Optical properties of aerosols in Titan's atmosphere: Large fluffy aggregates

摘要

Scattering properties of large (N > 2000) aggregates of monomers as found in Titan's atmosphere during the descent of the Cassini/Huygens probe are numerically calculated. We extend our previous quantitative analysis of small aggregates (N ≤ 256) to verify the obtained conclusions. To calculate the light scattering of such big non-spherical aggregates we use the DDSCAT code based on the discrete dipole approximation (DDA) technique introduced by Draine and Flatau (1994). The orientation averaged single-scattering phase function as well as the degree of linear polarization is calculated. Clusters with up to 2048 monomers are investigated in the optical wavelength range. We conclude that (a) the applied mathematical aggregation mechanism plays only a minor role, and it is sufficient to test particles formed by diffusion limited aggregation (DLA) with different cluster parameters; (b) the aggregate "fluffiness" can be qualitatively derived from polarization data; (c) for a fixed number of monomers the scattering function of intensity is sensitive to the size parameter of the aggregates (e.g. to the effective cluster density); (d) consequently, characterization of aggregates by their number of monomers only is generally insufficient and more detail cluster descriptions are needed; (e) even when an aggregate is described more explicitly significant intensity variations can be observed for "identical" model particles over the whole range of scattering angles, particular for large fluffy aggregates; (j) random generation averaging should be added to the standard orientation averaging procedure. Finally, we discuss our results in respect of the conclusions of the Tomasko et al. (2008) paper.