Submicron particle-size distributions were measured with 10-s resolution aboard the National Oceanic and Atmospheric Administration WP-3D Orion research aircraft in the plumes downwind of coal-fired power generation plants in the eastern United States and the urban areas of Nashville, Tennessee, and Atlanta, Georgia. Recently formed particles were present at the edges of some power plant plumes within 2 hours of emission, coinciding with OH maxima predicted by a numerical plume photochemistry model. Clearly detectable increases in particle volume (mass) concentration with increasing plume age, indicating gas-to-particle conversion, were found in only those plumes from sources that emit substantial quantities of SO2. The observed growth in particulate volume could be explained by the calculated oxidation of SO2 by OH. However, the amount of volume growth varied substantially from day to day, even in the plume of the same power plant. In the only case in which a quantitative growth law analysis could be performed, the observed growth of particle size with increasing plume age was consistent with condensation and inconsistent with aqueous oxidation. The advected particle volume flux in SO2-rich power plant plumes substantially exceeded that measured in the urban plumes downwind of Nashville and Atlanta.