Environmentally Persistent Free Radicals From Indoor Biomass Burning Under Systematically Varied Oxygen Levels: Implications for Altitude‐Associated Emissions

Abstract Environmentally persistent free radicals (EPFRs) can generate reactive oxygen species, leading to adverse health effects, with residential biomass burning recognized as a primary source. However, a significant knowledge gap remains regarding the effects of oxygen levels on EPFR emissions, a critical factor in emission dynamics and highly relevant for populations at varying altitudes. This study explored emission factors (EFs), formation mechanisms, and properties of EPFRs from biomass burning under varying oxygen proportions (18.2%–26.6%), simulating altitudes from sea level to 2,800 m. Significantly higher EPFR emissions from biomass burning were observed under lower oxygen proportion due to reduced combustion efficiency ( r = −0.789, p < 0.001). EPFR emissions from wood, straw, and corn cob combustion at 18.2% oxygen proportion were up to 29, 9.0, and 1.8 times higher than those observed under higher oxygen proportions (20.4%–22.3%), respectively. The lower sensitivity of corn cob combustion to oxygen variations suggested its suitability for high‐altitude regions. At 22.3%–26.6% oxygen proportions, EF EPFRs showed no significant variations, likely due to higher combustion temperatures promoting electron transfer, while radical diversity continued to increase. Notably, combustion under higher oxygen proportions might form more reactive oxygen‐centered EPFRs, raising potential toxicity risks and highlighting the trade‐off between emission reductions and changes in physicochemical properties. EPFRs formation, mediated by polycyclic aromatic hydrocarbons (PAHs) and trace element, was governed primarily by their intrinsic physicochemical properties rather than concentrations, with medium molecular weight PAHs identified as key precursors. This study provides critical oxygen‐specific insights into EPFR formation and a valuable reference for mitigating associated health risks.