Evolution in the Reactivity of Citric Acid toward Heterogeneous Oxidation by Gas-Phase OH Radicals

Chim, Man Mei; Lim, Christopher Y.; Kroll, J. H.; Chan, Man Nin

doi:10.1021/acsearthspacechem.8b00118

Cited by 19 publications

(23 citation statements)

References 36 publications

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“…The time between two successive collision events between gas-phase OH radical and the aerosol surface, τ coll , is estimated to be 4.8 µs at the highest OH radical concentration investigated in the work (i.e., the maximum OH exposure) using Eq. (5) (Chim et al, 2017(Chim et al, , 2018:…”

Section: The Effect Of Rh On the Heterogeneous Reactivitymentioning

confidence: 99%

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

Lam

Choczynski

et al. 2021

Atmos. Chem. Phys.

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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).

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Section: The Effect Of Rh On the Heterogeneous Reactivitymentioning

confidence: 99%

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

Lam

Choczynski

et al. 2021

Atmos. Chem. Phys.

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“…Given the hygroscopic data and erythritol particles were always exposed to high RH (i.e., 85 %) and did not pass through a diffusion dryer in our system, they were likely aqueous droplets before oxidation. At low RH, erythritol particles are known to be viscous (Song et al, 2016;Grayson et al, 2017;Chu et al, 2018). With high particle viscosity, the diffusion of erythritol molecules from the bulk phase to the particle surface for oxidation slows down, and the overall heterogeneous reaction rate can be controlled by the diffusion (Chim et al, 2018;Marshall et al, 2018).…”

Section: Physical State and Mixing Timescale Of Erythritol Particlesmentioning

confidence: 99%

“…where D org is the diffusion coefficient (m 2 s −1 ), k b is the Boltzmann constant, T is the temperature, η is the viscosity (η = 1.9 × 10 −3 Pa s at 293 K and 85 % RH) and R H is the hydrodynamic radius of a erythritol molecule (R H = 0.34 ± 0.01 nm) (Chu et al, 2018). Using Eq.…”

Section: Physical State and Mixing Timescale Of Erythritol Particlesmentioning

confidence: 99%

Effect of inorganic-to-organic mass ratio on the heterogeneous OH reaction rates of erythritol: implications for atmospheric chemical stability of 2-methyltetrols

Lam

Wilson

et al. 2020

Atmos. Chem. Phys.

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Abstract. The 2-methyltetrols have been widely chosen as chemical tracers for isoprene-derived secondary organic aerosols. While they are often assumed to be relatively unreactive, a laboratory study reported that pure erythritol particles (an analog of 2-methyltetrols) can be heterogeneously oxidized by gas-phase OH radicals at a significant rate. This might question the efficacy of these compounds as tracers in aerosol source-apportionment studies. Additional uncertainty could arise as organic compounds and inorganic salts often coexist in atmospheric particles. To gain more insights into the chemical stability of 2-methyltetrols in atmospheric particles, this study investigates the heterogeneous OH oxidation of pure erythritol particles and particles containing erythritol and ammonium sulfate (AS) at different dry inorganic-to-organic mass ratios (IOR) in an aerosol flow tube reactor at a high relative humidity of 85 %. The same reaction products are formed upon heterogenous OH oxidation of erythritol and erythritol–AS particles, suggesting that the reaction pathways are not strongly affected by the presence and amount of AS. On the other hand, the effective OH uptake coefficient, γeff, is found to decrease by about a factor of ∼20 from 0.45±0.025 to 0.02±0.001 when the relative abundance of AS increases and the IOR increases from 0.0 to 5.0. One likely explanation is the presence of dissolved ions slows down the reaction rates by decreasing the surface concentration of erythritol and reducing the frequency of collision between erythritol and gas-phase OH radicals at the particle surface. Hence, the heterogeneous OH reactivity of erythritol and likely 2-methyltetrols in atmospheric particles would be slower than previously thought when the salts are present. Given 2-methyltetrols often coexist with a significant amount of AS in many environments, where ambient IOR can vary from ∼1.89 to ∼250, our kinetic data would suggest that 2-methyltetrols in atmospheric particles are likely chemically stable against heterogeneous OH oxidation under humid conditions.

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“…Citric acid is a highly oxygenated organic molecule that has been used in several studies as a proxy for oxidized organic material in the atmosphere (Davies and Wilson, 2015;Grvzinic et al, 2015;Chim et al, 2018). It is highly soluble and will form a viscous aqueous solution at low RH (Davies and Wilson, 2016;Lienhard et al, 2014).…”

Section: Hygroscopicity Of Citric Acidmentioning

confidence: 99%

A Dual-Droplet Approach for Measuring the Hygroscopicity of Aqueous Aerosol

Choczynski¹,

Kohli²,

Sheldon³

et al. 2021

Preprint

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Abstract. Accurate characterization of the water activity and hygroscopicity of aqueous aerosol material allows us to predict the chemical and physical state of aerosol particles exposed to humid conditions in the environment. The hygroscopicity of aerosol determines the size, phase morphology, viscosity, chemical reactivity, and optical properties of constituent particles, and directly impacts their ability to form clouds in the atmosphere. In this work, we describe measurements of hygroscopicity using a linear quadrupole electrodynamic balance (LQ-EDB). We levitate two droplets, one droplet that acts as a relative humidity (RH) probe and one sample droplet, and expose them to controlled environmental conditions. We describe the development of a RH measurement using probe droplets of aqueous NaCl or LiCl, allowing for precise in-situ measurements of RH in the LQ-EDB chamber. We demonstrate that the RH may be determined with an accuracy of 0.5 % at 50 % RH and better than 0.1 % at 90 % RH using NaCl, and show that LiCl is effective at characterizing the RH from ~10 % RH up to ~90 %. We simultaneously measure the response of sample droplets containing aqueous material (including ammonium sulfate, citric acid, 1,2,6-hexanetriol, and tetraethylene glycol) and report hygroscopic growth via their radial growth factors. We use established thermodynamic models to validate the accuracy of the RH probe and to compare with the measured hygroscopicity of the samples. This approach shows significant advantages over other methods for accurately characterizing the hygroscopicity of samples with a range of characteristics, such as high viscosity and vapor pressure.

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Evolution in the Reactivity of Citric Acid toward Heterogeneous Oxidation by Gas-Phase OH Radicals

Cited by 19 publications

References 36 publications

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

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

Effect of inorganic-to-organic mass ratio on the heterogeneous OH reaction rates of erythritol: implications for atmospheric chemical stability of 2-methyltetrols

A Dual-Droplet Approach for Measuring the Hygroscopicity of Aqueous Aerosol

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