Numerical simulations of optical properties of nonspherical dust

aerosols using the T-matrix method

Numerical simulations of optical properties of nonspherical dust

aerosols using the T-matrix method

Hyung-Jin Choihyung-jin.choi@eas.gatech.eduSchool of Earth and Atmospheric SciencesGeorgia Institute of Technology

November 14, 2008

Graduate student symposium

Motivation: Importance of Mineral dustMotivation: Importance of Mineral dust

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Mineral wind-blown dust is the most abundant aerosol species that originates from diverse sources throughout the world

Assessment of radiative impact of dust remains highly uncertain because of the complexity associated with dust emission, variable transport and aging process

From: IPCC 2007

Dust particles play an important role in the Earth’s radiation budget.

Direct radiative forcing: absorption, scattering Indirect radiative forcing: affect clouds as CCN, IN

SeaWiFS image

10 source of dust

Satellite remote sensing provides the best tool for studying dust

Key limitation: passive remote sensing gives only column-integrated (2D) view

Based on the our observation of CALIPSO for Asian dust in spring 2007

The volume depolarization ratio shows high value over the source

During mid-range transport the depolarization ratio of Asian dust can remain as high as ~0.35 or be much lower (0.1~0.15) than that in the source region.

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The CALIPSO space lidar provides a new capability for improved understanding of dust impacts: measures the vertical distribution of aerosols over the whole Earth for 24 hours

=> provides 3D view determines height-resolved aerosol types:

=> measures the linear depolarization ratio δa which is indicative of the nonspherical particles (such as dust)

ex) Liu et al.(2008) found that δa remained constant during long-range transport of a Saharan dust outbreak => explained by little changes in the dust size distribution and shapes

Motivation : Previous study & Observation Motivation : Previous study & Observation

For passive remote sensing, many previous studies calculated the dust optical properties using

the T-matrix method, which approximates the shape of dust particles as spheroids.

=> How do changes in dust microphysical properties (size spectra, shape, and

composition) affect the optical properties (lidar ratio, depolarization ratio, and single

scattering albedo) measured by the CALIPSO lidar?

GoalsGoals

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Perform a modeling of the optical properties of nonspherical

dust particles to aid in the interpretation of CALIPSO data.

How do changes in dust microphysical properties (size spectra, shape, and

composition) affect the optical properties (lidar ratio, depolarization ratio,

and single scattering albedo) measured by the CALIPSO lidar?

Can we reproduce the observed optical properties of Asian dust from

CALIPSO using the T-matrix method?

ApproachApproach

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Observation Analyses & Previous Study(Optical properties)

T-matrix method(Dust particle = Spheroid)

randomly oriented nonspherical particles

• Reproduce the optical properties of Asian dust

Microphysical properties- Particle shape,- Size number distribution,- Refractive index

Optical properties- Single scattering albedo (ω0),- Lidar ratio (Sa),- Depolarization ratio (δa)

•Understanding of CALIPSO data

Based on a recent study by Lafon and Sokolick(2006), the refractive index of 1.56 + 0.003i at 532 nm (CALIPSO wavelength) was selected as representative for Asian dust.

Single scattering albedo: ω0 = Cs / Ce

Lidar ratio: Sa = 4π / ω0 P11(180°)

Depolarization ratio: δa

(δa = (P11(180°)-P22(180°))/(P11(180°)+ P22(180°))

Compare

Dust Particle

a

b

Spheroids are ellipsesrotated around one axis (b); if this axis (b) is the longer axis, they are called prolate, otherwise oblate.

Optical Modeling:Optical Modeling:

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3-2. Results

Input (Microphysical properties)

Refractive index: 1.56+0.003i at 532 nmSize distribution: lnσ2 = 0.5 - 0.1< r < 1 μm, rg1 = 0.5 μm for the fine mode - 0.1< r < 3 μm, rg1 = 1.0 μm for the coarse mode Aspect ratio: - Oblate: 1.05, 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0 - Prolate: 1/1.05, 1/1.1, 1/1.2, 1/1.4, 1/1.6, 1/1.8, 1/2.2, 1/2.4, 1/2.6, 1/2.8, 1/3.0

T-matrix method

Output (Optical properties for fine & coarse mode)

Extinction coefficient (Ce), Scattering coefficient (Cs),Single scattering albedo (ω0 = Cs / Ce)Phase function (P11(Θ))Asymmetry parameter (g)Lidar ratio (Sa) Linear depolarization ratio (δa)

Aspect ratio(ε’) 1.05 1.1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Mixture1a 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

Mixture2a 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083

Mix-

ture3 b

Fine 0.535 0.289 0.108 0.040 0.015 0.007 0.003 0.001 0.001 0.001

Coarse 0.103 0.234 0.218 0.157 0.101 0.065 0.041 0.027 0.018 0.026

Mixture4c 0.335 0.319 0.179 0.087 0.042 0.020 0.009 0.005 0.002 0.001

Mixture5c 0.141 0.173 0.230 0.219 0.123 0.060 0.029 0.014 0.006 0.003 0.001 0.001

5 Mixtures and 3 Cases

• Experiment: 100% Prolate (Wiegner et al., 2008)

• Varying proportion of fine and coarse mode - Case 1: 30% fine mode + 70% coarse mode - Case 2: 50% fine mode + 50% coarse mode - Case 3: 70% fine mode + 30% coarse mode

To see the relative distribution of each size mode

Mixtures- 1 & 2: by Dubovik et al.- 3 : by Wiegner et al.- 4 & 5: by Okada et al.

The comparison between fine & coarse mode and prolate & oblateThe comparison between fine & coarse mode and prolate & oblate

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Prolate vs. oblate spheroids: Distinct differences in all optical properties. Especially, the distributions of depolarization ratio

and lidar ratio in fine mode show different patterns.

Lidar ratio of prolate spheroids in coarse mode has higher values than that of oblate spheroids.

“Dust Mixture Experiment” : 100% prolate

Fine vs. coarse : Depolarization ratio in coarse mode changes little

with varying aspect ratio Single scattering albedo in fine mode has higher

values than that in coarse mode. Lidar ratio of prolate spheroids in coarse mode has

higher values than that in fine mode. Case 1(source area): 30% fine + 70% coarse mode Case 2(mid transport):

50% fine + 50% coarse mode Case 3(long transport):

70% fine + 30% coarse mode

a)

b)

c)

Results of “Dust Mixture Experiment”: Results of “Dust Mixture Experiment”:

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30% fine mode + 70% coarse mode

50% fine mode + 50% coarse mode

70% fine mode + 30% coarse mode

Coarser particles have lower values of ω0.

Dust may absorb more sunlight in the source area because of the relatively large fraction of coarse particles (ω0 : > 0.9 in Case 1).

Preferential removal of large particles during transport would result in less sunlight absorption (ω0 : > 0.93 in Case 3).

Lidar ratio varies with varying δa The CALIPSO lidar ratio for desert dust is

Sa=38.1 sr, which corresponds to δa ~ 0.24. δa higher than 0.3 are indicative of lower Sa.

None of cases can produce δa: <0.2 and >0.3 Limitations of the assumption on spheroid.

Range of δa: 0.21 ~ 0.28

Treatments of nonspherical dust particles as spheroids can reproduce some CALIPSO data.

Particle depolarization ratio (δa) has relatively low sensitivity to the size distribution.

The Liu’s conclusions are questionable..

• 1 & 2: Dubovik et al.

• 3 : Weigner et al.

• 4 & 5: Okada et al.

Summary and ConclusionSummary and Conclusion

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Our optical modeling with the T-matrix method showed

Treatments of nonspherical dust particles as spheroids can reproduce some

CALIPSO data.

However, this approach cannot provide the entire range of δa observed by

CALIPSO as well as ground-based lidars.

More realistic shapes of dust will need to be considered to improve the

interpretation of and aerosol retrievals from CALIPSO observations.

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Thank you!!