A method to determine the particle nucleation rate directly from atmospheric measurements is presented. During the Southern Ontario Oxidant Study (SONTOS) field campaign in rural Ontario, Canada, particle size distributions and concentrations of a range of photochemical species were measured. On 25 August 1993, the size distribution showed a pronounced peak in the concentration of nucleation mode particles. This correlated with, but lagged behind, a peak in the SO2 concentration. The data imply that nucleation occurred aloft as an SO2 plume was entrained into the growing boundary layer. The particle growth rates were determined from the evolution of the measured particle size distributions, while accounting for coagulation and dilution. In principle, measurements of precursor species are not needed. However, in this case study, the ground-based measurements did not reflect the aerosol concentrations in the plume aloft; as a result, extrapolation of the growth rate was necessary. This was accomplished by using a one-dimensional model to calculate the gas phase sulfuric acid concentration. The particle growth rate due to condensation of H2SO4 was calculated and used to extrapolate the observed growth backward to obtain the time of formation. From the particle number in a certain size interval, suitably corrected for losses by coagulation and dilution, and the time interval in which they formed, the nucleation rate can be determined. We obtained nucleation rates of 5--40 cm-3 s-1 for sulfuric acid mixing ratios of 3--10 pptv. These nucleation rates are higher than predicted by classical binary nucleation theory for H2SO4 and H2O.