General comments:

This manuscript presents an analysis of aerosol optical properties measured at three sites (urban, peri-urban, and forested) in the Paris region during summer 2022 as part of the ACROSS campaign. The authors retrieve complex refractive index (CRI) and single scattering albedo (SSA) values from measurements of aerosol absorption, scattering, and size distribution. They analyze the spatial and temporal variability of these properties under different meteorological conditions and air mass origins. Overall, this is a comprehensive and valuable dataset that provides new insights into aerosol optical properties and their variability in a major European urban area. The analysis is thorough and the manuscript is well-written. However, there are some areas that could be improved or clarified, as detailed in the specific comments below. I recommend publication after minor revisions to address these points.

Specific comments:

1.    The CRI retrieval method assumes homogeneous spherical particles. How valid is this assumption for the aerosol in this region, particularly at the urban site which likely has more fresh emissions? Some discussion of the potential impacts of particle morphology/mixing state on the retrievals would be helpful.

2.    Can the authors provide any additional context on the size/intensity of the fire plume event on July 19 and typical transport times to the measurement sites? This might help in interpreting the optical property changes observed. Additionally, maybe show aerosol transportation to and from the sites during biomass burning episode and heatwaves in Figure 1 using arrows.

3.    What do you see as the main novel contributions of this study? Specifically, how does the analysis in the Paris region advance our understanding of atmospheric aerosols as compared to previous field studies at other locations?

4.    Please consider the following suggestions in these sections:

a.    Methods:

i.    Consider adding a new section “2.4 Data Processing”  to provide step by step details of the data processing, including the quality control and data filtering procedures applied to the raw measurements before and after performing hourly averaging, to obtain the final data shown in the plots.

ii.    Please consider uploading the entire the code script used in the study to GitHub or Zenodo and attach its link/DOI in the code availability statement.

b.    Conclusions:

i.    The conclusions could be strengthened by more explicitly stating the implications of these results for aerosol radiative effects and air quality in the region. What are the key takeaways for modelers or policymakers?

ii.    Consider adding a brief paragraph on recommended potential future work which can build upon this study.

Technical corrections:

1.    Line 133: "a.g.l." should be defined on first use

2.    Eqn. 5 & 6: Missing space between "370" and "nm"

3.    Table 2 caption: "averages" should be "average"

4.    Throughout: Capitalization of "black carbon" as “Black Carbon”

5.    Figure 5: Missing n, k, and SSA text on the y-axis in the figures.

The authors describe observations of aerosols optical properties (complex refractive index and single scattering albedo) at three sites in France, representing an urban, semi-urban, and forested site during a field campaign in summer 2022. During that campaign, there were two heatwaves, one cleaner period, and one period of fire influence. These events allowed the authors to observed the urban-to-rural gradient under a number of conditions, and attribute changes in the optical properties to a combination of meteorological conditions as well as local influences.

This paper is very well written, clear, and concise, and provides the scientific community with an excellent case study of regional trends in aerosol properties. I recommend publication following some minor comments.

Page 3, line 112: The RambForest site is described as being “downwind” of Paris. However, the later analysis shows that this is often not true, depending on the wind conditions, and that it is often upwind of the city, or neither. I would address this here, to set up the reader to understand that there are various wind patterns that lead to urban outflow vs more stagnant air.

Page 6, line 221: The dynamic shape factor can be different than 1, particularly when the aerosol contains black carbon. Do you expect that accounting for this could close the gap between the results of the optical-iterative and the OPC-SMPS methods?

Page 10, line 379: It’s not clear whether the Saharan dust intrusion and the fire episode happened at the same time, or slightly offset in time. If the former, how can these two events be separated from each other?

Page 12, line 439: “as the latter one is located NW of Paris”. Should this read “SW”?

Figure 1: I suggest indicating on the map where the fire occurred.

Figure 2: The plots reference GRIMM, without indicating what this acronym stands for. I also recommend putting a PRG label over panels a and c and a RambForest label over panels b and d.