LAUSR

Widespread efforts to abate ozone (O 3 ) smog have significantly reduced nitrogen oxides (NO x ) emissions over the past two decades in the Southeast U.S. (SEUS), 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 SEAC 4 RS (August–September, 2013) and long-term ground measurement networks alongside a global chemistry-climate model to examine decadal changes in summertime reactive nitrogen species and ozone over the Southeast U.S. We find that most reactive nitrogen species, including NO x , peroxyacetyl nitrate (PAN) and nitric acid (HNO 3 ) decline proportionally with decreasing NO x emissions in this region, leading to a similar decline in exported NO y . This linear response is in part due to the nearly constant summertime supply of biogenic VOC emissions in this region. Our model captures the observed relative change of reactive nitrogen species 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 extreme ozone events.