Nitration of Phenols by Reaction with Aqueous Nitrite: A Pathway for the Formation of Atmospheric Brown Carbon

Nitrophenols are a major component of light-absorbing atmospheric organic aerosols, commonly referred to as brown carbon (BrC). Most nitrophenol formation pathways involve reactions of phenolic compounds with OH, NO3, and NO2 in the gas phase. In this study, an aqueous nitrophenol production pathway is investigated that can proceed in the dark without apparent OH radical formation. Using high-performance liquid chromatography–mass spectrometry, we demonstrate that catechol reacts in acidic solutions with dissolved nitrite to form nitrocatechol. The rate of nitration increases significantly from pH 4.4 to pH 3.4 such that nitrocatechol is susceptible to second-generation reactions under the most acidic conditions producing chromophores that absorb in the visible region (peak at 425 nm). Increases in the N:C ratio of the reaction solution, as detected by aerosol mass spectrometry, and enhanced absorption from 300 to 500 nm of wood and peat smoke aqueous extracts exposed to nitrite suggest that this is a general nitration pathway. The atmospheric conditions under which this BrC formation process may occur are discussed.