The formation of sulfur trioxide (SO3) during the combustion of sulfur-containing fuels, particularly coal, can increase significantly following the installation and operation of selective catalytic reduction (SCR) systems for reduction of nitrogen oxides (NOx). The increased SO3 formation can in turn lead to adverse environmental impacts, including visible near-stack plumes and increased fine PM emissions, primarily in the form of sulfuric acid (H2SO4) aerosols. The potential extent of the problem in the electric utility sector is estimated based on the population of coal-fired utility boilers, the sulfur content of coal burned by each unit, and the likelihood that units will install SCR and flue gas desulfurization (FGD) systems. Of the 363 large (= 250 MWe) generating plants in the eastern U.S., there is a significant potential that as many as 65 could experience visible H2SO4 aerosol plumes or more serious problems after installation of SCR and FGD systems, based on the sulfur content of the coal historically used at those plants. As use of FGD systems increases, it is also likely that utilities will turn to higher sulfur coal, which can exacerbate this problem. This report describes the mechanisms of SO3 and acid aerosol formation and removal across boiler convection sections, air preheaters, and wet FGD systems, and presents information from an exploratory study of the absorption of SO3 onto coal fly ash. A model of SO3 formation and emissions based on these mechanisms is shown to accurately predict the stack concentration of SO3 for a 1300 MWe pulverized coal-fired boiler, indicating that the mechanisms described have captured the fundamental behavior of SO3 in utility combustion and flue gas treatment systems. This information can provide the basis for developing mitigation approaches to reduce the impacts of SO3 formation across SCRs and the subsequent formation and emission of acid aerosols.