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