BACKGROUND AND AIM While levels of ambient air pollution have declined over time in the United States, studies have found persistent exposure disparities by race and ethnicity. Racial residential segregation may contribute to differential reductions in air pollution and perpetuate related health inequities. METHODS We analyzed harmonized data from three longitudinal cohort studies, spanning the years 1990-2017: Cardiovascular Health Study (CHS), Multi-Ethnic Study of Atherosclerosis (MESA), and Reasons for Geographic and Racial Differences in Stroke (REGARDS) (N=466,513 person-years). Annual average NO₂ and PM₂.₅ exposures for residential addresses were estimated from national spatio-temporal models. Neighborhood segregation indices of multi-race divergence, Black-White dissimilarity, and single-race isolation were created from Census tract-level data, with linear interpolation for intercensal years. We evaluated percent changes in NO₂ and PM₂.₅ over time by segregation z-score and race using log-linear repeated measures models adjusted for age, sex, individual and neighborhood socioeconomic status, urbanicity, and cohort. RESULTS From 1990 to 2017, mean (SD) NO₂ exposure decreased by 62.2% for Black participants [from 23.3 (9.6) to 8.8 (4.8) ppb] and 61.8% for White participants [17.0 (8.0) to 6.5 (4.1) ppb]; since 1999, PM₂.₅ respectively declined by 54.5% [16.5 (2.5) to 7.5 (1.0) μg/m3] and 54.4% [14.9 (2.8) to 6.8 (1.2) µg/m3]. Among Black participants, exposure levels were consistently higher in more segregated neighborhoods, where the rate of decline lagged: each standard deviation increase in divergence was associated with 0.19% (95% CI: 0.14-0.24) and 0.14% (0.12-0.16) slower average annual declines in NO₂ and PM₂.₅, respectively, with similar trends for dissimilarity and isolation. CONCLUSIONS Despite improvements in air quality, Black individuals continue to experience higher levels of air pollution than White individuals, particularly in more segregated neighborhoods. These trends may contribute to enduring racial inequities in health outcomes, warranting further research and action to improve environmental injustice. KEYWORDS Air pollution, segregation