A review of the dynamics of sulfate containing aerosols

Abstract

Sulfate containing aerosols are generated by irradiating mixtures of SO 2, olefins and NO–NO 2 in air. In smog chamber studies, three time domains are observed When the system is first irradiated, new particles form at a rate which depends on the concentration of the pre-existing aerosol. If the initial aerosol concentration is low. particles form rapidly and in high concentration. Increasing the initial aerosol concentration (and surface area) suppresses new particle formation because of scavenging in the size range d p< 100 A ̊ . Following new particle formation, the system passes into a transition period in which the onset of coagulation leads to a peaking and then a reduction in the number concentration. In the third time domain, the aerosol surface area per unit volume of gas. A, approaches a value sufficient to accommodate new condensable molecules and new particle formation becomes negligible. The asymptotic value of A results from the balance between condensation and coagulation, and is a function of the rate of gas to particle conversion. F. the gas temperature and particle density. For T= 300 K and a particle density of 1.46 g cm −3, theory predicts that A = 6.23 × 10 3 F 3,5, where A has dimensions of cm −1 and F s −1. This expression agrees well with smog chamber data for aerosols produced in a variety of chemical systems. Data for the Los Angeles smog aerosol are in fair agreement with this expression, too. For SO 2. NO–NO 2 and low molecular weight olefins the aerosol which forms by irradiation is primarily sulfuric acid. In smog chamber studies with a pre-existing aerosol, the mass distribution of sulfur with respect to particle size peaks in the size range between 0.1 and 0.2μm. A theoretical analysis which takes the particle growth law into account is in agreement with these experimental results. In the Los Angeles atmosphere, however, the peak occurs near 0.5 μm. Possible explanations for the discrepancy are discussed.