The Simulated Source Apportionment of Light Absorbing Aerosols: Effects of Microphysical Properties of Partially‐Coated Black Carbon

Abstract

Biomass burning (BB) fractions estimated with the aethalometer model differ from 0 even for pure aerosols from fossil fuel (FF) combustion. We used black carbon (BC) aerosols partially‐coated with non‐absorbing materials to represent typical FF aerosols, and the BB fractions were determined with the aethalometer model. Thus, the estimated BB fractions are the fractions that FF aerosols are incorrectly attributed to BB aerosols. The BC morphology and mixing state have significant effects on the estimation of BB fractions. For freshly emitted BC, the BB fractions do not deviate significantly from 0, and the BB fraction is generally in the range of −1%–10% based on an FF absorption Ångström exponent (AAE) of 1 and a BB AAE of 2. The BB fraction deviates substantially from 0 when BC becomes compact and is coated. The absolute values of the deviations sometimes can be close to 100% for heavily coated BC. The BB fraction is generally greater than 0 for fluffy BC while compact BC generally exhibits a negative BB fraction. The BB fractions of partially‐coated BC are very sensitive to the size distribution and coating ratio, which is consistent with the results of the core‐shell sphere model. We have also performed a series of studies with different configurations for BC morphologies, coating ratios, and mixing states, which show the variations of the estimated BB fractions with atmospheric aging. We also explain the reasons for the uncertainties in the BB fraction estimates and provide suggestions for using different AAE pairs.

Review of egusphere-2024-1155

“…As the particle ages, BC may collapse to a more compact structure (Zhang et al, 2008;Lack et al, 2014;Peng et al, 2016;Bhandari et al, 2019). A larger D f has often been used to represent BC with more compact structure (Liu and Mishchenko, 2005;Kanngießer and Kahnert, 2018;Cheng et al, 2014;Luo et al, 2019Luo et al, , 2023Luo et al, , 2024. Previous studies have shown that aged BC can sometimes have a D f of about 2.3 -2.6, assuming a fractal structure for aged BC (Adachi et al, 2010(Adachi et al, , 2007Chen et al, 2016).…”

“…We used the tunable algorithm developed by Woźniak (2012), which accurately reproduces the fractal law, to generate the fractal BC aggregates. Then, the coating materials were added similarly to previous studies (Zhang et al, 2018;Luo et al, 2023Luo et al, , 2024. We assumed that the coating structure is spherical.…”

“…Previous studies have shown that SP2 generally detects the scattering signal at scattering angles (Θ) of 45 (Moteki and Kondo, 2008;Wu et al, 2023). We used the following equation to represent the scattering signal returned by the coated BC:…”

“…The single particle soot photometer (SP2), an instrument for measuring the mass of individual BC particles, has recently been widely used to measure mixing states (Schwarz et al, 2006; R. S. Gao and Worsnop, 2007). The SP2 measures scattering of individual particles reflected from a 1064 nm laser, and the mass of the BC core is estimated from the incandescence signal (Moteki and Kondo, 2008;Wu et al, 2023). Based on an assumed BC mass density, we can calculate the mass-equivalent diameter of the BC cores.…”

“…In some studies, fully enveloped models have been used to determine the effects of BC morphology on the determination of mixing states (Wu et al, 2023;Liu et al, 2023). However, BC is often partially-coated, and the absorption of partially-coated BC is more complex than that of fully coated BC.…”