Nanoscale interfacial gradients formed by the reactive uptake of OH radicals onto viscous aerosol surfaces

Davies, James F.; Wilson, Kevin R.

doi:10.1039/c5sc02326b

Cited by 111 publications

(199 citation statements)

References 46 publications

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“…These reactodiffusive lengths indicate that at all relative humidities HO 2 radicals will be limited to the outermost molecular layers of the particle before reacting away, which is in agreement with the model. Note that it was shown previously that the uptake of gas-phase species generally increases with increasing reacto-diffusive length, which is consistent with our HO 2 uptake coefficient measurements (Slade and Knopf, 2014;Davies and Wilson, 2015;Houle et al, 2015). The red and blue lines in Fig.…”

Section: Ho 2 Uptake By Copper-doped Sucrose Aerosol Particlessupporting

confidence: 80%

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

et al. 2016

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Abstract. We report the first measurements of HO 2 uptake coefficients, γ , for secondary organic aerosol (SOA) particles and for the well-studied model compound sucrose which we doped with copper(II). Above 65 % relative humidity (RH), γ for copper(II)-doped sucrose aerosol particles equalled the surface mass accommodation coefficient α = 0.22 ± 0.06, but it decreased to γ = 0.012 ± 0.007 upon decreasing the RH to 17 %. The trend of γ with RH can be explained by an increase in aerosol viscosity and the contribution of a surface reaction, as demonstrated using the kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB). At high RH the total uptake was driven by reaction in the near-surface bulk and limited by mass accommodation, whilst at low RH it was limited by surface reaction. SOA from two different precursors, α-pinene and 1,3,5-trimethylbenzene (TMB), was investigated, yielding low uptake coefficients of γ < 0.001 and γ = 0.004 ± 0.002, respectively. It is postulated that the larger values measured for TMB-derived SOA compared to α-pinene-derived SOA are either due to differing viscosity, a different liquid water content of the aerosol particles, or an HO 2 + RO 2 reaction occurring within the aerosol particles.

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Section: Ho 2 Uptake By Copper-doped Sucrose Aerosol Particlessupporting

confidence: 80%

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

et al. 2016

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“…Organic compounds dissolved within liquid aqueous droplets have been found to be oxidized by gas-phase OH radicals at a much faster rate than within amorphous (semi)solid aerosol particles (Chan et al, 2014;Slade and Knopf, 2014;Davies and Wilson, 2015;Arangio et al, 2015). This is attributed to shorter mixing times in liquid submicron-sized droplets, allowing the condensed-phase material to diffuse rapidly within the droplet bulk and access the surface on the timescale of a reaction Berkemeier et al, 2013Berkemeier et al, , 2016Houle et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Abstract. Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particlephase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C 5 H 8 O 4 ) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a highresolution mass spectrometer reveal two major products: a five carbon atom (C 5 ) hydroxyl functionalization product (C 5 H 8 O 5 ) and a C 4 fragmentation product (C 4 H 6 O 3 ). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon-carbon bond scission) of tertiary alkoxy radicals resulting from the OH abstraction occurring at the methylsubstituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become 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 that partition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at highoxidation stages.

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“…Concerning the reaction with OH radicals, Davies and Wilson (2015) showed that reactions of citric acid (CA) aerosols with OH are slowed when the aerosol assumes a viscous state. The reacto-diffusive length of OH is very small.…”

Section: Atmospheric Outlookmentioning

confidence: 99%

“…As condensed-phase diffusivity is generally expected to be inversely related to viscosity, diffusion coefficients of water, organic molecules and oxidants are essential to understanding the influence of SOA physical state on aerosol processes. Slow diffusivities of organic molecules and oxidants can affect atmospheric chemistry, a point illustrated in a study by Davies and Wilson (2015), who observed the formation of interfacial gradients in the reactive uptake of OH radicals on viscous citric acid (CA) aerosol particles. Whereas particles remained well mixed when dilute, they showed that the surface region affected by the reaction (characterized by the depletion of CA and the formation of reaction products) is a strong function of relative humidity (RH) below 50 %, implying condensed-phase diffusion limitations.…”

Section: Introductionmentioning

confidence: 99%

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

et al. 2017

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Abstract. Field measurements indicating that atmospheric secondary organic aerosol (SOA) particles can be present in a highly viscous, glassy state have spurred numerous studies addressing low diffusivities of water in glassy aerosols. The focus of these studies is on kinetic limitations of hygroscopic growth and the plasticizing effect of water. In contrast, much less is known about diffusion limitations of organic molecules and oxidants in viscous matrices. These may affect atmospheric chemistry and gas-particle partitioning of complex mixtures with constituents of different volatility. In this study, we quantify the diffusivity of a volatile organic in a viscous matrix. Evaporation of single particles generated from an aqueous solution of sucrose and small amounts of volatile tetraethylene glycol (PEG-4) is investigated in an electrodynamic balance at controlled relative humidity (RH) and temperature. The evaporative loss of PEG-4 as determined by Mie resonance spectroscopy is used in conjunction with a radially resolved diffusion model to retrieve translational diffusion coefficients of PEG-4. Comparison of the experimentally derived diffusivities with viscosity estimates for the ternary system reveals a breakdown of the Stokes-Einstein relationship, which has often been invoked to infer diffusivity from viscosity. The evaporation of PEG-4 shows pronounced RH and temperature dependencies and is severely depressed for RH 30 %, corresponding to diffusivities < 10 −14 cm 2 s −1 at temperatures < 15 • C. The temperature dependence is strong, suggesting a diffusion activation energy of about 300 kJ mol −1 . We conclude that atmospheric volatile organic compounds can be subject to severe diffusion limitations in viscous organic aerosol particles. This may enable an important long-range transport mechanism for organic material, including pollutant molecules such as polycyclic aromatic hydrocarbons (PAHs).

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Nanoscale interfacial gradients formed by the reactive uptake of OH radicals onto viscous aerosol surfaces

Abstract: The reaction of hydroxyl radicals with viscous oxygenated organic aerosol forms nanometer-sized interfacial gradients.

Cited by 111 publications

References 46 publications

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

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

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

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Nanoscale interfacial gradients formed by the reactive uptake of OH radicals onto viscous aerosol surfaces

Abstract: The reaction of hydroxyl radicals with viscous oxygenated organic aerosol forms nanometer-sized interfacial gradients.

Cited by 111 publications

References 46 publications

The effect of viscosity and diffusion on the HO&lt;sub&gt;2&lt;/sub&gt; uptake by sucrose and secondary organic aerosol particles

The effect of viscosity and diffusion on the HO&lt;sub&gt;2&lt;/sub&gt; uptake by sucrose and secondary organic aerosol particles

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

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

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The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles