General Description and Recommendation:

The authors use the GEOS-Chem model to characterize reactive nitrogen (NOy) throughout the global troposphere. The characterization follows broad qualitative comparison of the model to NASA DC8 ATom aircraft observations over the remote ocean. The characterization provides details of the budget of prominent NOy compounds, a detailed depiction and overview of the magnitude of reaction pathways forming and destroying NOy compounds, and of emissions and deposition pathways. Some quantification of lifetimes of NOx and NOy against chemical sinks and deposition sinks is also given. The study is an incredibly valuable and much needed state of knowledge of NOy chemistry as represented by a model that keeps pace with the science. The paper is overall well written, the results are clearly depicted in figures throughout the manuscript, and the content is logically presented.

Included below are general concerns or requests for clarity that can be easily addressed by the authors, along with additional specific suggestions to further increase accessibility of the content to ACP readers.

General Comments:

[1] The model is assessed against ATom observations and determined to be robust. The comparison is for particular conditions sampled. Understandably, screening of the ATom observations removes instances like plumes that the grid resolution of GEOS-Chem can’t reproduce. But, there are other screening steps applied to remove nighttime observations that don’t fit the PSS assumption and to remove data collected over land. Does this then impact the ability to use the model to characterize tropospheric NOy? How consistent are the values in Figures 3-5 to values obtained from the model under the same conditions used to compare the model to ATom?

[2] The ATom comparison is mostly a qualitative description of the model performance. The assessment would benefit from providing a sense of scale of biases where the model and ATom observations disagree. This might merely require stating the model normalized mean bias in portions of the manuscript where biases are already identified.

[3] The assessment of the model throughout the results sections is missing reference to pertinent historical (Emmons et al., 1997; Bradshaw et al., 1998; Jaegle et al., 1998) and recent (Lee et al., 2025; Wei et al., 2025) literature that used ATom or predecessor NASA aircraft campaigns along with GEOS-Chem or similar state-of-knowledge models to characterize NOy in particular layers of the atmosphere or to characterize particular NOy compounds throughout the troposphere.

[4] Figure 3 is an incredibly valuable visualization of the representation of complex tropospheric NOy chemistry represented in a model like GEOS-Chem. The figure could be further enhanced by making it clearer in the caption that units are “Tg N” unless otherwise stated. The first sentence of the caption gives the impression that units throughout are “Gg N”. Another change could be to make clearer which pathway corresponds to thermal decomposition of PNs to NO2, as currently the pathway from PNs to NO2 is only labelled as photolysis. The NO to/from HONO seems to not follow the convention used for other reversible processes: a single reversible blue arrow and a black net arrow. Provide a supporting reference for the path from AERNIT (aerosol-phase organic nitrate) to gas-phase HNO3 via hydrolysis, as this seems on first glance like an unfavorable reaction, given it yields a sticky gas-phase compound that prefers to be in the aerosol phase.

[5] PPN turns out to be a prominent PNs compound, as indicated from the model budget values, but the model comparison to ATom is only given as a figure in the supplementary and the results from this figure are not mentioned in the main manuscript. Even though only 2 campaigns measured PPN, it would be useful to mention how the model compares to the observations in the main manuscript. Then to also say briefly whether the version of the model used in the study includes PPN photolysis, as Wei et al. (2025) suggested that, for the upper troposphere, the lack of PPN photolysis in the model causes a high bias in PPN and low bias in NO2. Horner et al. (2024) addressed this by adding PPN photolysis to the model.

[6] Provide consolidated definitions of AERNIT, pNO3, NIT, and NITs early in the main manuscript, as it’s confusing to follow what these represent. The definitions are in the supplementary, but for ease of following Figures 3-5 and accompanying text, these should also be stated in the main manuscript. Consider renaming AERNIT to orgNIT or NITorg. As written and given the pattern of naming other compounds in the manuscript, AERNIT suggests the sum of all aerosol phase nitrate, rather than the sum of all aerosol phase organic nitrates.

[7] Minor technical issue that appears in multiple locations: parentheses before citations are missing a space between the two (e.g., p. 2, line 45; p. 5, line 133).

[8] Replace all “em dash” instances with periods for ease of comprehension (e.g., p. 3 line 92) and especially p. 13, line 303 (initial interpretation is that em dash is shorthand for “to”, indicating a range of values).

[9] There are a few instances where compounds are defined multiple times throughout the manuscript. HONO and HNO4, for example, are both initially defined on p. 2, then redefined further down p. 2 and HNO4 is again defined on p. 5. Consider removing these redundancies.

Specific Comments:

p.1, line 21-22: Reword “at 500 hPa is 0.10” to “is 0.10 at 500 hPa”.



p.2, line 46 “biomass burning” is referred to as natural, but biomass burning is a combination of wildfires and intentional burning by humans for agricultural practices.



p.3, line 88: Delete “a” in “with a several studies noting”.



p.3, line 97: Should NOx as precursor of PM2.5 also be acknowledged in this sentence about the motivation for regulating NOx sources?



p.4, model description: What inventory is used for aircraft emissions?



p.4, line 117: Replace GEOS-Chem URL with the citation for the relevant Zenodo site used in the “Data Availability” statement.



p.5, lines 140-141: Provide a reason for not using the modified Luo et al. wet deposition scheme.



p.6, lines 174-175: This sentence is unclear as written. How are the “small number of points” determined? What is the “recommended range”? Which “two parameterizations” are these, as only one (PSS) is given?



p.6, line 185: How does the “filter out plumes” screening differ from the “fresh NOx emissions” filter?



p.7, Table 1: Convention is for table title to appear above the table, not below.



p.8, line 210: “shows a comparison” can simply be “compares”. Similarly, “shows a summary of” can simply be “summarizes” (p. 10, line 252).



p.9, Figure 2: Add timings of the individual campaigns for ease of following along. These are in Figure S1, but would be helpful to include in Figure 2 too.



p.9, line 247: Presumably aircraft and ships are mentioned because of the functionality of the diagnostics package in the model. Would be worth mentioning this, as otherwise it seems odd that these particular anthropogenic sources are mentioned and not others that make greater contributions (road vehicles, energy generation).



p.11, line 275: delete “of ” in “as well as their ratio of at the”



p.11, lines 280-281: What led to the conclusion that this lightning NOx and not dry season biomass burning NOx? Is this from investigating the modelled ratios of NOx:NOy in JJA and DJF, showing enhancements in the hemisphere where lightning occurs (rainy season) as opposed to where biomass burning occurs (dry season)?



p.14, line 320: Given the large production and los MPNs have a large production and loss, is it worth acknowledging the need to measure this? And under what conditions? And in what locations, based on your budget?



p.16, Figure 7: Inclusion of addition NOy compounds for GEOS-Chem only suggests these aren’t simulated by the other models. Is this the case? Not stated in the manuscript. If the case, say so in manuscript. If not the case, consider consider removing from Figure 7 or providing context for the reason for including these in Figure 7.



p.17, line 401: Typo in “issues in the simulation HNO3 and”

References:

Bradshaw et al. (1998) doi:10.1029/98JD00621.

Emmons et al. (1997) doi:10.1016/s1352-2310(96)00334-2.

Horner et al. (2024) doi:10.5194/acp-24-13047-2024.

Jaegle et al. (1998) doi:10.1029/97gl03591.

Lee et al. (2025) doi:10.1029/2025GL115001.

Wei et al. (2025) doi:10.5194/acp-25-7925-2025.

Dutta et al. present a comprehensive overview of the current understanding of the global budget of NOy as represented in the GEOS-Chem atmospheric chemistry model. The authors first compare the GEOS-Chem NOy simulation to observations from the ATom aircraft campaign, and then examine the global burden of key NOy species, quantify the reaction pathways controlling the cycling among the major NOy species, and identify the chemical processes that need improved representation in the model. In particular, the authors highlight the importance of organic nitrogen chemistry and the photolysis of particulate nitrate, and the need to better characterize these processes.

This is a valuable and timely study. NOy plays a central role in tropospheric chemistry and has been represented in global models since the 1970’s, although substantial differences among models persist. The representation of NOy chemistry in models has also evolved significantly in the past decade, with increasingly detailed treatment of organic nitrogen chemistry, heterogenous chemistry, and tropospheric halogen chemistry. The manuscript is well-written, the discussion is insightful, and the results are presented clearly through high-quality figures.

The manuscript will benefit from a more critical assessment of the representation of NOy chemistry and deposition in GEOS-Chem in the context of the the discussion of the NOy burden and chemical cycling (Sections 3.2 and 3.3). While some critical discussion of the model's representation of organic nitrogen chemistry is provided, this could be expanded to address other species, such as:

- HNO3: GEOS-Chem and many other models persistently overestimate HNO3 and it would be useful to discuss the implications of this bias for the global NOy budget.

- Halogen nitrates: The largest flux between NOx and NOz passes through halogen nitrates, yet there is very little discussion of the relative importance of the halogen radicals involved or of the uncertainties in halogen chemistry.

Technical comments:

- Line 128: Coarse-mode nitrate in GEOS-Chem is also formed by the titration of alkalinity in fresh sea salt aerosols by HNO3.

- Line 236: Please specify the months of the ATom deployments.

- Line 250: Not all emissions in the table are from CEDS.

- Table 2: For completeness, please include the stratospheric source of HNO3 in the table.

- Line 300: Should this read “aerosol organic nitrates”?

- Line 365: The NO2+NO3+M → N2O5+M reaction is termolecular.

- Line 368: Should this read ClNO2 instead of ClNO3?