Laboratory study of aerosol settling velocities using Laser Doppler velocimetry

Abstract As a method to experimentally study aerodynamic dragthe terminal settling velocities of aerosolized silica and glass microspheres(with sizes of 1–44 μm) have been measured at various gas pressures in the range 0.6–10 mbar and using various gas compositionsincluding He, Ar, Xe, Air, CO 2 , and water vapour. Within the molecular scattering regime, i.e. where the Knudsen numberKn > 10 and up to 500, reasonable agreement has been found with the model of Epstein (1924). Values of the scattering parameterδ were observed to be within the expected range of 1–1.44 for respectively specular/evaporative - diffuse molecular scattering processes. However, δ was seen to depend upon gas composition, specifically for H 2 O; δ = 0.96 ± 0.07, CO 2 ; δ = 1.16 ± 0.07, noble gasses; δ = 1.25–1.44 and in the case of Air δ = 1.18 ± 0.07 which is not in agreement with that conventionally used of around 1.3–1.4. These observations imply disagreement with the concept of a general or universal model of drag for all particle surfaces and atmospheres. Similarly, for Kn