Sulfate ( SO 4 2 − ) and nitrate ( NO 3 − ) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas–particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winter SO 4 2 − and NO 3 − . GEOS-Chem reproduces observed SO 4 2 − − NO 3 − − NH 4 + particulate concentrations (2.45 μg sm-3) and composition ( SO 4 2 − : 47%; NO 3 − : 32%; NH 4 + : 21%) during WINTER. Only 18% of SO₂ emissions were regionally oxidized to SO 4 2 − during WINTER, limited by low H₂O₂ and OH. Relatively acidic fine particulates (pH∼1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winter SO₂ emissions from 2007 to 2015 and find that the H₂O₂ limitation on SO₂ oxidation weakened, which increased the fraction of SO₂ emissions oxidizing to SO 4 2 − . Simultaneously, NOx emissions decreased by 35%, but the modeled NO 3 − particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled SO 4 2 − and no change in NO 3 − , as observed. Wintertime SO 4 2 − and NO 3 − are expected to change slowly between 2015 and 2023, unless SO₂ and NOx emissions decrease faster in the future than in the recent past.