An Exploration of the Effects of Rayleigh-Benard Turbulence on the Dry Deposition of Aerosols in the Pi Chamber
JACOB KUNTZLEMAN, Abu Sayeed Md Shawon, Ian Helman, Prasanth Prabhakaran, Raymond Shaw, Will Cantrell,
Michigan Technological University Abstract Number: 192
Working Group: Aerosol Physics
AbstractAerosol particles with diameters ranging from nanometers to micrometers are important in the study of many biological systems, clouds, and climate. Dry deposition of aerosols is one of the main mechanisms by which aerosols are removed from Earth’s atmosphere, and is therefore an important contribution to aerosol number and mass budgets. Recent work has highlighted uncertainties in the rate at which aerosols are deposited to a variety of natural surfaces under turbulent conditions. We measured the removal of aerosol particles ranging from 50 nm to 2 μm diameter in a dry, turbulent flow. The turbulence was maintained in the Pi Chamber via Rayleigh-Benard convection with a temperature gradient of 19 K across the 1 m height—i.e. a Rayleigh number of order 10
9. The results are qualitatively consistent with earlier studies in finding two deposition regimes: the characteristic settling time, τ, for the decay by deposition of large particles (d
p > 600 nm) is dominated by gravitational settling, as shown by the inverse square dependence of τ on particle diameter, d
p, while the characteristic deposition time of small particles (d
p < 600 nm) obeys τ = αd
p0.23, where α is independent of particle diameter but depends on chamber properties and turbulence intensity. The deposition of small particles is partly understood to be a result of diffusive effects, but the dependence of the characteristic settling time on particle diameter is weaker than that predicted by diffusion alone. The relative contributions of turbulent dispersion, thermophoresis, and electrostatics forces are considered. We finally present some preliminary evidence that small particle deposition rates are a function of turbulence strength.