The absorption Ångstrom exponent of black carbon with brown coatings: effects of aerosol microphysics and parameterization

Abstract. The aerosol absorption Ångstrom exponent (AAE) is a crucial optical parameter for apportionment and characterization. Due to considerable inconsistences associated with observations, numerical research is a powerful means to give a better understanding of the AAE of aged black carbon (BC) aerosols. Numerical studies of the AAE of polydisperse BC aggregates with brown coatings using the exact multiple-sphere T-matrix method (MSTM) are performed. The objective of the study is to thoroughly assess the AAE of coated BC influenced by their observation-based detailed microphysics and then provide a new AAE parameterization for application. At odds with our expectations, more large-sized BC particles coated by thin brown carbon can have an AAE smaller than 1.0, indicating that BC aerosols internally mixing with brown carbon can even show lower AAE than pure BC particles. The AAE of BC with brown coatings is highly sensitive to the absorbing volume fraction of the coating, coated volume fraction of BC, shell ∕ core ratio, and particle size distribution with a wide variation, whereas the impacts of BC geometry and BC position within the coating are negligible. The AAE of BC with brown coatings can be larger than 3.0 if there are plenty of small-sized coated BC particles, heavy coating, or a large amount of brown carbon. However, the AAE of BC with non-absorbing coating appears to be weakly sensitive to particle microphysics with values around 1.0 (i.e., 0.7–1.4), suggesting the substantial role of the absorbing volume fraction of the coating in AAE determination. With more realistic BC geometries, our study also indicates that the occurrence of brown carbon may not be confidently determined unless AAE > 1.4. The currently popular core–shell Mie model reasonably approximates the AAE of fully coated BC by brown carbon, whereas it underestimates the AAE of partially coated or externally attached BC and underestimates more for a lower coated volume fraction of BC. In addition, we present a parameterization of the AAE of coated BC with a size distribution on the basis of numerical results, which can act as a guide for the AAE response to the absorbing volume fraction of the coating, coated volume fraction of BC, and shell ∕ core ratio. The proposed parameterization of coated BC AAE generates a decent prediction for moderate BC microphysics, whereas caution should be taken in applying it for extreme cases, such as externally attached coated BC morphology. Our findings could improve the understanding and application of the AAE of BC with brown coatings.

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The absorption Angstrom exponent of black carbon with brown coatings: effects of aerosol microphysics and parameterization

10.5194/acp-2020-224 ◽

2020 ◽

Author(s):

Xiaolin Zhang ◽

Mao Mao ◽

Shihao Tang

Keyword(s):

Size Distribution ◽

Volume Fraction ◽

Optical Parameter ◽

Brown Carbon ◽

Angstrom Exponent ◽

Aerosol Absorption ◽

Highly Sensitive ◽

Ångström Exponent ◽

Bc Aerosols ◽

Ångstrom Exponent

Abstract. Aerosol absorption Angstrom exponent (AAE) is a crucial optical parameter for their apportionment and characterization. Due to considerable inconsistences associated with observations, a numerical research is a powerful means to give better understanding of the AAE of aged BC aerosols. Numerical studies of the AAE of polydisperse BC aggregates with brown coatings using the exact multiple-sphere T-matrix method (MSTM) are performed. The objective of the study is to thoroughly assess the AAE of coated BC influenced by their observation-based detailed microphysics and then provide a new AAE parameterization for application. At odds with our expectations, BC coated by thin brown carbon with more large particles can have an AAE smaller than 1.0, indicating that BC aerosols internally mixing with brown carbon can even show lower AAE than pure BC particles. The AAE of BC with brown coatings is highly sensitive to absorbing volume fraction of coating, coated volume fraction of BC, shell / core ratio, and particle size distribution with a wide variation, whereas the impacts of BC geometry and BC position within coating are trivial. The AAE of BC with brown coatings can be larger than 3.0, if there are more small coated BC particles, heavy coating, or more brown carbon. However, the AAE of BC with non-absorbing coating shows weakly sensitive to particle microphysics with values around 1.0 (i.e., 0.7–1.4), suggesting the substantial role of absorbing volume fraction of coating in the AAE determination. With more realistic BC geometries, our study also indicates that occurrence of brown carbon may be made confidently unless AAE > 1.4. In addition, we present a parameterization of the AAE of coated BC with a size distribution on the basis of numerical results, which can act as a guide for the AAE response to absorbing volume fraction of coating, coated volume fraction of BC, and shell / core ratio. Our findings can improve the understanding and application of the AAE of BC with brown coatings.

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On the attribution of black and brown carbon light absorption using the Ångström exponent

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-15493-2013 ◽

2013 ◽

Vol 13 (6) ◽

pp. 15493-15515 ◽

Cited By ~ 4

Author(s):

D. A. Lack ◽

J. M. Langridge

Keyword(s):

Black Carbon ◽

Light Absorption ◽

Absorption Efficiency ◽

Potential Range ◽

Brown Carbon ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Absorbing Material ◽

Visible Wavelengths ◽

Ångstrom Exponent

Abstract. The absorption Ångström exponent (åAbs) of black carbon (BC), or BC internally mixed with non-absorbing material (BCInt), is often used to differentiate the contribution of black carbon, dust and brown carbon to light absorption at low-visible wavelengths. This attribution method contains assumptions with uncertainties that have not been formally assessed. We show that the potential range of åAbs for BC (or BCInt) in the atmosphere can reasonably lead to +7% to −22% uncertainty in BC (or BCInt) absorption at 404nm derived from measurements made at 658 nm. These uncertainties propagate to errors in the attributed absorption and mass absorption efficiency (MAE) of brown carbon (BrC). For data collected during a biomass-burning event, the mean uncertainty in MAE at 404 nm attributed to BrC using the åAbs method was found to be 34%. In order to yield attributed BrC absorption uncertainties of ±33%, 23% to 41% of total absorption must be sourced from BrC. In light of the potential for introducing significant and poorly constrained errors, we caution against the universal application of the åAbs attribution method.

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The absorption Ångström exponent of black carbon: from numerical aspects

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-6259-2018 ◽

2018 ◽

Vol 18 (9) ◽

pp. 6259-6273 ◽

Cited By ~ 65

Author(s):

Chao Liu ◽

Chul Eddy Chung ◽

Yan Yin ◽

Martin Schnaiter

Keyword(s):

Particle Size ◽

Refractive Index ◽

Black Carbon ◽

Numerical Study ◽

Geometric Mean ◽

Optical Parameter ◽

Angstrom Exponent ◽

Wide Range ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. The absorption Ångström 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 µm, 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.

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The Absorption Ångström Exponent of black carbon: from numerical aspects

10.5194/acp-2017-836 ◽

2017 ◽

Cited By ~ 3

Author(s):

Chao Liu ◽

Chul Eddy Chul ◽

Yan Yin

Keyword(s):

Particle Size ◽

Refractive Index ◽

Particle Size Distribution ◽

Size Distribution ◽

Black Carbon ◽

Numerical Study ◽

Angstrom Exponent ◽

Wide Range ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. The Absorption Ångström Exponent (AAE) is an important aerosol optical parameter used for aerosol characterization and apportionment studies. The AAE of black carbon (BC) is widely accepted to be 1.0, although observational estimates give a quite wide range of 0.6~1.1. 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). 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 is quite insensitive to particle size distribution. BC AAE goes lower when BC particles are aged (compact structures or coated by other scattering materials). For coated BC, we prescribed the coating thickness distribution based on a published experimental study, where smaller BC cores were shown to develop thicker coating than bigger BC cores. Both Compact and Coated BC the AAE ranges, at realistic particle sizes. For both Compact and Coated BC, the AAE is highly sensitive to particle size distribution, ranging from approximately 0.8 to 1.0 for relatively large BC with wavelength-independent refractive index. When the refractive index is allowed to vary with wavelength, a feature with observational backing, the BC AAE shows a much wider range. We propose that the presented results herein serve as a comprehensive guide for the response of BC AAE to BC size, refractive index, and geometry.

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Measurements of optical properties of atmospheric aerosols in Northern Finland

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-11703-2005 ◽

2005 ◽

Vol 5 (6) ◽

pp. 11703-11728 ◽

Cited By ~ 1

Author(s):

V. Aaltonen ◽

H. Lihavainen ◽

V.-M. Kerminen ◽

M. Komppula ◽

J. Hatakka ◽

...

Keyword(s):

Optical Properties ◽

Size Distribution ◽

Seasonal Pattern ◽

Late Summer ◽

Aerosol Optical Properties ◽

Air Masses ◽

Angstrom Exponent ◽

Northern Border ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. Three years of continuous measurements of aerosol optical properties and simultaneous aerosol number size distribution measurements at Pallas GAW station, a remote subarctic site in the northern border of the boreal forest zone, have been analysed. The scattering coefficient at 550 nm varied from 0.2 to 94.4 Mm−1 with an average of 7.1±8.6 Mm−1. Both the scattering and backscattering coefficients had a clear seasonal cycle with an autumn minimum and a 4–5 times higher summer maximum. The scattering was dominated by submicron aerosols and especially so during late summer and autumn. The Ångström exponent had a clear seasonal pattern with maximum values in late summer and minimum values during wintertime. The highest hemispheric backscattering fraction values were observed in autumn, indicating clean air with few scattering particles and a particle size distribution strongly dominated by ultrafine particles. To analyse the influence of air mass origin on the aerosol optical properties a trajectory climatology was applied to the Pallas aerosol data. The most polluted trajectory patterns represented air masses from the Kola Peninsula, Scandinavia and Russia as well as long-range transport from Britain and Eastern Europe. These air masses had the largest average scattering and backscattering coefficients for all seasons. Higher than average values of the Ångström exponent were also observed in connection with transport from these areas.

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An Improved Absorption Ångström Exponent (AAE)-Based Method for Evaluating the Contribution of Light Absorption from Brown Carbon with a High-Time Resolution

Aerosol and Air Quality Research ◽

10.4209/aaqr.2017.12.0566 ◽

2019 ◽

Vol 19 (1) ◽

pp. 15-24 ◽

Cited By ~ 3

Author(s):

Guohua Zhang ◽

Long Peng ◽

Xiufeng Lian ◽

Qinhao Lin ◽

Xinhui Bi ◽

...

Keyword(s):

Time Resolution ◽

Light Absorption ◽

High Time ◽

High Time Resolution ◽

Brown Carbon ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Ångstrom Exponent

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The Ångström Exponent and Single-Scattering Albedo of Black Carbon: Effects of Different Coating Materials

Atmosphere ◽

10.3390/atmos11101103 ◽

2020 ◽

Vol 11 (10) ◽

pp. 1103

Author(s):

Jie Luo ◽

Yongming Zhang ◽

Qixing Zhang

Keyword(s):

Black Carbon ◽

Single Scattering ◽

Single Scattering Albedo ◽

Optical Measurements ◽

Coating Materials ◽

Total Size ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Mixing States ◽

Ångstrom Exponent

In this work, the absorption Ångström exponent (AAE), extinction Ångström exponent (EAE), and single-scattering albedo (SSA) of black carbon (BC) with different coating materials are numerically investigated. BC with different coating materials can provide explanations for the small AAE, small EAE, and large AAE observed in the atmosphere, which is difficult to be explained by bare BC aggregate models. The addition of organic carbon (OC) does not necessarily increase AAE due to the transformation of BC morphologies and the existence of non-absorbing OC. The addition of coating materials does also not necessarily decrease EAE. While the addition of coating materials can increase the total size of BC-containing particles, the effective refractive index can be modified by introducing the coating materials, so increases the EAE. We found that it is not possible to differentiate between thinly- and heavily-coated BC based on EAE or AAE alone. On the other hand, SSA is much less sensitive to the size and can provide much more information for distinguishing heavily-coated BC from thinly-coated BC. For BC with different coating materials and mixing states, AAE, EAE, and SSA show rather different sensitivities to particle size and composition ratios, and their spectral-dependences also exhibit distinct differences. Different AAE and EAE trends with BC/OC ratio were also found for BC with different coating materials and mixing states. Furthermore, we also found empirical fittings for AAE, EAE, SSA, and optical cross-sections, which may be useful for retrieving the size information based on the optical measurements.

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Estimating cloud condensation nuclei number concentrations using aerosol optical properties: role of particle number size distribution and parameterization

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-15483-2019 ◽

2019 ◽

Vol 19 (24) ◽

pp. 15483-15502 ◽

Cited By ~ 1

Author(s):

Yicheng Shen ◽

Aki Virkkula ◽

Aijun Ding ◽

Krista Luoma ◽

Helmi Keskinen ◽

...

Keyword(s):

Optical Properties ◽

Size Distribution ◽

Cloud Condensation Nuclei ◽

Size Distributions ◽

Condensation Nuclei ◽

Aerosol Optical Properties ◽

Average Bias ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. The concentration of cloud condensation nuclei (CCN) is an essential parameter affecting aerosol–cloud interactions within warm clouds. Long-term CCN number concentration (NCCN) data are scarce; there are a lot more data on aerosol optical properties (AOPs). It is therefore valuable to derive parameterizations for estimating NCCN from AOP measurements. Such parameterizations have already been made, and in the present work a new parameterization is presented. The relationships between NCCN, AOPs, and size distributions were investigated based on in situ measurement data from six stations in very different environments around the world. The relationships were used for deriving a parameterization that depends on the scattering Ångström exponent (SAE), backscatter fraction (BSF), and total scattering coefficient (σsp) of PM10 particles. The analysis first showed that the dependence of NCCN on supersaturation (SS) can be described by a logarithmic fit in the range SS <1.1 %, without any theoretical reasoning. The relationship between NCCN and AOPs was parameterized as NCCN≈((286±46)SAE ln(SS/(0.093±0.006))(BSF − BSFmin) + (5.2±3.3))σsp, where BSFmin is the minimum BSF, in practice the 1st percentile of BSF data at a site to be analyzed. At the lowest supersaturations of each site (SS ≈0.1 %), the average bias, defined as the ratio of the AOP-derived and measured NCCN, varied from ∼0.7 to ∼1.9 at most sites except at a Himalayan site where the bias was >4. At SS >0.4 % the average bias ranged from ∼0.7 to ∼1.3 at most sites. For the marine-aerosol-dominated site Ascension Island the bias was higher, ∼1.4–1.9. In other words, at SS >0.4 % NCCN was estimated with an average uncertainty of approximately 30 % by using nephelometer data. The biases were mainly due to the biases in the parameterization related to the scattering Ångström exponent (SAE). The squared correlation coefficients between the AOP-derived and measured NCCN varied from ∼0.5 to ∼0.8. To study the physical explanation of the relationships between NCCN and AOPs, lognormal unimodal particle size distributions were generated and NCCN and AOPs were calculated. The simulation showed that the relationships of NCCN and AOPs are affected by the geometric mean diameter and width of the size distribution and the activation diameter. The relationships of NCCN and AOPs were similar to those of the observed ones.

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