the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jun 2026
Perchloric acid (HClO4) Drives Atmospheric New Particle Formation Enhanced by Dimethylamine, ammonia and Sulfuric Acid: Mechanisms and Implications
Abstract. Recent studies have revealed observations of atmospheric perchloric acid (HClO4, PA) in the Arctic. There are few studies of PA forming aerosol particles in coastal marine regions. We use quantum chemical calculations and Atmospheric Clusters Dynamic Code (ACDC) to compare the enhancement potential of dimethylamine (DMA), ammonia (NH3), and sulfuric acid (SA) for PA-based new particle formation (NPF). The results show that DMA and NH3 can strongly interact with PA in both directions through hydrogen bonding and proton transfer. Halogen bonding is not found in PA-DMA and PA-NH3 clusters. Even if the concentration of NH3 exceeds that of DMA by 10–100 orders of magnitude, the cluster formation rate of PA-DMA cluster formation is much higher than that of the PA-NH3 cluster system. Clusters with the same number of PA molecules as DMA molecules play a key role in the growth of PA-DMA clusters. Compared with the nucleation of PA with SA, PA nucleates more easily with alkaline gas. The present results reveal the potential for new particle formation of PA in the Arctic boundary layer.
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Status: open (until 24 Jul 2026)
- RC1: 'Comment on egusphere-2026-1986', Anonymous Referee #1, 27 Jun 2026 reply
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RC2: 'Comment on egusphere-2026-1986', Jonas Elm, 30 Jun 2026
reply
Wang et al studies the role of perchloric acid (PA) in atmospheric cluster formation involving sulfuric acid (SA), ammonia (NH3) and dimethylamine (DMA). The cluster configurations are searched with the ABCluster program and the applied computational methods are state-of-the-art for the problem at hand.
Overall, while the present study is interesting and the quantum chemical data is calculated at a decent level. However, I am missing some further interpretation of the results. I have three major critiques of the manuscript, that I would like the authors to address before I can recommend publication:
1) The introduction does not cover our knowledge on cluster formation from quantum chemical studies. I.e. what have other people done and how does the current work fit in.
2) The methods (albeit state-of-the-art) are not justified. The authors should explain why certain functionals/methods are chosen based on benchmarks.
3) The discussion is written in a very observant style and does not properly put in the literature into context.Each of these points are further elaborated in the comments below.
Comments
Line 30: “The results show that DMA and NH3 can strongly interact with PA in both directions through hydrogen bonding and proton transfer.”
I do not understand the “both directions” part of the sentence here.
Line 32: “Even if the concentration of NH3 exceeds that of DMA by 10-100 orders of magnitude, the cluster formation rate of PA-DMA cluster formation is much higher than that of the PA-NH3 cluster system.”
This is a consistent finding with other acid-base clusters. Please elaborate on that this is not a unique finding and refer to the relevant literature.
Line 35: “Clusters with the same number of PA molecules as DMA molecules play a key role in the growth of PA-DMA clusters.”
Similar to above, this is a usual property of acid-base clusters. Please elaborate on this aspect in the manuscript.
Line 37: “The present results reveal the potential for new particle formation of PA in the Arctic boundary layer.”
I believe PA clusters were also studied in the recent work by Engsvang et al. (https://scholar.google.dk/citations?user=mVvvA3wAAAAJ&hl=da&oi=ao). The current work should be put into context with this study. See more below.
Line 46: “At least 50% of the overall concentration of aerosol particles in the atmosphere is believed to be attributed to new particle formation (NPF) (Ehn et al., 2014).”
I do not believe this statement is a finding of Ehn et al 2014. Please find the original reference and give proper credit to the original work.
Line 61: “NPF can occur through a variety of nucleation pathways and involves important precursors like sulfuric acid (H2SO4), iodine oxoacids (HIOx, HIO2, and HIO3), low-volatility organic compounds, ammonia (NH3), and amines, as shown by laboratory experiments and theoretical calculations (He et al., 2023; He et al., 2021;Kirkby et al., 2023).”
As this is a theoretical study there should be specific emphasis on what we know about cluster formation from calculations. What studies have there been on sulfuric acid-base cluster and clusters involving iodine oxides/oxoacids. You should put the current work into context of the existing literature.
Line 74: “Previous studies have found that chloric acid makes a relatively small contribution to new particle formation (Wang et al., 2025).”
Quite a bit earlier than Wang et al. Engsvang (https://scholar.google.dk/citations?user=mVvvA3wAAAAJ&hl=da&oi=ao) studied both chloric/perchloric acid clusters with bases and other acids. This should be mentioned in the current work.
Line 75: “Perchloric acid is a major component of chloric acid, …”
I do not understand this sentence. Do you meant that chloric acid is a precursor to perchloric acid?
Line 79: “The concentration of DMA surpasses 3 parts per trillion by volume, according to experiments, and the nucleation rates of DMA are significantly higher than those of ambient ammonia (Almeida et al., 2013).”
From the abstract and introduction, I get the impression that the authors are targeting the Arctic. The work by Almeida et al. is from CLOUD chamber experiments. Is the concentration of DMA in the Arctic also reaching 3 ppt? The authors should clearly convey what region they are interested in modelling.
Computational methods:
- Funelling approach: The applied computational workflow follows a standard routine, which is inspired by the funnelling approaches by Temelso et al. (https://doi.org/10.1021/acs.jpca.7b11236) and Kubecka et al. (https://doi.org/10.1021/acs.jpca.9b03853). It would be worth mentioning these works in the paper for justifying the workflow. What parameters were used in the ABCluster calculations.
- Choice of DFT functional: Why did you choose the ωB97X-D functional for your study? Please justify your choice by referring to potential benchmarks that demonstrate that this functional performs well for atmospheric molecular clusters.
- Choice of basis set: Please justify the choice of the 6-31++G(d,p) basis set for the final geometry optimization and vibrational frequency calculations.
- DLPNO-CCSD(T)/aug-cc-pVTZ: Please justify that DLPNO-CCSD(T)/aug-cc-pVTZ yield sufficiently accurate binding energies of the clusters. Which PNO convergence criteria were used in the calculations and were (T) fully iteratively calculated?
Line 120: “The results of the experiments employing the birth and death equations and the conclusions of the ACDC simulations correspond well(Mcgrath et al., 2012b).”
I do not understand this sentence. Which experiments are you referring to here? Also, the references Mcgrath et al., 2012a and Mcgrath et al., 2012b appears to be the same.
Line 126: “According to the ACDC manual (Mcgrath et al., 2012b), a system size of 6 molecules is large enough.”
To the best of my knowledge there is no manual in that paper. However, there is a manual at Tinja Olenius’ GitHub page. However, I cannot find the mentioning of 6 molecule clusters being “large enough”. Consider deleting this sentence.
Line 127: “The resulting PA-DMA systems (PA)5(DMA)5 cluster is set as boundary clusters.”
This seems like an odd choice for the boundary clusters and will have an impact on the dynamics. If only the 5-5 cluster is allowed to grow out and contribute to the nucleation rate, the simulations will artificially not allow monomer collisions to contribute to the rate. Perhaps the (PA)5(DMA)4 cluster should also be added to the boundary clusters.
Line 128: “The concentration ranges of [PA], [SA], [DMA] and [NH3] were defined at 106 −108 cm−3, 106 −108 cm−3, 0.1−100 ppt and 1−100 ppt, respectively”
Could the authors elaborate on why these concentration ranges were chosen and what region it may correspond to? For instance, as written in line 72 the PA concentration has been detected in an upper limit of 106 molecules cm-3, so why is this the lower limit investigated?
Line 133: Section 3 is very descriptive in nature, without many explanations about the implications of the findings. In each subsection, when describing your findings, please put it into context in the broader field of atmospheric chemistry.
Line 136-156: In Figure 1 and the surrounding text, how does these trends compare to the (PA)1-2(DMA)1-2 cluster structures obtained by Engsvang et al? Do you see large differences in the obtained cluster structures and stabilities?
Line 142 and 153: “N−H...Cl−O”
I guess this should be “N−H...O−Cl”
Line 160: “… indicating that pure PA molecules are thermodynamically less susceptible to forming clusters.”
Less susceptible compared to what in this context? Do you mean not susceptible?
Line 164: “Furthermore, the negative correlation between temperature and the ΔG values of PA−DMA clusters is discovered (Figure S7, S8 and S9), suggesting that the stability of PA−DMA clusters diminishes as the temperature rises.”
This is a natural consequence of the clustering process. Upon clustering the enthalpy and entropy change is negative. Hence, the free energy will always be lower at lower temperature. I suggest you remove this sentence.
Line 168: “,… which suggests the possibility of collisional growth of DMA with (PA)1−4 clusters.”
Did you not just establish in line 160 that the (PA)1-4 clusters will not form?
Line 172: “The PA−SA cluster system exhibits the greatest ΔG values among the PA−DMA, PA−NH3, and PA−SA cluster systems, suggesting that it is a thermodynamically challenging system.”
“Thermodynamically challenging“ sounds a bit odd. Perhaps say “thermodynamically least stable” instead?
Line 180: “The evaporation rate of DMA-rich clusters is higher than that of PA-rich clusters, as illustrated in Fig. 3, suggesting that clusters with a high percentage of PA molecules are more stable.”
I would perhaps not say “a high percentage of PA molecules are more stable”. Consider saying “with more PA molecules are more stable” instead.
Line 197: “At 278 K, for the PA−DMA cluster system, (PA)1(DMA)1, (PA)2(DMA)2, (PA)3(DMA)3, and (PA)4(DMA)4 clusters are the primary pathways and the growth process is unimpeded.”
The fact that the clusters on diagonal has the lowest actual free energy has also been seen in other acid-base systems (Olenius et al (https://doi.org/10.1063/1.4819024 ) and Elm et al (https://doi.org/10.1021/acs.jpca.7b08962)). It would be good to discuss that this phenomenon is consistent with the literature.
Line 223: “As the temperature decreases (from 298 K to 258 K), the J value of the PA-DMA cluster system increases.”
As also discussed above, this is a direct consequence of lower T leading to a lower free energy and in turn leading to a lower evaporation rate. Please elaborate on this aspect.
Line 226: “The J value of the PA-DMA system is 1.65 cm−3 s−1 at 258 K, [DMA]=1 ppt and [PA]=106 cm−3.”
Please elaborate on what this finding tells us about the role of PA nucleation in the Arctic atmosphere. Is this competitive with other nucleation schemes such as iodine/sulfuric acid nucleation?
Line 244: “Eventually (PA)5(DMA)4 and (PA)5(DMA)5 clusters are stable enough to grow from the PA-DMA system.”
I do not understand how the (PA)5(DMA)4 cluster can grow out of the system. In the method section it was stated that only (PA)5(DMA)5 clusters were allowed to grow out.
Line 264: “, … thus promoting the rapid formation of …”
Technically, the free energies do not tell us anything about the formation rate. Perhaps remove the word “rapid” here.
Line 284: “This study extends this paradigm to a overlooked ocean-derived chlorinated precursor: perchloric acid.”
And line 300: “… previously unrecognized atmospheric “sink” for HClO4.”
This gives the impression that the authors are the first to state that perchloric acid could be important for nucleation in the marine atmosphere. This was also stated in the work by Engsvang et al. Please tone down such statements.
Citation: https://doi.org/10.5194/egusphere-2026-1986-RC2
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General Comments
Utilizing a combination of quantum chemical calculations and cluster dynamics simulations (ACDC), Wang et al. investigated the clustering mechanisms of perchloric acid (HClO4, PA) with sulfuric acid (SA), dimethylamine (DMA), or ammonia (NH3). The cluster configurational space is explored using standard methodologies, relying on the global search algorithm of the ABCluster program and narrowing down the generated configurations using a funneling approach. Single-point energy calculations were then performed at the DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-31++G(d,p) level of theory, followed by the simulation of cluster formation rates and growth pathways using ACDC.
To further strengthen the work, I suggest expanding additional discussion to better emphasize the atmospheric implications of the findings. Additionally, some improvements to the formatting and language are needed. Overall, the study presents valuable computational results and provides useful insights into the chemical mechanisms of PA cluster formation and will be suitable for publication after minor revision. Therefore, I recommend a Minor Revision. Detailed comments are provided below.
Major Comments
Since halogen bonding is absent in all studied PA-containing clusters, repeatedly emphasizing this finding across multiple sections of the manuscript is unnecessary. Additionally, the observation that PA nucleates more easily with alkaline gases than with SA is an important and distinctive finding; this conclusion would be more impactful if introduced earlier in the abstract (e.g., at line 31).
The research rationale requires further elaboration: the introduction does not provide a sufficiently discussion of the potential atmospheric significance of PA in new particle formation (NPF). The authors should clarify why, given that chloric acid shows limited contributions to NPF, the individual component PA is worth investigating separately.
The manuscript describes the global minimum structure search and thermodynamic calculations but does not explain why specific methods, such as PM7 for pre-optimization, ωB97X-D for geometry optimization, and DLPNO-CCSD(T) for single-point energies, were selected. The authors should briefly justify the choice of each method and address their suitability for acid–base cluster systems. The authors are also encouraged to reference the supplementary information more systematically in the main text; while the supplementary material appears to include additional figures and computational details, these are not yet described or formally cited in the Methods section.
The study calculates ΔG values at 238 K, 258 K, and 278 K. However, the rationale for selecting these specific temperatures is not adequately explained in the context of the Arctic or mid-latitude marine boundary layer. Additionally, the figure caption for Figure 4 does not specify the concentrations of PA or DMA used when computing the actual Gibbs free energies.
One important issue in this study concerns the atmospheric representativeness of the simulated conditions. The PA concentration range used in the ACDC simulations (106 to 108 cm-3) is significantly higher than the concentrations recently observed in the Arctic atmosphere by Tham et al. (2023, Nature Communications, https://doi.org/10.1038/s41467-023-37387-y). As a result, the calculated nucleation rates likely overestimate what would occur under realistic atmospheric conditions. Furthermore, the concentration ranges assumed for SA and DMA are also not supported by observational references in the main text.
The paper notes that the PA-DMA cluster system exhibits the fastest nucleation rate among the three systems studied, but it does not provide an in-depth quantitative comparison with other atmospherically important nucleation mechanisms. To demonstrate the significance of PA as a nucleation precursor, the authors should compare the PA-DMA system’s nucleation rates with those of well-established systems such as SA-DMA, SA-NH3, and iodine oxoacid-driven nucleation (e.g., HIO3-DMA), as reported in recent literature.
Minor Comments