the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Aug 2026
Mechanistic Insights into Nitric Sulfuric Acid Formation and Its Enhancement of Sulfuric Acid-Ammonia Nucleation under Severe Urban Pollution
Abstract. Organosulfates (OSs) constitute an important component of secondary organic aerosols (SOA). Although previous studies have investigated the aerosol nucleation of OSs such as glycolic acid sulfate and methyl hydrogen sulfate, exploring the aerosol nucleation behavior of additional OSs remains essential. Herein, the formation mechanism of nitric sulfuric acid (NSA) from the reaction of sulfur trioxide (SO3) with nitric acid (NA)and its role in sulfuric acid (SA)-ammonia (A) aerosol nucleation was investigated using quantum chemical calculations combined with kinetic simulations. Our results demonstrate that NSA can be formed rapidly and stably in the gas phase with a low barrier of 3.61 kcal·mol-1. The SO3 + HNO3 reaction remains competitive with major atmospheric SO3 loss pathways, including reactions of SO3 + HCOOH, SO3 + C6H5COOH and NH3-catalyzed reactions, and even H2O-catalyzed hydrolysis under dry conditions. Kinetic simulations further demonstrate that NSA significantly accelerates SA-A nucleation, with rates reaching ~ 10-3 cm-3 s-1 under heavily polluted urban conditions (e.g., Beijing), corresponding to a five-order-of-magnitude enhancement over binary clusters and a 101-1011-fold increase relative to nitric acid. These findings underscore the importance of incorporating NSA into atmospheric aerosol models.
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RC1: 'Comment on egusphere-2026-3682', Anonymous Referee #3, 24 Aug 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3682/egusphere-2026-3682-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2026-3682-RC1 -
RC2: 'Comment on egusphere-2026-3682', Anonymous Referee #4, 24 Aug 2026
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The manuscript investigates the formation of nitric sulfuric acid (NSA) from the reaction of SOā with HNOā and its potential role in sulfuric acidāammonia nucleation using quantum-chemical calculations and ACDC simulations. The topic is relevant to atmospheric sulfur chemistry and new particle formation, and the attempt to connect molecular formation chemistry with cluster kinetics is potentially useful.
However, after examining the manuscript, I found several substantial inconsistencies in the atmospheric concentration estimates, kinetic comparisons, thermodynamic data, and reported nucleation rates. Some of these affect the physical basis of the main atmospheric conclusions. In particular, the assumed NSA concentrations used in the ACDC calculations are not convincingly connected to the atmospheric conditions discussed in the manuscript.
I therefore recommend Major Revision. A thorough numerical audit and a more conservative interpretation of the atmospheric implications are required before the conclusions can be evaluated reliably.
Major comments
1. The atmospheric concentration of NSA is not adequately constrained, and the treatment of equilibrium and steady state needs to be corrected.
The manuscript estimates [NSA] from the equilibrium relation, but describes the resulting quantity as a āsteady-state concentration.ā These are not equivalent. A steady-state atmospheric concentration requires an explicit balance between chemical production and all relevant loss processes. No such loss terms are included here. Are there any NSA atmospheric concentration observations? Why not? Furthermore, the authors should re-examine, and more importantly, the rationale for the coexistence of SO3 and HNO3 precursor concentrations in time and space. The authors should therefore re-establish the atmospheric range of [NSA], clearly distinguish equilibrium from steady state, and show how the selected ACDC concentrations are connected to realistic urban tropospheric conditions.
2. The chemical classification of NSA should be reconsidered.
The manuscript introduces nitric sulfuric acid (NSA, OāNOSOāH) within the context of organosulfates and later refers to the present results as providing insight into the role of OSs in aerosol nucleation. However, the NSA species studied here contains no carbon and therefore should not be classified as an organosulfate in the conventional sense. This is not merely a terminology issue, because the current Introduction and Conclusion use the organosulfate framework to motivate and interpret the study. The authors should revise this part of the manuscript, clearly distinguish NSA from true organosulfates such as methyl hydrogen sulfate and glycolic acid sulfate, and use a chemically appropriate description for NSA throughout.
3. The thermodynamic data do not support the general statement that NSA incorporation systematically stabilizes the clusters.
The manuscript repeatedly states that increasing NSA content lowers the formation free energy and systematically stabilizes SAāA clusters. However, Table S7 shows clear counterexamples. At 278.15 K, for example, the formation free energies of (SA)2ā A, SAā Aā NSA, and Aā (NSA)2 are approximately ā22.03, ā21.66, and ā20.46 kcal mol, respectively. Thus, replacing SA with NSA in this composition makes the formation free energy less negative rather than more negative.
4. The enhancement-factor analysis in Fig. 8 is currently not reproducible from the information provided.
The manuscript defines the NSA enhancement factor as the ratio between the cluster formation rates with and without NSA. However, how to get the values reported in Fig. 8? >10^10? In addition, the Fig. 8 caption refers to 238.15 K, any reported ACDC calculations?
The manuscript draws a strong quantitative conclusion that NSA enhances nucleation by many orders of magnitude more than NA, yet the corresponding NA thermodynamic data, evaporation rates, and ACDC results are not provided in sufficient detail for independent verification. The authors should provide the complete underlying data and clarify the temperature and calculation conditions used for Fig. 8.
5. The atmospheric interpretation is too broad relative to the actual simulation design.
The manuscript repeatedly associates individual simulation conditions with specific environments such as Beijing, HyytiƤlƤ, the Arctic, or polluted urban regions. However, the input parameters are assembled from different literature sources rather than representing simultaneous, site-specific observations. The SI also invokes stratospheric and geoengineering scenarios to justify very high SOā concentrations, while the main manuscript interprets the resulting NSA concentrations primarily in the context of urban tropospheric pollution. These are distinct atmospheric environments and should not be conflated. The authors should either provide genuinely site-specific input combinations or describe the calculations as idealized sensitivity scenarios. Statements such as a given pathway contributing a certain percentage āat HyytiƤlƤā or āin the Arcticā should be avoided unless the complete set of model inputs is representative of those environments.
Additional comments
1. The nomenclature of NSA should be checked and made consistent throughout the manuscript and SI. The SI tables refer to ānitrosulfonic acid,ā whereas the main manuscript uses ānitric sulfuric acid.ā
2. The authors should carefully check all cluster formulas and notation in the figures and captions, including inconsistent use of āNSA-SA-A,ā āSA-A-NS,ā and the definitions of acid/base labels.
3. The role of hydration and competing bases such as amines should at least be discussed as limitations when the results are extrapolated to heavily polluted urban NPF. The present ACDC system represents an unhydrated SAāNHāāNSA network and should not be interpreted as a complete description of urban nucleation chemistry.
Citation: https://doi.org/10.5194/egusphere-2026-3682-RC2
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