Abstract

The reaction of hydroxyl radicals (OH) with 4-nitrophenol (4-NP) in the aqueous solution was investigated at pH = 2 and 9. As a result, the molar yield of the phenolic products was measured to be 0.20 ± 0.05 at pH = 2 and 0.40 ± 0.1 at pH = 9. The yield of 4-nitrocatechol (4-NC) was higher at pH = 9; at the same time, a lower number of phenolic products was observed due to the hydrolysis and other irreversible reactions at pH > 7. Mineralization investigated with total organic carbon (TOC) technique showed that after 4-NP was completely consumed approx. 85 % of the organic carbon remained in the aqueous solution. Hence, up to 65 % of the organic carbon that remained in the aqueous solution accounted for the open-ring non-phenolic products. The light absorptivity of the reaction solution between 250 and 600 nm decreased as a result of OH reaction with 4-NP. At the same time, 4-NP solution showed some resistance to chemical bleaching due to the formation of the light-absorbing by-products. This phenomenon effectively prolongs the time-scale of chemical bleaching or 4-NP via reaction with OH by a factor of 3–1.5 at pH 2 and 9, respectively. The experimental data acquired indicated that both photolysis and reaction with OH can be important removal processes of the atmospheric brown-carbon from the aqueous particles containing 4-NP.

Highlights

  • Atmospheric brown carbon (BrC) is a subfraction of organic aerosols (OA) that is characterized by strong, wavelength-dependent absorption of the electromagnetic irradiation in the near ultraviolet (UV) and visible (VIS) regions (Laskin et al, 2015; Yan et al, 2018)

  • The first-order kbleaching coefficients derived via eq II (Table S3) show that the lifetimes of BrC chromophores are 3 and 1.5

  • The rates of bleaching of 4-NP solution due to reaction with OH and due to direct photolysis were compared (Fig. 4) using the kbleaching values derived in this work and the previously reported, average quantum yields (φ, molecules×photon-1) – see Table S3 and eq SIII and IV (Braman et al, 2020; Lemaire et al, 1985; Biswal et al, 2013; Garcıa Einschlag et al, 2003)

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Summary

Introduction

Atmospheric brown carbon (BrC) is a subfraction of organic aerosols (OA) that is characterized by strong, wavelength-dependent absorption of the electromagnetic irradiation in the near ultraviolet (UV) and visible (VIS) regions (Laskin et al, 2015; Yan et al, 2018). Nitrophenols are widespread nitroaromatic compounds that been identified among the major chromophores of atmospheric BrC (Harrison et al, 2005b; Laskin et al, 2015; Bluvshtein et al, 2017). 4-Nitrophenol (4-NP) is one of the most atmospherically abundant and environmentally widespread nitrophenols (Harrison et al, 2005b; Laskin et al, 2015) and is characterized by very high absorption cross-sections in the UV-Vis region (Jacobson, 1999). Chemical processing and decomposition (bleaching) of BrC can occur in air as well as in the atmospheric aqueous particles, which can involve direct photolysis and reactions with hydroxyl radicals (OH) (Hems et al, 2020; Laskin et al, 2015; Jiang et al, 2021; Li et al, 2020a; Forrister et al, 2015; Moise et al, 2015). The interplay between light absorbance and chemical composition of the aqueous solution of 4-NP that has been subjected to photolysis and oxidation by OH is poorly characterized (Zhao et al, 2015; Zhang et al, 2003)

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