Hydration of 3-Methyl-1,2,3-butanetricarboxylic Acid Evidenced by Matrix-Isolation Infrared Spectroscopy

The hydration mechanism of 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a relevant marker of secondary organic aerosol formation from the atmospheric oxidation of α-pinene, has been investigated using the matrix-isolation infrared spectroscopy technique. The experimental results were supported by theoretical calculations. Monomers of MBTCA and heterocomplexes MBTCA-n(H₂O) were identified. The presence of intramolecularly H-bonded monomers was experimentally identified, in accordance with the calculations. Heterocomplexes, i.e., aggregates with water, were found much more stable than homocomplexes. The results clearly demonstrate the affinity of MBTCA carboxylic functions for water molecules, and thus its ability to form an H-bond network, leading to the solvation of the molecule. This is the first experimental observation, evidencing the primary hydration stages of one relevant marker of secondary organic aerosol formation and aging. Although fundamental, such a molecular study helps in a better understanding of critical steps in cloud condensation nuclei formation.