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 nebulizer is estimated by means of a particle evaporation theory [Ferron and Soderholm (1990) J. Aerosol Sci. 21, 415] and a method to divide polydisperse distributions in monodisperse size fractions [Ferron, (1977) J. Aerosol Sci. 8, 407]. Each monodisperse size fraction is characterized by a mean particle size and a mass fraction. The evaporation is calculated by iteration using sufficiently small time steps. After each iteration step the new air and particle parameters are calculated. This iteration process is repeated until a preset time is reached. Input data for the calculations are the mass of water and solute in the aerosol supplied by the nebulizer, the humidity of the air delivered to the nebulizer and the final air temperature. The method is used to estimate the size distributions of aerosols produced by jet nebulizers as 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 geometric standard deviations (GSD) of about two are performed. It is found that the droplets smaller than 1 mu m evaporate rapidly and are in equilibrium with the relative humidity of the air within 0.1 s. Ninety-five percent of the total evaporated water mass took place within 0.1 s. However, equilibrium was not reached within 3 s. For all three nebulizers the largest increase (up to 30%) of the MMAD and GSD of the total mass distribution occurred in the first 0.1 s. Differences in the MMADs of the total mass and the salt mass distributions up to 60% were found. 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.