the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jan 2026
In Situ Real-Time Determination of SO2 Photochemical Oxidation in Nanoscale Sea Salt Aerosols based on Dark-Field Microscopy
Abstract. Heterogeneous reaction processes of aerosols play an important role in air quality and climate change. However, the lack of in-situ measurements of single-nanoparticle reactions results in large uncertainties in modeling the nanoparticle reaction kinetics. The study introduces a method to quantify reaction rates of single-nanoparticles using hygroscopic growth factors (GFs) and the Zdanovskii-Stokes-Robinson (ZSR) rule. Planar waveguide dark-field microscopy was employed to monitor sodium chloride (NaCl) aerosol GFs under ultraviolet (UV) irradiation and SO2 exposure in real time. The results revealed a first-order reaction rate constant of 0.6523 h⁻¹ for 100 nm NaCl aerosols. Moreover, the reaction rate constant exhibits a non-monotonic size dependence on particle diameter-increasing in the 50–200 nm range and decreasing for particle sizes larger than 200 nm. This reflects a competitive interplay between the surface curvature effect at small particle sizes and specific surface area effect at larger sizes, which is further validated by a combined analysis based on transition state theory and the double-film mass transfer approach. Subsequently, sodium octyl sulfate (SOS) was introduced to form binary NaCl-based nanoaerosols, where the organic coating content was systematically varied under constant surface curvature to modulate the specific surface area. An increase in organic volume fraction reduces the effective specific surface area and suppresses heterogeneous reaction rates, accompanied by a pronounced nonlinear transition from partial to complete coating. This further confirms the experimentally observed size-dependent nonlinearity in reaction rates and offers new insights into nanoscale sulfate formation, improving atmospheric chemical models and pollution-climate assessments.
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RC1: 'Comment on egusphere-2026-68', Anonymous Referee #1, 13 Feb 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-68/egusphere-2026-68-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2026-68-RC1 -
AC1: 'Reply on RC1', Zhibo Xie, 10 Mar 2026
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We thank the anonymous referees for their valuable and constructive comments/suggestions on our manuscript. We will revise the manuscript accordingly and please find our point-to-point responses below.
Comments by Anonymous Referee #1:
General Comments:
This study proposes an in situ real-time measurement method based on dark-field microscopy to investigate the photochemical oxidation of SO₂ in nanoscale sea-salt aerosols. By combining the hygroscopic GF with the ZSR mixing rule, the authors successfully retrieve single-particle reaction kinetics and reveal a non-monotonic dependence of the reaction rate on particle size. The method is innovative, and the research falls within the scope of ACP. However, the scientific significance and the completeness of the experimental design require further clarification and strengthening. Acceptance is recommended after addressing the following points.
Response: We thank the reviewer for the constructive comments and valuable suggestions. Following the reviewer’s recommendations, we will revise the manuscript to clarify the scientific significance of this study and strengthen the description of the experimental design. Point-to-point responses to the reviewer’s comments are provided below, and the corresponding revisions will be incorporated into the manuscript.
Major comments:
1)While this study achieves in situ observation of reaction kinetics for individual nano-aerosol particles—a notable methodological advance—what specific atmospheric chemistry problem does it aim to solve? For instance, does it target the mechanism of rapid sulfate formation during haze episodes? How do the findings directly improve existing atmospheric chemistry models or pollution assessments?
Response: We thank the reviewer for this important question. The primary objective of this study is to investigate the size-dependent kinetics of heterogeneous SO₂ oxidation in nanoscale sea salt aerosols. Rapid sulfate formation in atmospheric particles—such as that observed during haze episodes—has been widely reported, yet the controlling mechanisms and kinetic parameters remain uncertain, particularly at the nanoscale. Most previous studies derive heterogeneous reaction rates from bulk solutions or micrometer-sized droplets, while atmospheric aerosols frequently exist in the submicron and nanoscale range.
In this size regime, geometric factors such as surface-to-volume ratio and surface curvature may influence gas uptake and interfacial mass transfer, but experimental constraints on these effects remain limited. By combining dark-field microscopy with hygroscopic growth analysis, this work provides in situ kinetic measurements for individual NaCl particles in the 50–400 nm size range. The observed non-monotonic size dependence of sulfate formation rates suggests that nanoscale geometric effects can influence heterogeneous SO₂ oxidation. These findings provide experimental constraints on size-dependent sulfate formation processes and may help improve the representation of heterogeneous sulfur oxidation in atmospheric chemistry models.
2)In this study, the hygroscopic G) of particles is derived from grayscale intensity using dark-field microscopy, and reaction kinetics are inferred based on the ZSR rule. To verify the reliability of the optical method, please provide the measured hygroscopic GF results for pure NaCl particles under the same experimental conditions (e.g., RH range 25%–85%) and compare them with theoretical models (e.g., E-AIM) or classical HTDMA data. If direct measurements were not performed, please specify whether GF parameters for NaCl from validated literature were adopted and discuss their applicability within your optical measurement system.
Response: We thank the reviewer for this helpful suggestion. In response, we will add a comparison between the hygroscopic GF of monodisperse 100 nm NaCl particles measured using our dark-field microscopy method and the predictions from the thermodynamic model E-AIM under the same RH conditions.3)In Section 2.2 and Figure 3b, you state that IC measurements provided independent validation of the GF-based kinetic analysis, showing a "consistent linear trend" when plotting ln(C₀/C) versus time. However, the manuscript does not provide a clear, step-by-step explanation of how the IC data (presumably aqueous concentrations of anions like sulfate or chloride from filter extracts) were converted into the dimensionless ln(C₀/C) values used in Figure 3b
Response: We thank the reviewer for pointing out this lack of clarity. In response to this comment, wewill add a detailed description of the data processing procedure in the revised manuscript, including the step-by-step derivation showing how the ion chromatography (IC) measurements were converted into the dimensionless ln(C₀/C) values used in Figure 3b.
4)The SO₂ concentration used in the experiments (200 ppm) is much higher than typical atmospheric background levels (usually ppb). The authors explain this is necessary to obtain sufficient signal within the experimental timeframe. However, could this affect the representativeness of the reaction mechanism (e.g., surface adsorption, oxidation pathways)? Were validation experiments conducted at lower, more atmospherically relevant concentrations to confirm the applicability of the derived kinetic parameters?
Response: In this study, experiments were conducted at 85 % RH to ensure that NaCl particles were fully deliquesced and maintained a stable aqueous phase during UV irradiation. Under such conditions, gas–particle mass transfer and aqueous-phase chemistry can proceed without the additional complexity introduced by solid–liquid phase transitions. The selected RH therefore provides a controlled framework to examine size-dependent kinetic effects under well-defined aqueous conditions.An RH of ~85 % is atmospherically relevant, particularly in marine boundary layers, coastal regions, and humid polluted environments, where sea salt aerosols frequently exist in a deliquesced state. Such high-RH conditions are also representative of pre-cloud or near-cloud environments, where heterogeneous sulfur oxidation can contribute significantly to sulfate formation.We agree that reaction kinetics may differ at lower RH, where partial deliquescence, increased ionic strength, or viscosity effects could modify gas uptake and aqueous-phase reactivity. However, the curvature-related mass transfer considerations discussed in this work are fundamentally geometric and therefore expected to remain relevant across RH conditions, although their quantitative contribution may vary. Investigating RH-dependent transitions in kinetic regimes is an important direction for future work.Citation: https://doi.org/10.5194/egusphere-2026-68-AC1
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AC1: 'Reply on RC1', Zhibo Xie, 10 Mar 2026
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CC1: 'Comment on egusphere-2026-68', Pai Liu, 21 Apr 2026
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Report on the manuscript titled “In Situ Real-Time Determination of SO2 Photochemical Oxidation in Nanoscale Sea Salt Aerosols Based on Dark-Field Microscopy”
Recommendation: Major revision
Xiong et al. measured photochemical SO2 conversion in submicrometer droplets. I believe the analytical method is novel and potentially very valuable to the atmospheric chemistry community, as it enables in situ measurement of heterogeneous reaction kinetics on a single-particle basis in the nanoscale size range. Despite this methodological advancement, I have significant concerns regarding the proposed chemical mechanism, the experimental design, and the kinetic analysis presented in the current study. The clarity of the manuscript should also be improved substantially.
At its current stage, the manuscript does not yet meet the standard required for publication.
Detailed comments are provided below.
Major comments
- The authors need to clarify which photochemical mechanism is being investigated in this work. The manuscript uses terms such as “UV-catalyzed SO2 oxidation,” which are too vague. Is the SO2 conversion driven by photoactivation of chloride, such as that reported by Cao et al. (JACS 2024, 146, 1467–1475)? If so, I suggest that the authors explicitly state the targeted reaction mechanism in the Introduction or at the beginning of the Results section. The relevant chemical equations and the corresponding kinetic expression derived from the proposed mechanism should also be clearly presented.
- Does O2 exist in the gas phase of the multiphase reaction system? Based on the Methods section, I infer that the gas phase consists of SO2, N2, and water vapor. The authors should explain why O2 (or air) was excluded from the system. This exclusion does not reflect realistic atmospheric conditions. In addition, O2 may play an important role in chloride-mediated SO2 photooxidation. As shown by Cao et al. (2024), Cl radicals are generated through photoactivation of [Cl-…H3O+…O2] adducts rather than by direct activation of Cl- ions.
- How do the authors rule out the influence of the substrate? Both SiO2 and TiO2 can act as effective photocatalytic materials under UV irradiation. It is therefore possible that the substrate contributes to the conversion of S(IV). Moreover, if the reaction occurs partly or predominantly at the liquid–solid interface, for example in the contact region between the droplet and the substrate, the observed reaction rate could also increase with droplet size simply because larger droplets have a greater contact area with the substrate.
- The SO2 concentration used in the experiments (200 ppm) is extremely high. This is approximately three to four orders of magnitude higher than concentrations encountered even during severe air pollution episodes. The authors should consider repeating the experiments over a range of SO2 concentrations and examining how the kinetics scale with SO2 concentration. Without such parameterization, it is difficult to assess whether the measured kinetics can be meaningfully extrapolated to atmospherically relevant conditions.
- I am very confused by the derivation of first-order kinetics from the presented data. As stated in the manuscript, first-order kinetics should be reflected by an exponential decay in NaCl concentration. However, Figure 3d appears to show an exponential increase in C, rather than a decay. In addition, at the initial condition (t = 0), why is ln(C/C0) equal to zero rather than one? Furthermore, the sulfate concentration shown in Figure 3a increases approximately linearly with time, rather than following the type of exponential behavior expected for first-order kinetics. These observations seem inconsistent with the authors’ kinetic interpretation and should be clarified carefully.
Minor comments
Line 43: Do the authors mean oxidation of SO2 by H2O2?
Lines 47–48: The authors may wish to reconsider their interpretation of the work by Angle et al. In that study, the kinetics were derived from Raman measurements of changes in reactant or product concentrations, rather than from the evolution of ionic strength.
Figure 1: Please check the spelling of “diffusion dryer.”
Citation: https://doi.org/10.5194/egusphere-2026-68-CC1
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