Direct radiative forcing is a major impact of atmospheric dust aerosols.
Mineral dust is an important aerosol component that interacts with both
incoming and outgoing radiation, modulating radiative fluxes on Earth
and its atmosphere. Dust radiative impact directly depends on the
particles physico-chemical characteristics (e.g. mineralogy, shape,
size) which are the main source of uncertainties. Spectral signatures of
dust particles in the visible/short-wave infrared (V/SWIR) and long-wave
infrared (LWIR) are linked to mineral composition and variability.
Obtaining the precise spectral signature and size distribution of dust
particles can result in accurate derivation of refractive indices which
are used as inputs to model radiative forcing. In this work, V/SWIR and
LWIR reflectance spectra of heterogeneous dust samples from the United
States, East Asia, and Middle East were analyzed for their mineral
abundances. For this study we used global well-characterized soil
samples with comparable mineral compositions to windblown dust. The soil
samples cover a wide range of mineral compositions and represent both
arid and semi-arid regions [J P Engelbrecht et al., 2016]. Our
preliminary analysis used linear spectral mixing for both V/SWIR and
LWIR reflectance spectra. This approach is the simplest method to
determine mineral abundances from reflectance spectra. While this
resulted in a very low RMSE for the fit between the sample and modeled
spectra, modeled spectra did not match band centers and strengths for
all features. We also converted reflectance spectra to continuum removed
(CR) and mean optical path (MOPL) which have the potential to eliminate
nonlinear effects (e.g. multiple scattering) in spectral mixing. These
approaches, which modify the reflectance hull, significantly weakened or
removed the calcite absorption features at 2375 nm. Because these
samples are very fine grained (< 38 µm [J P Engelbrecht et
al., 2016]) multiple scattering effects are expected to be important
for both V/SWIR and LWIR spectral ranges and as our initial results show
linear mixing is insufficient to produce reasonable mineral abundances.
Our next efforts will include full radiative transfer models of the
measured spectra which we will present at the meeting.