Atmospheric aging modifies the redox potential and toxicity of humic-like substances (HULIS) from biomass burning

Abstract

This study investigated the redox potential and toxicological changes of wood smoldering emitted HULIS due to reactions in the atmosphere and in neutral lung fluids. Fresh HULIS aerosols exhibited substantial oxidative potential (OP) and antioxidant capacity (AOC). Nighttime oxidation via heterogeneous O₃ or NO₃˙ reactions impacted HULIS OP and AOC differently, with high humidity enhancing O₃ uptake and HULIS oxidation, causing a significant reduction in their redox potentials. The effective rate constants for HULIS redox-active components (RACs) by O₃ reaction increased with RH (<1.5% to 75%), with values of (3.2–12.7)×10⁻¹⁸ and (1.8–9.7)×10⁻¹⁸ cm³ mol⁻¹ s⁻¹ for antioxidant moieties and oxidant components, respectively. The corresponding rate constants for NO₃˙ reactions were 1.2 × 10⁻¹⁴ and 3.9 × 10⁻¹⁵ cm³ mol⁻¹ s⁻¹. The endpoint cytotoxicity of HULIS, studied with lung epithelial cells, correlated well with the changes in OP, suggesting OP as a good indicator of HULIS oxidative toxicity. Redox reactions of fresh and atmospherically aged HULIS in neutral lung fluid mimics decreased their OP and AOC accompanied by H₂O₂ generation. The H₂O₂ yields were determined by both OP and AOC of HULIS, and the addition of lung fluid antioxidants amplified the H₂O₂ yield. Atmospheric aging retarded the decay of the redox potential and abated H₂O₂ yield, demonstrating that the remaining RACs in aged HULIS have relatively longer lifetimes against intrinsic reactions in the lung fluid environment. This study underscores the need to consider atmospheric transformations and lung fluid aqueous reactions when evaluating the health impact of HULIS and related organic aerosols.