Mechanisms for the Enhancement of Nitrate Photodegradation by Brown Carbon

Abstract

Reactive nitrogen species (NO_y)─such as nitrogen dioxide (NO₂), nitric oxide (NO), and nitrous acid (HONO)─impact both the chemistry and oxidative capacity of the troposphere. Nitrate photodegradation, which releases NO_y, is enhanced by some light-absorbing organic molecules, i.e., brown carbon (BrC), but the mechanisms for enhancement remain poorly constrained. Here, we investigate how the photodegradation of aqueous nitrate is affected by five model BrC photosensitizers using illumination experiments and quantum chemical calculations. Nitrate photodegradation is not enhanced by oxidizing triplet excited states (3,4-dimethoxybenzaldehyde (DMB) and benzophenone (BP)) or by an efficient source of singlet oxygen (perinaphthenone (PN)), but is enhanced by N,N-dimethylaniline (NN-DMA) and vanillic acid (VA). Based on a kinetic model, electron-transfer reactions dominate the enhancement from NN-DMA, while solvated electrons make a smaller contribution. In contrast to NN-DMA, vanillic acid acts through a different, as-yet-unidentified mechanism that is second-order in the fully deprotonated form of VA. Based on ab initio calculations, energy-transfer reactions are not significant in our experiments because the energy of the nitrate triplet excited state is higher than the triplet energies of any of the five model chromophores. Overall, our results indicate there are at least three mechanisms by which brown carbon can enhance nitrate photodegradation.