Scientific objectives

The impacts of tropospheric aerosols on the common life (health, air visibility, pollution) justify their study. On the other hand, aerosols can impact on the climate evolution and represent an uncertainty in the radiation budget. Because of their variations in time and space, it is interesting to monitor the aerosols from satellites that allow a daily and global survey.
The POLDER products are used to generate aerosol tropospheric characteristics at global and regional scales, and to study the interactions between clouds and particles. They can also be used for atmospheric corrections to improve land surface products.

Introduction

The POLDER instrument concept allows measurements of the spectral, directional and polarized light reflected by the Earth-Atmosphere system. Data (measured radiances L*) are changed into normalized radiances L = L*/E_s, where E_s is the solar irradiance on the corresponding channel. After cloud masking and several corrections (gas absorptions, stratospheric aerosols), these data are used to derive every day some tropospheric aerosol characteristics (level 2 products) over ocean and land surfaces. These characteristics are produced at the 20x20 km2 resolution (super-pixel), as for the level 3 products(ten days and monthly syntheses).

Over ocean, the inversion scheme is based on the spectral normalized radiances L and the polarized normalized radiances in the 865 and 670 nm channels. The total aerosol optical thickness, the Angstrom exponent and the accumulation mode optical thickness are deduced.
Over land, where the surface contribution to the total radiance is generally large, the inversion scheme uses the polarized radiances in the 865, 670 and 443 nm channels. As the largest particles (radius > 0.5 µm) generate low polarization, only the optical characteristics of the accumulation mode are derived.
As inversion methods are different over ocean and land surfaces, two separate processing lines have been developed for the measurements, leading to separate products ( see flowchart over ocean and over land).

POLDER 2 advanced algorithms

The first version of the aerosol processing line was applied on the eight months of the ADEOS1-POLDER1 data. The algorithms and the first results concerning the aerosols over ocean and over land surfaces are described in Deuzé et al. (2000, 2001). From the result analysis, an improved version of the aerosol algorithms has been developed for the ADEOS2-POLDER2, which has been tested on the POLDER1 data.
For retrieval of aerosols over ocean, the main improvements are:

the use of bimodal particle size distributions.
the introduction of non-spherical large particles mixed with large spherical particles.

This allows a better restitution of the directional measurements (total and polarized radiances) and generally increases the Angstrom exponent often underestimated with a unique mode (Goloub et al., 1999).
For retrieval of aerosols over land, the main improvements are:

a better approximation of the polarized atmospheric signal
the use of the 443 nm channel
new particle size distributions
more models of polarized surfaces
a specific model for atmospheric corrections.

The new Level 2 (daily) and Level 3 (10 days and monthly) products are presented in: Products over ocean or Products over land surfaces

Development of the algorithms results from a joint effort of the Laboratoire d'Optique Atmosphérique (LOA) and the Laboratoire des Sciences et du Climat et de l'Environnement (LSCE). It has been support by CNES (Center National d'Etudes Spatiales), CNRS (Centre National de la Recherche Scientifique) and Région Nord-Pas de Calais.

For any question, please contact:      Mr. Deuzé at LOA