Characterizing Post-Fire Fine and Ultrafine Particles in the 2025 Eaton Fire Burn Zone and Nearby Areas Authors

Particle emissions may persist in wildland-urban interface (WUI) fire burn zones during the post-fire period, yet their characteristics remain poorly understood. Following full containment of the 2025 Eaton Fire, we conducted mobile and stationary aerosol measurements to characterize the spatial variability and temporal dynamics of fine and ultrafine particles within and around the burn zone. Our results show that the median PM 2.5 concentrations were slightly lower within the burn zone (inside the fire perimeter) than at locations >1000 m outside the perimeter (11.1 vs 12.0 μg/m 3 ), but the burn zone exhibited a higher frequency of PM2.5 spikes (2.8 vs 2.2 peaks/km) with greater peak prominence (9.0 vs 6.0 μg/m 3 ) and shorter durations (2.9 vs 7.5 s). These results indicate transient and intense PM2.5 emissions within the burn zone, which may not be captured by time-integrated measurements. In addition, particle number concentrations within the burn zone were higher than the urban background measured ~23 km downwind (1.2×10 4 vs 9.5×10 3 particles/cm 3 ). Distinct aerosol dynamics were observed within the burn zone that may contribute to elevated particle number concentrations, including (1) formation and growth of ultrafine particles and (2) shrinkage and disintegration of submicron particles. Together, these observations demonstrate that post-fire period represents an active and evolving phase of WUI particle emissions with important implications for human exposure and health.