Competition between functionalization and fragmentation pathways in the OH-initiated oxidation of aqueous tartaric acid droplets: Reaction products and model simulations

Abstract:

To gain better insights into the competition between functionalization and fragmentation pathways for oxygenated organic compounds, we investigate the OH-radical initiated oxidation of aqueous tartaric acid (C₄H₆O₆) droplets using an aerosol flow tube reactor. The molecular composition of reaction products is characterized by an atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high resolution mass spectrometer. The reaction produces four major products: a functionalization product (C₄H₄O₆) and three fragmentation products (C₃H₄O_4, C₃H₂O₄ and C₃H₂O₅), with a predominance of the functionalization product which supports the literature result that only less than 10% of carbon loss was observed for the OH oxidation of tartaric acid. The formation of the functionalization product (2-hydroxy-3-oxosuccinic acid, C₄H₄O₆) can be attributed to that the tertiary alkyl radical, formed after hydrogen abstraction, reacts with an O₂ molecule to form a hydroxyperoxyl radical which tends to quickly undergo intramolecular HO₂ elimination without fragmentation. The molecular transformation of aqueous tartaric acid droplets is stimulated using the kinetic multi-layer model of gas-particle interactions in aerosols and clouds (KM-GAP) and the Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model in order to take into account the change in particle-phase water and activities of reaction products during the oxidation. Results suggest that aqueous tartaric acid droplets become slightly less hygroscopic after oxidation due to the formation of less polar products. The formation of products with different hygroscopicities and volatilities largely determine the amount of particle-phase water, which in turn governs the size of the aqueous droplets and the concentration of the reactants. Consideration of the variation in water content in response to the chemical evolution in the aerosol is needed to better understand the heterogeneous oxidation of aqueous organic droplets, especially under humid environments. We will present the aerosol-DART mass spectrometric data, reaction pathways, and model results to address the role of water in the heterogeneous OH oxidation of this highly oxygenated organic compound.