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

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 O-3 or NO3 reactions impacted HULIS OP and AOC differently, with high humidity enhancing O-3 uptake and HULIS oxidation, causing a significant reduction in their redox potentials. The effective rate constants for HULIS redox-active components (RACs) by O-3 reaction increased with RH (<1.5% to 75%), with values of (3.2-12.7)x10(-18) and (1.8-9.7)x10(-18) cm(3) mol(-1) s(-1) for antioxidant moieties and oxidant components, respectively. The corresponding rate constants for NO3 reactions were 1.2 x 10(-14) and 3.9 x 10(-15) cm(3) 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.