Laboratory studies of water droplet evaporation kinetics

Quantifying the response of a population of cloud droplets to their changing environment is fundamental to our understanding of warm-cloud microphysics. The exchange of water mass between the condensed phase and the vapor depends on droplet size and ambient conditions, and it controls the evolution of droplet size spectra that contribute to the development of precipitation. In spite of the importance of this process, considerable disagreement remains between the results of prior experimental studies, and the details of its mechanism are only partially understood.

In this study, carefully designed experiments were combined with particle-scale model calculations to provide insight into the physics of water mass accommodation. We utilized an electrodynamic particle trap to examine the evaporation of individual high-purity water droplets. Droplets in the radius range from 15 to 50 microns were injected into the particle trap, where Mie-scattering techniques were used to provide high-precision measurements of their size as a function of time. The properties of the gaseous environment (T, total pressure, and dewpoint) were well-characterized. Least-squares fitting of a kinetic mass growth model was used to extract values of the mass accommodation coefficient from the experimental data.

The high quality of the least-squares fits demonstrates that the kinetic model provides an excellent description of the evaporation process. These new experimental results may also provide evidence for a molecular-level mechanism of the mass accommodation process, which may help to explain the disagreement between earlier measurements.