Optical and Chemical Analysis of Absorption Enhancement by Mixed Carbonaceous Aerosols in the 2019 Woodbury, AZ, Fire Plume

Abstract

Wildfires emit mixtures of light‐absorbing aerosols (including black and brown carbon, BC and BrC, respectively) and more purely scattering organic aerosol (OA). BC, BrC, and OA interactions are complex and dynamic and evolve with aging in the atmosphere resulting in large uncertainties in their radiative forcing. We report microphysical, optical, and chemical measurements of multiple plumes from the Woodbury Fire (AZ, USA) observed at Los Alamos, NM, after 11–18 hr of atmospheric transit. This includes periods where the plumes exhibited little entrainment as well as periods that had become more dilute after mixing with background aerosol. Aerosol mass absorption cross sections (MAC) were enhanced by a factor of 1.5–2.2 greater than bare BC at 870 nm, suggesting lensing by nonabsorbing coatings following a core‐shell morphology. Larger MAC enhancement factors of 1.9–5.1 at 450 nm are greater than core‐shell morphology can explain and are attributed to BrC. MAC of OA (MACOrg) at 450 nm was largest in intact portions of the plumes (peak value bounded between 0.6 and 0.9 m2/g [Org]) and decreased with plume dilution. We report a strong correlation between MACOrg(450 nm) with the fC2H4O2 (a tracer for levoglucosan‐like species) of coatings and of bulk OA indicating that BrC in the Woodbury Fire was coemitted with levoglucosan, a primary aerosol. fC2H4O2 and MACOrg(450 nm) are shown to vary between the edge and the core of plumes, demonstrating enhanced oxidation of OA and BrC bleaching near plume edges. Our process‐level finding can inform parameterizations of mixed BC, BrC, and OA properties for wildfire plumes in climate models.

Dilution impacts on smoke aging: evidence in Biomass Burning Observation Project (BBOP) data

“…We use the calculated f 60 and f 44 fractions (the unit mass resolution mass concentration ratios of m/z60 and 44 normalized by the total OA mass concentration) and O/C and H/C elemental ratios of OA as tracers of smoke and oxidative aging. Elevated f 60 values are indicative of "levoglucosan-like" species (levoglucosan and other molecules that similarly fragment in the AMS) (Aiken et al, 2009;Cubison et al, 2011;Lee et al, 2010) that are known tracers of smoke primary organic aerosol (POA) ; f 44 , the OA fractional component observed by the SP-AMS as the high-resolution ion fragment CO 2 + as well as some acid groups, is a proxy for SOA arising from oxidative aging (Alfarra et al, 2004;Cappa and Jimenez, 2010;Jimenez et al, 2009;Volkamer et al, 2006). Fractional components f 60 and f 44 have been shown to decrease and increase with photochemical aging, respectively, likely due to both evaporation and/or oxidation of semivolatile species that contribute to m/z60 in the SP-AMS and addition of oxidized species that contribute to m/z44 in the SP-AMS (Alfarra et al, 2004;Huffman et al, 2009).…”

Contrasting Stratospheric Smoke Mass and Lifetime From 2017 Canadian and 2019/2020 Australian Megafires: Global Simulations and Satellite Observations

“…In addition, in situ airborne measurements in the upper troposphere provide evidence of thick coatings (≳150 ηm on BC cores ≈70 ηm in radius, Ditas et al., 2018) on BC particles emitted by biomass burning (see also Dahlkötter et al., 2014), and of extremely thick coatings (≳250 ηm) in the BC17 plume sampled in the lower stratosphere (J. Katich, October 2021, personal communication). Organic coatings on BC cores can enhance light absorption by lensing (S. Liu et al., 2014; Lee et al., 2020), but the effect depends strongly on morphology (Cappa et al., 2012) and can be reduced by extruded structures. Moreover, the absorption enhancement does not increase linearly with coating thickness but it saturates at about twice the core size (Lack & Cappa, 2010) and can drop for extremely thick coatings (>3 times the core size), typical of the BC17 smoke injection.…”

“…The mixing state, determined by fuel composition, distribution, moisture and fire dynamics, is highly variable and complex. Ambient fire plumes (primarily warm tropospheric plumes) have been sampled over a range of ages and they start out with mean mass fractions of around 2% (Lee et al., 2020). Recent analysis of pyroCbs indicates that secondary organic aerosol formation, by condensation in cold updrafts of organic vapors (co‐emitted by fires), can reduce the BC mass fraction to about 1% as the plume rises toward the upper troposphere and lower stratosphere (Ditas et al., 2018).…”

Wildfire Smoke Demonstrates Significant and Predictable Black Carbon Light Absorption Enhancements

“…The very‐thick coatings are likely related to high condensation rates due to the larger thermogradients and colder absolute temperatures. High rates of photochemical processing at elevation (Hodzic et al., 2016; Lee et al., 2020; Zawadowicz et al., 2020) may mean that these plumes have aged faster than the Woodbury plumes that were of similar atmospheric age but transported at lower elevations. This could result in evaporation of coatings (D. Liu et al., 2017; Metcalf et al., 2013) and microphysical changes in particle shape and size that occur as a result of oxidation, removal of coatings, and particle freezing and re‐warming that alter BC optical properties.…”