Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model
By B. Scarnato, S. Vahidinia, D. Richard, Thomas W. Kirchstetter. 2012
174 citations recorded by the source corpus.
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.
Subfields: Atmospheric Science, Global and Planetary Change
View Paper: https://www.semanticscholar.org/paper/0a54e7e1128ae9430e5d718bcf2746237656df4d
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Citations in the corpus
- Variation of the radiative properties during black carbon aging: theoretical and experimental intercomparison (2015, methodology): “…atmosphere, which transforms BC from an external to internal mixing state (Schwarz et al., 2008; China et al., 2013) and increases its hygroscopicity (Zhang et al., 2008; Popovicheva et al., 2011) and light absorption (Jacobson, 2001; Shiraiwa et al., 2010; Qiu et al., 2012; Scarnato et al., 2013).”
- Aerosol Mixing State: Measurements, Modeling, and Impacts (2019, methodology): “Generally, particles contain different components and are not spherical, and the arrangement of the components is anisotropic (Adachi et al., 2010; Scarnato et al., 2013), so the selection of metrics varies between different studies.”
- Morphology and mixing state of aged soot particles at a remote marine free troposphere site: Implications for optical properties (2015, methodology): “Soot compaction changes the scattering and absorption cross sections depending on the refractive index, the monomer diameter, and the structural details [Liu et al., 2008; Scarnato et al., 2013].”
- Review of recent literature on the light absorption properties of black carbon: Refractive index, mass absorption cross section, and absorption function (2020, methodology): “The causes for the significantly higher MAC values predicted by Scarnato et al. (2013) remain unclear and deserves further investigations.”