Decadal changes in summertime reactive oxidized nitrogen and surface ozone over the Southeast United States

Abstract. Widespread efforts to abate ozone (O₃) smog have significantly reduced emissions of nitrogen oxides (NO_x) over the past 2 decades in the Southeast US, a place heavily influenced by both anthropogenic and biogenic emissions. How reactive nitrogen speciation responds to the reduction in NO_x emissions in this region remains to be elucidated. Here we exploit aircraft measurements from ICARTT (July–August 2004), SENEX (June–July 2013), and SEAC⁴RS (August–September 2013) and long-term ground measurement networks alongside a global chemistry–climate model to examine decadal changes in summertime reactive oxidized nitrogen (RON) and ozone over the Southeast US. We show that our model can reproduce the mean vertical profiles of major RON species and the total (NO_y) in both 2004 and 2013. Among the major RON species, nitric acid (HNO₃) is dominant (∼ 42–45 %), followed by NO_x (31 %), total peroxy nitrates (ΣPNs; 14 %), and total alkyl nitrates (ΣANs; 9–12 %) on a regional scale. We find that most RON species, including NO_x, ΣPNs, and HNO₃, decline proportionally with decreasing NO_x emissions in this region, leading to a similar decline in NO_y. This linear response might be in part due to the nearly constant summertime supply of biogenic VOC emissions in this region. Our model captures the observed relative change in RON and surface ozone from 2004 to 2013. Model sensitivity tests indicate that further reductions of NO_x emissions will lead to a continued decline in surface ozone and less frequent high-ozone events.