Review of "Tropospheric aerosols over the western North Atlantic Ocean during the winter and summer campaigns of ACTIVATE 2020: Life cycle, transport, and distribution" by Liu et al. (2024)

The manuscript provides valuable insights into the aerosol distributions and processes observed during the ACTIVATE 2020 campaign over the western North Atlantic Ocean. It effectively supports the campaign goal of studying aerosol-cloud-meteorology interactions by using data sets from all the different platforms, including GEOS-Chem model simulations, ground-based observations, and satellite data.

Moreover, the manuscript provides a comprehensive overview of the aerosol distribution, source, and transport coupled with meteorological parameters, along with an evaluation against ground-based and space-borne measurements. In addition, the authors perform sensitivity analyses with GEOS-Chem simulations, modifying parameters such as biomass burning height etc.

Overall, the manuscript demonstrates the satisfactory performance of the model against observations, especially in response to changing meteorological dynamics. It suggests potential improvements can be possible through finer resolution simulations.

I appreciate the authors for clearly defining the scientific objectives of the manuscript and for their valuable contributions to the understanding of aerosol dynamics, aerosol types, and associated meteorological drivers during the ACTIVATE campaign.

This manuscript falls well within the aim and scope of the ACP journal. I would recommend this manuscript for publication with minor corrections.

Total manuscript length is very high, authors need to work on reducing manuscript as well as abstract length. It would be helpful to future potential readers of this manuscript if the authors would consider these minor changes listed below:

General major questions:

1. The duration of the ACTIVATE campaign is six years (2019-2024), so why is this study focused only on the spring and summer of 2020?

2. In this manuscript, it has been mentioned several times that the vertical transport can be improved by applying high-resolution models/simulations. Thus, in this manuscript the study domain is very small (two box regions: the North ("N"; 36-39°N, 69-75°W) and the South ("S"; 32.5-36°N, 71-75.5°W)), so why this study has not used finer resolution nested grid (0.25*0.3125) simulations provided by GEOS-Chem model instead of global simulations (2*2.5)? This finer resolution simulation could be used as another sensitivity simulation.

3. This study is conducted during August Complex “Gigafire” took place in mid-August 2020 and the California Creek fire occurred in early September 2020, ranked among the top five in California wildfire history. Thus to see the GEOS-Chem model comparisons during other years of ACTIVATE campaign would be very interesting, and could bring some valuable knowledge.

4. I was unable to find the method used in this manuscript to spatio-temporally collocate the GEOS-Chem model with ground-based, satellite data sets for evaluation. As models provide spatio-temporally continuous data, while ground, airborne, and satellite data are very discontinuous. So, what is the collocation strategy between all the datasets?

Minor comments:

Abstract:

The abstract is very long, it needs to be shortened further to make it easier to follow. Furthermore, there is a single big sentence from line 30 to line 34 that can be reduced to one small sentence, and mentioning GEOS-Chem model driven by MERRA-2 does not bring any additional information to the abstract, I suggest to remove MERRA-2 from the abstract.

Similarly, from line 43 to 46, this large sentence must be reduced in length.

Introduction:

1. Please cite some references at line 65 and 66.

2. The introduction is very large at about 4 pages. I would strongly suggest to reduce it. From line 79 to 115 has the potential to be reduced as there is no need for such large discussions NAO+, NAO-, and synoptic scale impact of cyclones on wind pattern and consequent impact on aerosol transport in the introduction. Just mention the direction of the wind pattern created by NAO oscillation and cyclones, and the associated aerosol transport in 3-4 lines.

4. Furthermore, all these NAO and cyclonic transports have been well presented with figures in Section 4 "Meteorological Settings and Transport Pathways", so why discuss them twice? Once in the Introduction and again in Section 4.

After the end of the introduction, the second section starts as GEOS-Chem Model. I would suggest to have a section like Data and Methods. In this section all data sets from different platforms can be presented as sub-sections. Further, at the end of the Data and Methods section, write a paragraph about the collocation strategy used for GEOS-Chem model evaluation with ground-based and space-based datasets etc.

In Section 2 "Model Description", use only the information about the model simulations and the sensitivity simulations. Include the emission inventories used in an Appendix. This will help to reduce the length of the manuscript and will be easier for the readers.

At Section 4 “Meteorological Settings and Transport Pathways”, this section explains very well about the meteorological drivers for the aerosol transport and variability over WNAO region. So, please reduce the explanation in introduction about NAO+, NAO-, and synoptic impacts on aerosol distributions.

Furthermore, I would like to suggest to include figures 2 to 4 in the appendix, as the meteorological variability is not the result of this manuscript, but rather an established fact that is captured by the MERRA-2 and GEOS-Chem model. This will further help to shape the manuscript. Also keep figure 14 in the supplement, and also I was unable to read the color bar values, please improve it.

Section 5 "Simulated Aerosols over the WNAO and Model Evaluations" contains the most important result of this manuscript. This section is very well written and easy to understand.

It would be very helpful to segregate the section 7 “Summary and conclusions” into two sections, as this manuscript brings very valuable conclusion that supports the ACTIVATE campaign and future modelling aspects related to this campaign. However, the summary overshadows the conclusion of this manuscript.

Overall, although the manuscript is very long and took me a few days to read, I enjoyed reading it and appreciate the scientific motivation behind this manuscript to support ACTIVATE campaign.

This manuscript by Liu et al. compares GEOS-Chem model simulations against the observations conducted during the 2020 winter and summer campaigns of the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) campaign. The study examined the aerosol life cycle, composition, transport pathways, and distribution in the western North Atlantic Ocean (WNAO). 

The GEOS-Chem model results were compared against aircraft in situ and remote sensing measurements, ground-based observation and satellite observations. The study identified major transport mechanisms of aerosols over the WNAO; for example, the wintertime cold fronts can facilitate the continental outflow of aerosols, and boundary layer (BL) turbulent mixing can vertically transport sea salt aerosols. Based on the model simulation, biomass burning (BB) injection height was also found to influence the vertical distribution of aerosols. 

The manuscript is a comprehensive overview of the research findings during ACTIVATE and highlights the importance of model simulations in understanding aerosol properties and transport. The manuscript fits the scope of Atmospheric Physics and Chemistry. However, the manuscript can be improved further by focusing on the region where the aircraft campaign was conducted and replotting some of the figures. I recommend the publication of the manuscript after the following revisions. 

General comments: 

1. My major comment is that the manuscript can be more focused on comparing model results against aircraft and ground measurements, especially in the region where aircraft measurements were conducted (WNAO Central). The comparison between the model results and CALIOP observations was not well organized, and many subpanels of the figures were left undiscussed in the manuscript. Moreover, the quality of the figures and discussions in the section associated with the CALIOP comparison is not as good as the earlier sections. The authors may consider moving the CALIOP results to another manuscript or discussing the CALIOP WNAO Central results only in this manuscript. 

2. Overall, the figure quality in this manuscript is not high. It could be the issue of figure resolution, but the authors need to properly label the subpanels and place the figures side by side for comparison – especially comparing color maps. If the figures are not related to the main context of the manuscript, they need to be removed or relocated to the supplementary information. In several figures (e.g., Fig. 5 and Fig. 19), the letters cannot be read. The fonts of the figures are also not consistent. The color bar labels are overlapping (e.g., Figs. 14 and 16), the axis label does not have the proper unit (Figs. 14 and 16), or the unit is missing (Fig. 17). These issues need to be carefully addressed to maintain the scientific value of the manuscript. 

Detailed comments:  

1. Line 123: The authors provided a detailed literature review for the sulfate-nitrate-ammonium (SNA), sea salt, and aerosols, but the discussion of OA and SOA is relatively short. The southeast U.S. is known for the large SOA yield due to the interaction between biogenic and anthropogenic emissions. The SOA from this region is expected to contribute to the WNAO due to the continental outflow. 

2. Line 174: The authors mentioned that there are “two campaigns.” Is there another campaign apart from ACTIVATE? Or are the authors referring to “two deployments”? 

3. Line 313 to 336: I suggest moving this section to the supplementary information since it is not directly related to the discussions of the manuscript. 

4. Figs. 2a and 2b: Please reorganize the figures for a side-by-side comparison between winter and summer measurements. It is difficult to compare color maps, especially when they are in different figures. Please improve the figure resolution because the letters and symbols cannot be clearly recognized (see general comment #2). Also, can the authors explain why the BL height is higher in winter? 

5. Line 399: The authors discussed different transport mechanisms, both in the horizontal and vertical directions. However, in the horizontal direction, are there other mechanisms other than cold fronts? In the vertical direction, it is not clear how uplifting ahead of cold fronts and convective transport are different from each other. In addition, is entrainment considered in the vertical transport mechanisms? It is an efficient mechanism to control the aerosol population in the Eastern North Atlantic (ENA, see reference below), which is also affected by the continental outflow from North America. 

Zheng, G., Wang, Y., Aiken, A. C., Gallo, F., Jensen, M. P., Kollias, P., ... & Wang, J. (2018). Marine boundary layer aerosol in the eastern North Atlantic: seasonal variations and key controlling processes. Atmospheric Chemistry and Physics, 18(23), 17615-17635. 

6. Fig. 4c: The first panel (please label each panel inside the figure) shows that there is a high large-scale downward vertical flux for sea salt aerosols. Is this result true? What does it imply? 

7. Line 423: Why is biofuel considered in the model? Is it a major source of aerosols? Also, should it be considered as a part of anthropogenic source? 

8. Line 450: The model simulation shows that the sea salt aerosol mass concentration is higher in summer. Is it driven by the higher wind speed in the summer? Can the authors provide the average wind speeds during the deployments? 

9. Figs. 5 and 6: The letters cannot be recognized in the figure. Please improve the figure quality. 

10. Fig. 8 (last panel): Can the authors discuss why there are two peaks in OA under 0 to 5.5 km BB condition? 

11. Line 491: Why would a smaller CWC lead to a faster conversion of cloud water to precipitation? Should it be the opposite? 

12. Line 511: The authors stated that the reason why the model cannot reproduce the observed CO concentration is because of the model’s inability to retain the western U.S. BB plumes. However, the model agrees reasonably well with the observed BB aerosols, especially when using different injection heights. Are there additional reasons for the disagreement in the CO vertical profile? The authors may consider checking CO and BC measurements over the ENA, which is also under the influence of North America continental outflows (references below). 

Parrish, D. D., Trainer, M., Holloway, J. S., Yee, J. E., Warshawsky, M. S., Fehsenfeld, F. C., ... & Moody, J. L. (1998). Relationships between ozone and carbon monoxide at surface sites in the North Atlantic region. Journal of Geophysical Research: Atmospheres, 103(D11), 13357-13376.

Wang, Y., Zheng, G., Jensen, M. P., Knopf, D. A., Laskin, A., Matthews, A. A., ... & Wang, J. (2021). Vertical profiles of trace gas and aerosol properties over the eastern North Atlantic: variations with season and synoptic condition. Atmospheric Chemistry and Physics, 21(14), 11079-11098.

13. Line 531: The authors mentioned “weak entrainment” here. Is entrainment considered a vertical transport mechanism? 

14. Line 640: The authors can just use “Fig. 12” instead of “Fig. 12ab” because there are only two panels in the figure. Also, the left panels WNAO South and WNAO Bermuda in Fig. 12a are not discussed in the manuscript (please see general comment #2). 

15. Fig. 13: Please consider reorganizing the figures. The flight tracks and flight directions cannot be clearly seen. If GC BC and Dust are negligible, these two panels can be removed from the main body of the manuscript and simply discussed in the manuscript. 

16. Line 672: If the CALIOP retrieval results are in doubt, the discussion and figure should not be included in the manuscript since we cannot confidently compare the model and observations. 

17. Line 679: Please just use “Fig. 15” instead of “Fig. 15ab.”

18. Line 683: Please see the comment on Line 672. 

19. Section 5.4: This section can be restructured by having a subsection to (1) 5.4.1: show HSRL-2 and CALIOP results and (2) 5.4.2: conduct case analysis of land-ocean aerosol extinction gradient, SNA and sea salt mixing, transport of fire smoke, and so on. Please use subsections to divide the contents. 

20. Line 739: Since the transport event happened over several days, would the one-time surface weather map be able to support the transport trajectories? The FLEXPART results should be sufficient to support long-range transport.