Wim Vassen

Scattering matrices of mineral aerosol particles at 441.6 nm and 632.8 nm

We present measured scattering matrices as functions of the scattering angle in the range 5°–173° and at wavelengths of 441.6 nm and 632.8 nm for seven distinct irregularly shaped mineral aerosol samples with properties representative of mineral aerosols present in the Earths atmosphere. The aerosol samples, i.e., feldspar, red clay, quartz, loess, Pinatubo and Lokon volcanic ash, and Sahara sand, represent a wide variety of particle size (typical diameters between 0.1 and 100 μm) and composition (mainly silicates). We investigate the effects of differences in size and complex refractive index on the light-scattering properties of these irregular particles. In particular, we find that the measured scattering matrix elements when plotted as functions of the scattering angle are confined to rather limited domains. This similarity in scattering behavior justifies the construction of an average aerosol scattering matrix as a function of scattering angle to facilitate, for example, the use of our results for the interpretation of remote sensing data. We show that results of ray-optics calculations, using Gaussian random shapes, are able to describe the experimental data well when taking into account the high irregularity in shape of the aerosols, even when these aerosols are rather small. Using the results of ray-optics calculations, we interpret the differences found between the measured aerosol scattering matrices in terms of differences in complex refractive index and particle size relative to the wavelength. The importance of our results for studies of astronomical objects, such as planets, comets, asteroids, and circumstellar dust shells is discussed.

Experimental determination of scattering matrices of randomly oriented fly ash and clay particles at 442 and 633 nm

We present measurements of the scattering angle distribution of the whole scattering matrix for randomly oriented particles of three mineral samples: fly ash, green clay, and red clay at 442 and 633 nm. Fly ash consists of aggregates of nearly spherical particles while green clay and red clay particles represent irregular compact particles. We compare the measured results for fly ash with an experimentally determined average scattering matrix which is based on measurements for a broad selection of irregular mineral aerosol particles. We find that the scattering matrix of our polydisperse sample of aggregates of nearly spherical particles differs considerably from that of compact particles. In addition, the angular distribution of the elements of the scattering matrix (except F22(θ)/ F11(θ)) for fly ash particles seem to be dominated by the single monomers. The effects of small differences in composition on the scattering behavior have also been studied by comparing our experimental results for green clay particles with those obtained by Volten et al. [2001] for red clay particles at the same wavelengths.

Experimental determination of the phase function and degree of linear polarization of El Chichón and Pinatubo volcanic ashes

[1]xa0We present measurements of the phase function and degree of linear polarization for unpolarized incident light as a function of the scattering angle for El Chichon and Pinatubo volcanic ashes at 633 nm. The results are combined and compared with those for Pinatubo at 442 nm and Lokon volcanic ash at 442 and 633 nm [Volten et al., 2001]. The three samples of volcanic ashes consist of micron sized, irregularly shaped particles in random orientation. The results are presented in the form of figures and tables for all three samples. Similarities and differences between the results for the three samples are discussed. The measured phase functions and angular distributions of the polarization can be used to evaluate models used to calculate scattering properties of volcanic ash particles, to constrain scattering properties of volcanic ashes used in radiative transfer calculations, and to retrieve characteristics of (clouds of) volcanic ash particles in the atmosphere from remote sensing data.