Abstract
Abstract. Organic compounds residing near the surface of atmospheric aerosol particles are exposed to chemical reactions initiated by gas-phase oxidants, such as hydroxyl (OH) radicals. Aqueous droplets composed of inorganic salts and organic compounds can undergo phase separation into two liquid phases, depending on aerosol composition and relative humidity (RH). Such phase behavior can govern the surface characteristics and morphology of the aerosols, which in turn affect the heterogeneous reactivity of organic compounds toward gas-phase oxidants. In this work, we used an aerosol flow tube reactor coupled with an atmospheric pressure ionization source (direct analysis in real time) and a high-resolution mass spectrometer to investigate how phase separation in model aqueous droplets containing an inorganic salt (ammonium sulfate, AS) and an organic acid (3-methylglutaric acid, 3-MGA) with an organic-to-inorganic dry mass ratio (OIR) of 1 alters the heterogeneous OH reactivity. At high RH, 3-MGA/AS aerosols were aqueous droplets with a single liquid phase. When the RH decreased, aqueous 3-MGA/AS droplets underwent phase separation at ∼75 % RH. Once the droplets were phase-separated, they exhibited either a core–shell, partially engulfed or a transition from core–shell to partially engulfed structure, with an organic-rich outer phase and an inorganic-rich inner phase. The kinetics, quantified by an effective heterogenous OH rate constant, 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 slightly higher than that of aqueous droplets with a single liquid phase. This could be explained by the finding that when the RH decreases, higher concentrations of organic molecules (i.e., 3-MGA) are present at or near the droplet surface, which are more readily exposed to OH oxidation, as demonstrated by phase separation measurements and model simulations. This could increase the reactive collision probability between 3-MGA molecules and OH radicals dissolved near the droplet surface and secondary chain reactions. Even for phase-separated droplets with a fully established core–shell structure, the diffusion rate of organic molecules across the organic-rich outer shell is predicted to be fast in this system. Thus, the overall rate of reactions is likely governed by the surface concentration of 3-MGA rather than a diffusion limitation. Overall, understanding the aerosol phase state (single liquid phase versus two separate liquid phases) is essential to better probe the heterogenous reactivity under different aerosol chemical composition and environmental conditions (e.g., RH).
Highlights
Organic compounds present at or near the surface of atmospheric aerosols can be efficiently reacted with gas-phase oxidants, such as OH, ozone (O3) and nitrate radicals (George and Abbatt, 2010; Kroll et al, 2015; Chapleski et al, 2016), continuously changing aerosol properties through the alteration of surface and bulk composition (Lambe et al, 2007; Cappa et al, 2011; Harmon et al, 2013; Slade et al, 2015, 2017)
For most of the 3-MGA/an inorganic salt (AS) droplets, a morphological transition from core–shell to partially engulfed was observed, with the inner phase moving toward the edge of the droplet at relative humidity (RH) ranges from ∼ 73% to ∼ 67 % during dehumidification (Fig. 2a)
We investigated the heterogeneous kinetics of aqueous 3-MGA/AS droplets with an organic-to-inorganic dry mass ratio (OIR) of 1
Summary
Organic compounds present at or near the surface of atmospheric aerosols can be efficiently reacted with gas-phase oxidants, such as OH, ozone (O3) and nitrate radicals (George and Abbatt, 2010; Kroll et al, 2015; Chapleski et al, 2016), continuously changing aerosol properties (e.g., water uptake and cloud formation potential) through the alteration of surface and bulk composition (Lambe et al, 2007; Cappa et al, 2011; Harmon et al, 2013; Slade et al, 2015, 2017). Recent studies suggest that these organic–inorganic droplets can undergo liquid–liquid phase separation (LLPS) depending on environmental conditions (e.g., RH and temperature) and aerosol composition (including different types of inorganic salts, the average oxygento-carbon (O : C) elemental ratio of organic compounds and OIR) (Bertram et al, 2011; Zuend and Seinfeld, 2013; You et al, 2014; Qiu and Molinero, 2015; Charnawskas et al, 2017; Losey et al, 2018; Freedman, 2020). The molecular composition of 3-MGA/AS droplets before and after OH oxidation was investigated using an aerosol flow tube reactor coupled with a direct analysis in real time (DART) ionization source and a high-resolution mass spectrometer at different RH levels Based on these results, we attempt to explore and analyze quantitatively how phase separation in aqueous organic–inorganic droplets may determine the heterogeneous OH reactivity of methyl-substituted dicarboxylic acids
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