Retrieval of aerosol properties from MISR observations using GRASP algorithm: methodology, products and validation

Abstract:

MISR instrument on board of TERRA satellite has recorded unique dataset for aerosol studies. It is the longest available archive of a multi-angular and multi-spectral measurements. MISR is composed by 9 cameras capable to view the Earth at 9 different angles and measuring the light scattering at 4 different wavelengths (blue, green, red, and near-infrared). Official MISR products include extensive set of aerosol products including spectral aerosol optical depth, Angstrom exponent, absorption optical depth, as well as surface products such as directional hemispheric reflectance.

Generalized Retrieval of Atmosphere and Surface Properties (GRASP) is a versatile retrieval algorithm [Dubovik et al., 2011, 2014] that has been successfully applied to diverse remote sensing observations ranging from ground-based instruments such as nephelometers, photometers, sky-cameras or lidars to satellites like POLDER/PARASOL, MERIS/ENVISAT, TROPOMI/ Sentinel-5p, etc. GRASP is also designed to process synergies between two or several instruments as for example the lidar and sun-photometer inversion [e.g. Lopatin et al., 2013] that has been widely used by the community. To achieve such high versatility of applications, GRASP has been developed using fundamentally general and flexible concepts. The forward calculations of GRASP can be set up to employ the diverse aerosol models, from very detailed (e.g. binned size distribution, spectral complex refractive index, aerosol height, ...) to less sophisticated as an external mixture of different aerosol components.

Current work presents the results of applying GRASP to MISR observations. Specifically, MISR/GRASP retrieval models aerosol as an external mixture of 5 pre-defined aerosol components based on aerosol climatologies from observations of aerosol in smoke, oceanic, desert dust, urban clean and polluted environments. The surface BRDF is modelled using Ross-Li and Cox-Munk models over land/ocean respectively. The results of GRASP/MISR retrievals for year 2008, their validation with AERONET aerosol ground-based measurements, as well as comparisons with aerosol satellite products o from the official MISR and MODIS retrievals will be presented.