American Association for Aerosol Research - Abstract Submission

AAAR 38th Annual Conference
October 5 - October 9, 2020

Virtual Conference

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Ascorbate Oxidation by Iron, Copper and Reactive Oxygen Species: Review, Model Development, and Derivation of Key Rate Constants

JIAQI SHEN, Paul T. Griffiths, Steven J. Campbell, Battist Uttinger, Markus Kalberer, Suzanne E. Paulson, UCLA

     Abstract Number: 525
     Working Group: Health-Related Aerosols

Abstract
Ascorbic acid is among the most abundant antioxidants present in the lung, and the chemistry of ascorbic acid plays a key role in the mechanism by which air pollution particles elicit a biological response. Because ascorbic acid (AH2) is a highly redox active species, it engages in a far more complex web of chemical reactions than a typical soluble organic molecule, reacting not only with oxidants such as the hydroxyl radical, but also with redox-active transition metals such as iron and copper. The literature provides a solid outline of this chemistry, but there are several gaps and disagreements about reaction mechanisms, stoichiometries and reaction rates, particularly for the iron and copper reactions. Here we synthesize available data in the literature to develop a chemical kinetics model. We then use seven sets of laboratory measurements to constrain the mechanisms for iron and copper-mediated oxidation of ascorbic acid and derive key rate constants. The model built from existing literature is in poor agreement with the data without adjustment, although it does show that both ascorbic acid and ascorbate (AH-) are oxidized primarily by Fe(III) and Cu(II) rather than the reduced forms of these metals. Further, micromolar concentrations of the transition metal reactions are more important sinks for AH2 and AH- than the reactive oxygen species. We find that the evidence supports catalytic mechanism for the reactions, rather than a redox mechanism as suggested in some of the literature, with the stoichiometry for iron: Fe(III) + AH2 (or AH-) + O2 → Fe(III) + DHA + H2O2 (- 2H+ for AH-) and copper: Cu(II) + AH2 (or AH-) + O2 → Cu(II) + DHA + H2O2 (- 2H+ for AH-), and rate constants of 5.7×104 and 4.7×104 M-2s-1 for the Fe(III) + AH2/AH- reactions and 1.0×105 and 2.5×106 M-2s-1 for the Cu(II) + AH2/AH- reactions, respectively.