EPA R10-ORD Mobile Air Monitoring Laboratory: Deployment Results from the Yakima Valley

The Yakima Valley, WA experiences persistently poor wintertime air quality with regards to PM2.5 concentrations. Efforts to reduce wintertime PM2.5, including wood stove changeout programs, haven’t achieved the desired results. To address this concern, researchers from ORD’s PESD and EPA’s Region 10 deployed a mobile laboratory to the Yakima Valley in the winter of 23/24 to quantify the both the composition and precursors of PM2.5. Our results show that wind speed in the Yakima Valley is the single most important factor for determining total PM2.5 concentrations. When wind speeds are low, pollutants accumulate, and concentrations rise. Conversely, elevated winds dilute and move pollution away from the valley lowering concentrations. In addition, we have apportioned the organic fraction of PM2.5 using positive matrix factorization. This analysis shows that on typical days organics from cooking, traffic and biomass burning make up 41% of the PM2.5, while secondary organic aerosol makes up 35% of the PM2.5 and ammonium nitrate makes up 17% of the PM2.5.  This valley also experiences wintertime inversions. During these inversions, ammonium nitrate PM2.5 is enhanced relative to other sources suggesting a change in chemical formation. We examine the role that NOx concentrations play in forming ammonium nitrate and present empirical evidence of a NOx inflection point at around 10 to 20 ppbv with regards to ammonium nitrate production.

Impact/Purpose

Despite efforts to reduce wintertime fine particulate matter (PM2.5) concentrations in the Yakima Valley, the Washington State Department of Ecology, the Yakama Nation, and EPA Region 10 are concerned that the area will not attain the PM2.5 National Ambient Air Quality Standards (NAAQS) in coming years. The Yakima Valley is a community with environmental justice concerns; portions of the valley score at the 80th percentile or higher on EJ-Screen due to the high proportion of low income residents and people of color, including people of Native American and Hispanic descent. Because of the persistent and complex nature of wintertime air quality issues in many areas across Region 10, wintertime air quality is the Region 10 Air and Radiation Division's #1 science priority for 2022. Efforts to reduce wintertime PM2.5 concentrations in the Yakima Valley have not yielded expected results, which indicates that there are fundamental knowledge gaps for what is driving PM2.5 concentrations. We propose a joint field characterization and modeling study of PM2.5, its precursors, chemistry, and composition. This project will improve our understanding of the sources and chemistry of PM2.5 in the Yakima Valley and will assist the State, Yakama Nation, and EPA Region 10 with development of a control strategy and State/Tribal Implementation Plan, should the area become designated nonattainment with the NAAQS.

Citation

Lee, B., W. Wallace, J. Richardson, S. Waldo, AND R. Kotchenruther. EPA R10-ORD Mobile Air Monitoring Laboratory: Deployment Results from the Yakima Valley. Department of Ecology Briefing and the Friends of Toppenish Creek Briefing, Virtual, WA, July 15, 2025.