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Pure and mixed aerosols of ethane, ethylene, acetylene and carbon dioxide were generated in a collisional cooling cell and characterized by Fourier transform infrared spectroscopy between 600 and 4000 cm(-1). Pure ethane, pure ethylene, and mixed ethane/ethylene initially form supercooled liquid droplets, which over time crystallize to their stable solid phases. These droplets are found to be long-lived (up to hours) for pure ethane and mixed ethane/ethylene, but short-lived (up to seconds) for pure ethylene. Acetylene and carbon dioxide form solid aerosol particles. Acetylene particles have a partially amorphous structure, while carbon dioxide particles are crystalline. The structure of the infrared bands of carbon dioxide is strongly determined by the particles' shape due to exciton coupling. The comparison of various mixed systems reveals that acetylene very efficiently induces heterogeneous crystallization. As reported earlier, the co-condensation of acetylene and carbon dioxide can lead to the formation of a metastable mixed crystalline phase. Our preliminary calculations show that this mixed phase has a monoclinic rather than the cubic structure proposed previously.

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Strong evidence for ethane clouds in various regions of Titan's atmosphere has recently been found. Ethane is usually assumed to exist as ice particles in these clouds, although the possible role of liquid and supercooled liquid ethane droplets has been recognized. Here, we report on infrared spectroscopic measurements of ethane aerosols performed in the laboratory under conditions mimicking Titan's lower atmosphere. The results clearly show that liquid ethane droplets are significantly stabilized by methane gas which is ubiquitous in Titan's nitrogen atmospherea phenomenon that does not have a counterpart for water droplets in Earth's atmosphere. Our data imply that supercooled ethane droplets are much more abundant in Titan's clouds than previously anticipated. Possibly, these liquid droplets are even more important for cloud processes and the formation of lakes than ethane ice particles.

Aerosol particles composed of co-crystalline CO(2)·C(2)H(2) were generated in a bath gas cooling cell at cryogenic temperatures and investigated with infrared spectroscopy between 600 and 4000 cm(-1). Similar to results obtained for thin films of the co-crystal [T. E. Gough and T. E. Rowat, J. Chem. Phys. 109, 6809 (1998)], this phase was found to be metastable and decomposed into pure CO(2) and pure C(2)H(2). These decomposed aerosols were characterized through (i) a comparison to experimentally prepared aerosols of mixed CO(2) and C(2)H(2) of known architectures and (ii) the modeling of infrared spectra. A likely architecture after decomposition are C(2)H(2)-CO(2) core-shell particles with a disk-like shape. The co-crystalline CO(2)·C(2)H(2) aerosols prior to decomposition are modeled and analyzed in detail in the subsequent paper (Part II).