Significance Exposure to fine particulate matter is a leading cause of premature deaths and illnesses globally. In the eastern United States, substantial cuts in sulfur dioxide and nitrogen oxides emissions have considerably lowered particulate sulfate and nitrate concentrations for all seasons except winter. Simulations that reproduce detailed airborne observations of wintertime atmospheric chemistry over the eastern United States indicate that particulate sulfate and nitrate formation is limited by the availability of oxidants and by the acidity of fine particles, respectively. These limitations relax at lower ambient concentrations, forming particulate matter more efficiently, and weaken the effect of emission reductions. These results imply that larger emission reductions, especially during winter, are necessary for substantial improvements in wintertime air quality in the eastern United States.