https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018RG000615

Aerosol Mixing State: Measurements, Modeling, and Impacts

. The absorption Angstrom exponent (AAE) is an important aerosol
optical parameter used for aerosol characterization and apportionment
studies. The AAE of black carbon (BC) particles is widely accepted to be 1.0,
although observational estimates give quite a wide range of
0.6–1.3. With considerable uncertainties
related to observations, a numerical study is a powerful method, if not the
only one, to provide a better and more accurate understanding on BC AAE. This
study calculates BC AAE using realistic particle geometries based on fractal
aggregate and an accurate numerical optical model (namely the multiple-sphere
T-matrix method), and considers
bulk properties of an ensemble of BC particles following lognormal size
distributions. At odds with the expectations, BC AAE is not 1.0, even when BC
is assumed to have small sizes and a wavelength-independent refractive index.
With a wavelength-independent refractive index, the AAE of fresh BC is
approximately 1.05 and relatively insensitive to particle size. For BC with
geometric mean diameters larger than 0.12  µ m, BC AAE becomes
smaller when BC particles are aged (compact structures or coated by other
non-absorptive materials). For coated BC, we prescribe the coating fraction
variation based on a laboratory study, where smaller BC cores are shown to
develop larger coating fractions than those of bigger BC cores. For both compact and coated BC, the AAE is highly
sensitive to particle size distribution, ranging from approximately 0.8 to
even over 1.4 with wavelength-independent refractive index. When the
refractive index is allowed to vary with wavelength, a feature with
observational backing, the BC AAE may show an even wider range. For different
BC morphologies, we derive simple empirical equations on BC AAE based on our
numerical results, which can serve as a guide for the response of BC AAE to
BC size and refractive index. Due to its complex influences, the effects of
BC geometry is better to be discussed at certain BC properties, i.e., known
size and refractive index.

/pdf/the-absorption-angstrom-exponent-of-black-carbon-from-2rtlnu1akj.pdf

The absorption Ångström exponent of black carbon: from numerical aspects

The radiative properties of soot particles depend on their morphology and mixing state, but their evolution during transport is still elusive. Here we report observations from an electron microscopy analysis of individual particles transported in the free troposphere over long distances to the remote Pico Mountain Observatory in the Azores in the North Atlantic. Approximately 70% of the soot particles were highly compact and of those 26% were thinly coated. Discrete dipole approximation simulations indicate that this compaction results in an increase in soot single scattering albedo by a factor of <= 2.17. The top of the atmosphere direct radiative forcing is typically smaller for highly compact than mass-equivalent lacy soot. The forcing estimated using Mie theory is within 12% of the forcing estimated using the discrete dipole approximation for a high surface albedo, implying that Mie calculations may provide a reasonable approximation for compact soot above remote marine clouds.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014GL062404

Morphology and mixing state of aged soot particles at a remote marine free troposphere site: Implications for optical properties

Soot particles strongly absorb sunlight and hence act as a short-lived warming agent. Atmospheric aging of soot particles changes their morphology and mixing state and consequently alters their optical properties. Here we collected soot particles at tunnel, urban, mountaintop, and background sites in the North China Plain and analyzed their mixing structures and morphology using transmission electron microscopy. Soot particles were further classified into three types: bare-like, partly coated, and embedded. Bare-like soot particles were dominant at the tunnel site, while most soot particles were of the partly coated or embedded type at other sites. Fractal dimensions (Df) of different types of soot particles ranged from 1.80 to 2.16 and increased in the following order: bare-like < partly coated < embedded. Moreover, their average Df changed from 1.8 to 2.0 from the tunnel to the background site. We conclude that the Df can characterize the shape of soot aggregates reasonably well, and its variation refle...

/pdf/fractal-dimensions-and-mixing-structures-of-soot-particles-3znpqav2id.pdf

Fractal Dimensions and Mixing Structures of Soot Particles during Atmospheric Processing

. A theoretical black carbon (BC) aging model is developed to account for three typical evolution stages, namely, freshly emitted aggregates, BC coated by soluble material, and BC particles undergoing further hygroscopic growth. The geometric-optics surface-wave (GOS) approach is employed to compute the BC single-scattering properties at each aging stage, which are subsequently compared with laboratory measurements. Theoretical calculations are consistent with measurements in extinction and absorption cross sections for fresh BC aggregates with different BC sizes (i.e., mobility diameters of 155, 245, and 320 nm), with differences of ≤ 25 %. The measured optical cross sections for BC coated by sulfuric acid and for that undergoing further hygroscopic growth are generally captured (differences

/pdf/variation-of-the-radiative-properties-during-black-carbon-2wo3qy4dv2.pdf

Variation of the radiative properties during black carbon aging: theoretical and experimental intercomparison

. 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.

/pdf/effects-of-internal-mixing-and-aggregate-morphology-on-5dzronk3jv.pdf

S. vahidinia

Papers

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