the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Nov 2024
Base synergy in freshly nucleated particles
Abstract. Sulfuric acid (SA), ammonia (AM) and dimethylamine (DMA) are believed to be key contributors to new particle formation (NPF) in the atmosphere. NPF happens through gas-to-particle transformation via cluster formation. However, it is not obvious how small clusters grow to larger sizes and eventually form stable aerosol particles. Recent experimental measurements showed that the presence of mixtures of bases enhance the nucleation rate several orders of magnitude. Using quantum chemistry methods, this study explores this base synergy in the formation of large clusters from a mixture of SA, AM, and DMA. We calculated the binding free energies of the (SA)n(AM)x(DMA)n−x clusters, with n from 1 to 10, where x runs from 0 to n. The cluster structures were obtained using our recently developed comprehensive configurational sampling approach based on multiple ABCluster runs and metadynamics sampling via CREST. The structures and thermochemical parameters are calculated at the B97-3c level of theory. The final single point energy of the clusters is calculated at the ωB97X-DJB3/6-311++G(3df,3pd) level of theory.
Based on the calculated thermochemistry, we found that AM, despite being a weaker base, forms more intermolecular interactions than DMA and easily becomes embedded in the cluster core. This leads to the mixed SA-AM/DMA clusters being lower in free energy compared to the pure SA–AM and SA–DMA clusters. We find that the strong base DMA is important in the very initial steps in cluster formation, but for larger clusters an increased ammonia content is found. We also observed that the cluster-to-particle transition point for the mixed SA–AM–DMA clusters occurs at a cluster size of 14 monomers, which is notably smaller than the transition points for the pure SA-AM (16 monomers) or pure SA–DMA (20 monomers) systems. This indicates a strong synergistic effect when both AM and DMA are present, leading to the formation of stable freshly nucleated particles (FNPs) at smaller cluster sizes. These findings emphasize the importance of considering several base molecules, when studying the formation and growth of FNPs.
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RC1: 'Comment on ar-2024-28', Anonymous Referee #2, 21 Nov 2024
Galib Hasan et al. have conducted an in-depth study on the formation of freshly nucleated particles (FNPs) driven by gaseous sulfuric acid (SA), highlighting the synergistic stabilization effects of significant nitrogen bases, i.e., ammonia (AM) and dimethylamine (DMA) through quantum chemical calculations. Most prior theoretical research has been limited to small clusters, typically those with eight molecules or fewer, focusing on the nucleation stage. The cluster-to-particle transition, a critical yet underexplored process, presents significant challenges for both theoretical and experimental studies. This paper advances our understanding of FNP formation progress, with considerable atmospheric significance. The manuscript is well-organized, data-rich, and analytically rigorous, and hence I am delighted to recommend its publication in Aerosol Research journal after addressing my following comments.
Major Comments:
- The study examines the “cluster-to-particle transition point” and concludes that 14 monomers are needed for SA–AM–DMA clusters, 16 monomers for pure SA–AM, and 20 monomers for pure SA–DMA system. However, I am unclear about the criteria used to define the "point" of this transition. It seems not to be solely based on cluster size. So, I kindly suggest the authors clarify this to help other readers avoid similar confusion.
- In the Refined Single Point Energies section, the detailed discussion of errors caused by different calculation methods might be excessive for the main readership of Aerosol Research, who are likely more interested in the environmental impacts. Thus, selectively reducing or relocating some of the detailed content to the Supporting Information may be better, leaving the key results in the main text.
- The authors have systematically studied the SA–AM–DMA system under conditions of low and high temperature as well as low and high concentration, which is highly commendable. Yet, if feasible, relating these findings to specific environmental conditions and discussing their atmospheric significance would further strengthen the environmental implications of the article.
Minor Comments:
- Line 52: Dimethylamine à dimethylamine
- Line 115: To enhance clarity, please include the full term "Density Functional Theory" when the abbreviation "DFT" is introduced for the first time (line 91).
- Please note that in Figure 4's caption, yellow is incorrectly assigned to nitrogen. It should denote sulfur instead.
- Line 221: “single point refinement” is repeated twice in the text. Please correct this typographical error.
Citation: https://doi.org/10.5194/ar-2024-28-RC1 -
RC2: 'Comment on ar-2024-28', Anonymous Referee #1, 26 Nov 2024
Galib Hasan et al. have studied the formation of freshly nucleated particles from sulfuric acid and two bases, ammonia and dimethylamine, using quantum chemical methods. The manuscript shows the synergetic impact of these two bases on particle formation, and how, despite being a weaker base, larger clusters with addition of one to three ammonia monomers are more stable than clusters consisting purely of sulfuric acid and dimethylamine. The study also bring insight into the transition from clusters to particles, a question that has for long remained open. It is relatively well-written and easy to follow, and the analysis itself is well-made. Overall,, this study by Galib Hasan et al. furthers our understanding of the formation of freshly nucleated particles, and therefore I recommend it to be accepted for publication after the following comments have been addressed.
General comments
The transition of a cluster to a particle is a big part of the main conclusions and relevance of the paper. Therefore, it should be explained more than it is. Now it remains unclear for the reader how exactly the point of transition is defined, and what it is based on.
I am also missing some discussion on environmental impacts and the relation of these results to the real atmosphere. For example, conditions with two different temperatures and concentration regimes are studied, and these could be more clearly related to real atmospheric environments. In addition, the differences in the results for these different conditions and their impacts on atmospheric cluster formation could be discussed further.
There are some typos, formatting issues and minor grammatical issues, and I suggest the authors go over the text with care. Many of these issues are specified in the detailed comments.
Specific comments
Introduction
1. The first paragraph is half a page long, which makes it harder to read. It could be divided into at least two separate paragraphs.
2. L37: While this sentence does not really need a reference to begin with, referring to a study over 10 years old does not really fit the point it is trying to make.
3. L85: Would it be possible to explain a bit more in detail what this property-based criteria for determining the boundary between clusters and particles is?
Methods:
4. L134: Please define how this error is determined.
Results
5. Figure 3 and the respective text. The only clusters that are labeled are on the upper and lower edges. Therefore, on e.g., L194 (L197), where clusters (0,7) and (0,8) ( (0,9) and (0,10) ) are discussed, I cannot see what points refer to these clusters and where the free energy values referred to in the text are read from. This should be somehow clarified, the reader should also be able to see where the values come from.
6. L202: As this transition from cluster to particle is such a big part of the conclusions of the manuscript, the concept and why it is interpreted so should be explained a bit more. What is the threshold value of the averaged binding free energy change from cluster to cluster after which transition to particle is considered to have occurred? And why? In addition, I think a brief explanation on why the leveling out of the averaged binding free energy can be thought of as the cluster transitioning to a particle would also help in clarifying the concept and the relevancy of this paper for the readers who might be less familiar with the topic.
7. L219: “Hence, the cluster-to-particle transition point occurs at a cluster size of 14 monomers…” Please explain how this is determined from Figure 3. In addition, as such a large part of aerosol research is concerned with diameters of clusters/particles, could rough estimations of the diameter these clusters correspond to be given?
8. L229: In real world, sulfuric acid concentration also varies between environments. Would including that variation affect the results?
Technical comments
L46: Formatting issue with reference: “...base molecules Almeida et al. (2013).”
L59: “… in the stabilizing the initial SA clusters” → “… in stabilizing the initial SA clusters”
L155: “… the GFN1-xTB (Grimme et al., 2017) semi-empirical method.” → “… the GFN1-xTB semi-empirical method (Grimme et al., 2017).” When possible, placing the reference to the end of sentence, improves readability of the text.
L180: “We previously Wu et al. (2024) tested …” should be reformulated e.g., “In Wu et al. (2004), we previously tested …”
L185: “ … clusters are taken from (Wu et al., 2024),” → “… clusters are taken from Wu et al. (2024),”
L186: Figure 3a, not 2a, I presume.
Figure 3 caption:
“… ≤ n., under …” → “… ≤ n, under …”
“(298,15 k and 1 tm)” → “(298.15 k and 1 atm)”
Figure 4 caption: I would presume yellow is supposed to be sulfur, not nitrogen.
L210: “AM molecule, makes it”→ “AM molecule makes it”
L221: “single point refinement single point refinement.”
L236: “energies in Figure 5a, shows that” → “energies in Figure 5a show that”
L244: Please define MA and TMA.
L251: “around 4 monomers” Please clarify that this is for the SA-AM.
L235, L245, L257 and L265: “298,15 K” → “298.15 K” , “278,15 K” → “278.15 K”
Citation: https://doi.org/10.5194/ar-2024-28-RC2
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