Nighttime and daytime dark oxidation chemistry in wildfire plumes: an observation and model analysis of FIREX-AQ aircraft data
Authors: Decker Zachary C. J. , Robinson Michael A. , Barsanti Kelley C. , Bourgeois Ilann , Coggon Matthew M. , DiGangi Joshua P. , Diskin Glenn S. , Flocke Frank M. , Franchin Alessandro , Fredrickson Carley D. , Gkatzelis Georgios I. , Hall Samuel R. , Halliday Hannah , Holmes Christopher D. , Huey L. Gregory , Lee Young Ro , Lindaas Jakob , Middlebrook Ann M. , Montzka Denise D. , Moore Richard , Neuman J. Andrew , Nowak John B. , Palm Brett B. , Peischl Jeff , Piel Felix , Rickly Pamela S. , Rollins Andrew W. , Ryerson Thomas B. , Schwantes Rebecca H. , Sekimoto Kanako , Thornhill Lee , Thornton Joel A. , Tyndall Geoffrey S. , Ullmann Kirk , Van Rooy Paul , Veres Patrick R. , Warneke Carsten , Washenfelder Rebecca A. , Weinheimer Andrew J. , Wiggins Elizabeth , Winstead Edward , Wisthaler Armin , Womack Caroline , Brown Steven S. 
Published: November 2021

Abstract. Wildfires are increasing in size across the western US, leading to
increases in human smoke exposure and associated negative health impacts.
The impact of biomass burning (BB) smoke, including wildfires, on regional
air quality depends on emissions, transport, and chemistry, including
oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl
radical (OH), nitrate radical (NO3), and ozone (O3). During the
daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by
O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs
are oxidized mainly by O3 and NO3. This work focuses on the
transition between daytime and nighttime oxidation, which has significant
implications for the formation of secondary pollutants and loss of nitrogen
oxides (NOx=NO+NO2) and has been understudied. We present
wildfire plume observations made during FIREX-AQ (Fire Influence on Regional
to Global Environments and Air Quality), a field campaign involving multiple
aircraft, ground, satellite, and mobile platforms that took place in the
United States in the summer of 2019 to study both wildfire and agricultural
burning emissions and atmospheric chemistry. We use observations from two
research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed
chemical box model, including updated phenolic mechanisms, to analyze smoke
sampled during midday, sunset, and nighttime. Aircraft observations suggest
a range of NO3 production rates (0.1–1.5 ppbv h−1) in plumes
transported during both midday and after dark. Modeled initial instantaneous
reactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, and 99.6 %, respectively. Initial NO3 reactivity is 10–104
times greater than typical values in forested or urban environments, and
reactions with BBVOCs account for >97 % of NO3 loss in
sunlit plumes (jNO2 up to 4×10-3s-1), while
conventional photochemical NO3 loss through reaction with NO and
photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH
and O3 (11 %–43 %, 54 %–88 % for alkenes; 18 %–55 %, 39 %–76 %, for furans, respectively), but phenolic oxidation is split between
NO3, O3, and OH (26 %–52 %, 22 %–43 %, 16 %–33 %,
respectively). Nitrate radical oxidation accounts for 26 %–52 % of
phenolic chemical loss in sunset plumes and in an optically thick plume.
Nitrocatechol yields varied between 33 % and 45 %, and NO3
chemistry in BB plumes emitted late in the day is responsible for 72 %–92 % (84 % in an optically thick midday plume) of nitrocatechol
formation and controls nitrophenolic formation overall. As a result,
overnight nitrophenolic formation pathways account for 56 %±2 % of
NOx loss by sunrise the following day. In all but one overnight plume
we modeled, there was remaining NOx (13 %–57 %) and BBVOCs
(8 %–72 %) at sunrise.