OH-initiated oxidation reactions of nitroaromatics

Nitroaromatics are an incredibly toxic group of volatile compounds. They are both primary emissions from industrial sources [1] and biomass burning [2], as well as secondary emissions forming from the reactions between phenols and NO_x. Due to their toxicity and presence in the atmosphere, it is important to know their atmospheric fate. Nitrogen containing compounds have also been found to be abundant in organic aerosol [3], and thus nitroaromatics likely play a role in aerosol formation.  However, no mechanistic studies have been conducted on its gas-phase reactions under atmospheric conditions.

Most aromatics react in the atmosphere with the hydroxyl radical (OH) creating an alkyl radical that reacts with molecular oxygen producing a peroxy radical. This peroxy radical then undergoes subsequent unimolecular isomerization reactions and O₂ addition reactions in an autoxidation chain. Similar reactions have been shown to happen to a multitude of substituted aromatics, but not for nitro-substituted aromatics.

In this study, the reactions between the hydroxyl radical and three simple nitroaromatics (nitrobenzene, nitrophenol and nitrocatechol), as well as further possible reactions leading to termination and autoxidation are studied computationally. The three chosen compounds are among the simplest nitroaromatics and offer a range in both the toxicity of the reactant as well as atmospheric abundance. This study offers new insights into the atmospheric processes of nitroaromatics and elucidates their possible gaseous reaction mechanisms, which in turn gives insight on the effects of nitroaromatics in aerosol formation.

[1] Ahmed, M., Rappenglueck, B., Ganranoo, L., & Dasgupta, P. K. (2023). Source apportionment of gaseous Nitrophenols and their contribution to HONO formation in an urban area. Chemosphere, 338, 139499.

[2] Wang, H., Gao, Y., Wang, S., Wu, X., Liu, Y., Li, X., ... & Zhang, X. (2020). Atmospheric processing of nitrophenols and nitrocresols from biomass burning emissions. Journal of Geophysical Research: Atmospheres, 125(22), e2020JD033401.

[3] Wang, X., Hayeck, N., Brüggemann, M., Yao, L., Chen, H., Zhang, C., ... & Wang, L. (2017). Chemical characteristics of organic aerosols in Shanghai: A study by ultrahigh‐performance liquid chromatography coupled with Orbitrap mass spectrometry. Journal of Geophysical Research: Atmospheres, 122(21), 11-703.