Phenolic compounds are emitted in large amounts from biomass burning and can undergo aqueous-phase reactions in the atmosphere to form secondary organic aerosols (aqSOA). In this study, the kinetics and products of the reaction of vanillic acid (VA) with hydroxyl radicals (OH) were characterized, and their formation mechanisms were determined using ultrahigh-performance liquid chromatography ultraviolet/mass spectrometry (HPLC-UV/MS) and UV−vis absorption and fluorescence spectroscopies. The obtained rate constants of the VA + OH reaction are (9.8 ± 1.5) × 10 9 and (3.8 ± 0.7) × 10 9 M −1 s −1 at pH 2 and 10, respectively. A yellowish solution is obtained after illumination, and it absorbs in the UV−vis region and has an unusual fluorescence spectrum, which suggests formation of a humic-like substance (HULIS) by the C−C and C−O coupling of phenoxyl radicals. Additionally, hydroxylation of the aromatic ring occurs through OH addition, which increases the degree of oxidation of the products. This study indicates that the aqueous-phase OH oxidation of phenolic acid compounds may contribute to formation of HULIS in the atmosphere, especially in areas with active burning of biomass. The high-molecular-weight products would remain in the particle phase after fog/cloud evaporation and influence the chemical and optical properties of atmospheric particles.

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Despite the well-established role of small molecular clusters in the very first steps of atmospheric particle formation, their thermochemical data are still not completely available due to limitation of the experimental techniques to treat such small clusters. We have investigated the structures and the thermochemistry of stepwise hydration of clusters containing one bisulfate ion, sulfuric acid, base (ammonia or dimethylamine), and water molecules using quantum chemical methods. We found that water facilitates proton transfer from sulfuric acid or the bisulfate ion to the base or water molecules, and depending on the hydration level, the sulfate ion was formed in most of the base-containing clusters. The calculated hydration energies indicate that water binds more strongly to ammonia-containing clusters than to dimethylamine-containing and base-free clusters, which results in a wider hydrate distribution for ammonia-containing clusters. The electrical mobilities of all clusters were calculated using a particle dynamics model. The results indicate that the effect of humidity is negligible on the electrical mobilities of molecular clusters formed in the very first steps of atmospheric particle formation. The combination of the results of this study with those previously published on the hydration of neutral clusters by our group provides a comprehensive set of thermochemical data on neutral and negatively charged clusters containing sulfuric acid, ammonia, or dimethylamine.

Hydrogen bonded complexes of heterocyclic compounds with methanol were studied using matrix isolation FTIR spectroscopy and theoretical calculations.