Coagulation of fine aerosols

Abstract
The coefficient of coagulation in air at 1 atmosphere (≈ 10 5 Pa) has been measured for particles having mean radii ca . 1.6 and 3.0 nm. Ranges of concentration at constant radius were used so that wall loss could be allowed for. The values obtained were 62 and 113 x 10 -10 cm 3 particle -1 s -1 , respectively, which are 12 and 16 times the values calculated from Fuchs’s incorrect β -theory. At atmospheric pressure the coagulation coefficient is given by the classical Smoluchowski-Cunningham-Knudsen-Weber-Millikan expression when Kn < 20. Further reduction of particle size leads to lower values than the classical which rise to a maximum at Kn ≈ 27 and then fall towards the gas kinetic value which is not attainable at atmospheric pressure. The Nolan-Pollak condensation nucleus counter was reliable for particles of radius down to 1.5 nm, below which particles were missed increasingly down to the theoretical limit, 1.36 nm. A size distribution of particles was derived by using this instrument at different overpressures. The particle radii were measured by diffusion and by electron microscopy.