Environmentally persistent free radicals and other paramagnetic species in wildland-urban interface fire ashes

Wildland-urban interface (WUI) fires consume fuels, such as vegetation and structural materials, leaving behind ash composed primarily of pyrogenic carbon and metal oxides. However, there is currently limited understanding of the role of WUI fire ash from different sources as a source of paramagnetic species such as environmentally persistent free radicals (EPFRs) and transition metals in the environment. Electron paramagnetic resonance (EPR) was used to detect and quantify paramagnetic species, including organic persistent free radicals and transition metal spins, in fifty-three fire ash and soil samples collected following the North Complex Fire and the Sonoma-Lake-Napa Unit (LNU) Lightning Complex Fire, California, 2020. High concentrations of organic EPFRs (e.g., 1.4 × 10¹⁴ to 1.9 × 10¹⁷ spins g⁻¹) were detected in the studied WUI fire ash along with other paramagnetic species such as iron and manganese oxides, as well as Fe³⁺ and Mn²⁺ ions. The mean concentrations of EPFRs in various ash types decreased following the order: vegetation ash (1.1 × 10¹⁷ ± 1.1 × 10¹⁷ spins g⁻¹) > structural ash (1.6 × 10¹⁶ ± 3.7 × 10¹⁶ spins g⁻¹) > vehicle ash (6.4 × 10¹⁵ ± 8.6 × 10¹⁵ spins g⁻¹) > soil (3.2 × 10¹⁵ ± 3.7 × 10¹⁵ spins g⁻¹). The mean concentrations of EPFRs decreased with increased combustion completeness indicated by ash color; black (1.1 × 10¹⁷ ± 1.1 × 10¹⁷ spins g⁻¹) > white (2.5 × 10¹⁶ ± 4.4 × 10¹⁶ spins g⁻¹) > gray (1.8 × 10¹⁶ ± 2.4 × 10¹⁶ spins g⁻¹). In contrast, the relative amounts of reduced Mn²⁺ ions increased with increased combustion completeness. Thus, WUI fire ash is an important global source of EPFRs and reduced metal species (e.g., Mn²⁺). Further research is needed to underpin the formation, transformation, and environmental and human health impacts of these paramagnetic species in light of the projected increased frequency, size, and severity of WUI fires.