Preprints
https://doi.org/10.5194/acp-2021-267
https://doi.org/10.5194/acp-2021-267

  12 Apr 2021

12 Apr 2021

Review status: this preprint is currently under review for the journal ACP.

Nighttime and Daytime Dark Oxidation Chemistry in Wildfire Plumes: An Observation and Model Analysis of FIREX-AQ Aircraft Data

Zachary C. J. Decker1,2,3, Michael A. Robinson1,2,3, Kelley C. Barsanti4, Ilann Bourgeois1,2, Matthew M. Coggon1,2, Joshua P. DiGangi5, Glenn S. Diskin5, Frank M. Flocke6, Alessandro Franchin1,2,6, Carley D. Fredrickson7, Samuel R. Hall6, Hannah Halliday5,a, Christopher D. Holmes8, L. Gregory Huey9, Young Ro Lee9, Jakob Lindaas10, Ann M. Middlebrook1, Denise D. Montzka6, Richard H. Moore11, J. Andrew Neuman1,2, John B. Nowak11, Brett B. Palm7, Jeff Peischl1,2, Pamela S. Rickly1,2, Andrew W. Rollins1, Thomas B. Ryerson1, Rebecca H. Schwantes1,2, Lee Thornhill5,11, Joel A. Thornton7, Geoff S. Tyndall6, Kirk Ullmann6, Paul Van Rooy4, Patrick R. Veres1, Andrew J. Weinheimer6, Elizabeth Wiggins5,12, Edward Winstead5,11, Caroline Womack1,2, and Steven S. Brown1,3 Zachary C. J. Decker et al.
  • 1NOAA Chemical Sciences Laboratory (CSL), Boulder, Colorado 80305, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado 80309, USA
  • 3Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309-0215, USA
  • 4Department of Chemical and Environmental Engineering and College of Engineering – Center for Environmental Research and Technology (CE-CERT), University of California, Riverside, Riverside, CA 92507, USA
  • 5NASA Langley Research Center, MS 483, Hampton, VA 23681, USA
  • 6Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO 80301, USA
  • 7Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
  • 8Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL, 32304, USA
  • 9School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
  • 10Colorado State University, Department of Atmospheric Science, Fort Collins, CO, USA
  • 11Science Systems and Applications, Inc. (SSAI), Hampton, VA, USA
  • 12Universities Space Research Association, Columbia, MD, USA
  • anow at: EPA Office of Research and Development, RTP, NC 27711, USA

Abstract. Wildfires are increasing in size across the western U.S., 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 mid-day, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1–1.5 ppbv h−1) in plumes transported both mid-day and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, 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 ≥ 98 % of NO3 loss in sunlit plumes (jNO2 up to 4 x 10–3 s–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 mid-day 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 model, there is remaining NOx (13 %–57 %) and BBVOCs (8 %–72 %) at sunrise.

Zachary C. J. Decker et al.

Status: open (until 07 Jun 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Zachary C. J. Decker et al.

Zachary C. J. Decker et al.

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Short summary
To understand air quality impacts from wildfire smoke we need an accurate picture of how wildfire smoke changes chemically both day and night as sunlight changes the chemistry of smoke. We present a chemical analysis of wildfire smoke as it changes from midday through the night. We use aircraft observations from the FIREX-AQ field campaign with a chemical box model. We find that even under sunlight typical “nighttime” chemistry thrives and controls the fate of key smoke plume chemical processes.
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