La détermination de la composition chimique des aérosols atmosphériques est essentielle pour le climat terrestre et l’environnement. Néanmoins, les mesures in situ qui permettent d’accéder à cette composition sont limitées et les modèles de chimie-transport peuvent ne pas bien représenter la réalité. Notre travail de thèse a consisté à développer une nouvelle approche afin de remonter à la composition des aérosols à grande échelle par télédétection. Les modèles de mélange des composants des aérosols, soit par proportion en volume soit par l’approximation de Maxwell Garnett qui relient les propriétés optiques à la composition, ont été incorporés dans l’algorithme de restitution des propriétés atmosphériques (GRASP). La nouveauté du concept réside dans la restitution directe des composants chimiques de l’aérosol plutôt que dans une estimation indirecte à partir des propriétés optiques retrouvées. Les tests synthétiques ont montré une sensibilité des observations du satellite POLDER/PARASOL à la présence d’éléments chimiques clés des aérosols. La méthodologie a ensuite été appliquée aux mesures réelles. Les caractéristiques optiques dérivées de PARASOL en utilisant le module de composition chimique ont montré un bon accord (R de ~ 0,9 pour l’épaisseur optique) avec nos mesures de référence — le réseau AERONET. La méthodologie a aussi été appliquée aux mesures de AERONET. Les variabilités spatiale et temporelle de la composition de l’aérosol ainsi retrouvée correspondent bien à nos attentes. La composition obtenue a également été validée à l’aide de données de campagne de terrain et a pu être comparée avec les simulations réalisées avec le modèle chimie-transport GOCART. / Determination of atmospheric aerosol chemical composition has a great importance for Earth’s climate and environment. However, in situ measurements that enable determination of aerosol composition are limited in time and space, while simulations by chemical transport models may not accurately describe the reality. The current thesis presents a novel methodology for monitoring of aerosol composition by remote sensing on large spatial and temporal scale. Namely, the volume-weighted and Maxwell Garnett models, which link the aerosol optical properties and chemical composition, were incorporated into the first versatile algorithm (GRASP) that derives the atmospheric properties from remote sensing. The concept proposes the direct retrieval of fractions of aerosol chemical components instead of post-processing estimate of the aerosol composition from the retrieved optical properties (refractive index, aerosol sizes). The tests showed sufficient sensitivity of the POLDER/PARASOL satellite observations to presence of key aerosol chemical elements. Then, the methodology was applied for the real PARASOL measurements. The aerosol optical characteristics derived from PARASOL using the chemical composition module demonstrated a good agreement with our reference measurements – AERONET ground-based network (e.g., R of ~ 0.9 for aerosol optical thickness). The methodology was then applied to the AERONET measurements as well. The obtained spatial and temporal patterns of aerosol composition agree well with known physical expectations. The retried aerosol composition was validated using available field campaign data and inter-compared with GOCART chemical transport model simulations.

An accurate and generalized global retrieval algorithm from satellite observations is a prerequisite to understand the radiative effect of atmospheric aerosols on the climate system. Current operational aerosol retrieval algorithms are limited by the inversion schemes and suffering from the non-uniqueness problem. In order to solve these issues, a new algorithm is developed for the retrieval of non-spherical dust aerosol over land using multi-angular radiance and polarized measurements of the POLDER (POLarization and Directionality of the Earth’s Reflectances) and wide spectral high-resolution measurements of the MODIS (MODerate resolution Imaging Spectro-radiometer). As the first step to account for the non-sphericity of irregularly shaped dust aerosols in the light scattering problem, the spheroidal model is introduced. To solve the basic electromagnetic wave scattering problem by a single spheroid, we developed an algorithm, by transforming the transcendental infinite-continued-fraction-formeigen equation into a symmetric tri-diagonal linear system, for the calculation of the spheroidal angle function, radial functions of the first and second kind, as well as the corresponding first order derivatives. A database is developed subsequently to calculate the bulk scattering properties of dust aerosols for each channel of the satellite instruments. For the purpose of simulation of satellite observations, a code is developed to solve the VRTE (Vector Radiative Transfer Equation) for the coupled atmosphere-surface system using the adding-doubling technique. An alternative fast algorithm, where all the solid angle integrals are converted to summations on an icosahedral grid, is also proposed to speed-up the code. To make the model applicable to various land and ocean surfaces, a surface BRDF (Bidirectional Reflectance Distribution Function) library is embedded into the code. Considering the complimentary features of the MODIS and the POLDER, the collocated measurements of these two satellites are used in the retrieval process. To reduce the time spent on the simulation of dust aerosol scattering properties, a single-scattering property database of tri-axial ellipsoid is incorporated. In addition, atmospheric molecule correction is considered using the LBLRTM (Line-By-Line Ra- diative Transfer Model). The Levenberg-Marquardt method was employed to retrieve all the interested dust aerosol parameters and surface parameters simultaneously. As an example, dust aerosol properties retrieved over the Sahara Desert are presented.