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 their properties in thermodynamically metastable states, which is not possible with bulk experiments. For example a water cloud droplet in the atmosphere does not freeze at 0°C but at a much lower temperature (around -38°C), unless triggered by an "ice nucleus". At temperatures between 0°C and -38°C 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 same relative humidity it deliquesced, but only at much lower relative humidity. Thus again, for a considerable range of relative humidities upon drying, the particle is in a metastable (here oversaturated) state, which cannot be studied with bulk samples. Since aerosol particles under atmospheric conditions are often in such metastable states, experiments like ours allow determining thermodynamic properties otherwise not accessible.

Second, we are able to study the light scattering properties of individual particles. As the particles are in a size range comparable to the wavelength sunlight, their 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 ice, salts, solid organics and other species in atmospheric aerosol, or their vitrification (glass formation) can be studied using levitation techniques. This yields important input data for a better understanding of cloud processes.