Long-Term Source Apportionment at 12 Sites in the Western United States from 2000-2020

KAMALJEET KAUR, Jenna Krall, Cesunica Ivey, Heather Holmes, Kerry Kelly, University of Utah

     Abstract Number: 126
     Working Group: Source Apportionment

Abstract
Long-term source attribution is essential for evaluating the effectiveness of PM 2.5 reduction strategies. However, inconsistencies in chemical speciation methodologies over time present challenges in assessing source contributions. This study addresses these issues by combining Positive Matrix Factorization and Chemical Mass Balance to characterize PM 2.5 sources over a 20-year period (2000–2020) across 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). Between five and eight distinct source factors were identified at each site, 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. Across most sites, PM 2.5 concentrations declined significantly, with annual reductions ranging from -0.38 to -0.037 µg/m³ per year, except in Boise and Commerce City. Reductions in AN, AS, EC-rich, and EC-Cu-rich contributions reflect the impact of national regulations targeting mobile, point, and area sources, leading to decreased emissions of NOₓ, SO₂, and primary PM 2.5 . Despite increases in vehicle miles traveled (VMT), declining EC-rich (vehicle-related) concentrations at most sites (-0.099 to -0.028 µg/m 3 per year) highlight the benefits of reduced per-vehicle emissions. OC-rich concentrations, particularly in winter and associated with biomass burning, showed the most substantial seasonal declines (-0.238 to -0.045 µg/m³ per year), pointing to the effectiveness of local and national efforts to reduce residential wood combustion. In contrast, dust contributions generally increased (0.00067 to 0.024 µg/m³ per year), likely driven by growing regional aridity and VMT. These results demonstrate the long-term success of air quality policies in lowering PM 2.5 concentrations, while also highlighting emerging challenges such as dust.