Numerical simulations of optical properties of nonspherical dust aerosols using the T-matrix method Hyung-Jin Choi School of Earth and Atmospheric Sciences Georgia Institute of Technology November 14, 2008 Graduate student symposium

Motivation: Importance of Mineral dust 2  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. 4  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 δ a 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  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?

Goals 5  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?

Approach 6 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 (ω 0 ), - Lidar ratio (S a ), - Depolarization ratio ( δ a ) Understanding of CALIPSO data Based on a recent study by Lafon and Sokolick(2006), the refractive index of i at 532 nm (CALIPSO wavelength) was selected as representative for Asian dust. Single scattering albedo: ω 0 = C s / C e Lidar ratio: S a = 4π / ω 0 P 11 (180°) Depolarization ratio: δ a (δ a = (P 11 (180°)-P 22 (180°))/(P 11 (180°)+ P 22 (180°)) Compare Dust Particle a b Spheroids are ellipses rotated around one axis (b); if this axis (b) is the longer axis, they are called prolate, otherwise oblate.

Optical Modeling: Results Input (Microphysical properties) Refractive index: i at 532 nm Size distribution: lnσ 2 = < r < 1 μm, r g1 = 0.5 μm for the fine mode - 0.1< r < 3 μm, r g1 = 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, 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 (C e ), Scattering coefficient (C s ), Single scattering albedo (ω 0 = C s / C e ) Phase function (P 11 (Θ)) Asymmetry parameter (g) Lidar ratio (S a ) Linear depolarization ratio (δ a ) Aspect ratio(ε’) Mixture1 a 0.1 Mixture2 a Mix- ture3 b Fine Coarse Mixture4 c Mixture5 c 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 & oblate 8  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”: 9 30% fine mode + 70% coarse mode 50% fine mode + 50% coarse mode 70% fine mode + 30% coarse mode  Coarser particles have lower values of ω 0.  Dust may absorb more sunlight in the source area because of the relatively large fraction of coarse particles (ω 0 : > 0.9 in Case 1).  Preferential removal of large particles during transport would result in less sunlight absorption (ω 0 : > 0.93 in Case 3).  Lidar ratio varies with varying δ a  The CALIPSO lidar ratio for desert dust is S a =38.1 sr, which corresponds to δ a ~  δ a higher than 0.3 are indicative of lower S a.  None of cases can produce δ a : 0.3  Limitations of the assumption on spheroid.  Range of δ a : 0.21 ~ 0.28  Treatments of nonspherical dust particles as spheroids can reproduce some CALIPSO data.  Particle depolarization ratio (δ a ) 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 Conclusion 10  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 δ a 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.

11 Thank you!!