The science of ultrafine aerosols

Abstract

Ultrafine aerosols are gasdisperse systems containing particles with sizes considerably smaller than the wavelength of light and also smaller than the mean free path of gas molecules under standard conditions. Hence the particle diameters are smaller than 50nm and these particles behave like large molecules. Ultrafine aerosols form mainly by nucleation-condensation processes or, occasionally, by spray-drying. Formation by nucleation from supersaturated vapors yields primary particles in the l-10nm range at very high concentrations. Brownian motion (and rotation) is large and consequently coagulation dominates, hence the number concentrations come down to about 107 particles per cc within seconds. Ultrafine Aerosols have a large surface area per unit mass and are therefore chemically active and good absorbers. Their main feature is their large fraction of surface atoms per particle which have relatively high surface energies compared to the bond energies. Considering particles with diameters of lOnm and lnm respectively: they contain about 104 and 10 molecules per particle and the fraction of surface molecules is 25% and 100%. From lng material about 109 and 1012 particles of that sizes can be formed. The ratio of surface energy to bond energy is 2% and 20%. The fraction charged by unipolar charging is about 50% and 1% or less. The use of Ultrafine Aerosols for technical processes (production of unusual and/or very pure materials) as well as their importance for atmospheric processes like smog formation and due to air pollution induced intensive research during the past decade. The wide interest in turn led to the development of new instruments and new techniques. © 1992, IUPAC.