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. 

Ammonium nitrate (AN) is an important inorganic aerosol, impacting air quality and climate. However, the activity coefficient of AN that shapes the gas-particle partitioning has not been well examined. This paper investigated the dependence of the activity coefficient of AN on meteorological conditions and chemical composition by using three commonly used thermodynamic models. The critical role of RH or ionic strength was demonstrated through well-designed aerosol proxies and real ambient aerosols. The findings have important implications for improving the prediction of ammonium nitrate aerosols. Overall, the paper is well written and fits the scope of ACP. The paper can be recommended for publication after addressing the following comments.

Abstract: I would suggest the authors add 1-2 sentences to conclude the impact of uncertainties of the activity coefficient of AN on nitrate pollutions, given that the impact on nitrate pollutions has been highlighted in the title.

Lines 50-60: It is unclear how the different methods of calculating activity coefficients for three models would introduce intrinsic differences in activity coefficients. More discussions on this aspect would help understand the different performances of the models.

Lines 118-119: The amount of species in μg/m3 can be provided to have a straightforward connection with ambient conditions.

Line 155: As shown in Fig. S3, chloride is present in ISORROPIA, but not in EAIM. Why? How would chloride influence the calculations of activity coefficients?

Lines 186-187: Is the dependence on RH and T similar for other conditions with different fractions of nitrate?

Section 3.2.2: Have the authors examined the dependence of γ_AN² on sulfate concentrations? The presence of other soluble ions can largely mediate the calculation of activity coefficients.

Lines 371-372: The underestimation may also arise from the uncertainties of measured f_pNO3. The authors should provide more discussion on this aspect and offer additional insights on how to narrow the discrepancies between measurements and models.