DIRECT AND INDIRECT METHODS FOR
CORRECTING THE AEROSOL EFFECT ON REMOTE SENSING
Yoram J. Kaufman
Laboratory for atmosphères NASA /Goddard SFC, Greenbelt, MD 20771
and
Didier Tanré
Laboratoire d’Optique Atmosphérique, Université de Sciences et Techniques de Lille,
Villeneuve d'Ascq, France
ABSTRACT
Aerosol scatters solar radiation before it reaches the surface and scatters and absorbs it
again after it is reflected from the surface and before it reaches the satellite sensor. The
effect is spectrally and spatially dependent. Therefore atmospheric aerosol (dust, smoke
and air pollution particles) has a significant effect on remote sensing. Correction for the
aerosol effect was never achieved on an operational basis though several case studies were
demonstrated. Correction can be done in a direct way by deriving the aerosol loading from
the image itself and correcting for it using the appropriate radiative transfer model or by
an indirect way, by defining remote sensing functions that are less dependent on the
aerosol loading. To some degree this was already achieved in global remote sensing of
vegetation where a composite of several days of NDVI measurements, choosing the
maximal value, was used instead of a single cloud screened value. The Atmospheric
Resistant Vegetation Index (ARVI) introduced recently for EOS-MODIS is the most
appropriate example of indirect correction, where the index is defined in such a way that
atmospheric effect in a blue spectral channel cancels to a large degree the atmospheric
effect in the red channel in computations of a vegetation index. Atmospheric corrections
can also use aerosol climatology and ground-based instrumentation. These aspects of
aerosol studies and remote sensing are reviewed in this paper. New advances in groundbased
groundbased instrumentation and future satellite systems (including measurement of
polarization) are discussed.
KEY WORDS: Aerosol, Atmospheric corrections, radiative transfer, vegetation index
1. INTRODUCTION
The atmosphere affects remote sensing of the earth surface from space by scattering
and absorption by its aerosol particles (e.g. micron size dust particles or submicron pollution
particles suspended in the air) and molecules (nitrogen and oxygen in the atmosphere). The
effect of aerosol and molecular scattering is stronger in the shorter solar wavelengths where
the particle size is similar to the radiation wavelength. Absorbing gases (e.g. water vapor,
ozone and oxygen) absorb in specific spectral bands (e.g. Tanre et al., 1992). Aerosol scattering
increases the apparent surface reflectance over dark surfaces while aerosol absorption
reduces the apparent brightness of bright surfaces. While the concentration and profile of
atmospheric gases do not vary significantly, the concentration of aerosol and water vapor
can vary by more than an order of magnitude.
Correction for the effect of molecular scattering, ozone and oxygen absorption is
important, since even though their concentration does not change substantially from
climatologic values, their effect on the radiation detected by the satellite and on the
apparent surface reflectance will vary as a function of the view and illumination direction
(e.g. Tanre et al., 1992).
7
