Wildfires and postharvest crop residue burning release substantial amounts of black carbon (BC) into the atmosphere annually, posing significant environmental and public health challenges. Environmentally persistent free radicals (EPFRs) are abundant on BC surfaces, yet their reactivity and structural characteristics remain poorly understood. This study demonstrates that EPFRs on BC surfaces drive the rapid decay of coexisting phenolic acids, leading to the formation of more toxic products. Novel integrated approaches, including temperature-programmed desorption mass spectrometry and electrochemical capacitance measurements, indicate that these EPFRs are predominantly composed of oxygen-containing functional groups with superior electron storage and transfer capabilities. Statistically significant linear correlations are observed between phenolic acid decay rates and EPFR concentrations, oxygen-containing group abundances, and electron transfer capabilities. Furthermore, the presence of nitrate ions is found to accelerate phenolic acid degradation. Human body clearance and human-related organ toxicity predictions suggest that transformation products are less readily cleared by the human body than the parent compounds and are more likely to cause liver toxicity. These findings highlight the critical role of EPFRs in BC chemistry, emphasizing their potentially underestimated environmental and health impacts.
Keywords: biomass burning; biomass burning tracers; black carbon; electrochemical capacitance; environmentally persistent free radicals.