Chemical complexity and enrichment in wildland-urban interface fire emissions: A case study of particulate matter, gas-phase pollutants, and ash from the 2025 Los Angeles fires.

The Los Angeles (LA) wildland-urban interface (WUI) fires of January 2025, are among the most destructive natural disasters in U.S. history, generating complex emission profiles that remain insufficiently characterized. An intensive field sampling campaign was conducted during the active burn period to collect and characterize the complex physicochemical properties of size-segregated particulate matter (WUIF-PM), vapor-phase compounds (WUIF-VOC), and ash (WUIF ash). While regulatory air quality indices remained within moderate categories during the sampling period, chemical analyses revealed substantial enrichment of hazardous species relative to urban background and biomass-only wildfire conditions. Oxygenated and highly toxic polycyclic aromatic hydrocarbons (OPAHs) were found in ultrafine WUIF-PM0.1, (size < 100 nm) and the fractional concentration (µg/g) of US EPA 16 priority polycyclic aromatic hydrocarbons (PAHs) in WUIF-PM2.5 exceeded recent levels for LA background, and biomass-only wildfires by an order of magnitude due to the excessive burn of human-made structures. Non-crustal metals were predominantly concentrated in WUIF-PM0.1 and consistently enriched relative to both LA background (up to ∼30 ×) and biomass-only wildfire aerosols (10-1000 ×). Benzene, toluene, ethylbenzene, and xylene (BTEX) concentrations in WUIF-VOC air samples were 4.8-13-fold higher than background levels. More alarmingly, WUIF ash samples contained PAHs, metals, and per- and polyfluoroalkyl substances (PFAS) consistent with mixed combustion of structural materials and vegetation raising concerns for post-fire environmental health risks. These findings indicate that PM₂.₅ mass and criteria air pollutants alone may underestimate the toxicological burden of WUI smoke, and expanded physicochemical monitoring and characterization is needed to advance exposure assessment and health risk evaluation during complex WUI fire events.