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2.5. SATELLITE GROUP 85
2.5.14 Mie theory based characterization and modeling of atmospheric
aerosols
Participating scientist Suniti Sanghavi
Abstract We have developed a Mie model to generate the single scattering properties of a particle
given its size and complex refractive index. We put up a set of 24 aerosol scenarios to represent most
aerosols in the atmosphere. The Mie theory can be used to simulate these scenarios and thus examine
the optical behavior of most atmospheric aerosols.
Figure 2.48: Phase functions of different aerosol particles
Background Aerosols have gained increasing
importance in atmospheric sciences due to the
key role they play in regard to, amongst others,
the earth’s radiative budget, convective processes
and precipitation, and stratospheric chemistry
leading, eg., to the formation of the ozone
hole. They remain, however, difficult to quantify
due to their varied sources, lifetimes and transport
mechanisms. Thus they may occur over land or
water surfaces and at different heights in the atmosphere.
They also display inherent variations
in size, shape and chemical composition, leading
to different optical properties. We classify aerosols
to represent most situations found in the atmosphere
and characterize their optical properties
using the Mie theory. This can be used to simulate
the radiative properties of aerosols occuring in nature.
We intend to use the TRACY MC RTM for
this purpose. This should especially help quantify
the radiative effects of aerosols on the retrieval of
trace gas species from the satellite DOAS instrument
SCIAMACHY. Work is ongoing for other
applications that may include, in conjunction with
aerosol climatologies now available at sites such
as AERONET, aerosol inversion products from
SCIAMACHY radiances.
Funding See satellite group overview.
Methods and results We have adapted a compilation
of 24 distinct aerosol scenarios, consisting
of 5 main aerosol types, viz. urban industrial,
biomass burning, desert dust, marine and
volcanic, each associated with a typical complex
refractive index and vertical profile. Each is further
subclassified according to coarseness, absorptivity,
nonsphericity or height of occurence in the
atmosphere(km a.s.l.).
We obtain the optical properties, viz. extinction
cross section, single scattering albedo and phase
function of an aerosol particle of given size and
complex refractive index, using the Mie theory
(See figure). The Mie theory is a special case
of the Maxwell equations with boundary conditions
given by the interaction of electromagnetic
radiation of given wavelength at the surface of a
spherical particle.
Using the characteristic complex refractive indices,
size distributions and vertical profiles provided
by the the scenarios mentioned above, we
can use the Mie theory to determine bulk properties,
e.g. the Aerosol Optical Thickness (AOT)
at a given location. Adding surface reflection,
Rayleigh scattering, and molecular absorption allows
us to simulate fluctuations in radiances measured
by satellite due to variations in the aerosol
loading of the atmosphere.
Outlook/Future work Implementing aerosols
in TRACY RTM (Quantification of the effect
of aerosols on SCDs/AMFs in different spectral
regions), Aerosol retrieval/inversion from satellite/ground
based measurements.
Main publication Sanghavi [2003]