Levy, R. C., L. A. Remer, and Y. J. Kaufman, 2004: Effects of neglecting polarization on the MODIS aerosol retrieval over land. IEEE Trans. on Geoscience & Rem. Sens., 42 (11), 2576-2583.
Reflectance measurements in the visible and infrared
wavelengths, from the Moderate Resolution Imaging Spectroradiometer
(MODIS), are used to derive aerosol optical thicknesses
(AOTs) and aerosol properties over ocean and land surfaces,
separately. Both algorithms employ radiative transfer (RT) code
to create lookup tables, simulating the top-of-atmosphere (TOA)
reflectance measured by the satellite. Whereas the algorithm over
ocean uses a vectorRT code that includes the effects of atmospheric
polarization, the algorithm over land assumes scalar RT, thus
neglecting polarization effects. In the red (0.66 m) and infrared
(2.12 m) MODIS channels, scattering by molecules (Rayleigh
scattering) is minimal. In these bands, the use of a scalar RT code
is of sufficient accuracy to model TOA reflectance. However, in the
blue (0.47 m), the presence of larger Rayleigh scattering (optical
thickness approaching 0.2) results in nonnegligible polarization.
The absolute difference between vector- and scalar-calculated
TOA reflectance, even in the presence of depolarizing aerosols,
is large enough to lead to substantial errors in retrieved AOT.
Using RT code that allows for both vector and scalar calculations,
we examine the reflectance differences at the TOA, assuming
discrete loadings of continental-type aerosol. We find that the
differences in blue channel TOA reflectance (vectorÐscalar) may
be greater than 0.01 such that errors in derived AOT may be
greater than 0.1. Errors may be positive or negative, depending
on the specific geometry, and tend to cancel out when averages
over a large enough sample of satellite geometry. Thus, the neglect
of polarization introduces little error into global and long-term
averages, yet can produce very large errors on smaller scales and
individual retrievals. As a result of this study, a future version of
aerosol retrieval from MODIS over land will include polarization
within the atmosphere.
