Research report: Molecular representation of benzene and phenol secondary organic aerosols

A modeling study of benzene and phenol oxidation found that adding recently characterized chemical pathways substantially improved simulations of secondary organic aerosol, or SOA, compared with the standard Master Chemical Mechanism. The researchers expanded version 3.3.1 of the mechanism with successive hydroxyl additions, radical autoxidation, cyclic epoxide formation, and molecular rearrangements of bicyclic peroxy radicals. Quantum-chemical calculations were used to characterize pathways involving geminal-diol intermediates that were missing from commonly used aromatic oxidation schemes.

In box-model tests against 16 atmospheric chamber experiments, the original mechanism systematically underestimated aerosol concentrations, whereas the expanded schemes were generally within a factor of two and usually within 30% of observations. The study focused on benzene, the simplest aromatic compound, and phenol, the main product of benzene oxidation by the hydroxyl radical. Benzene is emitted by traffic, cigarette smoke, and industrial solvent use, including applications in textiles, pesticides, and pharmaceuticals.

Phenol is also emitted directly by biomass burning, traffic exhaust, and manure spreading in agriculture. Oxidation can transform these volatile organic compounds into more functionalized, lower-volatility products that enter the particle phase and form SOA. Existing three-dimensional air-quality models commonly parameterize this process with yields derived from atmospheric simulation chambers or with a limited set of representative surrogate compounds.

Although computationally efficient, those approaches do not explicitly resolve the molecular composition of the particle phase or the full network of reactions responsible for producing it. The team began with the quasi-explicit Master Chemical Mechanism, or MCM, which represented benzene degradation and first-generation oxidation products but omitted several pathways supported by recent experiments and theoretical calculations. The expanded mechanism permits repeated hydroxyl addition to phenolic rings, producing trihydroxybenzene, tetrahydroxybenzene, and pentahydroxybenzene in addition to the previously represented phenol and catechol.

It also incorporates autoxidation sequences for peroxy and alkoxy radicals, in which successive intramolecular reactions are followed by oxygen addition. Termination of these radical sequences can produce hydroperoxides, organonitrates, carbonyl compounds, and very low-volatility peroxide dimers formed by accretion between peroxy radicals. A separate pathway represents cyclic epoxides produced when hydroxyl oxidation of benzene-derived bicyclic peroxides and nitrates competes with fragmentation and other oxidation routes.

Several baseline assumptions were revised in light of more recent evidence. First-generation production of an open-chain epoxide was removed because experimental work indicates a very low yield, while the branching ratio leading to formation of a benzene-derived bicyclic peroxy radical was increased from 35.2% to 47%. The corresponding bicyclic alkoxy radical was also allowed to compete between decomposition and a hydrogen-shift pathway.

The latter produces a ketene–enol intermediate that rapidly oxidizes to formyl ketene, glyoxal, and formic acid, and an additional route accounts for acrolein formation. Because most end products introduced through the new pathways have low saturation vapor pressures, they partition almost entirely into the particulate phase. Loss reactions were retained instead for compounds with sufficiently high volatility to occur in both gas and particulate phases, using estimated reaction rates for photolysis and reactions with hydroxyl, nitric oxide, and nitrate radicals.

Particular attention was given to bicyclic peroxy radicals formed when hydroxyl addition occurs at a carbon already carrying a hydroxyl group, creating a geminal-diol structure. Such intermediates can rearrange rapidly and yield highly oxygenated organic molecules, including C6 carboxylic acids and, in the case of catechol, diacids. Because this minor reaction route can make a disproportionately large contribution to SOA mass, the relevant branching ratios and rate coefficients are important.

The researchers applied multiconformer transition-state theory, which incorporates multiple low-energy conformers of reactants and transition states, together with an Eckart treatment of quantum tunnelling. Calculations were performed at several density-functional and correlated levels of theory. A correction previously found to generate artefacts for hydrogen shifts from hydroxyl groups was deliberately not applied to that reaction class.

The calculations showed that endocyclization, which creates the five-membered oxygen bridge needed to form a geminal-diol bicyclic peroxy radical, competes with hydrogen shifts and, for catechol, with hydroperoxyl loss. The resulting endocyclization branching ratios were 10% for phenol and 59% for catechol. Because catechol can add oxygen at two different sites and quantum chemistry could not reliably resolve the relative probabilities, the researchers tested oxygen-addition branching ratios of 10% and 50% for the route leading toward endocyclization.

They also evaluated two possible ways of breaking a carbon–carbon bond during molecular rearrangement. Rearrangement of the phenol-derived radical could generate products containing eight oxygen atoms, while one catechol-derived route could proceed to products with as many as 11 oxygen atoms. Alternative rearrangements arising from hydroxyl addition at an ortho position were calculated to be too slow to compete under atmospheric conditions and were not included in the model.

Evaluation used version 2.0 of the SSH-aerosol multiphase box model, which represents gas-phase chemistry, aerosol dynamics, and partitioning among gas, organic, and aqueous phases. The simulation set covered eight phenol and eight benzene chamber experiments conducted over eight hours under natural daylight, with radical production driven by photolysis. The experiments varied nitrogen oxide levels and included conditions with no seed aerosol, acidic inorganic seeds, or neutral seeds.

Initial precursor concentrations ranged from 79 to 264 parts per billion for phenol and from 168 to 325 parts per billion for benzene, substantially above ambient concentrations so that oxidation and SOA formation could be measured over the chamber timescale. The original MCM produced SOA concentrations far below observations. Adding the newly implemented pathways greatly increased simulated particle mass, with autoxidation particularly influential at low nitrogen oxide levels and successive hydroxylation becoming more prominent under high-nitrogen-oxide conditions.

The addition of geminal-diol chemistry generally improved agreement with the chamber measurements, although it sometimes reduced mass that the preceding mechanism had overestimated. Simulations for phenol with neutral seeds remained as much as 1.9 times below observations. The researchers identified uncertainty in the gas–particle partitioning of intermediate-volatility carbonyls and quinones as one possible contributor, along with aqueous-phase hydration of light aldehydes when seed concentrations were high.

Sensitivity tests also showed that assumptions about the catechol cyclization branch can substantially alter the results: reducing the favored branching ratio from 50% to 10% lowered simulated SOA mass in most experiments and increased disagreement with observations. Across the experiments, the expanded mechanism had a mean relative error of 28.2%, compared with 83.2% for the original MCM. To make the detailed mechanism usable in three-dimensional air-quality models, the researchers reduced it with GENOA, an automated framework that removes, jumps, or lumps chemical species while retaining pathways important for SOA formation.

Across 9,433 European atmospheric conditions, the final scheme contains 30 reactions and 28 species, including 12 condensable compounds, compared with 1,772 reactions and 1,160 species in the detailed mechanism. It preserves successive hydroxylation, peroxy- and alkoxy-radical autoxidation, and geminal-diol bicyclic-peroxy-radical rearrangements. Its mean SOA-mass error relative to the expanded reference was 3.94%, with a maximum error of 22.5%; errors remained below 5% across most locations.

The study notes that intraparticle chemistry remains an important source of uncertainty, particularly for phenolic compounds that may undergo photochemical reactions within biomass-burning aerosol.

Subject of Research: Modeling benzene and phenol oxidation pathways that form secondary organic aerosol

Article Title: Molecular representation of benzene and phenol secondary organic aerosols

Article References: Le Bayon, A., Ojala, A., Iyer, S., Wang, Z., Lannuque, V., Couvidat, F., Ciuraru, R., & Sartelet, K. (2026). Molecular representation of benzene and phenol secondary organic aerosols. Atmospheric Chemistry and Physics, 26(20), 14749-14768. https://doi.org/10.5194/acp-26-14749-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14749-2026

Keywords: benzene oxidation, phenol oxidation, secondary organic aerosol, atmospheric chemistry, quantum chemistry, autoxidation, peroxy radicals, air-quality modeling, aerosol composition, GEMINAL diols, Molecular, representation