Titans stratospheric aerosol

The photochemical aerosol in Titan s stratosphere provides a good example. Several lines of evidence (Hunten et al, 1984) indicate that submicron size particles are responsible for the observed photometric and polarimetric properties of this aerosol. Refractive indices of a presumably comparable material were derived by Khare et al. (1984) by analyzing laboratory spectra of a polymeric residue obtained from electric discharge in an N2-CH4 gas mixture. Values of r = 1-8 and n = 0.17 appear to be reasonable averages over the 250-600 cm spectral range. This combination of large rii and small (a/k)(nr — 1) ensures that scattering is unimportant, and Eq. (8.3.9), modified to account for a finite field of view, is appropriate. 

Spectra of Titan s north polar limb, shown in Fig. 8.4.1, were obtained by the Voyager 1 spectrometer (IRIS). The continuum between gaseous emission features is due mainly to a stratospheric aerosol, although tropospheric emission also contributes to the spectmm shown in the lower panel. Because each field of view extends over several scale heights, it is necessary to convolve Eq. (8.3.9) with the field of view the onion peeling approach is not practical. Instead, the equation is solved directly, varying the aerosol parameters until the spectral continua are fit simultaneously for a given model. 

In practice, because of the large fields of view, vertical resolution in the stratosphere is limited to two levels. Continuity of particle number density N across the boundary z = zo between the two levels is required in the modeling, but dN/dz is allowed to be discontinuous. We replace Eq. (4.2.5) with 

The volume occupied by the aerosol particles per unit volume of atmosphere 

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Titan has the most extreme temperature inversion of all (see Fig. 8.2.2). The quantity in brackets in Eq. (9.1.57) has a value 10, while the factor multiplying it is 2. This leads to an unusually large thermal gradient in the lower stratosphere. The principal contributor to the large value of is a thick stratospheric aerosol, the product of photochemistry and charged particle bombardment in an atmosphere rich in organic gases. 

Samuelson, R. E. Mayo, L. A. (1991). Thermal infrared properties of Titan s stratospheric aerosol. Icarus, 91,207-19. 

The inference of cloud characteristics is based on much less sophisticated approaches than those for determining thermal stmcture and gas abundances. Clouds tend to be quite inhomogeneous compared with gaseous mixtures and require more parameters for adequate definition. Also, the appropriate equation of transfer [Eq. (2.1.40)] is considerably more complex than Eq. (8.2.1), and not nearly as amenable to inversion techniques. Even so, direct techniques are sometimes capable of leading to rather definitive conclusions about cloud and aerosol systems. We illustrate with an example concerning the abundance of the photochemical aerosol in Titan s stratosphere. 

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