Aerosol formation in reacting gases: Theory and application to the anhydrous NH3‐HCL system

Abstract

AbstractAerosol formation in reacting gases is treated in terms of the gas‐phase reaction, homogeneous nucleation of the reaction product, and particle growth by parallel mechanisms. In the absence of foreign nuclei, nucleation of the gas‐phase reaction product is described by classical homogeneous theory with a nonequilibrium correction factor included; particle growth by diffusion, surface reaction, and cluster scavenging are examined. The predicted contribution of the latter mechanism increases with increasing supersaturation ratio due to a shift in the distribution of cluster sizes in accordance with a Boltzmann‐type steady‐state distribution. Illustrative results predict a nucleation burst during which the formation of nucleating monomer by chemical reaction competes with the loss of monomer due to diffusion to the surface of the freshly formed particles. In addition, conditions under which cluster scavenging can dominate are shown.The particle‐size spectra predicted using the theory are compared with experimentally measured size spectra of NH4Cl particles formed by the gas‐phase reaction of NH3 and HCl. The size spectra were measured using an electrical aerosol size analyzer and an optical counter at the outlet of a continuous flow reactor after residence times ranging from 2 to 50 seconds. Reactant concentrations of 2.1 × 10−5 to 4.1 × 10−4 mol/m3 (0.5 to 10 ppm) were studied at 23 to 26°C and near atmospheric pressure. The predicted size spectra showed good agreement with the experimental results. At reactant concentrations below 4.1 × 10−4 mol/m3, the predominant particle growth was apparently by diffusion of the monomer in the noncontinuum range. Based on analysis of other data, cluster scavenging became more significant at reactant concentrations greater than about 8 × 10−4 mol/m3, and coagulation was not significantly important. Furthermore, these studies implied a microscopic surface free energy for NH4Cl of approximately 0.051 N/m.