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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