Review of “A double-box model for aircraft exhaust plumes based on the MADE3 aerosol microphysics (MADE3 v4.0)” by Sharma et al.

Sharma et al. has developed a box model for investigation of the role of aerosol microphysics in the exhaust plume stage for aviation-induced aerosol number concentrations, comparing this to the case of assuming instantaneous dispersion as is done in more complex but coarse atmosphere and climate models. Aerosol-cloud interaction is one of the key uncertainties in assessments of aviation-induced climate impacts with estimates currently exhibiting a very wide range and strong dependence on underlying assumptions. Moreover, advanced understanding of plume processes, both chemical and microphysical, is needed. The study thus provides a timely and important contribution to the literature with potential for broader application. I have some comments/suggestions/questions, mostly for clarification, that should be considered before publication.

Broader comments:

• The study briefly mentions that the difference between aerosol number concentrations between the instantaneous dispersion and plume approaches can have “significant effects implications for the calculation of the climate effect of aviation aerosol on low clouds”. I would however encourage a somewhat more detailed discussion of what that effect might look like, referencing different assumptions in literature estimates and relating to the sensitivity of results shown here, e.g. in what direction this could push estimates of the ERFaci, implications for studies exploring future aviation fuels or propulsion systems (e.g. given dependence on SO2 emissions, limitation that this study is initialized with contrail ice typical for jet fuel), potential changing geographic traffic pattern etc. This would strengthen the motivation and broader relevance of the study.
• There are quite a few experiments performed. While they are described in each sub-section of the Results, I think it could be helpful for the reader to rather have a separate section for experiments under Methods, including a table with the experiment names
• Some figures, e.g. 11, 13 are a bit difficult to read – is it possible to increase font size?
• The abstract, introduction, and conclusion sections are quite repetitive, more or less listing the same results. Consider shortening the introduction and/or sharpening the abstract?

Specific comments:

Line 1 and elsewhere: I would encourage different wording than “significant” when there is no statistical basis for the statements.

Line 318: does this initialization also differ between regions or is it fixed?

Line 335: it would be helpful with a bit more detail in this paper about what “typical background concentrations” mean in EMAC, for the different regions

Line 342: If I understand correctly (otherwise, please clarify), this means that there is a range of estimates of the correction factors for each region? Given the sensitivity of the results to different parameters, this would seem like valuable information to add to the percentage numbers given throughout the text. Moreover, does “multiple ensemble simulations” here correspond to the number of ensembles given in table 1? If so, the number of ensembles differ substantially, does that affect the results?

Line 366: “such as SO2” – what else? It does not seem that this study considers species other than sulfate and soot?

Line 406: what does “typical conditions” mean – annual means? Based on frequency or occurrence? Do the authors expect strong seasonal differences given the regional variation found? Please define/specify.

Line 449: Maybe a column could be added to table 2 given the initial background concentration in the different regions? (And temperature and RH differs in table 2 – these differences are included? Would these differences be expected to influence the results? Via sulfate production, but likely small effect…)

Line 551-554: It could maybe be good with some absolute numbers as well, not only percentage?

Line 618: “remarkable” – is this the right word here?