the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jul 2025
Identification of Micro-dynamics Phase Transition processes for Ammonium Sulfate aerosols by Two-dimensional Correlation Spectroscopy
Abstract. Phase transitions of particles are importance because it could influence reactive gas uptake, multiphase chemical reactions pathway, ice and polar stratospheric cloud formation. The traditional understanding assumes that phase transitions are thermodynamically equilibrium, yet this is not the case at the molecular level. Current understanding can not account for these phenomena, since the interaction with water vapor induces modifications in both the composition and local chemical microenvironment of aerosols. Our findings demonstrate that these inconsistencies can be reconciled through elucidation of the microscopic dynamic processes governing phase transformation for aerosol. We propose a novel method which is accurate in determining the phase transition point and identification of micro-dynamics phase transition processes for ammonium sulfate aerosols by using two-dimensional correlation spectroscopy. During efflorescence transition processes, we measured the phase transition point at 39 % ± 0.8 % (RH), and its start and end points at 41 %± 0.8 % (RH) and 36 %± 0.8 % (RH), respectively. We also explore that there are four distinct micro-dynamics steps during the efflorescence processes. Initially, there was a gradual loss of liquid water for the solution droplets. Subsequently, it formed the supersaturated ammonium sulfate (AS) particles. Furthermore, hydrogen bonds between liquid water and sulfate dissociate, reducing liquid sulfate concentration. Sulfate and ammonium ions in the bulk phase gradually approach each other, further expelling residual water. The efflorescence occurs and forms crystal/solid AS. Eventually, the remaining liquid water molecules eventually detach from the AS system, completing the liquid-to-solid phase transition. This method will help improve comprehending of the transport and deposition of inhaled aerosol. Moreover these insights will spur fundamental research into the formation and transformation mechanisms of atmospheric aerosols.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2662', Anonymous Referee #1, 21 Jul 2025
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RC2: 'Comment on egusphere-2025-2662', Anonymous Referee #2, 03 Oct 2025
The paper proposes a new method for investigating ammonium sulfate (AS) efflorescence using two-dimensional correlation infrared spectroscopy (2D-IR). Coupling this method with FTIR and PCMW2D, the authors attempt to validate the microdynamic mechanisms behind AS efflorescence. Although the method has potential and the topic is timely, the manuscript in its current form falls short of the standards expected for AMT, particularly in terms of methodological detail, validation, and clarity of interpretation. I therefore recommend rejection at this stage. With substantial additional work and restructuring, however, I believe this research could form the basis of a stronger future submission.
First, I want to say that I initially was happy to receive this review, as it is a topic in which I want to deepen my knowledge. The authors would excuse me for the long time I took to properly review their manuscript as I first wanted to be fully familiar with literature. After reading the introduction, I noted the absence of several key references on efflorescence, which makes it difficult for a reader to appreciate the state of the art and the potential contribution of 2D-IR. Including these references and clarifying the recent advances would greatly strengthen the paper. Importantly, because of several vague sentences, the paper often posits ‘old’ state of the art as traditional understanding, without clearly explaining the more recent results. This substantially weakens the introduction. Unlike what is stated, the paper does not provide a kinetic mechanism of AS efflorescence which has been proposed through literature (Onasch 1999, Takahama 2007, Ciobanu 2010, Wang 2017, Xu 2022). This should be clearly stated in introduction. Several sentences are overstated and should be revised for accuracy. For instance: “The traditional understanding assumes that phase transitions are thermodynamically equilibrium,” (L15), “the molecular-scale dynamical processes governing aerosol phase transitions remain hitherto uncharacterized” (L77) should be revised to reflect more accurately the current understanding in the literature. Thus, I would recommend to have an introduction that is in 3 to 4 paragraphs: the first one, presenting the importance of understanding aerosol transition, the 2nd one on equilibrium and kinetic mechanisms that are currently known, the 3rd would present the methods that arrive to these conclusions and highlight the current limitation, and a 4th one could introduce 2D-IR as a promising method that needs to be investigated and validated as done in the paper.
In my opinion, the manuscript oscillates between two positions. Either it is a “method paper”, which aims to demonstrate the value and robustness of an approach (in this case 2D-COS applied to aerosols). Or it is a “process paper”, which seeks to provide new scientific insights into the efflorescence of AS. However, in its current form, it mixes the two approaches without clarifying its angle. Many of the results presented are already known generalities (efflorescence values, qualitative sequence of the process), while the methodological contribution is not sufficiently validated (next point). I recommend that the authors clearly refocus their contribution. In AMT, it seems to me that the methodological contribution is the most relevant: it should therefore be highlighted, demonstrating the reproducibility, validation and advantages of the method compared to existing approaches, rather than claiming to offer a scientific (re)interpretation of efflorescence.
Accordingly, we would expect from a “method paper” to read many details on the method, which currently lacks. In its current format, the paper lacks sufficient detail for reproducibility. To make the method accessible to readers, I suggest considering the following points. But I want to stress that this list is probably not exhaustive, and any detail the authors think would be necessary should be integrated.
- The paper refers to Wei et al., 2022 for the description of the experimental setup. Where elements of the setup and analysis are literally identical to Wei et al. (2022), a concise reference is acceptable (e.g., ‘atomizer/dryer/DMA layout as in Wei 2022’). However, parameters that are specific to the present study must be reported explicitly here, as they directly affect FTIR intensities, 2D-COS outcomes, and the inferred efflorescence thresholds. For instance, we need details on the 300 nm selection and its distribution, mass/areal loading on ZnSe (and resulting optical thickness), deposition morphology and homogeneity, the exact RH program and dwell strategy during dehumidification
- No validation tests are presented. It would be important to show, for example, tests on standard salts or blank spectra. Finally, it is absolutely necessary to carry out repeatability tests and to test the method under different conditions. I understand that the comparison with the literature on RH values can be seen as a comparison test, but in my view, this is not sufficient.
- Regarding section 1.2, the presentation is a little too theoretical and not practical enough. For example, the size and overlap of the window used in PCMW2D are not specified, even though they directly affect the resolution and robustness of the results. Similarly, the description of spectrum pre-processing (baseline correction, noise correction, normalisation) is insufficient to guarantee reproducibility. Finally, no significance criteria are mentioned to distinguish real correlative peaks from noise. These details are necessary if we are to evaluate the robustness of the conclusions drawn from the 2D maps.
- L146, it is stated that linear baseline corrections and smoothing are performed. Such preprocessing can have major implication on the signal. The paper should clearly state which correction is applied and for which reason. In particular, I think that sensitivity tests should be done to show the impact on the conclusion.
I have some questions regarding the results and discussion. As these sections will likely need to be revised in light of my previous comments, the following comments should not be read as blocking, but rather as points that will likely need to be clarified later on:
- Section 2.1: The authors restrict their analysis to the regions 1000–1500 and 2500–3550 cm-1, but do not show the complete spectrum. However, the evolution of other bands (e.g., lattice modes <900 cm-1, NH4+ bending ≈1680 cm-1) could provide additional clues about the liquid-solid transition and better support the proposed four-step sequence. I recommend presenting the complete FTIR spectra (before and after pre-treatment) in order to verify the absence of spurious peaks and identify any additional spectral signatures.
- Typically, one would expect point 5 to be discussed in several sections, including 2.1: for example, does it have an impact on the estimated value of RH? Or when the authors state: ‘Since asynchronous spectra reflect differing rates of spectral changes, the sequence of molecular bond transformations during efflorescence can be deduced as: (1084 cm-1) > (1097 cm-1) > (1463 cm-1) > (1417 cm-1).’ (L306) How would preprocessing impact this point?
In this way, I would recommend to show the raw and post-processed results.
- Discussion remains general and mostly hinge on visual observation. The interpretation would benefit from data-based interpretations, using statistical tests, spectrum analysis techniques etc. The values would then be thoroughly compared with literature, allowing for a proper validation of the method
- I have probably missed one point, but when I look at the Figure 2, I would say that the maximum in correlation occurs at RH = 33 – 34 %. This is below the reported value of (39 ± 0.8) %. Can you clarify this point please? In the same way, I read maxima for the Fig 2B at RH = 32 and 35%. Section 2.2 needs a similar revision.
Finally, I would recommend using more cautious language and not overinterpreting the results. If the method is sufficiently reliable, it should speak for itself, and the overly superlative language disturbed me much more than it convinced me of the method's merits. Here are some examples, but again, it is true for most of the paper.
- “This study could provide critical insight about redefining atmospheric heterogeneous chemistry.” (L87)
- Lines 293-298: "Conversely, three negative cross-peaks […] causing these ions to move closer together." The authors directly interpret the negative cross-peaks as evidence that ammonium and sulphate ions move closer together during water loss. However, in my opinion, a negative cross-peak only indicates that the spectral intensities at the two wavelengths vary in opposite directions with RH. The translation into structural terms (“ions move closer together”) is a plausible hypothesis, but it should be formulated more cautiously and supported by other evidence (e.g., band shifts (section 2.1) or comparison with previous work).
- Similarly, the presentation of the sequence of AS efflorescence (1084 > 1097 > 1463 > 1417 cm-1) might be too strong. A more appropriate phrasing would be that the spectral variations at 1084 cm-1 appear earlier in the humidity-driven process than those at 1097 cm-1, and so on. The physical interpretation of these sequences (e.g. as bond dissociation or ion rearrangements) should remain a hypothesis rather than a demonstrated fact. What the asynchronous 2D-COS actually shows is that spectral variations at 1084 cm-1 occur earlier in the RH-driven process than those at 1097 cm-1, and so on. I recommend reformulating in that sense, to avoid over-interpretation.
- Following the latter point, the concluding paragraph of section 2.3 should be revised. The mechanistic details proposed (hydrogen bond dissociation, ions approaching) go beyond what FTIR/2D-COS can directly demonstrate, and should be framed as hypotheses rather than conclusions. The final statement on NH4+ surface enrichment is particularly problematic: this cannot be inferred from the present data and appears to be borrowed from Tian et al. (2011). I recommend the authors either remove this claim or clearly state it as literature context, not as a conclusion of their own work.
- Section 2.4 concludes that the microscopic kinetic evolution of AS efflorescence is elucidated ‘at the molecular level.’ This phrasing is too strong. 2D-COS analysis does reveal sequential spectral changes and can suggest a two-step efflorescence pathway, but it does not directly provide molecular-level kinetics. Such interpretations require careful assumptions and should be stated more cautiously, e.g. as ‘the data are consistent with a multi-step efflorescence mechanism’ rather than as a direct elucidation of molecular dynamics.
More minor mistakes are present along the manuscript such as English wording, capital letters or citation format. Although this is not in itself problematic, these small issues accumulate and could distract readers. Careful proofreading would help improve the overall readability and presentation.
In conclusion, I think this manuscript illustrates the difficulty of writing method-focused papers, which require a careful balance between presenting the technique itself and showing how it is validated. In its present form, the introduction and framing currently do not meet the standards expected for AMT. This, in my view, is less a reflection of the first author’s effort than of the need for closer guidance from the supervisors in positioning the work. With stronger supervision and restructuring, I believe this study could evolve into a much more solid contribution, and I encourage the authors to pursue this line of research.
Citation: https://doi.org/10.5194/egusphere-2025-2662-RC2
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The manuscript entitled "Identification of Micro-dynamics Phase Transition processes for Ammonium Sulfate aerosols by Two-dimensional Correlation Spectroscopy" investigates the micro-dynamic mechanisms of ammonium sulfate (AS) aerosol phase transitions using two-dimensional correlation infrared spectroscopy (2D-IR), coupled with relative humidity (RH) control. By employing 2D-IR (including generalized 2D-IR and perturbation-correlation moving window 2D (PCMW2D) spectroscopy), the authors successfully elucidate non-equilibrium micro-dynamic processes during AS efflorescence, revealing four distinct sequential steps at the molecular level. This approach advances beyond conventional methods (e.g., ESEM, H-TDMA) that primarily characterize physical parameters (size, shape), offering unprecedented insights into intermolecular interactions (e.g., hydrogen bond dissociation, ion reconfiguration). The manuscript may be suitable for publication after major revisions and addressing the following concerns.