Comment on acp-2021-267

doi:10.5194/acp-2021-267-rc1

Abstract

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 (<math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mi>x</mi></msub></mrow><mo>=</mo><mrow class="chem"><mi mathvariant="normal">NO</mi></mrow><mo>+</mo><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="85pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="f0add4bbe2151ecfa7cd944e28fa7e9e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-16293-2021-ie00001.svg" width="85pt" height="13pt" src="acp-21-16293-2021-ie00001.png"/></svg:svg>) 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 (0.1â1.5âppbvâhâ1) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1â%, 87.7â%, and 99.6â%, respectively. Initial NO3 reactivity is 10â104 times greater than typical values in forested or urban environments, and reactions with BBVOCs account for >97â% of NO3 loss in sunlit plumes (jNO2 up to <math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">4</mn><mo>Ã</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><msup><mi mathvariant="normal">s</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="59pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="75fd83c3fc1e7202c7ef5bff89e9ecd3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-16293-2021-ie00002.svg" width="59pt" height="14pt" src="acp-21-16293-2021-ie00002.png"/></svg:svg>), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O3 (11â%â43â%, 54â%â88â% for alkenes; 18â%â55â%, 39â%â76â%, for furans, respectively), but phenolic oxidation is split between NO3, O3, and OH (26â%â52â%, 22â%â43â%, 16â%â33â%, respectively). Nitrate radical oxidation accounts for 26â%â52â% of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33â% and 45â%, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72â%â92â% (84â% in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56â%Â±2â% of NOx loss by sunrise the following day. In all but one overnight plume we modeled, there was remaining NOx (13â%â57â%) and BBVOCs (8â%â72â%) at sunrise.

Highlights

It is well known that biomass burning (BB), including wildfires, can have large impacts on air quality at local, regional, and global scales (Jaffe et al, 2020)
This study details the competitive oxidation of biomass burning volatile organic compound (VOC) (BBVOCs) in four near-sunset or low-photolysis smoke plumes sampled by NOAA Twin Otter or NASA DC-8 aircraft during the FIREX-AQ 2019 field campaign
– In most cases, BBVOC comparisons show that the model and observations agree within a factor of ∼ 2 if not within observation errors

Summary

Introduction

It is well known that biomass burning (BB), including wildfires, can have large impacts on air quality at local, regional, and global scales (Jaffe et al, 2020). Wildfires emit NOx, nitrous acid (HONO), biomass burning VOCs (BBVOCs), and particulate matter (PM) that evolve chemically on a range of timescales, from seconds to weeks downwind (Akagi et al, 2011; Andreae and Merlet, 2001; Decker et al, 2019; Hatch et al, 2015, 2017, 2018; Koss et al, 2018; Palm et al, 2020). These emissions and their chemical products influence air quality through ozone (O3) production, emitted PM, and secondary organic aerosol (SOA) formation (Brey et al, 2018; Jaffe et al, 2020; Jaffe and Wigder, 2012; Lu et al, 2016; Palm et al, 2020; Phuleria et al, 2005). The evolution of the smoke downwind is influenced by several variables such as fuel type, burn conditions, moisture content, nitrogen content, meteorology, and time of day

Methods

Results

Conclusion