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

Abstract

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.

Aqueous-Phase Photooxidation of Vanillin in the Presence of Nitrite: Characteristics, Products, and Mechanism

“…The aqSOA formed in the initial stages of photoreaction exhibits significant light absorption in the near-UV and visible regions, indicating the formation of BrC chromophores, which can influence the optical properties and radiative balance of atmospheric aerosols. 28,36 Moreover, the toxicity of the generated products, such as nitrophenols, is often higher than that of their precursors, underscoring the importance of our findings for understanding the health impacts of atmospheric particulates. 44 Additionally, solution pH and the VL/NO 2 − molar ratio significantly influences the decay rate of VL, as well as the composition and intensity of the products.…”

“… − Therefore, NO 2 – not only serves as a significant source of ·OH but also contributes to the pool of RNS in the atmosphere. The aqueous-phase photooxidation reactions between phenols and NO 2 – are considered pivotal pathways for the formation of aqSOA and BrC. − For instance, Pang et al has reported that NO 2 – -initiated VL photooxidation reactions can generate nitrophenols, with the absorbance of the products in the visible light region increasing with increasing pH . Additionally, NO 2 – , despite being less abundant than nitrate (NO 3 – ) in the atmosphere, is characterized by a higher molar absorptivity coefficient. , Despite these significant properties, the role of NO 2 – in atmospheric chemistry is frequently underappreciated, and there exists a notable dearth of research concerning the aqueous-phase photooxidation between phenols and NO 2 – .…”