An aerosol evolution model that incorporates explicit chemical changes for gas and aqueous phase sulfur reactions and equilibrium with gaseous precursors is used to simulate the change in coal-fired power station source emissions for a variety of conditions. These "aged" profiles are used in a chemical mass balance (CMB) source apportionment to estimate the amount of PM2.5 and sulfate contributed by local power stations vs regional sources at locations near the Mt. Zirkel Wilderness Area in northwestern Colorado. On average, local power station contributions were negligible because gas-phase conversion created insufficient sulfate during reasonable plume aging periods. However, when plumes were combined with fogs in the nearby Yampa Valley prior to transport to the Wilderness, substantial fractions of sulfate, with corresponding contributions to light extinction, were estimated by the CMB. These contributions were qualitatively consistent with independent hourly measurements of particle light scattering, black carbon, and sulfur dioxide near the Wilderness and with observations of plume trapping and haze movements from time-lapse videos.