Dynamics of sulfate particle production and growth in smog chamber experiments

The dynamics of sulfate particle production from diluted and filtered coal-fired power plant flue gases and from similar NO_x-SO₂-air mixtures were studied in a smog chamber. The results revealed that the rate of SO₂ to SO₄⁴-conversion in clean air is slower than the rate measured in the atmosphere. Similar SO₂ to SO₄²-conversion rates were measured for both filtered stack gas and synthetic stack gas mixtures. An excellent correlation was found between the rate of particulate sulfate formation and the rate of particle volume growth as measured with electrical aerosol size analyzer (EASA). From this correlation, sulfuric acid concentration in the aerosols was estimated to be 4.6 ± 0.3 molar. These two rates showed first order dependence on SO₂ at the sub-part per million range. Above 1 ppm, the reaction rates were independent of SO₂ concentration. Particle formation was interpreted in terms of the following major processes: nucleation, condensation and coagulation. The values of the nucleation and coagulation rate coefficients, k₄ and k₁₄, respectively, were found to be k₄ = 6.9 ppm^-1 min^-1, and k₁₄ = 1.4 × 10⁷ ppm^-1 min^-1 respectively. The value of the condensation rate constant (k₁₀) was estimated to be 2.3 × 10⁷ ppm^-1 min^-1 in the initial stages of particle production and is expected, according to the collision theory, to increase with particle size. Computer simulations of the reaction sequence involving particle formation and growth were conducted. Very good agreement was obtained between the particle number densities predicted by the simple model and those determined experimentally.