Chemical and toxicological impacts of the 2025 Southern California wildfires on urban air quality in Los Angeles

The January 2025 wildfires (Palisades, Eaton, and Hughes) in Southern California represented the increasing severity of wildfire impacts within densely populated urban regions, illustrating how extreme wildfire episodes can directly affect air quality in major metropolitan areas. This study aimed to characterize the chemical composition, mass size distribution, and oxidative potential (OP) of ambient particulate matter with an aerodynamic diameter below 10 μm (PM₁₀) during and following these extensive wildfires in Los Angeles. Peak PM₁₀ concentrations during the active wildfire phase reached 152 μg/m³, exceeding the National Ambient Air Quality Standards (NAAQS). Chemical analysis revealed pronounced elevations in biomass burning tracers, notably potassium (K) (0.47 μg/m³) and levoglucosan (664 ng/m³), as well as polycyclic aromatic hydrocarbons (PAHs) with concentrations peaking at 48 ng/m³, significantly higher than urban background levels. Elemental and organic carbon (EC and OC) exhibited maximum concentrations of 9.3 μg/m³ and 43.2 μg/m³, respectively, during the peak wildfire period. Size distribution analysis demonstrated a distinct bimodal pattern with accumulation mode maxima (0.3-0.4 μm) and coarse mode peaks (4-6 μm), indicative of fresh biomass emissions and resuspended ash particles. The OP, assessed via the dithiothreitol (DTT) assay, correlated strongly (r = 0.94) with biomass burning markers, such as OC, EC, and K, underscoring the significant oxidative burden imposed by biomass smoke relative to background conditions. Our findings underscore the significant impacts of wildfire emissions on the air quality and their potential health implications of wildfire-derived PM₁₀ in a densely urbanized environment.