Enhanced Rates of Transition-Metal-Ion-Catalyzed Oxidation of S(IV) in Aqueous Aerosols: Insights into Sulfate Aerosol Formation in the Atmosphere

Angle, Kyle; Neal, Erin E.; Grassian, Vicki H.

doi:10.1021/acs.est.1c01932

Cited by 37 publications

(74 citation statements)

References 54 publications

(140 reference statements)

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“…Similarly, the spontaneous oxidation induced by the water radical cation has several implications for atmospheric and aerosol chemistry, topics that involve sub-micrometer size droplets (e.g., spray and organic aerosols) ( Gantt and Meskhidze, 2013 ; Ault and Axson, 2017 ; Kirpes et al, 2018 ). Although an enhanced rate of oxidation has been reported in aerosols via other mechanisms and other reactive species (e.g., oxygen, ozone, hydroxyl radicals or hydrogen peroxide) ( Glasius et al, 2000 ; Richards-Henderson et al, 2016 ; Liu et al, 2020 ; Angle et al, 2021 ), we suggest that the possible role of the water radical cation chemistry is worthy of attention. The effects of this radical species might be significant considering the generality here demonstrated for X=Y containing compounds (common moieties in VOCs).…”

Section: Discussionmentioning

confidence: 59%

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

et al. 2022

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Spontaneous oxidation of compounds containing diverse X=Y moieties (e.g., sulfonamides, ketones, esters, sulfones) occurs readily in organic-solvent microdroplets. This surprising phenomenon is proposed to be driven by the generation of an intermediate species [M+H2O]+·: a covalent adduct of water radical cation (H2O+·) with the reactant molecule (M). The adduct is observed in the positive ion mass spectrum while its formation in the interfacial region of the microdroplet (i.e., at the air-droplet interface) is indicated by the strong dependence of the oxidation product formation on the spray distance (which reflects the droplet size and consequently the surface-to-volume ratio) and the solvent composition. Importantly, based on the screening of a ca. 21,000-compound library and the detailed consideration of six functional groups, the formation of a molecular adduct with the water radical cation is a significant route to ionization in positive ion mode electrospray, where it is favored in those compounds with X=Y moieties which lack basic groups. A set of model monofunctional systems was studied and in one case, benzyl benzoate, evidence was found for oxidation driven by hydroxyl radical adduct formation followed by protonation in addition to the dominant water radical cation addition process. Significant implications of molecular ionization by water radical cations for oxidation processes in atmospheric aerosols, analytical mass spectrometry and small-scale synthesis are noted.

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Section: Discussionmentioning

confidence: 59%

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

et al. 2022

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“…The mechanisms of heterogeneous SO 2 oxidation in aerosols, however, remain a matter of considerable debate. ,− Cheng et al explained the missing sulfate source with oxidation of SO 2 by NO 2 in aerosol water, which can occur rapidly within a 5.4 < pH < 6.2 range . Hung et al identified that the SO 2 oxidation by O 2 can occur via an uncatalyzed route at the surface of acidic aerosol droplets. , Liu et al found that the aqueous SO 2 oxidation by H 2 O 2 can be accelerated 50-fold by the salt effects in aerosol microdroplets .…”

Section: Introductionmentioning

confidence: 99%

“…This enhanced reaction rate is independent of aerosol acidity and was regarded as a major contributor to the missing sulfate . Angle et al observed that the aqueous TMI-catalyzed oxidation of S(IV) by O 2 was faster in the optically levitated aerosol droplets than in the bulk solution, although the TMI-catalyzed reaction is unfavorable at the high-ionic-strength condition . Wang et al reported that the Mn(II)-catalyzed oxidation of SO 2 by O 2 at aerosol surface was 2 orders of magnitude faster than the other known reaction mechanisms .…”

Section: Introductionmentioning

confidence: 99%

“…In the case of smoke chamber experiments, fine-tuning the background conditions can be challenging, and uncertainties may arise from wall loss . With the advent of aerosol optical tweezers (AOT), kinetic experiments can be conducted in a single levitated aerosol droplet by using Raman spectroscopy. , This technique can facilitate a continuous kinetic experiment with a long reaction time and well-controlled background conditions. For example, Angle et al utilized an AOT and spontaneous Raman spectroscopy to measure the TMI-catalyzed autoxidation of S(IV) in levitated Na 2 SO 3 droplets .…”

Section: Introductionmentioning

confidence: 99%

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Rapid Sulfate Formation via Uncatalyzed Autoxidation of Sulfur Dioxide in Aerosol Microdroplets

Chen

Liu

Wang³

et al. 2022

Environ. Sci. Technol.

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Severe winter haze events in Beijing and North China Plain are characterized by rapid production of sulfate aerosols with unresolved mechanisms. Oxidation of SO2 by O2 in the absence of metal catalysts (uncatalyzed autoxidation) represents the most ubiquitous SO2 conversion pathway in the atmosphere. However, this reaction has long been regarded as too slow to be atmospherically meaningful. This traditional view was based on the kinetic studies conducted in bulk dilute solutions that mimic cloudwater but deviate from urban aerosols. Here, we directly measure the sulfate formation rate via uncatalyzed SO2 autoxidation in single (NH4)2SO4 microdroplets, by using an aerosol optical tweezer coupled with a cavity-enhanced Raman spectroscopy technique. We find that the aqueous reaction of uncatalyzed SO2 autoxidation is accelerated by two orders of magnitude at the high ionic strength (∼36 molal) conditions in the supersaturated aerosol water. Furthermore, at acidic conditions (pH 3.5–4.5), uncatalyzed autoxidation predominately occurs on droplet surface, with a reaction rate unconstrained by SO2 solubility. With these rate enhancements, we estimate that the uncatalyzed SO2 autoxidation in aerosols can produce sulfate at a rate up to 0.20 μg m–3 hr–1, under the winter air pollution condition in Beijing.

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“…39 The pH at surfaces of aerosols can also be different from those in aerosol bulks, 40 and a recent aerosol kinetics study observed the enhanced rates of catalyzed oxidations in aerosols and attributed this enhancement to the pH gradient at the air-water interfaces of aerosols. 41 In the present work, we choose the oxidation reaction of sodium thiosulfate (STS), Na 2 S 2 O 3 , with O 3 as a model system of sulfur oxidation in aerosols, as such model reaction has been utilized in several studies associated with atmospheric aerosol chemistry. [42][43][44] In particular, the present work exploited aerosol optical tweezers (AOT) combined with Ra-…”

Section: Introductionmentioning

confidence: 99%

Accelerated Sulfur Oxidation by Ozone on Surfaces of Single Optically Trapped Aerosol Particles

Hsu

Lin

Huang

et al. 2022

Preprint

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The sulfur oxidation in mixed sodium thiosulfate/sucrose/aqueous micro-droplets by gaseous ozone is studied in this work via aerosol optical tweezers coupled with Ra- man spectroscopy, which can simultaneously determine various physicochemical proper- ties and the heterogeneous reaction kinetics of single optically trapped micro-droplets, allowing for elucidating their complicate interplay. According to the kinetics measure- ment results at different relative humidities, ozone concentrations and stoichiometries of inorganic and organic solutes, this work finds that a high aerosol ionic strength can accelerate the ozone oxidation of thiosulfate at air-water interfaces, while a high aerosol viscosity prolongs the reaction timescales due to diffusion-limited kinetics. The kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB) is utilized to inves- tigate the observed heterogeneous kinetics results and to retrieve the surface reaction rate coefficients. The KM-SUB simulations results indicate that the observed kinet- ics of sulfur oxidation in binary sodium thiosulfate aqueous micro-droplets with high ionic strengths is dominated by interfacial reactions, and the fitted surface reaction rate coefficients increase one order of magnitude when the droplet ionic strength increases around 40 M. Furthermore, this work demonstrated that including the effects of inter- facial ion depletion in the kinetics simulations can lead to an excellent agreement with the experimental results, implying its potential role in the interfacial kinetics.

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Enhanced Rates of Transition-Metal-Ion-Catalyzed Oxidation of S(IV) in Aqueous Aerosols: Insights into Sulfate Aerosol Formation in the Atmosphere

Cited by 37 publications

References 54 publications

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

Rapid Sulfate Formation via Uncatalyzed Autoxidation of Sulfur Dioxide in Aerosol Microdroplets

Accelerated Sulfur Oxidation by Ozone on Surfaces of Single Optically Trapped Aerosol Particles

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