Long-term PM_2.5 source apportionment at 12 sites in the western United States from 2000-2020

Long-term source attribution studies help evaluate the effectiveness of PM_2.5 reduction strategies, but inconsistencies in chemical speciation methods complicate source attribution. This study addresses these challenges by integrating Positive Matrix Factorization and Chemical Mass Balance to assess source contributions to PM_2.5 over a 20-year period (2000-2020) at 12 monitoring sites in five western U.S. states: Utah (Bountiful, Hawthorne, Lindon), California (Fresno, Bakersfield, Modesto, Visalia, Sacramento), Nevada (Reno, Las Vegas), Idaho (Boise), and Colorado (Commerce City). At each site, this study identified five to eight source factors including secondary ammonium nitrate (AN), secondary ammonium sulfate (AS), dust, chloride, organic carbon (OC) rich, elemental carbon (EC) rich, EC Cu rich, Cl Zn rich, and aged sea salt. Over two decades, PM_2.5 concentrations significantly declined (-0.38 to -0.037 μg/m³ per year) at all sites except Boise and Commerce City. Declines in AN, AS, EC rich, and EC Cu rich concentrations suggest the role of stringent national regulations on mobile, point, and area sources, resulting in reductions in NO_x, SO₂, and direct PM_2.5 emissions. Despite increasing vehicle miles traveled (VMT), EC rich (vehicle) concentrations decreased, indicating the importance of lower per-vehicle emissions. Winter OC rich concentrations, linked to biomass burning, saw the largest seasonal decline due to national and local efforts to curb residential wood combustion emissions. In contrast, dust concentrations generally increased, likely driven by rising regional aridity and VMT. These findings underscore the long-term effectiveness of air-quality policies in reducing PM_2.5 concentrations.