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Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, whichalter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects thehygroscopicity and leads to a concomitant response in the equilibrium amount of particle-phase water. The variation in the aerosolwater content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. Toattain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates theheterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C5H8O4) droplets, a model compound forsmall branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental andmodeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time,DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C5) hydroxylfunctionalization product (C5H8O5) and a C4 fragmentation product (C4H6O3). These two productslikely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon–carbon bond scission) oftertiary alkoxy radicals resulting from the OH abstraction occurring at the methyl-substituted carbon site. Based on theidentification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the AerosolInorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositionalchanges of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the dropletsbecome slightly more hygroscopic after oxidation, as the mass fraction of water is predicted to increase from 0.362 to 0.424; however,the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products thatpartition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller dropletsize. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining theevolution of aerosol water content and droplet size at high-oxidation stages

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols