Transition from high- to low-NOx control of night-time oxidation in the southeastern US

The influence of nitrogen oxides (NOx) on daytime atmospheric oxidation cycles is well known, with clearly defined high- and low-NOx regimes. During the day, oxidation reactions—which contribute to the formation of secondary pollutants such as ozone—are proportional to NOx at low levels, and inversely proportional to NOx at high levels. Night-time oxidation of volatile organic compounds also influences secondary pollutants but lacks a similar clear definition of high- and low-NOx regimes, even though such regimes exist. Decreases in anthropogenic NOx emissions in the US and Europe coincided with increases in Asia over the last 10 to 20 years, and have altered both daytime and nocturnal oxidation cycles. Here we present measurements of chemical species in the lower atmosphere from day- and night-time research flights over the southeast US in 1999 and 2013, supplemented by atmospheric chemistry simulations. We find that night-time oxidation of biogenic volatile organic compounds (BVOC) is NOx-limited when the ratio of NOx to BVOC is below approximately 0.5, and becomes independent of NOx at higher ratios. The night-time ratio of NOx to BVOC in 2013 averaged 0.6 aloft. We suggest that night-time oxidation in the southeast US is in transition between NOx-dominated and ozone-dominated. The influence of NOx levels at night on atmospheric oxidation is unclear. Analyses of aircraft observations suggest that night-time oxidation is transitioning from a high- to low-NOx regime in the southeast US due to declines in NOx levels.