Oxidation of Phenolic Aldehydes by Ozone and Hydroxyl Radicals at the Air–Solid Interface

Abstract

Biomass burning emissions contain abundant phenolic aldehydes (e.g., syringaldehyde, vanillin, and 4-hydroxybenaldehyde) that are oxidized during atmospheric transport, altering the physicochemical properties of particulates. Herein, the oxidative processing of thin films made of syringaldehyde, vanillin, and 4hydroxybenaldehyde is studied at the air−solid interface under a variable O 3 (g) molar ratio (410 ppbv−800 ppmv) and relative humidity (0−90%). Experiments monitored the absorption changes of C�C, C�O, and �COOH vibration changes during the oxidation of thin films by transmission Fourier transform infrared spectroscopy (FTIR). Selected spectroscopic features of aromatic ring cleavage by O 3 (g) revealed the production of carboxylic acids. Instead, monitoring O�H stretching provided a comparison of a hydroxylation channel from in situ produced hydroxyl radical. The overall oxidation reactivity trend syringaldehyde > vanillin > 4hydroxybenzladehyde can be explained based on the additional electron density from methoxide substituents to the ring. The reactive uptake coefficient of O 3 (g) increases for higher relative humidity, e.g., for syringaldehyde by 18 and 215 times at 74% and 90% relative humidity (RH), respectively, as compared to dry conditions. A Langmuir−Hinshelwood mechanism fits well the kinetics of oxidation under a variable O 3 (g) molar ratio at 74% RH, providing useful information that should be included in atmospheric chemistry models.