Review for ‘Marine Organic Aerosols at Mace Head: Effects from Phytoplankton and Source Region Variability’ by Chevassus et al., submitted for publication in Atmospheric Chemistry and Physics (ACP)

Chevassus et al. present high-resolution time-of-flight aerosol mass spectrometer measurements at Mace Head to investigate the chemical composition of marine submicron aerosol particles and applied positive matrix factorization to identify sources contributing to coastal PM. Overall, the manuscript is well-written. Below, I provide some suggestions for further improving the manuscript.

Quality of the figures:

The resolution of the figures in the document is rather poor, making several texts difficult to read. While this applies to multiple examples, I will specifically address Fig. 6, where the x- and y-axis labels, for instance, appear quite small and blurry. To improve readability, either the font size should be increased or, more effectively, the resolution of the figures in the document should be enhanced.

Additionally, there are some details in the figures that seem to result from a lack of thorough editing. For example, in Fig. 6c under "Chlorophytes," remnants of previous text can still be seen, which were not entirely removed from the image. A similar issue is present under ‘Cyanobacteria.’

Furthermore, in Fig. 1b, the total percentage of components exceeds 100%, which does not seem logical. In Fig. 1c, ‘2%’ is partially cut-off. Another issue in Fig. 1d concerns the labeling of the x-axis with dates. The date labels are positioned directly on the ticks, making it unclear what specific time they represent. Ideally, I recommend shifting the date labels slightly to the right or left so that each date label appears between two ticks. This would clarify that the ticks represent midnight. As it stands, it is not entirely clear what time the ticks indicate.

Introduction:

Overall, the introduction of the manuscript is well-written; however, it does not adequately prepare the reader for the subsequent content. For instance, it lacks any mention of the current knowledge on peat-derived organic aerosols, methane sulfonic acid organic aerosols, and oxidized oxygenated organic aerosols. Including a few sentences summarizing the current understanding of these aerosol groups would be beneficial.

On the other hand, the current manuscript includes a fairly detailed discussion of cloud condensation nuclei (CCN). However, since the manuscript does not provide further measurements or establish a significant connection to CCN, I would recommend shortening this section.

Additionally, it would be helpful to briefly explain the principle of Positive Matrix Factorization (PMF) and highlight how it has been utilized in previous studies by other research groups. This would provide readers with better context for understanding its application in the manuscript.

Further comments:

L54: You mention that sea spray aerosol particles are enriched in biogenic organic aerosol. Could you please clarify relative to, or in comparison with, what this enrichment is observed?

L120: In section 2.2, you mention that submicron aerosol particles have been measured. Does ‘submicron’ strictly refer to particles smaller than 1 micrometer, or is there a more specific cutoff for this classification?

L176: Maybe "arrival height" might be more appropriate than 'starting height'?

L235 I think it would be easier to read if this part were rephrased as: ‘….where 77% of the time exceeds the for the white cap threshold of 4 m s^-1 hinting at strong sea spray influences.’

L243 As I'm not a meteorologist, this question might seem basic, but could you please clearly define how you distinguish between the marine boundary layer, marine free troposphere, and planetary boundary layer? As I understood it, the marine boundary layer is a subset of the planetary boundary layer, yet here you present them as two separate categories. Could you elaborate on this distinction?

L275 Does this fragment C₂H₄O₂⁺ clearly identify the molecule ‘cellulose’, or does it refer to carbohydrates or polysaccharides in general? Could you elaborate on your conclusion regarding cellulose?

L421 Please rephrase to ‘carbohydrates and derivatives’ This is necessary because arabitol and mannitol are not strictly carbohydrates, but rather ‘sugar alcohols’.

Please check to what extent the paper by Paglione et al. (2024) can be used for comparison here. In my opinion, positive matrix factorization was also performed in that study. A thorough comparison or at least a mention in the paper seems appropriate.

Paglione, M., Beddows, D. C. S., Jones, A., Lachlan-Cope, T., Rinaldi, M., Decesari, S., Manarini, F., Russo, M., Mansour, K., Harrison, R. M., Mazzanti, A., Tagliavini, E., and Dall’Osto, M.: Simultaneous organic aerosol source apportionment at two Antarctic sites reveals large-scale and ecoregion-specific components, Atmospheric Chemistry and Physics, 24, 6305–6322, https://doi.org/10.5194/acp-24-6305-2024, 2024.

The study by Chevassus et al. on Marine Organic Aerosols at Mace Head effectively highlights the influence of phytoplankton and source variability on different organic aerosols, analyzed using AMS. The manuscript is well-written and presents valuable findings, making it deserving of publication in the journal after addressing a few revisions.

1. The legibility of the text in the figures requires improvement, particularly in Figures 5 and 6. Ensure that labels and the fonts are clearly readable. Additionally, the labeling across all figures should be harmonized for consistency. For example, compare the font sizes of the labels in Figures 1 and 2. Introduce adequate spacing between panels to clearly distinguish factors and profiles.

2. The abstract does not mention whether the expected aim of obtaining elemental ratios for parameterization was achieved. If successful, explicitly state the derived ratios in the abstract and clarify their relevance. For which specific parameterizations can these ratios now be utilized?