Basis of the algorithm for remote sensing over land surfaces

Aerosol remote sensing over land from visible radiance measurements is more difficult than over ocean because the surface reflectances are generally much greater than the aerosol ones, except over dark surfaces (vegetation in the blue channel, lakes in near infrared). Airborne experiments have shown that the relative contribution of the surface compared to the atmosphere, is less important in polarized light than in total light. So, the aerosol algorithm over land is based on a best fit between polarized POLDER measurements and data simulated for different atmospheres (aerosol model and optical thickness) and ground surfaces conditions. In a given direction, the simulated polarized radiances are computed as follow:

L_p = L_p^mol + L_p^surf exp[-m(c_aa + c_mm)] + L_p^aer exp[-m c_mm] Eq.1

L_p^mol is the molecular contribution depending on the wavelength and on the pressure or pixel elevation, L_p^surf is the surface contribution depending on the kind of surface (Nadal and Bréon, 1999) and L_p^aer is the aerosol one which forms the useful part to extract aerosol properties. The exponential terms take into account the attenuation of the aerosol signal through the molecular slab and the attenuation of the ground contribution through the total atmosphere.
_a (resp. _m) is the aerosol (resp. molecular) optical thickness. The air mass m is equal to 1/cos _s + 1/cos _v, where _s (resp. _v) is the solar (resp. zenith view) angle.

Over land, ground based measurements show that the aerosol polarization mainly comes from the small spherical particles (Vermeulen et al., 2000) with radii less than about 0.5 µm corresponding to the accumulation mode. So, the aerosol models used in the algorithm consist in lognormal size distributions of spherical particles: their characteristics are close to the oceanic small mode ones.
Note that, a bimodal aerosol model is included for atmospheric corrections. See Land surfaces level2: Atmospheric corrections

Algorithm principle

Knowing the super-pixel characteristics (altitude, surface classification, NDVI), the molecular (L_p^mol) and the surface (L_p^surf) polarized radiances are computed in the 865 and 670 nm channels, for the viewing directions. Given an aerosol model n, the optical thickness ₈₆₅ is adjusted to fit the polarized radiances, simulated with Eq.1, and the measured ones. The root mean square _n(₈₆₅) indicates the fit quality.
The _n minimum gives the aerosol model and the corresponding optical thickness ₈₆₅.

Mode parameters over land surfaces

The lognormal size distributions of spherical particles depend on the standard deviation and on the modal radius r_m ,

Only the mode of small particles is considered: we assume a standard deviation = 0.403, a set of ten modal radii r_m, from 0.05 to 0.15 µm and a refractive index m=1.47-0.01i.

Reference :

Vermeulen A., Devaux C., and Herman M. :
Retrieval of the scattering and microphysical properties of the aerosols from ground-based optical measurements including polarization.
I, Method, Appl. Opt., 39(33), 6,207-6,220, 2000.

Products over land surfaces

Main aerosol parameters are illustrated below. A complete list of parameters is also available for level 2 and level 3 products.

Daily products (Level 2)

Quality indicator for the pixel (DQS).
Optical thickness ₈₆₅ at 865 nm, corresponding to the polarized particles (mainly anthropogenic aerosols).
Angstrom exponent between 865 and 670 nm channels.
Aerosol Index A.I. =
Refractive Index m.
Indexes of Quality
Optical thickness ' at 865 nm used for atmospheric corrections for land surfaces applications (fixed aerosol model).

Level 2 Products over Bengal Golf on November 14, 1996.

Optical thickness over land compared to the optical thickness of the small mode over Ocean at 865 nm wavelength. Both optical thicknesses, corresponding to the small particle mode, are close in the northern part of the Golf, where the anthropogenic aerosols leave the continent, and along the coast of Burma	Aerosol Index This index exhibits a good continuity over the Indian and Burmese coastal regions. Note that, over ocean, close to the Burmese coast, the Aerosol Index is quite equal to zero, but negative like the Angstrom exponent (black color).
Angström exponent over land.

 

Syntheses (Level 3)

There are 2 kinds of syntheses, the first on a ten-day period, and the second on the month.
The way to obtain the mean parameters is explained in the Section "Aerosols over Ocean: syntheses".

For each 10 days period:
Number of POLDER observations, Mean values of ₈₆₅ , , A.I. and '.

For the monthly synthesis:
Number of POLDER observations,
Mean values of ₈₆₅ , , ' and A.I.
Quartiles on ₈₆₅ , A.I. and '.
Relative frequencies of occurrences on and m.

The syntheses below are made on the same period than over Ocean, from the 11th to the 30th of November 1996. Examples with POLDER 2 data are presented in "Preliminary results".

Aerosol optical thickness at 865 nm wavelength.

Angström exponent.

Aerosol Index.

The optical thickness is rather low, compared to the oceanic one, because over land, only the small particles are detected using polarized measurements (anthropogenic aerosols). The maximum values are found in the Ganja Valley (urban and industrial particles) and over South Africa and Sahelian region (biomass burning aerosol). The Aerosol Index (A.I.), which indicates the presence of small polarizing particles, makes possible the comparison between oceanic and continental aerosol retrievals.

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