Phenomenological kinetics of aerosol formation and growth in N2,O2-SO2 mixtures by irradiation of high energy electron beam

Nitrogen gas and/or air containing small amounts of SO2 and H2O were irradiated by an electron beam of 1.2 MeV. The resultant aerosol was collected and identified as sulfuric acid mist by its infrared spectrum. The formation and growth of aerosol were monitored by observing the attenuation of He-Ne laser light through the irradiated gas. The turbidity change determined by log (I0/I), in which I0 and I represent the light intensities before and after irradiation, respectively, was described by the empirical equation (1). log (I0/I)=Const x [Beam current]sup(1.8)[Irradiation time]sup(2.0) (1) If the Rayleigh scattering is assumed in the early stage of the reaction, the above empirical equation can be derived on the basis of the following kinetic model. The irradiation of electron beam has produced a gaseous precursor material (sulfuric acid), which forms embryos or small particles immediately. After the accumulation of a certain amount of embryos, sulfuric acid produced by further irradiation deposits on the particles and the particles coagulate with each other with a slower rate. This coagulation was observed as a growth of aerosol after the interruption of irradiation. The coagulation rate constant was estimated as about 1 x 10-10 cm3.particle-1.sec-1. The observed spectrum of the turbidity was very similar to the one of monodisperse spherical particles of radius of 200 -- 300 nm according to the Mie theory. Growth of particles to this size is expected if the particles coagulate with the above estimated rate. According to the above kinetic model, the constant in equation (1) should be proportional to the rate of formation of the precursor material. The dependency of this constant on the concentrations of H2O, NO and CO in mixed gases suggests that an oxidation process of SO2 by radicals, mainly OH, precede the aerosol formation. (auth.)