Oxidation of Substituted Catechols at the Air–Water Interface: Production of Carboxylic Acids, Quinones, and Polyphenols

Anthropogenic activities contribute benzene, toluene, and anisole to the environment, which in the atmosphere are converted into the respective phenols, cresols, and methoxyphenols by fast gas-phase reaction with hydroxyl radicals (HO^•). Further processing of the latter species by HO^• decreases their vapor pressure as a second hydroxyl group is incorporated to accelerate their oxidative aging at interfaces and in aqueous particles. This work shows how catechol, pyrogallol, 3-methylcatechol, 4-methylcatechol, and 3-methoxycatechol (all proxies for oxygenated aromatics derived from benzene, toluene, and anisole) react at the air–water interface with increasing O₃(g) during τ_c ≈ 1 μs contact time and contrasts their potential for electron transfer and in situ production of HO^• using structure–activity relationships. A unifying mechanism is provided to explain the oxidation of the five proxies, which includes the generation of semiquinone radicals. Functionalization in the presence of HO^• results in the formation of polyphenols and hydroxylated quinones. Instead, fragmentation produces polyfunctional low molecular weight carboxylic acids after oxidative cleavage of the aromatic bond with two vicinal hydroxy groups to yield substituted cis,cis-muconic acids. The generation of maleinaldehydic, maleic, pyruvic, glyoxylic, and oxalic acids confirms the potential of oxy aromatics to produce light-absorbing aqueous secondary organic aerosols in the troposphere.