Heterogeneous Nitrate Radical Oxidation of Catechol Produces Nitrocatechols, Oligomers, and Brown Carbon.

Catechol is an important biomass-burning phenolic compound and a precursor of secondary organic aerosol. This work reports the molecular products generated during heterogeneous oxidation of catechol thin films by nitrate radicals under dry conditions. Fourier transform infrared spectroscopy (FTIR), UV-visible spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and ultrahigh-pressure liquid chromatography coupled with mass spectrometry (UHPLC-MS) were used to characterize condensed-phase products and light-absorbing chromophores. FTIR spectra showed the appearance of nitroaromatic absorptions near 1590 and 1350 cm-1. UV-visible spectra revealed a broad absorption feature centered near 349 nm characteristic of brown-carbon chromophores. ESI-MS identified 4-nitrocatechol, dinitrocatechol species, catechol dimers, and catechol trimers. UHPLC-MS confirmed the presence of nitroaromatic and oligomeric products through chromatographic separation and mass-spectral assignments. Results support a mechanism involving semiquinone radical intermediates followed by nitration and radical coupling. These findings establish heterogeneous NO3 oxidation of catechol as an important nighttime pathway linking biomass-burning emissions to the formation of light-absorbing secondary organic aerosol.