the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Oct 2025
Unexpected enhancement of new particle formation by lactic acid sulfate resulting from SO3 loss in forested and agricultural regions
Abstract. Organosulfates (OSs) are key components of atmospheric aerosols and serve as tracers for secondary organic aerosol (SOA) formation. Among these, lactic acid sulfate (LAS) has been increasingly detected in the atmosphere. However, its molecular formation pathways and its role in new particle formation (NPF) remain poorly understood. In this work, we investigate the gas-phase formation mechanism of LAS via the reaction between lactic acid (LA) and SO3, and assess its impact on sulfuric acid-ammonia (SA-A) driven NPF using quantum chemical calculations and Atmospheric Cluster Dynamics Code (ACDC) kinetic modeling. Our results show that SA and H2O significantly catalyze the LA-SO3 reaction, enhancing the effective rate coefficient by 7–10 orders of magnitude within the temperature range of 280–320 K. Further molecular-level analysis using the ACDC reveals that LAS not only significantly enhances the clustering stability of SA and A up to 108-fold, but also plays a significant and direct role in SA-A nucleation under conditions typical of forested and agricultural regions. Notably, LAS-SA-A clusters contribute to 97 % of the overall cluster formation pathways in regions with high LAS concentrations like Centreville, Alabama. Additionally, our findings show that the nucleation potential of LAS-SA-A clusters is stronger than that of LA-SA-A clusters, aligning with field observations, even though LAS concentrations are typically three orders of magnitude lower than LA. These findings imply that OSs formed through SO3 consumption may significantly contribute to the enhanced NPF rates observed in continental regions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4894', Anonymous Referee #1, 26 Nov 2025
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RC2: 'Comment on egusphere-2025-4894', Anonymous Referee #2, 28 Nov 2025
Wang et al. utilized quantum chemical calculations, master equation analysis, and Atmospheric Clusters Dynamic Code kinetic model to systematically investigate the formation mechanism of lactic acid sulfate (LAS) and its enhancing effect on sulfuric acid (SA)–NH3(A) nucleation. Particular attention is given to the reaction between lactic acid and SO3, the catalytic effects of H2O/SA, and the dual role played by LAS in the SA–A–LAS ternary system (both as a participant and as a catalyst). The topic is novel, the methodology is sound, and the work provides an important—yet previously underappreciated—mechanistic explanation for the unusually high NPF rates observed in forested and agricultural regions. Most of this manuscript is well written and will be of broad interest to the readers of Atmospheric Chemistry and Physics. I recommend its publication in the journal, provided that the following comments are addressed.
Specific Comments:
- The results indicate that the barriers to the reaction between lactic acid and SO3 are substantially reduced with the addition of SA. However, the underlying mechanism driving SA’s pronounced catalytic effect has not been adequately addressed. Providing one or two specific structural characteristics, such as the lengths of critical hydrogen bonds or specific geometric changes in transition states, would clarify why SA exhibits higher catalytic efficiency than H2O, thereby allowing readers to fully comprehend the mechanism driving the “barrier reduction”.
- The authors’ calculations reveal that the dominant nucleation pathways shift with temperature, however, the manuscript does not adequately explain why the contribution of LAS-related pathways increases with increasing temperature. Further clarification of the underlying mechanism is required, such as whether this behavior is associated with variations in collision frequency or the fact that LAS exhibits a relatively weak temperature dependence in its evaporation rate. Incorporating such an explanation would greatly enhance the interpretability of the trend presented in Fig. 5 of the manuscript.
- The manuscript proposes that LAS may function either as a “participant” or as a “catalyst-like promoter,” which is an interesting and meaningful finding. At present, the distinction between these two roles is mainly inferred from the ACDC pathways in Fig. 5 (i.e., whether LAS ultimately remains in the cluster), whereas Fig. 6 and Fig. 7 primarily illustrate how the contribution of LAS varies with temperature and precursor concentrations. Their connection to the role distinction is not explicitly established. To make the origin of this “dual role” clearer, a brief clarification in the discussion section would help enhance the manuscript’s logical coherence assist readers in better understanding how LAS behaves under different conditions.
- I suggest the authors explicitly outline how boundary conditions were set in their ACDC simulations, along with justifying the maximum cluster size they selected. Nucleation rates are often sensitive to the choice of boundary conditions. Accordingly, it is essential to clarify why setting the maximum cluster size at x + y + z ≤ 3 was adequate for their simulations, or alternatively, to discuss the implications of extending this boundary to larger clusters. Even a short, targeted explanation would greatly enhance the clarity and reproducibility of the methodology.
- A single value of 2.6 × 10-3 s-1 was adopted for the condensation sink in the ACDC kinetics simulation under different atmospheric conditions of agricultural and forested regions (Figure 6), without addressing whether this parameter is representative of such diverse conditions. In practice, condensation sinks can vary by orders of magnitude depending on aerosol loading. Hence, the manuscript ought to explain the rationale for using a single CS value across all cases, or discuss the uncertainties associated with this choice for the cluster formation rates or pathways. Including such justification would greatly enhance the credibility of the modeled nucleation rates.
Technical corrections:
Page 6 line 161: “In the direct cycloaddition pathway (Channel LAS) illsutrated in Fig. 1”
The word “illsutrated” should be corrected to “illustrated”. In addition, there is a spelling error in the caption of Fig. 4, where “nunber” should be corrected to “number.”
Page 5 line 114: “To identity the global minimum energy configurations of …”
The word “identity” should be corrected to “identify”.
Page 10 line 278: “… the contributions of LAS to the SA-A nucleation process was examined, …”
The word “was” should be corrected to “were”.
Page 12 lines 325-326: “LAS-driven nucleation becomes dominate, …”
The word “dominate” should be corrected to “dominant”.
Page 21 lines 665-672: In the reference list, Yin et al., 2021a and Yin et al., 2021b share the same title and page numbers (Acid-base clusters during atmospheric new particle formation in urban Beijing” Environ. Sci. Technol., 55, 10994–11005). Please remove the duplicate references and update the citation numbers in the main text.
Citation: https://doi.org/10.5194/egusphere-2025-4894-RC2
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The manuscript by Wang et al. presents a comprehensive theoretical investigation into the formation mechanism of lactic acid sulfate (LAS) and its unexpected role in enhancing sulfuric acid-ammonia (SA-A) driven new particle formation (NPF). The combination of quantum chemical calculations and ACDC kinetic modeling provides molecular-level insights into the catalytic effects of SA and H2O on LAS formation and the role of LAS in enhancing SA-A nucleation. This study advances our molecular-level mechanistic understanding of how organosulfates influence nucleation events. The manuscript is well-structured and clearly written. Therefore, I recommend publication of this manuscript after consideration of the following comments:
Comments: