Wildfire Smoke Volatile Organic Compound Emission Factors, OH Reactivity, and Secondary Organic Aerosol Formation Potential Across Western U.S. Fuel Types: A 2020-2024 Airborne Campaign

This study investigates VOC emission factors, OH radical reactivity, and secondary organic aerosol formation potential of fresh and aged wildfire smoke from six major Western U.S. fuel types quantified by airborne campaign measurements 2020-2024 within the context of atmospheric chemistry and wildfire smoke research, an area of growing scientific importance given its implications for wildfire smoke air quality modeling, emission factor database update for CMAQ and WRF-Chem, and health impact assessment of secondary aerosol formation in affected communities. Using whole-air canister and proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) VOC measurements from NOAA WP-3D aircraft with fire CO tracer normalization to emission factors (g/kg dry fuel) and Lagrangian aging analysis, we examine biomass combustion releasing fuel-type-dependent VOC mixtures (furans, phenols, terpenoids, aromatic hydrocarbons) with high OH reactivity driving rapid photochemical secondary organic aerosol formation during daytime smoke aging, amplifying PM2.5 population exposure downwind in 48 wildfire events sampled (2020-2024) with 284 valid plume transects: 142 fresh plume (<30 min) and 142 aged (6-18 hr), measuring >200 VOC species across 6 fuel type categories drawn from California chaparral, California mixed conifer, Oregon/Washington Douglas fir, Idaho sagebrush, Colorado subalpine, and Colorado pinyon-juniper fuel types sampled by NOAA WP-3D from 2020-2024 summer campaigns. Results indicate that total VOC emission factors range from 4.8 g/kg (sagebrush) to 28.4 g/kg (chaparral) dry fuel; chaparral and pinyon-juniper smoke show 3.2x higher SOA formation potential per kg fuel than conifer smoke; aged smoke SOA mass exceeds primary PM2.5 at 8 hours in all fuel types (p < 0.001), with 28.4 vs. 4.8 g VOC/kg; chaparral SOA 3.2x higher than conifer; SOA exceeds primary PM2.5 at 8 hours as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to atmospheric chemistry and wildfire smoke research and carry actionable implications for the design of programs and policies targeting wildfire smoke air quality modeling, emission factor database update for CMAQ and WRF-Chem, and health impact assessment of secondary aerosol formation in affected communities.