Abstract
Abstract. Widespread efforts to abate ozone (O3) smog have significantly reduced emissions of nitrogen oxides (NOx) 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 NOx 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 (NOy) in both 2004 and 2013. Among the major RON species, nitric acid (HNO3) is dominant (∼ 42–45 %), followed by NOx (31 %), total peroxy nitrates (ΣPNs; 14 %), and total alkyl nitrates (ΣANs; 9–12 %) on a regional scale. We find that most RON species, including NOx, ΣPNs, and HNO3, decline proportionally with decreasing NOx emissions in this region, leading to a similar decline in NOy. 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 NOx emissions will lead to a continued decline in surface ozone and less frequent high-ozone events.
Highlights
Since the 1990s, the US Environmental Protection Agency (US EPA) has targeted emissions of nitrogen oxides (NOx) to improve air quality by lowering regional photochemical smog
We investigate the response of NOy speciation to the significant NOx emission controls in this region over the past decade in light of the fast-evolving understanding of isoprene photooxidation
We use extensive aircraft and ground observations combined with a global chemistry– climate model (GFDL AM3) to examine decadal changes in NOy abundance and speciation as well as in surface ozone mixing ratios over the Southeast US between the summers of 2004 and 2013
Summary
Since the 1990s, the US Environmental Protection Agency (US EPA) has targeted emissions of nitrogen oxides (NOx) to improve air quality by lowering regional photochemical smog (the 1990 Clean Air Amendment). The average of the annual fourth highest daily maximum 8 h average (MDA8) ozone over 206 sites has decreased by 31 % from 101 ppb in 1980 to 70 ppb in 2016 across the continental US, with more significant reductions in rural areas of the eastern US in summer (Simon et al, 2015; Cooper et al, 2012) We use both aircraft- and ground-based datasets, combined with a high-resolution chemistry–climate model, to evaluate responses of reactive oxidized nitrogen (RON) and surface ozone to the NOx emission reductions in the Southeast US. We demonstrate the sensitivity of RON and MDA8 ozone to a hypothetical NOx emission reduction over the decade
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