Scientific Background

There are several reasons why one wants to study single levitated particles in atmospheric science.

First, experiments on micro-meter sized particles allows us to study properties of them in thermodynamically metastable states. For example a water cloud droplet in the atmosphere does not freeze at 0°C but at a much lower temperature (around -38°C). At temperatures in between the droplet is in a supercooled, metastable state. Or consider the hygroscopic growth an aqueous solution particle as it may occur in the troposphere. Under very dry conditions such a particle may be completely solid. Upon humidifying the particle will take up water from the gas phase and eventually deliquesce (becomes a liquid aqueous solution droplet) at some higher relative humidity. When drying again, the particle in general does not crystallize (effloresce) at the very relative humidity it deliquesced, but only at much lower relative humidity. Thus again, for a considerable range of relative humidities upon drying, the particle will be in a metastable (here oversaturated) state, which can not be studied with bulk samples. Since aerosol particles under atmospheric conditions are often in such a metastable state, experiments like ours allow to determine thermodynamic properties otherwise not accessible to experiments.

Second, we are able to study the light scattering properties of particles. As the particles are in a size range comparable to the wavelength sunlight, there scattering properties can not be in general described by geometric optics, but more elaborate schemes taking account of diffraction etc. have to be used (e.g. Mie theory in case of spherical particles). For particles of complex morphologies experiments with levitated particles allow a comparison with theoretical models of the light scattering properties.

Third, homogeneous and heterogeneous nucleation of atmospheric aerosol can be studied using levitation techniques. This yields important input data for a better understanding of cloud processes.