Droplet Nucleation In a Rapid Expansion Aerosol Chamber

Abstract

We present a new experimental facility to investigate the nucleation and growth of liquid droplets and ice particles under controlled conditions and characterize processes relevant to cloud microphysics: the rapid expansion aerosol chamber (REACh). REACh is an intermediate size chamber (~0.14 m$^3$) combining the principle of an expansion chamber with the ability to probe the influence of turbulent flows. Nucleation is achieved via a sudden pressure drop accompanied by a temperature drop, which can cause humid air to condense into a cloud of droplets under the appropriate thermodynamic conditions. REACh features tight control and monitoring of the initial saturation ratio of water vapor, identity and concentration of seeding aerosol particles, temperature, pressure, and air flow mixing, together with high speed real time measurements of aerosol and droplet size and number. Here, we demonstrate that the minimum temperature reached during each expansion can be reasonably described by the thermodynamics of dry or moist adiabats, for a wide range of initial relative humidity. The size and number of droplets formed, and the overall lifetime of the cloud, are characterized as a function of the aerosol concentration and initial water vapor saturation ratio. The total droplet concentration scales linearly with the seeding aerosol concentration, suggesting that all injected aerosol particles serve as condensation nuclei. While the total number of droplets formed increases with aerosol concentration, the mean droplet size decreases with the concentration of seeding aerosols as a result of competition for the available water vapor. Theoretical considerations provide a quantitative prediction for the mean droplet size for a wide range of conditions.