| Summary: | A special challenge posed by mineral dust aerosols is associated with their
predominantly nonspherical particle shapes. In the present study, the scattering and
radiative properties for nonspherical mineral dust aerosols at violet-to-blue (0.412, 0.441,
and 0.470 μm) and red (0.650 μm) wavelengths are investigated. To account for the
effect of particle nonsphericity on the optical properties of dust aerosols, the particle
shapes for these particles are assumed to be spheroids. A combination of the T-matrix
method and an improved geometric optics method is applied to the computation of the
single-scattering properties of spheroidal particles with size parameters ranging from the
Rayleigh to geometric optics regimes. For comparison, the Mie theory is employed to
compute the optical properties of spherical dust particles that have the same volumes as
their nonspherical counterparts. The differences between the phase functions of
spheroidal and spherical particles lead to quite different lookup tables (LUTs) involved in
retrieving dust aerosol properties. Moreover, the applicability of a hybrid approach based on the spheroid model for the phase function and the sphere model for the other phase
matrix elements is demonstrated. The present sensitivity study, employing the Moderate
Resolution Imaging Spectroradiometer (MODIS) observations and the fundamental
principle of the Deep Blue algorithm, illustrates that neglecting the nonsphericity of dust
particles leads to an underestimate of retrieved aerosol optical depth at most scattering
angles, and an overestimate is noted in some cases.
The sensitivity study of the effect of thin cirrus clouds on dust optical depth retrievals
is also investigated and quantified from MODIS observations. The importance of
identifying thin cirrus clouds in dust optical depth retrievals is demonstrated. This has
been undertaken through the comparison of retrieved dust optical depths by using two
different LUTs. One is for the dust only atmosphere, and the other is for the atmosphere
with overlapping mineral dust and thin cirrus clouds. For simplicity, the optical depth and
bulk scattering properties of thin cirrus clouds are prescribed a priori. Under heavy dusty
conditions, the errors in the retrieved dust optical depths due to the effect of thin cirrus
are comparable to the assumed optical depth of thin cirrus clouds.
With the spheroidal and spherical particle shape assumptions for mineral dust
aerosols, the effect of particle shapes on dust radiative forcing calculations is estimated
based on Fu-Liou radiative transfer model. The effect of particle shapes on dust radiative
forcing is illustrated in the following two aspects. First, the effect of particle shapes on
the single-scattering properties of dust aerosols and associated dust direct radiative
forcing is assessed, without considering the effect on dust optical depth retrievals.
Second, the effect of particle shapes on dust direct radiative forcing is further discussed
by including the effect of particle nonsphericity on dust optical depth retrievals.
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