Aerosol Correction for Satellite Images
Rudolf B. Husar
CAPITA, Washington University, St. Louis, MO 63130, rhusar@me.wustl.edu
ABSTRACT
The intensity of reflected radiation from the Earth’s surface is spectrally attenuated by the
blanket of atmospheric aerosols which also act as reflectors, adding backscattered
radiation to the satellite images. The quantity and quality of aerosol interference is thus
influenced by aerosol optical depth. Deteoration of contrast and excess radiance over
natural vegetation can be used to estimate these parameters extracted from LANDSAT
images. Using the aerosol data extracted from the image, “haze free” images are
reconstructed pixel by pixel. The aerosol correction scheme is most reliable over land
with much inherent structure and will be illustrated for Multispectral Scanner and
Thematic Mapper images.
INTRODUCTION
Atmospheric aerosols constitute nuisance to satellite and other remote sensing of surfaces
for the same reason as clouds and water vapor do: their perturbation of the image is
unpredictably variable in space and time. Cloud scattering and absorption completely
obscures the ground for satellite vision. Water vapor absorption is significant but it is
confined to specific spectral bands which can be avoided. The role of haze aerosols
perturbation stretches over the entire visible and near IR radiation , but the haze is
partially transparent vertically and for many conditions the perturbation on the image can
be corrected for. The condition for a successful haze correction or removal scheme from
a satellite image is that its key optical properties of the haze are either known a priori, or
they can bee obtained from the image. These are the wavelength dependent optical depth,
τ(tau), the scattering phase function, and the single scatter albedo. Unfortunately, due to
strong and unpredictable spatial-temporal variability of τ(tau), and to the lesser degree of
the scattering phase function and the single scatter albedo, determining their values, a
priori, is not feasible. One is forced, therefore to seek methods of aerosol optics
characterization from the distorted image itself. While such a ‘bootstrap’ approach may
appear hopelessly undefined physically, one draws encouragement from the fact that if
such a scheme is successful then remote sensing offers means of detecting relevant
optical aerosol parameters, which in turn have application in many branches of
atmospheric physics: most notably in radiative climate and weather modification.
In fact, the primary motivation for this work was drawn from the possible quantitative
detection by remote sensing of aerosol optics in a vertical column and our interest in
haze correction of images was only secondary. In the approach described below, the two
applications are tightly coupled: one needs: one needs a haze perturbed image to estimate
the aerosol optics parameters: conversely, a ‘perfectly’ reconstructed haze aerosol
parameter is achieved only if one is able to reconstruct a haze free image. A major source
of encouragement was the remarkably success if semi-empirical haze correction scheme
of Gordon and Clark (1980), applied to the Coastal Zone Color Scanner data.
Furthermore, the aerosol perturbation of an image is rather subtle: it acts as an attenuator
(filter) of the radiation originating from the surface which is characterized by the aerosol
optical depth ( ??). It also acts, however, as a source of light thus adding light to the
surface reflected radiation (1-??)?? se section ???. Thus a dark section of an image may
become brighter while the brighter sections may darken depending on the relative values
of the haze attenuation of source functions. These subtle features of haze effects on the
transmitted images combined with the data inadequately on the phase function, ??? and to
a lesser degree, ??? hamper the development of haze correction schemes for haze-free
image.
In what follows, the physics of photon transmission through a hazy atmosphere will be
expressed via the radiative transfer equation and leading to the haze image perturbation
as a function of three aerosol parameters: optical depth, ?? scattering phase function, ??
and the single scatter albedo, ??. In Part ?? specific procedures are outlined to estimate
these parameters from hazy LANDSAT images and for the reconstruction of the hazefree image.
Vision through the hazy atmosphere may that be human or electronic, can be subdivided
into four components as illustrated in Figure1. : (1) the light source, usually the sun (2)
the reflecting objects; (3) the radiatively active hazy atmosphere and (4) the image
sensing detector.
In the absence of clouds, solar radiation first passes through the ozone layer (10-40km)
optically defined by its absorption optical depth, ?? (Figure 2) For remote sensing only
the kopius ozone absorption band (0.52-0.74 µm) is of interest with its peak optical depth
of ??-?? at ?? µm. The absorbed shortwave radiation by ozone contributes only to the
??ing of the upper atmosphere but none to the backscattered radiation.
Much of the backscattering or ‘airlight’ arises from the molecular or Rayleigh
atmosphere which also attenuates the incoming solar radiation with its strongly
wavelength dependent optical depth: ?? = 0.00888 ?4?.?0?5? (Robinson 1966)’ The
phase function of Rayleigh scatter ??= ¾(1+cos??2?).
The most severe perturbation to the incoming solar radition occurs in the lowest 3 km of
the atmosphere which contains light scattering/absorbing aerosol and water vapor.
Indirect??
The vertical aerosol optical depth may range from 0.01 to 1.0 and its phase function may
also be variable. Some general features of the haze aerosol were documented by
Barteneva (1960) and are of substantial utility. The aerosol phase function measured
over several locations exhibited well defined continental USSR pattern as a function of
visual range (inversely proportional to the extinction coefficient (///). At low ???=0.02
km-1 the measured phase function closely resembled the Rayleigh phase function, with
the same amount scattered. However, as the haziness (???/) increased the scattering
phase function became systematically skewed toward the forward direction. At visual
range about 50 km, measured light scattering in the forward direction was 3 times that of
the backscattering , and at visual range of 10 km, the forward/backscattering ratio was
about 7.0, i.e. the backscattering vonstituted about 12.5% of the total scattering. For
these conditions, over 50% of the scattered radiation was confined to the forward
scattering angles 0-30 degrees.
AEROSOL SCATTERING AND ABSORPTION
For the visible and near IR of the spectrum the degradation of optical environment arises
from the scattering and absorption of haze particles. These may be liquid droplets or
solid crystals sharing the common name. The important role of these trace constituents
arises from the extreme high light scattering efficiency per unit mass. Within the aerosol
population, the particles compatible in size to the wavelength of light are those that
dominate the deterioration of optical environment.
It is worth noting that scattering is not restricted to the visible part of the electromagnetic
radiation and that the scattering laws apply to all wavelengths. The scattering of radio ??
waves by satellites , microwaves by raindrops, thermal radiation by clouds, light by fine
particles and electron scattering by molecules are all similar phenomena. In each case, the
wavelength is of the same magnitude as the scatterer. Thus, a natural scaling factor for
scattering is the wavelength, of the incoming radiation and is used in the dimensionless
optical size parameter.