Formation and growth of photochemical aerosols in Titan's atmosphere

Abstract

Recent developments in our understanding of the morphology (shape and size) of haze aerosols in Titan's atmosphere, aggregate particles and their associated optical properties (West 1990, West and Smith 1991), have been considered in light of a microphysical modeling of aerosols. Using an Eulerian model rather similar to the one of Toon et al. (1980), it is shown that the growth of particles may be divided in two stages. The first one corresponds to the initial growth near the formation altitude by accretion of very small elementary particles (synthetized oligomers). Due to the large number of accreted small particles, this stage leads to the formation of nearly spherical particles (called “monomers” by West and Smith). In the second stage, these particles (“monomers”) settle in the atmosphere and stick together forming aggregates, of the same kind as those obtained experimentally by Bar-Nun et al. (1988). The classical assumption of spherical particles, which is made in all existing models, is shown to be valid during the first growth stage. The mean ratio between the masses of two particles colliding together is quantified as a function of altitude using a specific index (mass sharing index, MSI). From this criterion, the two previously defined regions (the formation region inside which monomers are generated, the settling region where aggregates build up) may be clearly separated and the boundary between them precisely defined and located. Identifying the monomer radius with the mass-averaged radius at the altitude of the boundary, it is shown that this radius is extremely sensitive to the altitude where aerosols are created. The reason is that the residence time of particles near the formation altitude, which determines to a large extent the size of monomers, is an increasing function of the pressure. The strong dependence of the monomer radius on the pressure (thus the altitude) is calculated using different sets of parameters (eddy diffusion coefficient, electrical charge, mass production rate, …) in order to cover the widest range of possibilities. Using parameters favored by West in his analysis of the polarizing properties of Titan's haze, the formation altitude of aerosols is found to lie in the range from 350 to 400 km.