Photosensitizer Inhibition of HONO Yield from Nitrophenols: A Study of Isomeric Effects and Ionic Strength Dependence

Gas-phase nitrous acid (HONO) is a key atmospheric component that drives daytime hydroxyl radical production, significantly impacting atmospheric oxidation. A significant daytime source of HONO is the photolysis of nitrophenols, compounds released during biomass burning. In reality, biomass burning also releases photosensitizers like methoxyphenols, which may interact with nitrophenols under light to either enhance or suppress HONO formation. Additionally, variations in atmospheric ionic strength could influence these co-photolytic reactions. However, the detailed contributions of these interactions remain poorly understood. In this study, we investigated HONO formation from two nitrophenol isomers, 2-nitrophenol (2NP) and 4-nitrophenol (4NP), in the presence of the photosensitizer vanillic acid (VA) under aqueous aerosol-mimicking conditions (pH 2). We also examined the effect of ionic strength on these co-photolysis reactions.

Our results reveal two key findings: a strong isomeric effect and a complex role for the photosensitizer VA. First, 4NP released substantially more HONO than 2NP, demonstrating that molecular structure critically influences yield. Second, VA suppressed HONO formation from both isomers. Over 5 hours, maximum HONO levels from pure 2NP and 4NP were 1.5 ppb and 55 ppb, respectively. With VA present, the HONO yields from 2NP and 4NP decreased to 0.7 ppb and 40 ppb, respectively. The influence of ionic strength further distinguished these pathways. Increasing ionic strength (0 to 5 M NaClO₄) significantly enhanced HONO production from both pure isomers, raising 2NP yield from 1.5 to 13 ppb and 4NP yield from 35 to 55 ppb. However, VA dramatically altered this response: it not only lowered the overall HONO yield but also reversed the enhancing effect of ionic strength for 2NP (yield decreased from 0.7 to 0.25 ppb) and greatly attenuated it for 4NP (yield increased only from 35 to 40 ppb). This indicates that VA fundamentally modifies the reaction environment, likely through competing ion-dipole and solvation effects. Density functional theory calculations indicate that VA-NP dimers in aqueous solution stabilize the nitro group via water molecules and VA hydroxyl groups, making HONO dissociation harder. For example, the triplet-state binding energy of the VA-2NP dimer (-53.8 kcal/mol) is significantly more stable than that of the 2NP-2NP dimer (-37.0 kcal/mol). Similarly, the VA-4NP dimer shows stronger stabilization of the aci-nitrophenol intermediate due to hydrogen bonding with VA's functional groups, hindering H₃O⁺-assisted HONO release. By revealing the distinct roles of photosensitizers and ionic strength, this work provides a clearer mechanistic framework for HONO production from nitrophenol photolysis and its atmospheric significance.