Identification and Quantification of 4‑Nitrocatechol Formed from OH and NO<sub>3</sub> Radical-Initiated Reactions of Catechol in Air in the Presence of NO<sub><i>x</i></sub>: Implications for Secondary Organic Aerosol Formation from Biomass Burning

Catechol (1,2-benzenediol) is emitted from biomass burning and produced from a reaction of phenol with OH radicals. It has been suggested as an important secondary organic aerosol (SOA) precursor, but the mechanisms of gas-phase oxidation and SOA formation have not been investigated in detail. In this study, catechol was reacted with OH and NO<sub>3</sub> radicals in the presence of NO<sub><i>x</i></sub> in an environmental chamber to simulate daytime and nighttime chemistry. These reactions produced SOA with exceptionally high mass yields of 1.34 ± 0.20 and 1.50 ± 0.20, respectively, reflecting the low volatility and high density of reaction products. The dominant SOA product, 4-nitrocatechol, for which an authentic standard is available, was identified through thermal desorption particle beam mass spectrometry and Fourier transform infrared spectroscopy and was quantified in filter samples by liquid chromatography using UV detection. Molar yields of 4-nitrocatechol were 0.30 ± 0.03 and 0.91 ± 0.06 for reactions with OH and NO<sub>3</sub> radicals, and thermal desorption measurements of volatility indicate that it is semivolatile at typical atmospheric aerosol loadings, consistent with field studies that have observed it in aerosol particles. Formation of 4-nitrocatechol is initiated by abstraction of a phenolic H atom by an OH or NO<sub>3</sub> radical to form a β-hydroxyphenoxy/<i>o</i>-semiquinone radical, which then reacts with NO<sub>2</sub> to form the final product.