| 25 Nov 2025
Driving factors for the activity coefficient of atmospheric ammonium nitrate: discrepancies among thermodynamic models and impact on nitrate pollutions
Abstract. Semi-volatile NH4NO3 is a major component of atmospheric aerosols, and its environmental and climate effects are largely regulated by the gas-particle partitioning. The activity coefficient of NH4NO3, γAN, is one key parameter controlling the gas-particle partitioning, yet its dependence on meteorological condition and chemical profile remains uncertain. Here we investigated into this issue with comprehensive simulations and ambient observations, based on results of three widely-used thermodynamic models, i.e. ISORROPIA, E-AIM, and AIOMFAC. Across all models, γ2AN ranges between 10-2 and 10-1, with AIOMFAC results ~ 33 % lower than E-AIM and ISORROPIA. Correspondingly, AIOMFAC estimate higher particle phase nitrate values. The correlates positively with relative humidity (RH) and temperature, while RH generally contributes larger variations under typical scenarios. The effect of chemical composition on is more complex and is strongly modulated by RH. Furthermore, γ2AN responds more strongly to changes of particle chemical profile in E-AIM, whereas in ISORROPIA and AIOMFAC γ2AN is more sensitive to meteorological variations. As E-AIM is typically considered as the benchmark thermodynamic model, these results suggest the potential under-representation of chemical profiles in predicting γ2AN for ISORROPIA and AIOMFAC. The corresponding influence on 3-D chemical-transport model predictions of NH4NO3 are encouraged in future studies.
Competing interests: At least one of the co-authors is editor of ACP
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This study investigates the driving factors of ammonium nitrate activity using comprehensive simulations and global ambient observations, comparing three widely used thermodynamic models (ISORROPIA, E-AIM, and AIOMFAC) to clarify the impacts of meteorological conditions and chemical profiles.
The manuscript is acceptable for publication following the implementation of the following key revisions:
1. How does this study interpret the activity coefficient of ammonium nitrate (NH4NO3)? Aerosols are complex systems, and the individual activity coefficients of NH4+ and NO3- are objectively existing concepts. How is the activity coefficient of NH4NO3 defined to clarify its differences from those of other compounds such as sodium nitrate (NaNO3) and ammonium sulfate ((NH4)2SO4)? Additionally, why does the study use γAN in some places and its square in others?
2. I understand that the authors did not decouple meteorological factors and chemical composition in the study design. However, the expression in lines 14–16 of the abstract may need further refinement to more clearly convey the interdependence and relative contributions of these two types of factors, thus avoiding potential ambiguity for readers regarding the study's core findings on γAN.
3. The title mentions the "impact on nitrate pollutions", yet the relevant description in the abstract is overly brief. It is recommended to supplement a concise statement explaining how discrepancies in ammonium nitrate activity coefficients among thermodynamic models affect the prediction, assessment, or mitigation of nitrate pollution. This will help readers quickly grasp the real-world relevance of the research beyond theoretical parameter analysis.
4. It is recommended that 2–3 sentences be added in the Introduction to summarize the comparisons of the three thermodynamic models (ISORROPIA, E-AIM, and AIOMFAC) regarding pH and hydrogen ion activity. This supplementation will help better highlight the research gap in the comparative analysis of γAN and clarify the necessity of the current study.
5. The statement "AIOMFAC consistently underestimates..." in line 147 is overly absolute.