Relative Humidity Modulates Nitration and Ring Fragmentation during Heterogeneous Oxidation of Catechol by Nitrate Radicals

Abstract

Catechol is an important biomass-burning-derived phenolic compound and a precursor to secondary organic aerosols (SOAs). Although nitrate radicals (NO3) are recognized as major nighttime oxidants, the influence of the aerosol water content on heterogeneous NO3 oxidation of phenolic compounds remains poorly understood. Here, we investigate the impact of relative humidity (RH = 0–90%) on the heterogeneous oxidation of catechol thin films by nitrate radicals generated from the reaction of O3 and NO2. Product formation was characterized using Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, electrospray ionization mass spectrometry (ESI-MS), ultrahigh-pressure liquid chromatography coupled with mass spectrometry (UHPLC–MS), and ion chromatography–mass spectrometry (IC–MS). Under dry conditions, oxidation favored nitration and oligomerization pathways, producing mono-, di-, and trinitrocatechols together with catechol dimers and trimers. These products exhibited enhanced absorption in the 300–500 nm region, characteristic of brown carbon chromophores. Increasing RH progressively suppressed the relative nitration capacity of the NO3/N2O5 system while promoting oxidative ring-opening reactions, leading to the formation of dicarboxylic acids, including cis,cis-muconic acid, 2,3-dihydroxymaleic acid, and oxalic acid. Product distributions revealed a transition from functionalization-dominated chemistry at low RH to fragmentation-dominated chemistry at high RH. These results indicate that RH strongly controls nighttime nitrate radical oxidation of biomass-burning phenolics by shifting product formation from light-absorbing nitroaromatic brown carbon toward highly oxygenated fragmentation products. The findings provide new mechanistic insights into the role of aerosol water in governing the chemical aging and optical evolution of biomass-burning aerosols.