Understanding the Recent Stagnation in PM _2.5 Concentrations Across the United States: A Seasonal Composition Perspective

Abstract Long‐term declines in concentrations of fine particulate matter (PM 2.5 ) in the United States (U.S.) have been disrupted in recent years, with recent trends stagnating or reversing. In this study, we analyze surface observations of PM 2.5 composition from 2002 to 2022 to identify the chemical components driving this shift. We find that PM 2.5 concentrations plateau across seasons and regions in the contiguous U.S. since 2016, even after excluding estimated wildfire impacts, suggesting that the rise in wildfire activity alone does not account for these trends. The stagnation is primarily driven by a slowdown in the reduction of sulfate and a non‐significant increase in organic aerosols. In the Eastern and Central U.S., sulfate concentrations generally mirror decreasing anthropogenic SO 2 emissions, except in winter, where chemical feedbacks related to oxidant limitations weaken the response of sulfate. We find that nitrate and NO 2 concentrations decrease slower than anthropogenic nitrogen oxides (NO x ) emissions, particularly in fall and winter, suggesting a potential overestimate in the decrease of NO x emissions in the U.S. Environmental Protection Agency National Emission Inventory (NEI) and/or an increasing role of natural and non‐U.S. sources. In the Southeast, the decline in organic aerosol concentrations has stalled since 2015, possibly due to weaker decreases in sulfate‐induced secondary organic aerosol (SOA) formation from isoprene, combined with increases in monoterpene‐derived SOA as the climate warms. Despite continued decreases in the NEI black carbon (BC) emissions, BC concentrations have stagnated since 2015, even after removing the estimated influence of wildfire smoke, indicating a possible underestimate in emissions.