Quantifying Wildfire Heavy-Dense Smoke’s Persistent Effects on PM 2.5 Speciation across U.S. Regions: Evidence from 2007 to 2023 Smoke Events

Wildfire smoke substantially degrades air quality by increasing total PM2.5 concentrations and altering their chemical speciation. Although existing research has documented PM2.5 during active fires, the spatiotemporal persistence of these pollutants across U.S. regions remains insufficiently quantified. Leveraging quasi-experimental variation arising from 2007 to 2023 wildfire smoke events, we employ difference-in-differences (DID) and event study approaches to estimate the causal effects on both PM2.5 concentrations and their chemical speciation, while assessing their duration. Our analysis reveals that smoke events increase PM2.5 concentrations by 35 percent nationally, with pronounced regional heterogeneity: Western states experience the most severe impacts (+50 percent persisting for three days), whereas northeastern and southern regions exhibit weaker and shorter lived effects. Chemical speciation analysis reveals distinct temporal dynamics: Heavy metals (chromium, manganese) and inorganic compounds exhibit transient spikes (one day), whereas lead, vanadium, and organic carbon remain elevated for three days. Our findings underscore the urgent need to integrate chemical speciation into air quality monitoring systems to better assess composition-specific health risks under climate-driven intensification of wildfires.