the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jul 2026
Trifluoroacetic acid enhances sulfuric acid–ammonia nucleation in the cold atmosphere: Molecular mechanism and atmospheric implications
Abstract. New particle formation (NPF) is a major source of atmospheric particulates and cloud condensation nuclei (CCN), yet conventional sulfuric acid (SA)–ammonia (NH3) nucleation cannot fully explain the observed NPF and CCN generation. While recent CLOUD studies have shown that nitric acid can enhance SA–NH3 nucleation in the cold upper troposphere [Nature, 605, 483-489, 2022], this mechanism can only explain particle formation under certain cold atmospheric environments. Here, we use trifluoroacetic acid (TFA) as a model perfluorocarboxylic acid (PFCA) to investigate the stabilizing effect of PFCAs on SA–NH3 clusters, given their atmospheric nucleation relevance, long lifetime, and widespread distribution. Using quantum chemical calculations with Atmospheric Cluster Dynamics Code simulations, we find that TFA forms stable cage-like SA–NH3–TFA clusters via strong hydrogen bonds and proton transfer. At 220 K (favourable cold conditions), TFA enhances the SA–NH3 nucleation rate by up to 950-fold and contributes up to 93 % of the main simulated growth flux at low SA. TFA assisted stabilization represents a potentially efficient pathway for SA–NH3 nucleation in cold atmospheres. This mechanism is most directly relevant to the cold boundary layer environments where TFA has been measured. In the upper troposphere, its role serves as a critical low-temperature mechanistic insight, pending direct observations of gas-phase TFA aloft. This study provides a molecular-level foundation for understanding general PFCA-enhanced nucleation mechanisms in cold atmospheric environments.
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Status: open (until 31 Aug 2026)
- RC1: 'Comment on egusphere-2026-2877', Anonymous Referee #1, 30 Jul 2026 reply
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RC2: 'Comment on egusphere-2026-2877', Anonymous Referee #2, 02 Aug 2026
reply
This study investigates the potential role of trifluoroacetic acid (TFA), a representative perfluorocarboxylic acid (PFCA), in atmospheric new particle formation by combining quantum chemical calculations combined with Atmospheric Cluster Dynamics Code (ACDC) simulations. The potential involvement of persistent fluorinated organic acids in atmospheric nucleation has received increasing attention. However, their roles in molecular clustering and early-stage particle formation remain poorly constrained. This work provides molecular-level insights into TFA-containing sulfuric acid–ammonia (SA-NH3) clusters and their potential influence of these clusters on nucleation processes under cold atmospheric conditions. The combination of quantum chemical calculations and kinetic cluster simulations provides an appropriate framework for linking molecular-scale interactions with atmospheric nucleation processes. The analyses of cluster structures, thermodynamic properties, kinetic behavior, and formation pathways contribute to a better understanding of TFA-assisted nucleation mechanisms. Taken together, the findings provide valuable molecular-scale insights into the potential role of fluorinated organic acids in atmospheric nucleation chemistry. The manuscript would be suitable for publication after the authors carefully consider and address the following comments.
Specific Commnets:
- The authors investigate hydrated clusters and assess the influence of relative humidity on cluster stability. Because water molecules can significantly affect hydrogen-bonding networks and the thermodynamic properties of atmospheric clusters, the structural and energetic information of these hydrated clusters is important for evaluating the calculated results. The authors should provide the Gibbs free energies of formation and optimized Cartesian coordinates of the studied hydration clusters in the Supplement. Including these data would improve the reproducibility of the calculations and allow more detailed evaluation of the hydration effects discussed in the manuscript.
- The reliability of the predicted nucleation enhancement depends strongly on the completeness of conformational sampling. The authors should provide additional details on the sampling strategy and clarify whether selecting 100 low-energy structures after PM7 optimization is sufficient to identify the global minimum structures of the investigated clusters.
- Since UFF may not accurately capture proton transfer or bond rearrangement, the authors should clarify whether ionic configurations or structures involving proton transfer were considered during the initial sampling. This information is important for assessing the reliability of the predicted cluster structures and subsequent nucleation simulations.
- The authors should further justify the boundary conditions used in the ACDC simulations and discuss their atmospheric representativeness. Since simulated nucleation rates are sensitive to precursor concentrations and environmental parameters, the basis for selecting the concentrations of sulfuric acid, ammonia, and TFA, as well as temperature and relative humidity, should be clarified. This information would help evaluate the relevance of the predicted nucleation enhancement under different atmospheric conditions.
- The relevant statement should be further specified to better reflect the atmospheric conditions under which the conclusion applies. For example, the statement that “the SA-NH3-TFA ternary mechanism emerges as a particularly critical contributor to the clustering process” could be revised to “the SA-NH3-TFA ternary mechanism emerges as a critical contributor to the clustering process under cold atmospheric conditions.”
- The caption of Fig. 5 should clearly indicate that pathways contributing less than 5% to the total flux are excluded from the pathway analysis.
- The manuscript compares the roles of TFA and nitric acid in SA-NH3 A quantitative comparison would strengthen the mechanistic interpretation of the proposed analogy. For example, differences in cluster formation free energies, evaporation rates, and collision coefficients between TFA- and HNO3-containing clusters could provide further insight into their respective thermodynamic and kinetic effects on nucleation.
- The treatment of the sticking coefficient in the ACDC simulations should be clarified. Since the sticking coefficient can influence cluster formation kinetics and the resulting particle formation rates, a brief discussion of its uncertainty and potential impact on the simulated results would be helpful.
Technical issues:
- The description should be made more precise in terms of terminology. The statement that “SA, NH3, and TFA can form cage-like clusters stabilized by the hydrogen-bond network” should be revised to “SA, NH3, and TFA can form cage-like structures stabilized by the hydrogen-bond network.”
- The terms “nucleation rate” and “cluster formation rate” should be used consistently throughout the manuscript.
Citation: https://doi.org/10.5194/egusphere-2026-2877-RC2
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- 1
The authors report a molecular-level investigation of how trifluoroacetic acid (TFA), used as a model perfluorocarboxylic acid with long lifetime and widespread distribution, enhances sulfuric acid (SA)–ammonia (NH3) nucleation under cold atmospheric conditions, using quantum chemical calculations combined with Atmospheric Cluster Dynamics Code simulations. The results show that TFA can form stable cage-like SA–NH3–TFA clusters through strong hydrogen-bond networks and proton-transfer-induced electrostatic interactions. Thermodynamic and kinetic analyses further indicate that these clusters become increasingly stable at lower temperatures. The simulated particle formation rates and cluster concentrations are enhanced by TFA, particularly under low-SA, high-NH3, high-TFA, and low-temperature conditions, and the pathway analysis suggests that TFA-containing growth routes can dominate the simulated clustering flux under favorable cold conditions. Importantly, the authors appropriately frame the atmospheric relevance of this mechanism differently for the cold boundary layer and the upper troposphere: the former is better supported by existing TFA observations, whereas the latter remains an exploratory sensitivity scenario due to the lack of direct gas-phase TFA measurements aloft. The calculations are performed at an adequate computational level, and the mechanistic analysis is generally thorough. The work addresses acid-assisted atmospheric nucleation involving halogenated organic acids and provides molecular-level insight into a potentially important pathway for new particle formation in cold environments, while appropriately acknowledging the need for further observational constraints. Overall, this study falls within the scope of Atmospheric Chemistry and Physics. I recommend publication after the authors adequately address the following comments.
Scientific issues:
1. The manuscript uses TFA as a “model PFCA” to explore the general role of perfluorocarboxylic acids in atmospheric nucleation. The authors emphasize that TFA is the shortest-chain and most abundant PFCA and possesses strong acidity and hydrogen-bonding ability. However, it remains unclear whether the observed enhancement originates from general PFCA characteristics or from unique properties of TFA. A more cautious statement is recommended in the manuscript.
2. The simulations consider TFA concentrations ranging from 1.0 × 106 to 1.0 × 108 molecules cm-3. Since the predicted enhancement reaches nearly three orders of magnitude at high TFA abundance, the atmospheric importance depends critically on whether such concentrations occur frequently. The authors state that these values are based on boundary-layer observations. However, under what environments were these measurements obtained should be more clarified.
3. The authors focus on clusters containing up to six molecules. Please clarify why clusters larger than six molecules were not considered and whether the selected cluster size is sufficient to capture the nucleation enhancement. A brief discussion based on computational limitations and previous ACDC studies would improve transparency.
4. The manuscript appropriately acknowledges that direct gas-phase TFA measurements in the upper troposphere are unavailable. However, some statements still imply that TFA may represent an important upper tropospheric nucleation pathway. Given that the UT simulations are based on assumed TFA concentrations rather than observational constraints, the authors should consistently describe these results as “potential mechanistic scenarios” rather than “atmospherically relevant pathways”. The conclusion should clearly separate: (1) observationally supported cold boundary-layer implications. (2) hypothetical upper tropospheric sensitivity tests.
5. The introduction explains that nitric acid enhances SA–NH3 nucleation under cold conditions. However, the logical transition from nitric acid to TFA is somewhat abrupt. The authors should explain why TFA is expected to behave similarly to nitric acid considering molecular properties and atmospheric concentrations. This would make the hypothesis more scientifically grounded.
6. The manuscript defines the enhancement factors only in figure captions. This definition should be introduced in the main text before discussing enhancement values.
Technical issues:
1. “enhances SA–NH3 nucleation by up to 950-fold” in the Abstract should be changed to “enhances the simulated particle formation rate by up to 950-fold” because nucleation itself is not directly measured.
2. Line 42. “… Wang et al., 2023; Zhang et al., 2018b) However, …” should be corrected as “… Wang et al., 2023; Zhang et al., 2018b). However, …”
3. Page 3: “upper tropospheric sensitivity tests” should be used consistently instead of “upper tropospheric conditions”.
4. “3.1 Cluster Structures and Intermolecular Interactions” should be corrected as “3.1 Cluster structures and intermolecular interactions”.
“3.2 Thermodynamic and Kinetic Stability of Clusters” should be corrected as “3.2 Thermodynamic and kinetic stability of clusters”.
“3.3 Particle Formation Rates Enhanced by TFA” should be corrected as “3.3 Particle formation rates enhanced by TFA”.
“3.4 Cluster Concentrations and Enhancement” should be corrected as “3.4 Cluster concentrations and enhancement”.
“3.5 Dominant Cluster Formation Pathways” should be corrected as “3.5 Dominant cluster formation pathways”.
5. Reference “Wang, M., Xiao, M., Bertozzi, B., Marie, G., Rörup, B., Schulze, B., and Bardakov, R.: Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation, Nature, 605, 483–489, https://doi.org/10.1101/pdb.caut449, 2022.” should be “Wang, M., Xiao, M., Bertozzi, B., Marie, G., Rörup, B., Schulze, B., and Bardakov, R.: Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation, Nature, 605, 483–489, https://doi.org/10.1038/s41586-022-04605-4, 2022.”
6. Reference “Stewart, J. J. P.: MOPAC2016, Stewart Computational Chemistry: Colorado Springs, n.d.” should be “Stewart, J. J. P.: MOPAC2016, Stewart Computational Chemistry: Colorado Springs, 2016.”