Understanding Reductions of PM2.5 Concentration and Its Chemical Composition in the United States: Implications for Mitigation Strategies

Motivated by the recent tightening of the US annual standard of fine particulate matter (PM_2.5) concentrations from 12 to 9 μg/m³, there is a need to understand the spatial variation and drivers of historical PM_2.5 reductions. We evaluate and interpret the variability of PM_2.5 reductions across the contiguous US using high-resolution estimates of PM_2.5 and its chemical composition over 1998–2019, inferred from satellite observations, air quality modeling, and ground-based measurements. We separated the 3092 counties into four characteristic regions sorted by PM_2.5 trends. Region 1 (primarily Central Atlantic states, 25.9% population) exhibits the strongest population-weighted annual PM_2.5 reduction (−3.6 ± 0.4%/yr) versus Region 2 (primarily rest of the eastern US, −3.0 ± 0.3%/yr, 39.7% population), Region 3 (primarily western Midwest, −1.9 ± 0.3%/yr, 25.6% population), and Region 4 (primarily the Mountain West, −0.4 ± 0.5%/yr, 8.9% population). Decomposition of these changes by chemical composition elucidates that sulfate exhibits the fastest reductions among all components in 2720 counties (76% of population), mostly over Regions 1–3, with the 1998–2019 mean sulfate mass fraction in PM_2.5 decreasing from Region 1 (29.5%) to Region 4 (11.8%). Complete elimination of the remaining sulfate may be insufficient to meet the new standard for many regions in exceedance. Additional measures are needed to reduce other PM_2.5 sources and components for further progress.