MEASUREMENT OF THE COAGULATION RATE OF A HIGH KNUDSEN NUMBER AEROSOL WITH ALLOWANCE FOR WALL LOSSES

A thermal generator for CaCO3 aerosols is described which produced compact, fairly homogeneous (σg ≅ 1.5) particles with radii from 0.005 to 0.04μm; fewer than 10% of the particles were electrically charged. Contributions to the generated aerosol from the flame and the diluting carrier gas were negligible. Coagulation coefficients for the aerosols were measured in two ageing balloons of different sizes over periods up to 30 min at atmospheric pressure. The particle concentrations were measured with a Nolan-Pollak counter and the particle size was determined by electron microscopy. After making allowance for particle loss by diffusion to the walls of the balloons, the coagulation coefficient was calculated from the decrease in particle concentration as a function of time; the initial concentration of the coagulating aerosols was varied by factors up to 30. The mean coagulation coefficient for the particles of 0.005μm median radius was 54 × 10-10 cm3/particle sec. This value is about 17% below the Smoluchowski coagulation coefficient multiplied by the Cunningham-Knudsen-Weber-Millikan factor for a Knudsen number equal to 13; if the Fuchs'β factor is included the theoretical value is reduced to 9 cm3/particle sec. It is concluded that this factor is erroneous. © 1979.