Wildland–urban interface co-combustion of biomass, synthetic polymeric materials, and lithium-ion batteries generates a new class of ultrafine soot–metal–PAH hybrid particles

Wildland-urban interface (WUI) fires increasingly involve the co-combustion of biomass with synthetic polymers such as polystyrene (PS) and lithium-ion batteries (LIBs); yet the resulting particulate emissions, including ultrafine particles (≤0.1 μm), remain insufficiently quantified and mechanistically unresolved. Here, we present a size- and chemistry-resolved analysis of particulate matter (PM) covering ultrafine particles (≤0.1 μm), fine particles (0.1-2.5 μm) and coarse particles (2.5-10 μm), trace elements, and polycyclic aromatic hydrocarbons (PAHs) emitted under controlled, near-source flaming conditions (50 kW/m² radiant heat flux; 20.95% O₂) for four representative fuel combinations (Pine, Pine + PS, Pine + LIB, and Pine + PS + LIB). Pure pine combustion produced ultrafine-dominated emissions (~81% by number) with low PM mass (16 μg/m³), trace metals (0.41 μg/m³), and PAHs (13 ng/m³). In contrast, LIB and/or polymer involvement induced firm number-mass decoupling, shifting PM mass to the fine mode and increasing total PM up to 3.3-fold. Battery involvement led to a > 19-fold enrichment of particulate trace elements, dominated by nickel, lithium, phosphorus, cobalt, and aluminum, and to the formation of compact metal-soot hybrid particles during thermal runaway. PAHs increased concurrently, with preferential partitioning of carcinogenic high-molecular-weight species into ultrafine and fine particles. These results show that battery- and polymer-involved WUI fires generate a chemically distinct class of respirable particles enriched in toxic metals and PAHs that cannot be inferred from biomass combustion alone and are poorly captured by mass-based air-quality metrics, highlighting an emerging exposure risk for firefighters and nearby populations.