Summary: Large cities with large concentrations of population are facing serious environmental challenges. This study investigates the spatial and temporal characteristics of aerosol states in megacities using up-to-date spaceborne aerosol retrievals (AOD and Dust Optical Depth - DOD) with fine spatial resolution (0.1°x 0.1°). The author found that the spatial and temporal variability of AOD is closely related to the population agglomerations and the adoption of emission policies, which also is of significance for assessing the effectiveness of air pollution emission regulations.

I have some general and minor comments (please see below) which should be addressed prior to publication.

General comments:

1. Line 128-131: The author first mentioned the use of the MIDAS dataset with high resolution to analyze the spatial and temporal variability of AOD, and this dataset appears several times in the paper, so I think it is necessary to give specific details about MIDAS when it first appears.
2. As the author mentioned in the paper, a uniform pixel-based approach is applied for all cities. However, because different cities are located at different latitudes, increasing the uncertainty of pixel size in the east-west direction. Maybe the author could implement a distance conversion based on the relationship between latitude and pixel size to ensure that the city-centric division of space remains consistent? Based on the same thoughts, to reduce uncertainty, is it more reasonable to express the single measure of the AOD gradients in terms of distance as well?
3. Line 191-195: Why did the author construct the daily AOD time-series for urban domain by calculating the AOD median rather than AOD mean?

minor comments：

1. Line 99: Replace “surface-based” with “ground-based”.
2. Line 206: Add title number “2.6”, and “Intra-annual” should be replaced by “intra-annual”.
3. Line 217: Add title number “2.7”, “Intra-annual” should be replaced by “intra-annual”.
4. Line 227: Replace the title number “2.6” with “2.8”.
5. Line 322: Punctuation is in red
6. Line 440: Note the order in which the abbreviations of Ground Based (GB) appear.
7. Figure 17 and figure 18 Figures 17, 18 seem to lack the serial numbers “(a)” and “(b)”.
8. There is a spelling error in the header of table A1 and “Agglomiration” should be changed to “Agglomeration”.

The manuscript by Papachristopoulou et al., "Aerosol optical depth regime over Megacities of the world", presents a study to investigate the spatial and temporal variability of urban aerosol state of 81 cities with population over 5 million, relying on daily satellite-based aerosol optical depth data. The focus and objectives of this study were quite straightforward and, probably at least partly for that reason, no actual problem points or weaknesses could be found in the approach and analysis. Because of this, my comments are also general or specific but minor. But of course, it should be mentioned in favor of the manuscript that the description and analysis was quite thorough, and these results bring useful information about the spatial and temporal variability of aerosols in the context of Megacities. I consider the topic and results of this manuscript to fit the scope of ACP and think it could be published if the minor comments below are addressed.

Line 146, here you mention about the number of days or seasons required for seasonal or annual means. Nine days out of 30 or three seasons out of four sound like somewhat loose limit. In place where there is a strong seasonal variability, it likely has a significant impact in the monthly mean whether the nine days of the month are in the very beginning or in the end of the month. Could you describe how did you arrive at these limits, and whether you tested more strict ones and the influence of these in the results.

Line 287, you write "... revealed that the 4x4 pixels area is a domain well fitted ...". How it was “revealed”, could you describe your analysis/conclusions in somewhat more detail.

Line 345, your finding that the city of Los Angeles resulted in different trend than overall western US was interesting. How about the results in the Figure 16, did they perhaps give a similar indication that also in the scale of city center vs surrounding area, a similar difference in the trend was found? Related to Los Angeles (LOSA), is there a reason why it was not included in the Figure C1b?

Line 410, I would think that the large interannual variability in North America, most notably in Atlanta is related to the significant impact by biogenic aerosols, discussed in many papers (one of the best known is Goldstein et al. 2009). Could you elaborate on this matter.

REFERENCES

Goldstein, A. H., Koven, C. D., Heald, C. L., and Fung, I. Y.: Biogenic carbon and anthropogenic pollutants combine to form a cooling haze over the southeastern United States, P. Natl. Acad. Sci. USA, 106, 8835–8840, 2009.