ESTIMATION OF THE SIZE DISTRIBUTION OF AEROSOLS PRODUCED BY JET NEBULIZERS AS A FUNCTION OF TIME

The evaporation of polydisperse aerosol droplets produced by a jet neb ulizer is estimated by means of a particle evaporation theory [Ferron and Soderholm (1990) J. Aerosol Sci. 21, 415] and a method to divide p olydisperse distributions in monodisperse size fractions [Ferron, (197 7) J. Aerosol Sci. 8, 407]. Each monodisperse size fraction is charact erized by a mean particle size and a mass fraction. The evaporation is calculated by iteration using sufficiently small time steps. After ea ch iteration step the new air and particle parameters are calculated. This iteration process is repeated until a preset time is reached. Inp ut data for the calculations are the mass of water and solute in the a erosol supplied by the nebulizer, the humidity of the air delivered to the nebulizer and the final air temperature. The method is used to es timate the size distributions of aerosols produced by jet nebulizers a s a function of time. Calculations for three jet nebulizers with mass median aerodynamic diameters (MMAD) of 1.0, 2.8 and 9.0 mu m and geome tric standard deviations (GSD) of about two are performed. It is found that the droplets smaller than 1 mu m evaporate rapidly and are in eq uilibrium with the relative humidity of the air within 0.1 s. Ninety-f ive percent of the total evaporated water mass took place within 0.1 s . However, equilibrium was not reached within 3 s. For all three nebul izers the largest increase (up to 30%) of the MMAD and GSD of the tota l mass distribution occurred in the first 0.1 s. Differences in the MM ADs of the total mass and the salt mass distributions up to 60% were f ound. These results indicate that the aerosol distribution critically depends on the time after production and that the total mass and salt mass distributions differ substantially. (C) 1997 Elsevier Science Ltd .