Abstract
In planetary atmospheres, adsorption of volatile molecules occurs on aerosols prior to nucleation and condensation. Therefore, the way adsorption occurs affects the subsequent steps of cloud formation. In the classical theory of heterogeneous nucleation, several physical quantities are needed for gas condensing on a substrate like aerosols, such as the desorption energies of the condensing gases on the substrate and the wetting parameters of the condensed phases on the substrate. For most planetary atmospheres, the values of such quantities are poorly known. In cloud models, these values are often approximately defined from more or less similar cases or simply fixed to reproduce macroscopic observable quantities such as cloud opacities. In this work, we used the results of a laboratory experiment in which methane and ethane adsorption isotherms on tholin, an analog of photochemical aerosols, are determined. This experiment also permits determination of the critical saturation ratio of nucleation. With this information we then retrieved the desorption energies of methane and ethane, which are the quantitative functions describing the adsorption isotherms and wetting parameters of these two condensates on tholin. We find that adsorption of methane on tholin is well explained by a Langmuir isotherm and a desorption energy ΔFo = 1.519 ± 0.0715 × 10−20 J. Adsorption of ethane tholin can be represented by a Brunauer-Emmett-Teller isotherm of type III. The desorption energy of ethane on tholin that we retrieved is ΔFo = 2.35 ± 0.03 × 10−20 J. We also determine that the wetting coefficients of methane and ethane on tholin are m = 0.994 ± 0.001 and m = 0.966 ± 0.007, respectively. Although these results are obtained from experiments representative of the Titan case, they are also of general value in cases of photochemical aerosols in other planetary atmospheres.
Highlights
Cloud models for planetary atmosphere are essentially based on the classical theory of nucleation and condensation as proposed, for instance, in Pruppacher & Klett (1997)
For cloud formation, when the saturation ratio exceeds S = 1, nucleation corresponds to the agglomeration under Maxwellian statistics of enough molecules to form embryos of radius r beyond a critical radius rcrit
The desorption energy of methane on tholin evaluated in this work is a factor of 2 lower than the values generally used in previous works (e.g., ∆Fo = 2.88 × 10−20 J for Seki & Hasegawa 1983 and ∆Fo = 3.03 × 10−20 J for Himeno et al 2005)
Summary
Cloud models for planetary atmosphere are essentially based on the classical theory of nucleation and condensation as proposed, for instance, in Pruppacher & Klett (1997). We are especially interested in the experiment with ethane and methane on tholin since clouds appear on several planets with nucleation on this type of nuclei (Moses et al 1992; Barth & Toon 2003; Rannou et al 2006) These two cases are the only examples for which all the information is available in Curtis et al (2008) for an in-depth study. The value aaotisdfojnSuinssctraitttnhhdseetrcevooxanmplguelpeyraiomrdifneempgnetw.tnodPitsorhobotctnhaeieenmdoiJ.bnHUsgSesDriivnn=egdtJhtcvihrsiatelawuetexatsyphoeewrfvimiSatlhcoeurbniesttt,haStelh=eceyoxSnpccoderabiristn--, imental data of methane and ethane condensation on tholins, each author proposed two values of m, which are reported later for comparison with our own results These two works omit that the nucleation rate depends on the desorption energies, which can be retrieved from experiments.
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