Specific Comments

1)            Title

In the title the emphasis is given both to natural and urban topography. The effect of urban topography is taken into account by the microscale model, but it is not discussed in the paper. For this reason, the Authors should either improve the discussion of the effects of urban topography on air pollutant dispersion or remove urban topography from the title.

2)            Paper organization

I found that the Section on principal-component analysis is not well connected with the rest of the paper. In particular, it is not clear what is the added value of this additional analysis in characterizing the effect of topography on pollutant dispersion in the Dresden basin. Therefore, the motivation and the added value of this additional analysis should be better explained.

Minor and technical remarks

Line 60 and line 66: use lowercase after colons.

Line 81: “mean age of stratospheric age”: repetition.

Line 109: “for a MORE realistic simulation…”

Equation 1: c is not introduced.

Equation 3: c_ais not introduced.

Line 247: delete the full stop after “planes”.

Caption Figure 4: change “Section A” with “Appendix A”.

Title Section 3.1.2: change “March” with “May”.

Some Section titles are repeated: “Age-concentration modeling – proof of concept”, “Principal-component analysis”.

Line 466: “with generally lower wind speeds than in cases…” probably something is missing in this sentence.

Line 498: change “vise versa” with “vice versa”.

Line 536: I would change “reflects the real distribution of air pollutant sources” with something similar to “reflects the presence of accumulation or recirculation areas”.

Figure 5: it would be interesting to have also a map of the total emissions, and not only of the emissions within the first vertical layer.

Figure7 and 8: in my opinion for the interpretation of the results it would be clearer to plot the time series of wind speed and direction and not of the two horizontal wind components.

This manuscript used CAIRDIO to investigate the impacts of urban and natural topography on the local winds and air pollution dispersion in the Dresden Basin in the Eastern German Elbe Valley. Overall, the manuscript is well-written, and the conclusions are well-supported. This model can benefit the community for microscale prediction of the environment. However, I have a few comments that I think are worth to be discussed in the manuscript. Overall, I recommend publishing after minor revision. Please see my comments below:

General comments:

1. It is a little bit bothering me when you use the term “age” in the manuscript. In my understanding, aerosol’s degree of aging should be referred to not only the lifetime in the atmosphere but also how many reactions they involved (both physical and chemical). Or do you simply refer to how many days after aerosol is generated? Could you make it clear in the manuscript?
2. For equation 2, I do not understand why you remove the source and sink for aging. The aging process of BC in the atmosphere is very complicated (see “Bounding the role of black carbon in the climate system: A scientific assessment”). I suggest adding some discussion about the atmospheric aging processing of BC and why you decide not to include that.
3. This manuscript is too long. Since it is an extension of a previously published work, maybe you can remove common parts and refer to that paper.
4. For your equations, could you define all parameters clearly and put their unit? Maybe you can make a table.
5. When you discuss other models for estimating aerosol aging, I suggest adding a discussion of the FLEXPART and CO tracer method since a recently published paper (“Particle phase-state variability in the North Atlantic free troposphere during summertime is determined by atmospheric transport patterns and sources”) used this method to estimate the aging of the aerosol air mass.

Specific comments:

1. I suggest improving the title by mentioning this is a model study and the model name since the current title sounds like a field measurements
2. L144-150, “The first two … of their age.” You can either mark the terms in the equation or show the terms in the manuscript. It is not very clear to me when I read it.
3. L272-273, “For the air-chemistry … 14km resolution.” Could you explain what you mean by air chemistry? Do you mean chemical reactions in the gas phase or aerosol phase or both (heterogeneous reactions)? Moreover, what reactions you included (e.g., ozonation, aqueous phase reaction, photolysis, NOx reaction, etc.,).
4. L275, “Domains … respectively.” How do you choose the location and size of D1 to D3? Moreover, I think you should not use the resolution since that means how many ticks per length. Maybe you should use distance.
5. Figure 3. Where is D4? It is not clear to me. Also, I think you should label D2. It is not clear to me which box is D1 since D1 is labeled between two boxes.
6. Figure 6. I suggest using a color different from the color bar for the mark of Dresden. Moreover, do you have any spots that have MSL pressure below 996 hPa? If not, you can reduce the range of the color bar to cover the min and max of pressure.
7. Figure 7. It is not clear to me how you calculate the b and c color bars and why b and c have different Z ranges.
8. Figure 8. I suggest adding a caption even if it will be the same as Figure 7’s.
9. L380-381, “3.3 … concept.” Please check all your section numbers.
10. Figure 12. What is the color bar?

The authors studied the impacts of urban and natural topography on air pollution in the Dresden Basin region using the CAIRDIO modeling. Overall, the paper is well-written with minor edits required for this Special issue on Air quality research at street level – Part II (ACP/GMD inter-journal SI).

Specific comments:

Introduction: it is unclear as to why the Dresden Basin is an important region to study for the Special Issue. There are also no discussions on the health impacts of air pollution in this region. Observations from either satellite or ground-based measurements should be addressed to support the significance of air pollution in this region.

Figure 3. The map does not indicate the absolute coordinates of the study region, so readers who are unfamiliar with the region would not be able to efficiently locate the exact region. I suggest that the authors either add a separate map to show the geographic location of Dresden or modify the coordinates of the current map to absolute coordinates. Also, “hmsl” needs to be defined.

P33 Line 724: It is unclear how aerosols are represented in the RRTM since they will affect the radiative fluxes. It would be helpful to readers if the authors describe the aerosol optical properties if the air pollution in the Dresden Basin in terms of single scattering albedo, asymmetry parameter and Ångström exponent.

Technical comments:

P2 Line 37: Remove “In fact”

Figure 2: “in experiment two” should be a new sentence

Sections 3.3 and 3.5 do not have contents. Please fix the section numbers of the whole manuscript.