Why atmospheric organosulfur still needed a source explanation
Organosulfur compounds have been recognized as ubiquitous aerosol constituents that can make up roughly 5% to 30% of fine particulate organic matter, and their formation pathways have been studied through field observations, laboratory experiments, and models [1][4]. A foundational concern is that some measurements may overestimate particulate organosulfate abundance when sampling occurs in the presence of SO2, which means field evidence itself can carry artifacts that complicate source attribution [2]. Competing work has shown that organosulfates are not inert endpoints: an alpha-pinene-derived organosulfate reacts with aqueous hydroxyl radicals at (2.2 +/- 0.2) x 10^9 L mol^-1 s^-1, giving atmospheric lifetimes from minutes in remote aerosol conditions to about 2 days in urban cloud conditions, and producing smaller and more oxygenated organosulfur products [4]. Other kinetic work reports that organosulfates react with OH at 10^8 to 10^9 M^-1 s^-1, corresponding to lifetimes of minutes in aqueous aerosol to days in cloudwater, and argues that loss pathways must be included in models [8]. Together, these studies establish that organosulfur abundance reflects a balance of formation, sampling, and rapid aqueous transformation, leaving room for a missing rapid formation route [1][2][4][8].
The microdroplet interface as a catalyst-free reactor
The anchor paper sprayed aqueous Na2SO3 at an atmospherically relevant 40 micromolar concentration into microdroplets and observed HSO3-, SO3 radical anion, HSO4-, and SO4 radical anion in the mass spectrum, indicating rapid spontaneous one-electron oxidation of sulfite without catalysts, external potential, or radiation [1]. Control experiments reduced dissolved oxygen from 7.07 to 2.43 mg L^-1 with no significant effect on SO3 radical anion intensity, trace metals in ultrapure, deionized, and distilled water were below 0.1 ppb, and no SO3 radical anion was detected with conventional electrospray ionization or applied external voltage, ruling out dissolved oxygen, trace transition metals, and ionization-induced processes as major contributors [1]. When sulfite was sprayed with glyoxal, alpha-pinene oxide, methacrolein, or 3-methyl-2-butenal, organosulfur products appeared, including m/z 138.9702, 156.9807, 233.0842, 165.0232, 183.0338, 151.0063, and 169.0166, with MS/MS fragment ions such as HSO3- at m/z 81 supporting sulfonate structures [1]. The inferred reaction time was about 220 microseconds at a 20 mm reaction distance, and product yield reached up to 50% at that distance, which places the chemistry on a microsecond timescale rather than in the mass spectrometer inlet [1].
Interfacial electric fields lower the barrier
The mechanistic claim is that the air-water interface of microdroplets supplies a strong electric field that makes sulfite oxidation and subsequent organosulfur formation feasible without bulk-phase catalysts or strong oxidants [1]. Theoretical calculations for SO3 radical anion and HSO3- addition to the C=C bond of methacrolein show lower Gibbs free energy barriers at the air-water interface than in bulk solution, supporting the interpretation that the interfacial field facilitates the reaction by lowering activation barriers [1]. Pressure-dependent experiments showed that increasing sheath gas pressure from 40 to 120 psi reduces droplet diameter from about 20 to 4 micrometers and increased SO3 radical anion signal intensity, consistent with enhanced interfacial electric field at higher surface curvature [1]. The paper is careful that this trend may also reflect evaporation-driven concentration, reactant enrichment, altered solvation, and non-equilibrium interfacial dynamics, so the electric-field explanation is a strong interpretation rather than a uniquely isolated cause [1]. A precursor study on amine-carbon dioxide reactions in microdroplets similarly proposed that trace water creates superacid or superbase character at the interface and accelerates carbamic acid formation, showing that interfacial acid-base extremes are an established microdroplet concept [3].
From laboratory formulas to ambient aerosol detections
The anchor paper reports that m/z 151.0070 (C4H7O4S-), m/z 165.0227 (C5H9O4S-), and m/z 233.0853 (C10H17O4S-) were consistently detected in eight aerosol samples collected from Tianjin and Shanghuang Station, while m/z 169.0176 (C4H9O5S-) and m/z 183.0333 (C5H11O5S-) were observed only in four Tianjin samples [1]. MS/MS analysis of a Tianjin aerosol sample found fragment ions consistent with microdroplet products, including HSO3- at m/z 81, supporting structural analogy between laboratory-generated and ambient organosulfur compounds [1]. The absence of m/z 169.0176 and 183.0333 at Shanghuang Station may be associated with higher relative humidity of 67-69% there versus 53-59% in Tianjin, because increased aerosol liquid water could promote particle growth and weaken interfacial electric field strength through decreased curvature, reducing OH generation and further oxidation of the m/z 151.0070 and 165.0227 products [1]. Lower oxygenated volatile organic compound precursor abundance at Shanghuang Station may also contribute, and the higher intensity of m/z 233.0853 in Tianjin may reflect higher anthropogenic oxygenated volatile organic compound concentrations and more favorable interfacial oxidation conditions [1]. This field comparison is a validation test of generalizability, but it is based on limited urban and mountain samples and cannot quantify a global organosulfur contribution [1].
What remains uncertain before model inclusion
The main boundary is that atmospheric relevance rests on detections at one urban site and one high-altitude mountain station, with no global budget or source apportionment, so the pathway should not yet be treated as a quantified global organosulfur source [1]. A limitation review of interfacial chemistry of atmospheric microdroplets cautions that laboratory microdroplets are highly controlled and near-monodisperse, whereas natural microdroplets span submicron to tens of micrometers and are polydisperse, and that controversy remains over whether laboratory microdroplets accurately represent cloud droplets [5]. That review also notes that current theoretical models often use static droplet sizes and constant interfacial parameters, ignoring evaporation-driven changes in concentration, curvature, surface charge density, and built-in electric field, which can misclassify reaction rates and even switch reaction pathways over time [5]. Competing aqueous-phase work shows that organosulfates are rapidly oxidized by OH and can fragment into smaller organosulfur products, meaning ambient concentrations reflect formation minus loss and cannot be used alone to estimate production rates [4][8]. Sulfate modeling studies also show that aqueous and gas-phase oxidation updates change modeled sulfate over Asia and Tokyo, so any new sulfur pathway would need to be evaluated against existing heterogeneous and radical chemistry rather than added in isolation [6][7]. Finally, catalyst-free interfacial oxidation is not unique to sulfite: ozone micro-nanobubbles coupled with H2O2 can oxidize toluene under mild conditions via interfacial reactive oxygen species, reinforcing that interfacial oxidant chemistry is a broader phenomenon whose atmospheric significance still requires quantitative field and model testing [9].
About These Sources
This research page is built on 9 peer-reviewed studies — published from 2019 to 2026, 5 from 2024 or later, collectively cited 249 times — selected as the most relevant from 13 studies that passed quality screening, drawn from 77 papers retrieved from a database of over 500 million.
Sources used in this answer
Rapid spontaneous generation of organosulfur from inorganic sulfur in atmospheric microdroplets
The anchor paper shows that sulfite and oxygenated volatile organic compounds react in microdroplets to form organosulfur compounds without catalysts or external energy, attributes the acceleration to interfacial electric fields lowering activation barriers, and detects matching organosulfur formulas in urban and high-altitude aerosol samples [1].
Overestimation of Monoterpene Organosulfate Abundance in Aerosol Particles by Sampling in the Presence of SO2
This foundational study warns that monoterpene organosulfate abundance in aerosol particles can be overestimated when sampling occurs in the presence of SO2, establishing a measurement-artifact baseline for field organosulfur evidence [2].
Accelerated reactions of amines with carbon dioxide driven by superacid at the microdroplet interface
This precursor microdroplet study shows that amine-carbon dioxide reactions are accelerated at the droplet interface, with trace water proposed to create superacid or superbase character, providing prior support for interfacial electric-field and acid-base effects in microdroplet chemistry [3].
Rapid aqueous-phase oxidation of an α-pinene-derived organosulfate by hydroxyl radicals: a potential source of some unclassified oxygenated and small organosulfates in the atmosphere
This competing aqueous-phase study reports that an alpha-pinene-derived organosulfate reacts rapidly with OH radicals at (2.2 +/- 0.2) x 10^9 L mol^-1 s^-1, with lifetimes from minutes to about 2 days, and can produce smaller and more oxygenated organosulfur products [4].
Interfacial chemistry of atmospheric microdroplets
This limitation review cautions that laboratory microdroplets differ from natural polydisperse atmospheric droplets and that static theoretical models may misclassify dynamic interfacial reaction rates and pathways [6].
Year-round modeling of sulfate aerosol over Asia through updates of aqueous-phase oxidation and gas-phase reactions with stabilized Criegee intermediates
This competing modeling study shows that updating aqueous-phase oxidation and gas-phase stabilized Criegee intermediate chemistry improves modeled sulfate over Asia in winter, indicating that sulfur budget estimates depend on which oxidation pathways are included [7].
Differences in Model Performance and Source Sensitivities for Sulfate Aerosol Resulting from Updates of the Aqueous- and Gas-Phase Oxidation Pathways for a Winter Pollution Episode in Tokyo, Japan
This competing modeling study for a Tokyo winter pollution episode finds that updated aqueous and gas-phase oxidation pathways change sulfate model performance and source sensitivities, reinforcing that sulfur chemistry updates can alter source attribution [8].
OH Radical Oxidation of Organosulfates in the Atmospheric Aqueous Phase.
This competing kinetic study reports OH radical oxidation rate constants of 10^8 to 10^9 M^-1 s^-1 for five atmospherically relevant organosulfates, corresponding to lifetimes of minutes in aqueous aerosol to days in cloudwater, and argues that organosulfate loss pathways must be included in models [9].
Catalyst-free aqueous-phase oxidation of toluene by ozone micro-nanobubbles coupled with H2O2 via interfacial reactive oxygen species.
This competing study demonstrates catalyst-free aqueous-phase oxidation of toluene by ozone micro-nanobubbles coupled with H2O2 via interfacial reactive oxygen species, showing that catalyst-free interfacial oxidation is a broader phenomenon beyond organosulfur formation [13].