 m−3. Random coefficient models show regional results ranging from 32% to 39% declines, and measured results show a 35–46% decline in dry-NO3 from 1997–1999 to 2006–2008. These results suggest that regulating NOx emissions has been highly effective in reducing both ozone and dry-NO3 concentrations during the OS.Overall, a 32% reduction in total annual anthropogenic NOx emissions from 1997 to 2005 has been accompanied by a 7–8 ppb, or 12–13% decline in OS dm8h O3 concentration, and a 34% decline in dry-NO3 concentration in the 20-state NBP region. Model results indicate a 50% further reduction in NBP regulated NOx emissions will reduce O3 concentrations an additional 3–5% and dry-NO3 concentrations by 13–16%.

Research highlights
► NBP and ARP NOx emissions have declined 70% from 1997 to 2008 in the Eastern US during O3 season.
► At the same time, daytime ozone in the Eastern US has declined 8 ppb or 14%.
► Dry-NO3 (particulate NO3 + HNO3(g)) has declined from 3.1 to 1.8 μg NO3 m−3.
► Both Dry-NO3 and daytime ozone are highly correlated to the NOx emission declines.
► Strong relations between O3 and NOx emissions exist for both rural and urban sites in the Eastern US.