Measurements of saturation ratio and water droplet development in a rapid expansion chamber

Cloud properties in Earth’s atmosphere have many influences, including the saturation ratio, aerosol concentrations, drop size distributions, and turbulence. It is possible to measure these properties in-situ, but only instantaneously. Cloud models require a temporal component to predict their behavior. In our laboratory facility, we generate clouds within a vacuum chamber using rapid expansion, and we measure the resulting changes in temperature, saturation ratio, and drop size distribution at high frequency. If the chamber is seeded with some CaC₃ particles (up to ~400 cm^-3), a similar number of droplets form, holding the vapor content to equilibrium. When the aerosol concentration is larger, proportionally fewer droplets form, since the vapor is quickly depleted. Interestingly, if the environment within the chamber is clean (<2 particles cm^-3), numerous droplets nucleate. The saturation ratio often maximizes around 2, never reaching the value of 4 that theory shows should be the lower limit for homogeneous nucleation of water. However, if we calculate the saturation ratio from the temperature that would be produced by a dry adiabatic expansion (S_adiabatic), the threshold value to nucleate droplets is S_adiabatic~4.