Biomass burning releases a significant amount of particles, which impact human health and climate. But biomass burning also releases large amounts of gases, such as phenols, that can form additional particulate mass, either through gaseous or aqueous reactions. For example, phenols have moderate to high Herry’s Law constants, so they partition significantly into aqueous phases including cloud, fog drops and aerosol water. They then undergo rapid reactions with oxidants to form low-volatility products, i.e., aqueous secondary organic aerosol (aqSOA), which might absorb light and possibly affect climate. While hydroxyl radical is typically thought to be the major oxidant in atmospheric drops and particles, recent work has shown that triplet excited states can be dominant. Our goals in this work are to determine if a triplet excited state is an important oxidant for aqueous phenols and whether the aqSOA formed absorbs significant sunlight. To address these goals we measured rate constants for the aqueous reactions of the triplet state of 3,4-dimethoxybenzaldehyde (3DMB*) with six phenols: tyrosol, guaiacyl acetone, ferulic acid, vanillyl alcohol, syringyl acetone, and syringic acid. We find that the second-order rate constants for these reactions are all fast, with values in the range of (0.26 to 4.1) ×109 M-1s-1, and that rate constants increase with increasing methoxy substitution. For a given phenol, rate constants are 1.6 to 5.5 times higher at pH 2 than at pH 5. We developed a quantitative structure-activity relationship between redox potentials of the phenols and their rate constants, which enables us to predict rate constants for a wide range of phenols. We also determined the mass absorption coefficients (MACs), i.e., the amount of light absorbed, for phenols and their SOA products. Compared with the precursor phenols, SOA exhibits enhanced light absorption at longer wavelengths (&gt; 300 nm), indicating that these phenol reactions form brown carbon. Based on our measured rate constants, previously measured oxidant concentrations, and estimated aqSOA yields, our work shows that organic triplet excited states can be important aqueous oxidants for phenols and that this can be a significant source of SOA.

Kinetics for the aqueous oxidation of atmospheric phenols with an organic triplet excited state

2019 Fall National Meeting

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