Calculation of optical properties of light-absorbing carbon with weakly absorbing coating: A model with tunable transition from film-coating to spherical-shell coating

Kanngießer, Franz; Kahnert, Michael

doi:10.1016/j.jqsrt.2018.05.014

Cited by 23 publications

(21 citation statements)

References 62 publications

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“…A similar effect was previously reported (Kahnert, 2017). According to Kanngießer and Kahnert (2018), the speed of transition from film coating to spherical growth (Pei et al, 2018) is a morphological parameter that strongly affects the depolarization ratio. A rapid transition to spherical growth may cause a smaller depolarization ratio of internally mixed soot particles.…”

Section: Single-scattering Propertiessupporting

confidence: 84%

“…Surface tension is the result of intermolecular forces at the microscopic scale, described as the Lennard-Jones potential (Becker et al, 2014). Although some approaches to simulate surface tension in a discrete particle system, such as smoothed particle hydrodynamics, have been proposed (Tartakovsky and Meakin, 2005;Leinonen and von Lerber, 2018), a general approach to reproduce the variety of fluid effects has not yet been developed (Akinci et al, 2013). In this study, we examined a virtual and simple potential field on the surface of the modeled particle to simulate the morphological effects of surface tension.…”

Section: Artificial Surface Tension Of Mixed Soot and Water-soluble Cmentioning

confidence: 99%

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A shape model of internally mixed soot particles derived from artificial surface tension

2019

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Abstract. To retrieve the physical properties of aerosols from multi-channel ground-based and satellite measurements, we developed a shape model of coated soot particles and created a dataset of their optical properties. Bare soot particles were assumed to have an aggregate shape, and two types of aggregates with different size–shape dependences were modeled using a polyhedral Voronoi structure. To simulate the detailed shape properties of mixtures of soot aggregates and adhered water-soluble substances, we propose a simple model of surface tension derived from the artificial surface potential. The light-scattering properties of the modeled particles with different volume fractions of water-soluble material were calculated using the finite-difference time-domain method and discrete-dipole approximation. The results of the single-scattering albedo and asymmetry factors were compared to those of conventional internally mixed spheres (i.e., effective medium spheres based on the Maxwell-Garnett approximation and simple core-shell spheres). In addition, the lidar backscattering properties (i.e., lidar ratios and linear depolarization ratios) of the modeled soot particles were investigated. For internally mixed soot particles, the lidar backscattering properties were sensitive to the shape of the soot particles and the volume mixing ratio of the assumed water-soluble components. However, the average optical properties of biomass smoke, which have been reported from in situ field and laboratory measurements, were difficult to explain based on the individually modeled particle. Nonetheless, our shape model and its calculated optical properties are expected to be useful as an alternative model for biomass smoke particles in advanced remote sensing via multi-channel radiometer and lidar measurements.

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Section: Single-scattering Propertiessupporting

confidence: 84%

Section: Artificial Surface Tension Of Mixed Soot and Water-soluble Cmentioning

confidence: 99%

A shape model of internally mixed soot particles derived from artificial surface tension

2019

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“…Aged soot has often been found to be internally mixed with sulfate aerosols [93,94]. Sulfates represent the host or coating material most commonly used in modeling studies [19,44,45,47,78,84,86,95]. Soot can also be mixed with or embedded in weakly absorbing carbonaceous materials derived from combustion of biomass and biofuels and termed brown carbon, or BrC [69].…”

Section: Model Aerosol-particle Morphologiesmentioning

confidence: 99%

Scattering and Radiative Properties of Morphologically Complex Carbonaceous Aerosols: A Systematic Modeling Study

Liu

Mishchenko

2018

Remote Sensing

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This paper provides a thorough modeling-based overview of the scattering and radiative properties of a wide variety of morphologically complex carbonaceous aerosols. Using the numerically-exact superposition T-matrix method, we examine the absorption enhancement, absorption Ångström exponent (AAE), backscattering linear depolarization ratio (LDR), and scattering matrix elements of black-carbon aerosols with 11 different model morphologies ranging from bare soot to completely embedded soot-sulfate and soot-brown carbon mixtures. Our size-averaged results show that fluffy soot particles absorb more light than compact bare-soot clusters. For the same amount of absorbing material, the absorption cross section of internally mixed soot can be more than twice that of bare soot. Absorption increases as soot accumulates more coating material and can become saturated. The absorption enhancement is affected by particle size, morphology, wavelength, and the amount of coating. We refute the conventional belief that all carbonaceous aerosols have AAEs close to 1.0. Although LDRs caused by bare soot and certain carbonaceous particles are rather weak, LDRs generated by other soot-containing aerosols can reproduce strong depolarization measured by Burton et al. for aged smoke. We demonstrate that multi-wavelength LDR measurements can be used to identify the presence of morphologically complex carbonaceous particles, although additional observations can be needed for full characterization. Our results show that optical constants of the host/coating material can significantly influence the scattering and absorption properties of soot-containing aerosols to the extent of changing the sign of linear polarization. We conclude that for an accurate estimate of black-carbon radiative forcing, one must take into account the complex morphologies of carbonaceous aerosols in remote sensing studies as well as in atmospheric radiation computations.These fresh soot aerosols are mostly hydrophobic and externally mixed with other aerosol constituents, but tend to become semi-externally and internally mixed (according to the mixing classification in Ref.[21]) as they age in the atmosphere. The aging processes include condensation of sulfates, nitrates, organics, and water on soot particles' surfaces; coagulation with preexisting aerosols; and heterogeneous reactions with gaseous oxidants [22][23][24][25]. Meanwhile, the soot aggregate morphology changes and exhibits compaction from fluffy to more densely packed formations [26][27][28].According to Ueda et al. [29,30], carbonaceous aerosols can be classified into seven types based on their dominant morphology: soot aggregates, spherical particles, coccoid particles, dome-like particles, clusters of spherical and coccoid bodies, crystalline coarse bodies, and re-crystallized droplet particles. China et al. [31,32] classify the soot component of carbonaceous aerosols into four major morphological types: embedded, with soot fully engulfed by, e.g., a sulfate host; partly coated, wherein the coating ma...

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“…A heterogeneous internal mixture is usually represented by a core containing BC surrounded by a shell of less absorbing material (e.g., Kanngiesser & Kahnert, ). Although this core‐shell model seems to be in agreement with our mental picture of the aged biomass burning aerosols, its disadvantage is an increasing number of free parameters, and results which do not seem to rank this model much better than a simple homogeneous sphere model (Cappa et al, ; China et al, ; Curci et al, ; Kahnert et al, ; Lesins et al, ; Schuster et al, ).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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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.

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Calculation of optical properties of light-absorbing carbon with weakly absorbing coating: A model with tunable transition from film-coating to spherical-shell coating

Cited by 23 publications

References 62 publications

A shape model of internally mixed soot particles derived from artificial surface tension

A shape model of internally mixed soot particles derived from artificial surface tension

Scattering and Radiative Properties of Morphologically Complex Carbonaceous Aerosols: A Systematic Modeling Study

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

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