the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Feb 2024
Technical note: A theoretical study on the mechanism of citric acid-driven multi-component nucleation of sulfuric acid-base-water clusters
Abstract. New particle formation (NPF) is one of the important sources of aerosol and an important reason for the rapid increase of PM2.5 mass concentration in polluted areas. It has been shown that citric acid, present in atmosphere, is a potential precursor of new particles and may play a role in the nucleation process of new particles. However, the exact mechanism by which citric acid contributes to nucleation remain unclear. The thermodynamically stable geometry of SA·AM·Wn·CAm, SA·DMA·Wn·CAm, and SA·AM·DMA·Wn·CAm (n = 0–4, m=0–1) clusters were optimized and the Gibbs free energy and hydration distribution were calculated at the M06-2X/6-311+G(2d, p) level in this study. The results demonstrate that three carboxyl groups (-COOH) and one hydroxyl group (-OH) of citric acid can act as both hydrogen donors and acceptors through hydrogen bonding interactions with sulfuric acid-base-water clusters. This interaction lowers the nucleation barriers (△G﹤ 0), indicating an energetically favorable reaction. At three relative humidities (RH), anhydrous and monohydrate forms dominate in SA·AM·Wn·CA clusters, as well as SA·AM·DMA·Wn·CA (n = 0–4) clusters; while anhydrous form dominates in SA·DMA·Wn·CA (n = 0–4) clusters. These findings suggest that citric acid reduces the hydrophilicity of these clusters. In conclusion, the involvement of citric acid in atmospheric processes is conductive to cluster formation, thereby facilitating the multicomponent nucleation of sulfuric acid-base-water clusters. Overall, our study highlights how citric acid participates in and enhances new particle formation processes.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2023-3113', Jonas Elm, 13 Mar 2024
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RC2: 'Comment on egusphere-2023-3113', Anonymous Referee #2, 15 Mar 2024
The manuscript presents quantum chemical calculations addressing the role of citric acid in aerosol nucleation. The authors calculate Gibbs free energies of molecular clusters consisting of sulfuric acid (SA), ammonia (AM) and/or dimethylamine (DMA) and citric acid (CA), one molecule each, with 0-4 water (W) molecules. Also the level of cluster hydration with and without CA is studied.
While the topic is relevant for Atmospheric Chemistry and Physics, quantitative effects of citric acid on molecular cluster stability, especially with respect to other precursor gases participating in nucleation, are not shown. Only free energies don’t give information of cluster kinetics parameters relevant to nucleation, namely cluster evaporation rates or nucleation rates. Also, the language of the manuscript is very unclear, and the text should preferably be revised thoroughly by a native English speaker. Therefore, the work can’t be recommended for publication in its current state, and needs substantial revisions to be considered.
Main comments
The authors study the thermodynamics of the addition of CA to the SA•base•Wn clusters, finding the reaction to be favorable. In general, it’s not surprising that the addition of an acid molecule to an acid•base cluster is favorable. However, the more relevant questions are:
(1) How does the stability of the SA•base•CA•Wn cluster compare to the stability of e.g. the (SA)2•base•Wn cluster? That is, when different precursors such as SA and CA are available, which types of clusters are more likely to form?
(2) How does the addition of CA affect the total evaporation rate (i.e. lifetime) of SA•base•Wn? (See e.g. Ortega et al., 2012; https://doi.org/10.5194/acp-12-225-2012.) Cluster evaporation rates give direct information of the potential effect of CA. Strong conclusions (L338-341) can’t be drawn solely based on the presented Gibbs free energies.
(3) While hydrate distributions are presented, they don’t give information on absolute cluster stabilities, as hydration may stabilize or destabilize clusters (Henschel et al., 2016; https://doi.org/10.1021/acs.jpca.5b11366). It’s stated that the addition of a CA molecule lowers the hydrophilicity of some clusters, but how does this affect cluster evaporation (lifetime)?
(4) In general, the studied molecular clusters are too small for assessing the nucleation potential of CA. Cluster stabilization may, and often does, occur at larger cluster sizes consisting of several acid and base molecules, and these sizes are also important for determining the particle formation rate. This should be made clear.
Computational methods:
(5) L90: genmer tool: How does the procedure compare with other currently used configurational sampling methods (e.g. Kubečka et al., 2023; https://doi.org/10.1021/acsomega.3c07412)?
(6) Density functional theory (here M06-2X) is not the most reliable quantum chemical method for single-point energies. In Table 1, there don’t seem to be comparisons to higher-level methods. The last column does not give any information of method or source for the reference values “Refs”. What are the superscript numbers in the Table?
Comparing results from different density functionals generally doesn't tell much about the accuracy of any of them. What does the expression “basic reactions” in the table caption mean or refer to?
(7) There are unclear statements in the Methods section, and the text should be carefully revised. For example:
L113: The expression “global minimum structure on the potential energy surface, which is a steady state” is bizarre. How is it a steady state? Furthermore, the authors state to have calculated the “infrared spectra” which is not mentioned anywhere else in the manuscript.
L124: The expression “For a chemical reaction in which a number of molecules form a cluster” is not exactly correct; this is a chain of reactions, and the quantity (formation free energy) is not an actual reaction energy but a standard measure of cluster stability.
Other:
(8) It’s often not clear what the energy quantities discussed in the text refer to, i.e. reaction energies (of which reaction, e.g. hydration or CA addition) or formation free energies. Please revise the text for these occurrences.
(9) Figure 5 shows data for larger clusters not included in the study. Where do these suddenly introduced cluster data come from?
Minor comments and language
In the manuscript title, “driven” should be changed to “enhanced” since the clustering is presumably mainly driven by SA.
Large part of the Results section is devoted to describing the cluster structures. This part should be shortened substantially (or moved to the supplement) as it’s not very relevant for assessing CA effects on nucleation.
L32: Background: the Yao et al. reference is not suitable for the statement; there are plenty of general references on the topic, some of which are cited later on in the text and should be cited here, too.
The English is very bad: there are lots of grammatically erroneous, repetitive and even incomprehensible sentences that are in some cases simply incorrect. The text needs a very thorough revising. To give a couple of examples:
L35: “Nucleation is evident in the creation of thermodynamically stable molecular clusters…” How is it evident? Formation of clusters is called nucleation.
L37: “New particles exhibit a wide range of shapes and sizes …” The shape of newly formed ~1 nm particles is hardly relevant, and the particles certainly don’t exhibit a wide range of sizes.
L59: “Citric acid (C6H8O7) is a tricarboxylic acid containing one hydroxyl group, which have been detected in sea salt particles along the Atlantic coast, being present in atmospheric particles and may be directly emitted from tangerine is likely adsorbed on pollens emitted from Japanese cedar, suggesting that citric acid may be involved in new particle formation, and thus citric acid is a potential precursor for new particles.” The sentence is rather incoherent, and the existence of CA in sea salt particles doesn’t suggest that it’s a nucleation agent.
Citation: https://doi.org/10.5194/egusphere-2023-3113-RC2
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