741 research outputs found
Modelling mineral dust using stereophotogrammetry
Real, three-dimensional shape of a dust particle is derived from a pair of scanning-electron microscope images by means of stereophotogrammetry. The resulting shape is discretized, and preliminary discrete-dipole-approximation computations for the single dust particle reveal that scattering by such an irregular shape differs notably from scattering by a sphere or a Gaussian random sphere which both are frequently used shape models for dust particles
Single scattering by realistic, inhomogeneous mineral dust particles with stereogrammetric shapes
Light scattering by single, inhomogeneous mineral dust particles was
simulated based on shapes and compositions derived directly from measurements
of real dust particles instead of using a mathematical shape model. We
demonstrate the use of the stereogrammetric shape retrieval method in the context
of single-scattering modelling of mineral dust for four different dust types
– all of them inhomogeneous – ranging from compact, equidimensional shapes
to very elongated and aggregate shapes. The three-dimensional particle shapes
were derived from stereo pairs of scanning-electron microscope images, and
inhomogeneous composition was determined by mineralogical interpretation of
localized elemental information based on energy-dispersive spectroscopy.
Scattering computations were performed for particles of equal-volume
diameters, from 0.08 μm up to 2.8 μm at 550 nm wavelength, using the
discrete-dipole approximation. Particle-to-particle variation in scattering
by mineral dust was found to be quite considerable and was not well
reproduced by simplified shapes of homogeneous spheres, spheroids, or
Gaussian random spheres. Effective-medium approximation results revealed that
particle inhomogeneity should be accounted for even for small amounts of
absorbing media (here up to 2% of the volume), especially when considering
scattering by inhomogeneous particles at size parameters 3<<i>x</i><8. When
integrated over a log-normal size distribution, the linear depolarization
ratio and single-scattering albedo were also found to be sensitive to
inhomogeneity. The methodology applied is work-intensive and the
light-scattering method used quite limited in terms of size parameter
coverage. It would therefore be desirable to find a sufficiently accurate but
simpler approach with fewer limitations for single-scattering modelling of
dust. For validation of such a method, the approach presented here could be
used for producing reference data when applied to a suitable set of target
particles
Photoxidation Processes in Normal Green Chlorella Cells
peer reviewedThe authors describe the kinetics of pigment bleaching in Chlorella cells exposed to very intense light (100,00 lux). They distinguish two phases: an induction phase preceding bleaching, and a bleaching phase. Both phases require oxygen. During the bleaching phase, carotene disappears, then chlorophyll (a), and finally chlorophyll (b) and the carotenols. The induction of bleaching is dark-reversible and seems to reflect important changes in the cell metabolism. It has been suggested that the induction is concerned with some modifications of the properties of the chloroplast proteins that are essential for the stability of the pigments
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