scholarly journals The absorption Ångström exponent of black carbon: from numerical aspects

2018 ◽  
Vol 18 (9) ◽  
pp. 6259-6273 ◽  
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.

scholarly journals The Absorption Ångström Exponent of black carbon: from numerical aspects

2017 ◽  
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.

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

2020 ◽  
Vol 20 (16) ◽  
pp. 9701-9711 ◽  
Author(s):  
Xiaolin Zhang ◽  
Mao Mao ◽  
Yan Yin ◽  
Shihao Tang
Keyword(s):  
Size Distribution ◽  
Black Carbon ◽  
Volume Fraction ◽  
Optical Parameter ◽  
Brown Carbon ◽  
Angstrom Exponent ◽  
Aerosol Absorption ◽  
Ångström Exponent ◽  
Bc Aerosols ◽  
Ångstrom Exponent

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.

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

Atmosphere ◽  
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.

scholarly journals Determination of the refractive index of insoluble organic extracts from atmospheric aerosol over the visible wavelength range using optical tweezers

2018 ◽  
Vol 18 (8) ◽  
pp. 5235-5252 ◽  
Author(s):  
Rosalie H. Shepherd ◽  
Martin D. King ◽  
Amelia A. Marks ◽  
Neil Brough ◽  
Andrew D. Ward
Keyword(s):  
Refractive Index ◽  
Wavelength Range ◽  
Atmospheric Aerosol ◽  
Organic Aerosol ◽  
Angstrom Exponent ◽  
Organic Extracts ◽  
Ångström Exponent ◽  
Aqueous Core ◽  
Ångstrom Exponent ◽  
Real Refractive Index

Abstract. Optical trapping combined with Mie spectroscopy is a new technique used to record the refractive index of insoluble organic material extracted from atmospheric aerosol samples over a wide wavelength range. The refractive index of the insoluble organic extracts was shown to follow a Cauchy equation between 460 and 700 nm for organic aerosol extracts collected from urban (London) and remote (Antarctica) locations. Cauchy coefficients for the remote sample were for the Austral summer and gave the Cauchy coefficients of A = 1.467 and B = 1000 nm2 with a real refractive index of 1.489 at a wavelength of 589 nm. Cauchy coefficients for the urban samples varied with season, with extracts collected during summer having Cauchy coefficients of A=1.465±0.005 and B=4625±1200 nm2 with a representative real refractive index of 1.478 at a wavelength of 589 nm, whilst samples extracted during autumn had larger Cauchy coefficients of A = 1.505 and B = 600 nm2 with a representative real refractive index of 1.522 at a wavelength of 589 nm. The refractive index of absorbing aerosol was also recorded. The absorption Ångström exponent was determined for woodsmoke and humic acid aerosol extract. Typical values of the Cauchy coefficient for the woodsmoke aerosol extract were A=1.541±0.03 and B=14800±2900 nm2, resulting in a real refractive index of 1.584 ± 0.007 at a wavelength of 589 nm and an absorption Ångström exponent of 8.0. The measured values of refractive index compare well with previous monochromatic or very small wavelength range measurements of refractive index. In general, the real component of the refractive index increases from remote to urban to woodsmoke. A one-dimensional radiative-transfer calculation of the top-of-the-atmosphere albedo was applied to model an atmosphere containing a 3 km thick layer of aerosol comprising pure water, pure insoluble organic aerosol, or an aerosol consisting of an aqueous core with an insoluble organic shell. The calculation demonstrated that the top-of-the-atmosphere albedo increases by 0.01 to 0.04 for pure organic particles relative to water particles of the same size and that the top-of-the-atmosphere albedo increases by 0.03 for aqueous core-shell particles as volume fraction of the shell material increases to 25 %.

scholarly journals Absorption Angstrom Exponent in AERONET and related data as an indicator of aerosol composition

2009 ◽  
Vol 9 (5) ◽  
pp. 21785-21817 ◽  
Author(s):  
P. B. Russell ◽  
R. W. Bergstrom ◽  
Y. Shinozuka ◽  
A. D. Clarke ◽  
P. F. DeCarlo ◽  
...  
Keyword(s):  
Black Carbon ◽  
Optical Depth ◽  
Aerosol Layer ◽  
Ozone Monitoring Instrument ◽  
Aerosol Composition ◽  
Cluster Analyses ◽  
Angstrom Exponent ◽  
Near Ultraviolet ◽  
Ångström Exponent ◽  
Ångstrom Exponent

Abstract. Recent results from diverse air, ground, and laboratory studies using both radiometric and in situ techniques show that the fractions of black carbon, organic matter, and mineral dust in atmospheric aerosols determine the wavelength dependence of absorption (expressed as Absorption Angstrom Exponent, or AAE). Taken together, these results hold promise of improving information on aerosol composition from remote measurements. The purpose of this paper is to show that AAE values for Aerosol Robotic Network (AERONET) retrievals from Sun-sky measurements describing the full aerosol vertical column are also strongly correlated with aerosol composition or type. In particular, we find AAE values near 1 (the theoretical value for black carbon) for AERONET-measured aerosol columns dominated by urban-industrial aerosol, larger AAE values for biomass burning aerosols, and the largest AAE values for Sahara dust aerosols. Ambiguities in aerosol composition or mixtures thereof, resulting from intermediate AAE values, can be reduced via cluster analyses that supplement AAE with other variables, for example Extinction Angstrom Exponent (EAE), which is an indicator of particle size. Together with previous results, these results strengthen prospects for determining aerosol composition from space, for example using the Glory Aerosol Polarimetry Sensor (APS), which promises retrievals of multiwavelength single-scattering albedo (SSA) and aerosol optical depth (and therefore aerosol absorption optical depth (AAOD) and AAE), as well as shape and other aerosol properties. Cluster analyses promise additional information content, for example by using the Ozone Monitoring Instrument (OMI) to add AAOD in the near ultraviolet and CALIPSO aerosol layer heights to reduce height-absorption ambiguity.

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

2013 ◽  
Vol 13 (6) ◽  
pp. 15493-15515 ◽  
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.

scholarly journals Can the Aerosol Absorption Ångström Exponent Represent Aerosol Color in the Atmosphere: A Numerical Study

Atmosphere ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 187
Author(s):  
Dapeng Zhao ◽  
Yan Yin ◽  
Chao Liu ◽  
Chunsong Lu ◽  
Xiaofeng Xu
Keyword(s):  
Numerical Study ◽  
Solar Spectrum ◽  
Transfer Model ◽  
Blue Color ◽  
Angstrom Exponent ◽  
Aerosol Absorption ◽  
Model Aerosol ◽  
Ångström Exponent ◽  
Human Eyes ◽  
Ångstrom Exponent

The aerosol absorption Ångström exponent (AAE) is widely used to indicate aerosol absorption spectrum variations and is an important parameter for characterizing aerosol optical absorption properties. This study discusses the relationship between aerosol AAEs and their colors numerically. By combining light scattering simulations, a two-stream radiative transfer model, and an RGB (Red, Green, and Blue) color model, aerosol colors that can be sensed by human eyes are numerically generated with both the solar spectrum and human eye response taken into account. Our results indicate that the responses of human eyes to visible light might be more significant than the incident spectrum in the simulation of aerosol color in the atmosphere. Using the improved numerical simulation algorithm, we obtain the color change of absorption aerosols with different AAEs. When the AAE value is small, the color of the aerosol is generally black and gray. When the AAE value increases to approximately 2 and the difference between the light transmittances at wavelengths of 400 nm and 730 nm is greater than 0.2, the aerosol will appear brown or yellow.

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