Mechanistic Insights into Heterogeneous Oxidation: Product-Resolved Analysis of Phenolic Aldehyde Films

Abstract

Phenolic aldehydes emitted during biomass burning undergo multiphase oxidative aging that can transform the chemical composition and optical properties of organic aerosols. Previous work established that ozone exposure of syringaldehyde, vanillin, and 4-hydroxybenzaldehyde solid films at elevated relative humidity produces competing fragmentation and functionalization pathways, with additional evidence for coupled products. Here, we extend that mechanistic foundation by using scaled-up solid-film oxidation experiments to obtain sufficient product mass for structural analysis by 1H NMR, 13C NMR, HSQC NMR, UHPLC-UV-MS, and IC–conductivity–MS. The combined workflow enables cross-validation of key aromatic acid products, including syringic acid, vanillic acid, and 4-hydroxybenzoic acid, and supports assignment of ring cleavage products and higher-molecular-weight species that are difficult to resolve using a single analytical method. Standard addition and quantitative NMR approaches provide mutually consistent product concentrations for major products, strengthening confidence in the product distribution and mechanistic interpretation. The results show that ozone aging of phenolic aldehyde films establishes a unified reaction framework linking the molecular structure to competing oxidation pathways leading to aromatic acids, C6 ring-opening products, truncated C5 and smaller multifunctional carboxylic acids, and higher-molecular-weight products consistent with oligomer formation. These results reveal how molecular substitution systematically controls pathway branching among fragmentation, functionalization, and coupling during heterogeneous oxidation. The findings provide new molecular-level understanding of how biomass-burning phenolic aldehydes evolve at particle surfaces and provide product-level evidence for pathways that contribute to secondary organic aerosol composition and brown carbon chromophores.