Large quantities of reactive methoxyphenols are emitted to the atmosphere during biomass burning. Understanding the processing of methoxyphenols at atmospheric interfaces is a matter of major interest. This presentation will display information gathered by multiple spectroscopies and separation techniques utilized to study the oxidation of model thin films prepared with the methoxyphenols vanillin and syringaldehyde. The reactivity of both methoxyphenols at the air-solid interface is compared to the parent phenolic aldehyde 4-hydroxybenzaldehyde. A broad range of ozone O3(g) molar ratios (from 200 ppbv to 800 ppmv) and variable relative humidity (0-90%) was explored. Interestingly, competitive oxidation pathways of 1) direct ozonolysis and 2) indirect oxidation operate simultaneously. Direct ozonolysis produces the cleavage of aromatic bonds monitored in the infrared at ~1608 cm-1, generating new stretchings for unsaturated hydrocarbon (1600-1680 cm-1), carboxylic acid (1700-1725 cm-1) and ester (1730-1750 cm-1) bonds. Indirect oxidation is mediated by in situ generated hydroxyl radicals (HO●) as demonstrated by the early production of polyhydroxymethoxyphenols that can also undergo ring fragmentation. A reactivity trend for the generation of secondary organic aerosol precursors can be correlated with the molecular structure and their functional groups. The work will provide mechanistic information of atmospheric interest.

Atmospheric processing of biomass burning methoxyphenols

ACS Spring 2022

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