Mie Scattering Captures Observed Optical Properties of Ambient Biomass Burning Plumes Assuming Uniform Black, Brown, and Organic Carbon Mixtures

Abstract

We use a simple model of a spherical biomass burning aerosol particle containing an internal mixture of black carbon (BC) and organic carbon (OC) with an effective refractive index calculated as a volume fraction‐weighted mean of refractive indices. Brown carbon (BrC) is considered to be an OC with an imaginary part of refractive index at the blue end of the solar spectrum higher (in absolute value) than at the red end. Mie‐scattering formalism is employed to calculate absorption Ångström exponent (AAE) as a function of single‐scattering albedo (SSA) for a set of BC refractive indices, BC volume fraction, and a set of effective refractive indices of BC and BrC mixtures. Ambient plumes are characterized by their mean SSA at 405 nm wavelength and two‐wavelength (405 and 781 nm) AAE values. Comparing observed and model‐calculated AAE and SSA values identifies the BC refractive index and its volume fraction and an effective refractive index of the BC and BrC mixture. From these values, the imaginary part of BrC refractive index is calculated. For observed southwestern ambient fires, the imaginary part of BrC refractive index is ≤0.016. In contrast, fires in the Amazon and from Africa are dominated by BC. We also use the Mie scattering model to establish the upper limit on AAE of BC (AAE ≤ 1.4) and the upper limit of the BC and OC mixture (AAE ≤ 1.7). Any AAE(405/781) > 1.7 requires the presence of BrC. We derive several relationships between AAE, SSA, BC fractions, and an imaginary part of BrC refractive index of ambient fires.

Modeling the smoky troposphere of the southeast Atlantic: a comparison to ORACLES airborne observations from September of 2016

“…This inference is at first glance contradicted by the model comparisons to BC and OA mass concentrations. Further model evaluation of the model refractive indices is beyond the scope of this study, and deductions of appropriate values from the measurements remain a topic of ongoing re-search (Chylek et al, 2019;Taylor et al, 2020). The HSRL-2 aerosol typing algorithm, based on Sugimoto et al (2006), did not indicate contributions from dust to the extinction of more than 5 %-10% on most flights, so that dust can be discounted as a significant influence on the observed absorption Ångström exponents.…”

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

“…Lower values of ΔrBC/ΔCO during Plume B (2.6 μg·m −3 ·ppm −1 , Figure 2d) than the other plumes are consistent with mixed smoldering‐flaming conditions indicated by its lowest mean MCE value (Figure 2a). Furthermore, Plume B exhibited the highest values of AAE 450/870 (2.4, Figure 2c) among all plumes, indicating that it contained the most BrC (Chylek et al, 2019; Kirchstetter et al, 2004). Plume B also had a higher fraction of OA internally mixed with rBC particles (m coat /m Org = 0.05, Figure 2b).…”

“…Figure S5 shows the Mie scattering model results of the refractive indices of BrC with real part of 1.46 (Chylek et al, 2019; Schmid et al, 2009). Model results indicate k Org at 870 nm is a maximum of 0.010i ± 0.005i for all plumes assuming the upper bound scenario and 0.050i ± 0.005i for Model iii.…”

Unexpected Biomass Burning Aerosol Absorption Enhancement Explained by Black Carbon Mixing State

“…Negligible enhancements ( E abs ~ 1) have been reported in BBA originating from forest fires in North America, even when substantial coatings were present (Cappa et al, 2012; Healy et al, 2015), whereas E abs values up to 1.6 have been observed in the Amazonia region (Lack et al, 2012; Liu et al, 2017). Several hypotheses have been put forward to explain this variability: (1) A restructuring of rBC‐containing particles from fractal morphologies towards a compact core could counteract the lensing effect (Kahnert & Kanngießer, 2020; Shingler et al, 2016); (2) rBC‐containing particles are not entirely coated but instead exhibit an aggregate structure with non‐rBC material (Liu & Mishchenko, 2007), thus leading to overall little change in absorption by rBC‐containing particles when internally mixed (Wu et al, 2018); (3) BBA absorption enhancement is partly attributed to the presence of light‐absorbing organic aerosols termed brown carbon (BrC) (Brown et al, 2018; Chylek et al, 2019); (4) accounting for mixing‐state diversity within the rBC‐containing particle population (i.e., the distribution of coating material across the ensemble of rBC‐containing particles), as has been done by Fierce et al (2016) and Liu et al (2017), is necessary to accurately quantify the average absorption. In part because of the paucity of comprehensive ambient measurements of rBC mixing state in BBA plumes, the mechanisms responsible for the large variation of E abs remain elusive.…”