Exploring Aerosol Removal Mechanisms in the Pi Cloud Chamber

Though clouds play an essential role in our climate, their remote and transient nature makes them challenging to study in situ. As such, cloud chambers are often utilized to investigate cloud behavior in a more simple, stable, and accessible manner. For many cloud chamber experiments, understanding the means of aerosol activation and removal is of particular interest. Though many instruments exist to monitor aerosol and droplet concentration and size distribution within the chamber, complete Lagrangian insight is usually beyond reach, making the quantification of unintentedl removal mechanisms, such as sidewall loss, difficult. As such, a more detailed understanding of how chamber characteristics unintentionally impact cloud behavior is required, especially given future cloud chamber designs could increase these effects.

The methods utilized by cloud chambers are varied, but this study focuses on the Pi Cloud Chamber housed at Michigan Tech University. The Pi Chamber generates Rayleigh-Bénard turbulence and, upon introducing water vapor and aerosols into the chamber, can create steady-state clouds lasting from hours to days. Here, we investigate the lifetime of aerosols subjected to Rayleigh-Bénard turbulence but free of the effects of humidity (i.e., dry conditions) by injecting solid, non-evaporating spheres (namely oil, glass, and hollow glass) into the chamber. We estimate particle removal rates by tracking the decrease in bulk concentration over time and compare these rates to theoretical estimates based on gravitational settling alone. Our results indicate that mechanisms other than gravitational settling, such as sidewall deposition, might be at play. Finally, we use direct numerical simulations to explore and explain the discrepancy between the measured removal rates and those typically assumed.