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

Abstract

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.

Emissions of organic compounds from western US wildfires and their near-fire transformations

“…The average nighttime concentration of nitroaromatic compounds were higher by 66 % (although not statistically significantly higher, p > 0.05) than in the daytime samples, possibly due to the higher yield of NO 3 oxidation (Finewax et al, 2018). This is in agreement with the airborne plume study in FIREX-AQ (Decker et al, 2021), which also found the strong oxidation of phenolic compounds by NO 3 . In our study, the daytime oxidation of phenolic compounds by NO 3 is probably less important than in those airborne plumes, possibly due to NO emission on the ground that consumed NO 3 .…”

Complexity in the Evolution, Composition, and Spectroscopy of Brown Carbon in Aircraft Measurements of Wildfire Plumes

“…Catechol was selected due to its rapid reaction with OH (see Supporting Information ). Three plume intercepts from 24 August 2019 and five plume intercepts from 28 August 2019 are omitted from Figure 2e, because they occurred after sunset and would not be dominated by OH loss (Decker, Robinson, et al., 2021; Decker, Wang, et al., 2021). The majority of the flight legs show constant or increasing BrC MAC values as a function of chemical plume age, but there is no overall consistent trend.…”

“…The FIREX‐AQ field campaign included four aircraft and several mobile laboratories to examine wildfire emissions and chemistry in the western U.S. This study focuses on the “Chemistry” Twin Otter, which was based in Boise, ID and Cedar City, UT during 3 August–5 September 2019 and included instruments for gas species, aerosol species, and meteorology (Decker, Robinson, et al., 2021; Robinson et al., 2021). The Twin Otter completed 39 science flights with durations of 2.5–3 hr, and sampled nine unique wildfires in Idaho, Oregon, Nevada, Arizona, and Utah during a mixture of afternoon, evening, and night conditions.…”

Photochemical evolution of the 2013 California Rim Fire: synergistic impacts of reactive hydrocarbons and enhanced oxidants

“…We also update rate coefficients for reaction of peroxyacetic acid with OH (Berasategui et al, 2020) and peroxyacyl radicals with HO 2 (Jenkin et al, 2019), which are slower/faster than MCM default values by factors of 123/1.33, respectively. The oxidation of some additional biomass burning VOC (furans, syringol, and guaiacol) is incorporated using an MCM extension developed following recent laboratory and field studies (Coggon et al, 2019;Decker et al, 2019Decker et al, , 2021aRobinson et al, 2021).…”