Hannakaisa Lindqvist

Optical properties of light absorbing carbon aggregates mixed with sulfate: assessment of different model geometries for climate forcing calculations

Light scattering by light absorbing carbon (LAC) aggregates encapsulated into sulfate shells is computed by use of the discrete dipole method. Computations are performed for a UV, visible, and IR wavelength, different particle sizes, and volume fractions. Reference computations are compared to three classes of simplified model particles that have been proposed for climate modeling purposes. Neither model matches the reference results sufficiently well. Remarkably, more realistic core-shell geometries fall behind homogeneous mixture models. An extended model based on a core-shell-shell geometry is proposed and tested. Good agreement is found for total optical cross sections and the asymmetry parameter.

Models for integrated and differential scattering optical properties of encapsulated light absorbing carbon aggregates

Optical properties of light absorbing carbon (LAC) aggregates encapsulated in a shell of sulfate are computed for realistic model geometries based on field measurements. Computations are performed for wavelengths from the UV-C to the mid-IR. Both climate- and remote sensing-relevant optical properties are considered. The results are compared to commonly used simplified model geometries, none of which gives a realistic representation of the distribution of the LAC mass within the host material and, as a consequence, fail to predict the optical properties accurately. A new core-gray shell model is introduced, which accurately reproduces the size- and wavelength dependence of the integrated and differential optical properties.

Small Irregular Ice Crystals in Tropical Cirrus

Images acquired by a Cloud Particle Imager (CPI) are analyzed to compile a statistical covariance function of radius for an ensemble of small, irregular, quasi-spherical ice crystals in tropical cirrus measured during the Department of Energy Atmospheric Radiation Measurement Program’s Tropical Warm Pool International Cloud Experiment. The infrequent occurrence of multiple particles in single CPI frames suggests that most crystals sampled were natural ice crystals rather than artifacts from large particles shattering on probe tips. The covariance function is used in conjunction with the Gaussian random sphere geometry to generate threedimensional model ice particles for ray-optics simulations at 550-nm wavelength. The crystal shapes and single-scattering properties are compared with those obtained by the same methodology for midlatitude cirrus sampled over Oklahoma. The small tropical ice crystals are closer to spherical than their midlatitude counterparts and, consequently, their asymmetry parameters are larger, but the differences are not significant from thestandpointofclimatestudies.Becausesomequasi-spherical icecrystalsseem partiallyfaceted,a convex hull transformationisintroducedthatincreasestheasymmetryparameterfrom0.785to0.808.Furthermodifyingthe covariance function to promote sixfold symmetry in the model crystals increases the asymmetry parameter to 0.818. Theintroductionofinternalscatterers,suchas airbubbles,has a larger impact, decreasing theasymmetry parameter by up to tens of percent, depending on their amount and characteristics. Unfortunately, no data are available to determine realistic values for the internal scatterers to assess their likely actual impact.

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.

Stereogrammetric Shapes of Mineral Dust Particles

This chapter considers shape models which are retrieved from scanning electron microscope (SEM) images of real dust particles. These models describe the real shape of the particle as measured from the images with automated image processing techniques. A general description of methods for retrieving the shape from SEM stereo images is provided together with our own developments based on dense image matching techniques. Light scattering results using the realistic shape models are presented and compared with those obtained using simplified mathematical shape models such as spheres, spheroids, and Gaussian random spheres. The impact of surface roughness on light scattering is addressed and ideas for further improvement of the shape retrieval algorithms in view of the light scattering computations are discussed.