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. Organic compounds residing near the surface of atmospheric aerosol particlesare exposed to chemical reactions initiated by gas-phase oxidants, such ashydroxyl (OH) radicals. Aqueous droplets composed of inorganic salts andorganic compounds can undergo phase separation into two liquid phases,depending on aerosol composition and relative humidity (RH). Such phasebehavior can govern the surface characteristics and morphology of theaerosols, which in turn affect the heterogeneous reactivity of organiccompounds toward gas-phase oxidants. In this work, we used an aerosol flowtube reactor coupled with an atmospheric pressure ionization source (directanalysis in real time) and a high-resolution mass spectrometer toinvestigate how phase separation in model aqueous droplets containing aninorganic salt (ammonium sulfate, AS) and an organic acid (3-methylglutaricacid, 3-MGA) with an organic-to-inorganic dry mass ratio (OIR) of 1 altersthe heterogeneous OH reactivity. At high RH, 3-MGA/AS aerosols were aqueousdroplets with a single liquid phase. When the RH decreased, aqueous 3-MGA/ASdroplets underwent phase separation at ∼75 % RH. Once thedroplets were phase-separated, they exhibited either a core–shell,partially engulfed or a transition from core–shell to partially engulfedstructure, with an organic-rich outer phase and an inorganic-rich innerphase. The kinetics, quantified by an effective heterogenous OH rateconstant, was found to increase gradually from 1.01±0.02×10-12 to 1.73±0.02×10-12 cm3 molec.−1 s−1 when the RH decreased from 88 % to 55 %. The heterogeneous reactivity of phase-separated droplets is slightlyhigher than that of aqueous droplets with a single liquid phase. This couldbe explained by the finding that when the RH decreases, higherconcentrations of organic molecules (i.e., 3-MGA) are present at or near thedroplet surface, which are more readily exposed to OH oxidation, asdemonstrated by phase separation measurements and model simulations. Thiscould increase the reactive collision probability between 3-MGA moleculesand OH radicals dissolved near the droplet surface and secondary chainreactions. Even for phase-separated droplets with a fully establishedcore–shell structure, the diffusion rate of organic molecules across theorganic-rich outer shell is predicted to be fast in this system. Thus, theoverall rate of reactions is likely governed by the surface concentration of3-MGA rather than a diffusion limitation. Overall, understanding the aerosolphase state (single liquid phase versus two separate liquid phases) isessential to better probe the heterogenous reactivity under differentaerosol chemical composition and environmental conditions (e.g., RH).

atmospheric chemistry and physics

Effects of liquid–liquid phase separation and relative humidity on the heterogeneous OH oxidation of inorganic–organic aerosols: insights from methylglutaric acid and ammonium sulfate particles