The spectroscopy of aerosols is developing into an active and important field. It allows us to characterize aerosols in a nonintrusive way, in real time, and on site. Understanding the spectroscopic features of these highly complex systems requires the development of novel experimental as well as theoretical methods. This review focuses on infrared extinction spectra. The main goal is to summarize how information about intrinsic particle properties (such as size, shape, and architecture) can be gathered from observed spectroscopic patterns. We discuss the limitations of standard continuum approaches, which have been used for decades to analyze infrared spectra, and we demonstrate the importance of molecular models for the analysis of spectroscopic data.

The present study puts an end to the ongoing controversy regarding volume versus surface nucleation in freezing aerosols: Our study on nanosized aerosol particles demonstrates that current state of the art measurements of droplet ensembles cannot distinguish between the two mechanisms. The reasons are inherent experimental uncertainties as well as approximations used to analyze the kinetics. The combination of both can lead to uncertainties in the rate constants of two orders of magnitude, with important consequences for the modeling of atmospheric processes.

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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.

Phase transitions and shape changes of aerosol particles play a fundamental role in atmospheric as well as technical processes involving aerosols. For the example of fluoroform particles, we demonstrate how information about both processes can be extracted from time-dependent infrared spectra by comparison with vibrational exciton calculations. We find volume crystallization rate constants for fluoroform particles in the range of J(v) = 10(8)-10(10) cm(-3) s(-1) at a temperature of T = 78 K. Furthermore, our investigation reveals that supercooled fluoroform droplets crystallize to the most stable monoclinic bulk crystal structure. Immediately after crystallization, the particles have a cube-like shape which evolves with increasing time to an elongated shape. The present results provide new data for a better understanding of the Rapid Expansion of Supercritical Solutions when fluoroform is used as a supercritical solvent under expansion conditions which lead to fluoroform aerosol formation.