From total aerosol concentration data obtained with a condensation nuclei counter (CNC) and size distribution information obtained with an electrical aerosol analyzer (EAA). in and adjacent to the Labadie power plant plume near St. Louis, Missouri, nucleation rates, QN and the fraction of aerosol mass from gas-to-particle conversion going into the nuclei mode and accumulation mode have been calculated. Nucleation rates in the plume ranged from 1700cm-3s-1 at 0630 in the morning close to the stack to 1 at night and long distances. Background rates ranged from 2 to 4 in the middle of the day to essentially zero at night. Approximately 5% of the new mass from gas-to-particle conversion in the plume formed new nuclei while the remainder condensed directly on accumulation mode particles. Nucleation rates calculated from the photooxidation of SO2 in smog chamber experiments were found to compare favorably with those in the plume. Furthermore, the number of new nuclei formed per unit volume of aerosol formed by the gas-to-particle conversion were found to be comparable in the plume and in the smog chamber experiments, being about 5 × 106 μ-3 in the smog chamber and 0.6 to 2.6 × 106 μm-3 in the plume. The ratio of the volume formation rate of aerosol to the SO2 concentration in the plume and in the smog chamber were found to be comparable, 38 μm3cm-3h-1ppm-1 in the smog chamber compared to 57 in the plume. These ratios were somewhat smaller than those calculated for aerosol formed in a sulfur-containing propane flame by Barsic (1977) and alongside of a Los Angeles freeway by Clark et al. (1976). A simple correlation between the total number concentration of the aerosol and the number of particles in the accumulation mode (Na) has been found to be NT = 10 (Na)0.904 (NT. Na cm-3). Using the lumped mode model, a further simple relationship between NT and the nucleation rate, QNn, has been found to be NT = 1.725 × 104 (QNn) 1 2. These later correlations would only apply to well-aged aerosol several hours away from fresh injections of combustion nuclei (QNn no. cm-3s-1).