Laboratory Measurements of Cloud Scavenging of Interstitial Aerosol in a Turbulent Environment

Principal Investigator(s):
Will Cantrell, Michigan Technological University

Aerosol particles and cloud droplets are inextricably intertwined in Earth's atmosphere. Every cloud droplet forms on an aerosol particle while clouds are the primary removal mechanism for aerosol particles with diameters in the range ~ 50 to 1000 nm – particles which are too big to be efficiently removed via diffusion and too small to have an appreciable settling velocity. Traditionally, those connections have been considered in the absence of turbulence, but recent measurements, both laboratory and field, have shown that size and chemical composition may not be enough to predict which particles will activate, or serve as the condensation centers for cloud droplets. Diffusional capture of interstitial aerosol by cloud droplets is another feedback/removal mechanism that has recently been highlighted as important for understanding aerosol effects in remote regions and in predicting primary ice concentrations in clouds.

We will measure aerosol removal rates, through both activation and diffusion, in a cloudy, turbulent environment as a function of size and chemical composition, using the newly developed cloud chamber at Michigan Tech, called the Pi Chamber. These measurements are possible in our facility because cloud formation is initiated and sustained through Rayleigh-Bénard convection. This technique allows us to sustain a cloudy environment for days at a time, if needed. The long measurement times that the Pi Chamber enables will allow us to 1) separate the particles which have served as the basis for cloud formation and those which have not, 2) determine the rate at which cloud droplets collect interstitial aerosol (i.e. those which have not become cloud droplets), and 3) quantify these processes in the presence of effects due to turbulence and phoretic effects.

This work supports DOE's goal of improving our understanding of the effects of aerosol composition and physical processes on removal processes.