Performance of the discrete dipole approximation for optical properties of black carbon aggregates

2018 ◽  
Vol 221 ◽  
pp. 98-109 ◽  
Author(s):  
Chao Liu ◽  
Shiwen Teng ◽  
Yingying Zhu ◽  
Maxim A. Yurkin ◽  
Yuk L. Yung
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Dipole Approximation ◽  
Discrete Dipole Approximation

scholarly journals Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model

2013 ◽  
Vol 13 (10) ◽  
pp. 5089-5101 ◽  
Author(s):  
B. V. Scarnato ◽  
S. Vahidinia ◽  
D. T. Richard ◽  
T. W. Kirchstetter
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Radiative Forcing ◽  
Dipole Approximation ◽  
Discrete Dipole Approximation ◽  
Approximation Model ◽  
Mixing State ◽  
Internal Mixing ◽  
Aggregate Morphology ◽  
Nm Range

Abstract. According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT is flexible in simulating the geometry and refractive index of particle aggregates. DDSCAT predicts a higher mass absorption coefficient (MAC), lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.16 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. The MAC of BC (averaged over the 200–1000 nm range) is amplified when internally mixed with NaCl (100–300 nm in radius) by factors ranging from 1.0 for lacy BC aggregates partially immersed in NaCl to 2.2 for compact BC aggregates fully immersed in NaCl. The SSA of BC internally mixed with NaCl is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle size and morphology. This study shows that DDSCAT predicts complex morphology and mixing state dependent aerosol optical properties that have been reported previously and are relevant to radiative transfer, climate modeling, and interpretation of remote sensing measurements.

scholarly journals Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model

2012 ◽  
Vol 12 (10) ◽  
pp. 26401-26434 ◽  
Author(s):  
B. Scarnato ◽  
S. Vahidinia ◽  
D. T. Richard ◽  
T. W. Kirchstetter
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Linear Polarization ◽  
Incident Radiation ◽  
Dipole Approximation ◽  
Approximation Model ◽  
Mixing State ◽  
Internal Mixing ◽  
Aggregate Morphology ◽  
Nm Range

Abstract. According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT predicts a higher mass absorption coefficient, lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.18 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. In the 300 to 550 nm range, AAE values ranged in this study from 0.70 for compact to 0.95 for lacy aggregates. The SSA of BC internally mixed with NaCl (100–300 nm in radius) is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle morphology. The bare BC (with a radius of 80 nm) presents in the linear polarization a bell shape feature, which is a characteristic of the Rayleigh regime (for particles smaller than the wavelength of incident radiation). When BC is internally mixed with NaCl (100–300 nm in radius), strong depolarization features for near-VIS incident radiation are evident, such as a decrease in the intensity and multiple modes at different angles corresponding to different mixing states. DDSCAT, being flexible on the geometry and refractive index of the particle, can be used to study the effect of mixing state and complex morphology on optical properties of realistic BC aggregates. This study shows that DDSCAT predicts morphology and mixing state dependent optical properties that have been reported previously and are relevant to radiative transfer and climate modeling and interpretation of remote sensing measurements.

scholarly journals Using the DDA (Discrete Dipole Approximation) Method in Determining the Extinction Cross Section of Black Carbon

2015 ◽  
Vol 22 (1) ◽  
pp. 153-164 ◽  
Author(s):  
Krzysztof Skorupski
Keyword(s):  
Refractive Index ◽  
Black Carbon ◽  
Cross Section ◽  
Main Parameter ◽  
Complex Refractive Index ◽  
Dipole Approximation ◽  
Discrete Dipole Approximation ◽  
Extinction Cross Section ◽  
Quality Of Results

AbstractBC (Black Carbon), which can be found in the atmosphere, is characterized by a large value of the imaginary part of the complex refractive index and, therefore, might have an impact on the global warming effect. To study the interaction of BC with light often computer simulations are used. One of the methods, which are capable of performing light scattering simulations by any shape, is DDA (Discrete Dipole Approximation). In this work its accuracy was estimated in respect to BC structures using the latest stable version of the ADDA (vr. 1.2) algorithm. As the reference algorithm the GMM (Generalized Multiparticle Mie-Solution) code was used. The study shows that the number of volume elements (dipoles) is the main parameter that defines the quality of results. However, they can be improved by a proper polarizability expression. The most accurate, and least time consuming, simulations were observed for IGT_SO. When an aggregate consists of particles composed of ca. 750 volume elements (dipoles), the averaged relative extinction error should not exceed ca. 4.5%.

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