Online Measurements during Simulated Atmospheric Aging Track the Strongly Increasing Oxidative Potential of Complex Combustion Aerosols Relative to Their Primary Emissions

Abstract

Oxidative potential (OP) is increasingly recognized as a more health-relevant metric than particulate matter (PM) mass concentration because of its response to varying chemical compositions. Given the limited research on the OP of complex combustion aerosols, the effects of aging processes on their OP remain underexplored. We used online instruments to track the evolution of OP [via dithiothreitol (DTT) assays] during the aging of wood burning and coal combustion emissions by hydroxyl-radical-driven photooxidation and dark ozonolysis. We observed very substantial increases in the intrinsic OP (OP_m^DTT) of complex combustion aerosols (e.g., OP_m^DTT up to 100 pmol min^–1 μg^–1 for OH-aged wood burning emissions) within 1 day of equivalent aging. Further analysis in relation to the degree of oxidation revealed a potential for generalizing the OP of carbonaceous aerosols with average carbon oxidation state ${\overline{OS}}_c$ values ranging from −1.5 to −0.5 by assuming they have a constant OP_m^DTT value of ∼10 ± 6 pmol min^–1 μg^–1. Additionally, we uncovered a strong dependency of OP_m^DTT on both the source/precursor and aging pathway with ${\overline{OS}}_c$ above ∼−0.5. OH photooxidation was identified as an exceptionally efficient pathway for generating highly oxidized, multifunctionalized, and DTT-active products, particularly from wood burning emissions.