Understanding Reductions of PM 2.5 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/m 3 , 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.