An electrodynamic levitation system for studying individual cloud particles under upper-tropospheric conditions

A laboratory system composed of an electrodynamic levitation cell within an environmental control chamber has been designed and built. The system is ideal for studies of individual particles, such as pure water droplets, aqueous solution droplets, solid salt particles, and ice crystals, under mid- and upper-tropospheric conditions. The experimental system has several features that make it particularly useful for studies of cloud physics. The levitation cell has a cubic geometry with transparent electrodes, thus allowing for full, three-axis positioning of a levitated particle, as well as a large range of viewing angles for optical access and light-scattering measurements. Particles in the approximate diameter range of 10 to 100 μm can be suspended indefinitely with minimal wall influences. The levitation cell is housed within an environmental control chamber capable of operating at temperatures (T), pressures (p), vertical velocities (w), and saturation ratios (with respect to ice, S(i)) in the ranges -70 ≤ T ≤ -20°C, 200 ≤ p ≤ 1000 hPa, 0 ≤ w ≤ 0.2 m s^-1, and 0 ≤ S(i) ≤ 1. The design allows for a continuous flow of gas vertically through the levitation cell during experiments, thereby maintaining a constant and well-characterized environment around the levitated particle. A grid-injection technique, whereby two flows with different trace gas (including water vapor) concentrations are mixed upstream at small spatial scales, allows for independent and rapid control of trace gas concentration near the levitated particle. Finally, for liquid droplets, particle size is continuously monitored by measuring scattered laser light from the particle. The light scattering measurements also allow droplet freezing to be clearly observed. The system has been used for studies of homogeneous freezing nucleation of liquid water and surface kinetic properties on water droplets. The nucleation data are well described by the standard statistical description of homogeneous nucleation and are consistent with previously reported measurements. Droplet evaporation data obtained at low pressures illustrate the utility of the system in studying mass and energy transfer in the transition regime. The evaporation data derived from this system are consistent with a condensation coefficient of 0.06 if the thermal accommodation coefficient is assumed to be unity.