Ph Modifies the Oxidative Potential and Peroxide Content of Biomass Burning Hulis Under Dark Aging

Abstract

Humic-like substances (HULIS) account for a major redox-active fraction of biomass burning organic aerosols (BBOA). During atmospheric transport, fresh acidic BB-HULIS are subject to complex atmospheric aging processes. Aging through neutralization by mixing with alkaline dust particles or with alkaline gases can modify their properties in droplets and humid aerosols. In this study, the pH of solutions containing HULIS isolated from wood smoldering emissions was adjusted with NaOH and NH 3 to values in the range of 3-9. The solutions aged under oxic dark conditions. Evolution of the oxidative potential (OP) and the total peroxide content were measured together with changes in the optical properties and chemical composition. Notable immediate responses such as peroxide generation, HULIS autoxidation, and an increase in OP and light absorption were observed under basic conditions. Initial total peroxides, OP, and absorption increased exponentially with pH, regardless of the alkaline species added. Dark aging further increased the oxidation state of HULIS and led to pH-dependent toxic and chemical changes, exhibiting an alkaline-facilitated initial increase followed by a decrease of OP and peroxides. Although highly correlated with OP, the contribution of the total peroxide to OP is minor. Alkalinity-assisted autoxidation of phenolic compounds and quinoids with concomitant formation of H 2 O 2 and other alkalinity-favored peroxide oxidation reactions are proposed here for explaining the observed HULIS OP and chemical changes in the dark. Our findings suggest that alkaline neutralization of BB-HULIS represents a previously overlooked peroxide source and pathway for modifying aerosol toxicity and chemical composition. Additionally, these findings indicate that the lung fluid can modify the OP and peroxide content of inhaled BB-HULIS. The results also suggest that common separation protocols of HULIS using basic extraction methods should be treated with caution when evaluating and comparing their composition, absorption, and relative toxicity.

Keywords: biomass burning HULIS, pH influence, autoxidation, oxidative potential, peroxides, aqueous aging