GADS Optical Properties

The MLSM version 1.0 estimates a background mode Martian aerosol backscatter profile plus an additional enhanced mode aerosol backscatter if desired. The simple aerosol model uses optical property databases from available literature (Ockert-Bell et al., 1997, Forget, 1998).

Single-Scattering Properties of the Martian Dust Particles (Ockert-Bell et al., 1997).

Wavelength	Single Scattering Albedo	Asymmetry Parameter	Extinction Coefficient	Index of refraction (real)	Index of refraction (img.)
0.21	0.72	0.81	2.60	1.47	0.008
0.30	0.61	0.88	2.58	1.48	0.038
0.35	0.61	0.86	2.61	1.50	0.039
0.40	0.63	0.84	2.65	1.51	0.034
0.50	0.78	0.73	2.82	1.52	0.011
0.60	0.91	0.67	2.98	1.51	0.004
0.67	0.93	0.65	3.04	1.51	0.003
0.70	0.94	0.65	3.06	1.51	0.003
0.80	0.95	0.64	3.13	1.50	0.003
1.02	0.95	0.63	3.24	1.50	0.003
1.21	0.95	0.63	3.32	1.50	0.004
1.39	0.96	0.63	3.36	1.50	0.004
2.20	0.95	0.63	3.25	1.49	0.006
2.49	0.95	0.63	3.11	1.49	0.007
2.90	0.80	0.67	2.77	1.50	0.045
3.00	0.81	0.67	2.77	1.51	0.039
3.19	0.88	0.65	2.76	1.52	0.022
3.40	0.93	0.64	2.69	1.52	0.013
3.60	0.95	0.63	2.59	1.51	0.009
3.78	0.96	0.63	2.49	1.51	0.008
3.98	0.95	0.63	2.36	1.50	0.008
4.15	0.88	0.65	2.23	1.50	0.025

Since 2.2 mm wavelength is the closest to the on and off channels, 2.2 mm optical properties are used in the MLSM. The aerosol backscatter phase function is approximated using the asymmetry parameter in the Henyey-Greenstein equation (L. Henyey and J. Greenstein,1941).

A modified gamma size distribution (Deirmendjian, 1969) is used to compute particle size distribution as follows

Surface layer aerosol backscatter, scattering and extinction is computed respectively by integrating over particle radius size from 0.01 to 10.0 mm as follows

The MLSM uses atmosphere density from the GADS to scale the vertical distribution of the aerosols either assuming a constant dust mixing ratio or a varying dust mixing ratio that is a function of pressure.

b_p(z) = Q/Q_o * r(z)/r_o * b_p b_s(z) = Q/Q_o * r(z)/r_o * b_s b_e(z) = Q/Q_o * r(z)/r_o * b_e

Zmax is computed from the Forget empirical formula based upon the solar longitude, L_s and latitude, q.

zmax = 60 + 18 * sin(L_s - 160) - (32 +1 8 * sin(L_s - 160)) * sin(q)⁴ - 8.0 * sin(L_s - 160) * sin(q)⁵

A sensitivity simulation is shown for the potential Mars backscatter (2.2 mm) with a constant dust mixing ratio as a function of altitude and various mode radius. A mode radius of 0.1 mm represents a very clean atmosphere and a mode radius of 0.4 mm represents a moderate dusty atmosphere.

An additional enhanced backscatter mode can be added to the background mode via user supplied particle concentration or a particle concentration suggested by the TES data for low, moderate and extreme dust. SWA computed total column optical depths (Tau at 0.6 mm) for a wide range of backscatter profiles and compared them to total column optical depths from the visible (0.6 mm) TES (Justus et. al., 1996; Justus et al., 2002; Clancy et al., 2003) data. Assuming a dust mode radius of 0.4 mm (Toon et al., 1977), the comparison suggests that a particle concentration of around 1.1e6 cm^-3 should be used for a low dust background event, 2.1e7 cm^-3 should be used for a moderate dust event and 5.31e7 cm^-3 should be used for an extreme dust events. Currently these enhanced options are applied to the global statement of backscatter. In a future version it is intended that the variability of the global distribution of TES backscatter be tied to the nature run atmosphere.

The MLSM allows the user to choose the median profiles or to use the median profiles with aerosol backscatter randomly distributed with log normal variability. However, the user should use the variability option with caution, since the MLSM aerosol variability is based upon Earth's aerosols. The MLSM uses the same random data seed for an entire lidar line of sight path. The aerosol attenuation is not adjusted for the varying backscatter.

For molecular CO² attenuation, the MLSM needs per km attenuation coefficients databases. Since it is not practical to compute the attenuation for every instantiation of each lidar shot, SWA decided to build bracketing databases. The AFGL line-by-line optical property model, FASCODE, was run with minimum and maximum expected Mars temperature and pressure profiles from the MCD atmospheres. The variability is very little for pressure, however there is a small variability in some temperature. SWA generated CO² attenuation coefficients for the following on channel wavelengths; 2.0531997, 2.0532002, 2.0532007, 2.0532012, 2.0532018, 2.0532023, 2.0532028, 2.0532033, 2.0532040 (center), 2.0532044, 2.0532049, 2.0532054, 2.0532059, 2.0532065, 2.0532070, 2.0532075, 2.0532080 mm and the off channel; 2.053472 mm .

The MLSM spatially and temporally interpolates upon the attenuation databases using the temperature and pressure profiles from the GADS. SWA performed a sensitivity study on two way nadir attenuation effects on expected aerosol backscatter as a function of altitude. As the figure below shows, there is a quick fall off in attenuated backscatter around 70 km for the on line (center). Using an on line shifted the the far left or right of the center line will allow the DIAL to make measurement further down into the Martian atmosphere.