Homogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paul-trap. The size of a single droplet is continuously measured by analyzing the angle-resolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleation theory in order to obtain the size and the formation energy of the critical nucleus.

Supercooled sulfuric acid solution droplets are a major constituent of the stratospheric background aerosol. We report the rate of homogeneous freezing and the crystallization dynamics of single levitated sulfuric acid solution droplets with 0 to 57.6 wt.%The energy of formation of the critical nucleus and the H 2 SO 4 . self-di †usion activation energy are deduced from the experiments as a function of temperature and concentration. The results are compared to both theoretical and experimental work of other groups.

The uptake dynamics of HCl by single sulfuric acid microdroplets under stratospheric conditions is investigated. The droplets are typically 30-70 µm in diameter and weakly charged so that they can be stored in an electrodynamic trap. The gas uptake of the droplets is monitored either by measuring their size by angle resolved Mie-scattering patterns or by electrostatic balancing of the droplets in the trap. At low temperatures and high sulfuric acid concentration (T < 190 K for 48 wt % H 2 SO 4 and T < 195 K for 56 wt % H 2 SO 4 , respectively), liquid-phase diffusion inside the droplet is the rate-limiting step in the overall uptake process. In this regime, the diffusion coefficients D liq of HCl in supercooled sulfuric acid solutions are found to increase strongly with temperature and H 2 O concentration. The results are discussed with respect to diffusion models that have been proposed recently. In contrast, at higher temperatures and lower sulfuric acid concentrations (30-40 wt % H 2 SO 4 , 185-207 K) gas-phase diffusion with subsequent accommodation/dissolution at the liquid surface determines the observed uptake velocity. A new method to deduce accommodation coefficients R is proposed.

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