This study examines the role of NH₃ emissions in global aerosol chemical composition and acidity. The study uses the EMAC model with three different emissions scenarios to quantify effects on size-resolved inorganic aerosol composition and pH. The study observes a link between NH₃ emissions and pH, with complex effects that vary regionally.  The study is the first, to my knowledge, to examine predictions of aerosol pH and the sensitivity of pH to different emissions scenarios on a global scale.  The study highlights some current model limitations that contribute to challenges predicting pH.  The manuscript is very well written and nicely organized. The scope is certainly a fit for ACP and I believe it will be of interest to a broad audience once a number of issues are addressed.  

Specific Comments:

1.    A significant issue with the manuscript is the predictions of pH in arid regions of the globe.  Thermodynamic equilibrium models, including ISORROPIA, are challenged to represent aerosol pH when relative humidity is low (e.g., see the extensive discussion in Pye et al. (2020)).  The manuscript contains significant discussion of results in dry, arid regions – e.g., the Middle East, and over desert regions.  The physical interpretation of these results is ambiguous unless the predictions of ALWC and aerosol pH in these regions are more closely scrutinized.  The associated discussions likely need substantial revision, or at least more discussion about the potential problems of such predictions under low RH conditions.

2.    The title and the abstract are misleading because the different emission scenarios vary SO₂ and NO_x emissions at the same time as NH₃. Therefore, interpreting the results is more complex than a typical modeling sensitivity analysis where one factor is varied while all other factors are held constant. In the case of pH, it is not straightforward to attribute the observed changes to the differences in NH₃ because the precursors for aerosol sulfate and nitrate also changed simultaneously.  With the emissions in SO₂, NO_x, and NH₃ changing in different directions (some regions, these go up, in other regions, they go down, and not always together), the results were quite complex and not easy to interpret. For example, a conclusion of the study succinctly stated in Line 707 is: “pH changes closely correspond to variations in NH₃ emissions”, however, the study is not really able to derive the quantitative relationship in each region because of the concurrent changes in SO₂ and NO_x. Ultimately, I think the authors need to do more to facilitate interpretation of the results and isolate the effects of individual species on the observed changes in pH, though this is not easy.   

3.    The authors acknowledge that the size-resolved pH predictions do not follow the expected trend in many cases, as pH does not systematically decrease with decreasing particle diameter for the different size bins in many locations.  Only when NH₃ emissions are eliminated does the model predict more acidic smaller particles. The model errors in predictions of NH₃/NH₄⁺ partitioning suggest there are associated errors in the predictions of aerosol pH. Much more discussion of this point is warranted.  

4.    A comparison of stable and metastable mode results from thermodynamic models has been done before. However, it has never been done for global simulations, so the present results are quite important because of their scale. I encourage the authors to expand on this discussion and to consider moving Fig. S4 to the main manuscript. 

5.    Section 2.2.2: it is really not accurate to frame the discussion around H⁺ and H₂O, only.  Other particle components can affect the aerosol pH by affecting the H⁺ activity coefficient. Although ISORROPIA assumes an H⁺ activity coefficient of unity, other models that solve for γ_H+ would have an effect on pH from other aerosol components.

6.    Table 7: why is the SE-USA case from Pye et al. (2020) not included in this comparison? Aerosol pH in this region has been studied extensively and should provide a good point of comparison for the present study.

7.    For the comparison to the field-derived pH in Xi’an shown in Table 7, the authors are encouraged to consult Guo et al. (2017), who provide a different estimate of aerosol pH in Xi’an. It is not reasonable to request the present manuscript to arbitrate this disagreement, however, the authors should be aware of different pH estimates for this region.

8.    Overall comment: reconsider the number of sig figs used in many cases. E.g., in Section 5.3 reporting NH₃ emissions to 0.01 Tg and reporting NH₃ lifetimes to the 0.01 day do not likely reflect uncertainties in these values. 

9.    Lines 72 – 74: this sentence needs revision – what does “excess NH₃ released to the atmosphere” really mean. 

10.    Color scale of Fig. 1 was quite difficult to determine the magnitude of the changes in many regions.

11.    Line 411 and 430 (and elsewhere): best not to use phrases like this…global pH values show that sulfuric acid is rarely fully neutralized. See also Guo et al. (2017).

12.    Paragraph lines 549 – 555: I do not follow the discussion in this paragraph. 

13.    Line 562: the effect of NH₃ emissions on SNA formation has been studied for decades. Also, I would not categorize the effect of NH₃ emissions on pH as the “subject of debate,” but rather understudied. 

14.    Line 704: this is not true in terrestrial regions where highly acidic (e.g., pH < 2)) particles are observed or predicted.

Technical Corrections:

Line 67: trend should be plural.

Line 133: should ‘of’ be added after ‘number’?

Table 2 header: ‘cases’ is repeated.

Line 653: do the authors mean ‘marine aerosol’ instead of ‘oceanic’?

References:

Guo, H., Weber, R. J., and Nenes, A.: High levels of ammonia do not raise fine particle pH sufficiently to yield nitrogen oxide-dominated sulfate production, Scientific Reports, 7, 12109, 10.1038/s41598-017-11704-0, 2017a.

Pye, H. O. T., Nenes, A., Alexander, B., Ault, A. P., Barth, M. C., Clegg, S. L., Collett Jr, J. L., Fahey, K. M., Hennigan, C. J., Herrmann, H., Kanakidou, M., Kelly, J. T., Ku, I. T., McNeill, V. F., Riemer, N., Schaefer, T., Shi, G., Tilgner, A., Walker, J. T., Wang, T., Weber, R., Xing, J., Zaveri, R. A., and Zuend, A.: The acidity of atmospheric particles and clouds, Atmos. Chem. Phys., 20, 4809-4888, 10.5194/acp-20-4809-2020, 2020.