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/m 2 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/m 3 ), trace metals (0.41 μg/m 3 ), and PAHs (13 ng/m 3 ). 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.