Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin

1. Depolarization of polarized light by atmospheric molecules and aerosols Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin • Introduction • Objective • Effect of anisotropic air molecules on radiation polarization • Depolarization of linearly polarized light by aerosols • Height of GSLC site on laser depolarization at TOA • Conclusion CLARREO Science Definition Team Meeting, Hampton, VA, April 10-12, 2012

2. Introduction Polarized light is depolarized by atmospheric components E10 E1 E10 E1 E2 For single scattering, if particle shape is symmetric to the incidence direction, the scattered light is not depolarized; but multiple scattering can cause depolarization for any particle shapes. The depolarization of the linearly polarized light by atmospheric components will incur uncertainty in the calibration of space-borne sensors for polarization with ground to space laser calibration (GSLC) system.

3. Objective • In this study, we firstly examine the effect of molecular anisotropy on the polarization of Earth-atmosphere solar radiation. • We also calculated the depolarization of light by small sphere aggregates and irregular Gaussian-shaped particles, to reveal the effect of aerosols on the depolarization of linearly polarized light. • By doing these, we aim to achieve an accurate modeling of polarized radiation for CLARREO PDM and GSLC applications.

4. Effect of anisotropic air molecules on radiation polarization For isotropic molecule Rayleigh scattering (Chandraskhar 1950) For randomly oriented anisotropic molecule Rayleigh scattering (Hansen and Travis 1974) For air How does the air molecule depolarization affect the polarization of upward radiation?

5. WL = 490 nm SZA = 21.72 deg Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin

6. WL = 490 nm SZA = 41.58 deg Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin

7. WL = 532 nm SZA = 21.72 deg Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin

8. WL = 532 nm SZA = 41.58 deg Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin

9. Comparison of Pristine-sky DOP and reflectance at 490 nm and 532 nm, SZA = 21.72 deg Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin

10. Comparison of Pristine-sky DOP and reflectance at 490 nm and 532 nm, SZA = 41.58 deg Wenbo Sun, Bruce Wielicki, David Young, and Constantine Lukashin

11. Depolarization of linearly polarized light by aerosols CALIPSO-measured depolarization ratios of different aerosols

12. Calculation of the depolarization of linearly polarized light by aerosol particles For any light scattered by any particles For linearly polarized light scattered by randomly oriented particles We define I1 and I2 as parallel and perpendicular intensity of scattered light; I01 and I02 as parallel and perpendicular intensity of incident light, respectively. For linearly polarized incidence, in a proper coordinate system, we can have Depolarization ratio for linearly polarized incidence is In this study, depolarization ratios of 3 particle habits are calculated. Refractive index of smoke aerosol (1.53+0.001i) is used. Note: This is only for scattered light. For total field, we must add the transmitted light.

13. Phase matrix elements of irregular aerosols are calculated by the 3D UPML FDTD light scattering model The FDTD is a direct numerical solution of the source-free Maxwell’s equations discretized both spatially and temporarily UPML Scattered field only Incident + scattered field UPML UPML . Incidence Inner surface for wave source Scattered field only UPML

14. Validation of the light scattering model Comparison of phase matrix elements from Mie theory and the FDTD

15. Depolarization ratios of irregular aerosols have common features Randomly Oriented Randomly Oriented Depolarization ratio at 532 nm as function of scattering angle for sphere aggregates of smoke particles

16. Depolarization ratio at 532 nm as function of scattering angle for Gaussian-shaped aerosol particles

17. Why do depolarization ratios of irregular aerosols have common features? Phase matrix elements of Gaussian particles

18. Height of GSLC site on laser depolarization at TOA Received = Direct + Forward-Scattered AOT = 0.0 AOT = 0.1 3 km 532 nm DOP and normalized forward-scattered radiance at TOA for GSLC site at 0 km and 3 km altitude (AOT = 0.1 below 3 km only)

19. Conclusion Aerosol is the primary component of clear atmosphere to depolarize light. Air molecules are secondary issue. Randomly oriented small irregular particles have some common depolarization properties as functions of scattering angle and size parameter. Depolarization ratio of scattered light in the forward-scattering direction is very small, generally smaller than ~0.3% for aerosols. Lager particles result in smaller forward-scattering depolarization ratio but larger backscattering depolarization ratio. Over mountain > 3km, linearly polarized laser beam is little depolarized by the atmosphere. The laser intensity is also little affected by the atmosphere.