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. Wildfires are increasing in size across the western US, leading toincreases in human smoke exposure and associated negative health impacts.The impact of biomass burning (BB) smoke, including wildfires, on regionalair quality depends on emissions, transport, and chemistry, includingoxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxylradical (OH), nitrate radical (NO3), and ozone (O3). During thedaytime, when light penetrates the plumes, BBVOCs are oxidized mainly byO3 and OH. In contrast, at night or in optically dense plumes, BBVOCsare oxidized mainly by O3 and NO3. This work focuses on thetransition between daytime and nighttime oxidation, which has significantimplications for the formation of secondary pollutants and loss of nitrogenoxides (NOx=NO+NO2) and has been understudied. We presentwildfire plume observations made during FIREX-AQ (Fire Influence on Regionalto Global Environments and Air Quality), a field campaign involving multipleaircraft, ground, satellite, and mobile platforms that took place in theUnited States in the summer of 2019 to study both wildfire and agriculturalburning emissions and atmospheric chemistry. We use observations from tworesearch aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailedchemical box model, including updated phenolic mechanisms, to analyze smokesampled during midday, sunset, and nighttime. Aircraft observations suggesta range of NO3 production rates (0.1–1.5 ppbv h−1) in plumestransported during both midday and after dark. Modeled initial instantaneousreactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, and 99.6 %, respectively. Initial NO3 reactivity is 10–104times greater than typical values in forested or urban environments, andreactions with BBVOCs account for &gt;97 % of NO3 loss insunlit plumes (jNO2 up to 4×10-3s-1), whileconventional photochemical NO3 loss through reaction with NO andphotolysis are minor pathways. Alkenes and furans are mostly oxidized by OHand O3 (11 %–43 %, 54 %–88 % for alkenes; 18 %–55 %, 39 %–76 %, for furans, respectively), but phenolic oxidation is split betweenNO3, O3, and OH (26 %–52 %, 22 %–43 %, 16 %–33 %,respectively). Nitrate radical oxidation accounts for 26 %–52 % ofphenolic chemical loss in sunset plumes and in an optically thick plume.Nitrocatechol yields varied between 33 % and 45 %, and NO3chemistry in BB plumes emitted late in the day is responsible for 72 %–92 % (84 % in an optically thick midday plume) of nitrocatecholformation and controls nitrophenolic formation overall. As a result,overnight nitrophenolic formation pathways account for 56 %±2 % ofNOx loss by sunrise the following day. In all but one overnight plumewe modeled, there was remaining NOx (13 %–57 %) and BBVOCs(8 %–72 %) at sunrise.

Nighttime and daytime dark oxidation chemistry in wildfire plumes: an observation and model analysis of FIREX-AQ aircraft data