Review of Sulo et al. – Coral reef exposure increases aerosol and cloud condensation nuclei over the Great Barrier Reef

This manuscript presents an analysis of multiple atmospheric measurement datasets collected over the Great Barrier Reef (GBR) between 2016 and 2023. Overall, this is a valuable contribution for several reasons. First, the Southern Hemisphere remains significantly under-sampled compared to the Northern Hemisphere, limiting our understanding of background aerosol concentrations.

More importantly, the role of marine emissions and marine ecosystems in shaping atmospheric composition remains one of the largest uncertainties in climate models. This is especially relevant in the Southern Hemisphere, where anthropogenic influences are generally lower than in the Northern Hemisphere. Studies such as this are therefore essential for improving our understanding of aerosol–cloud interactions in relatively pristine environments.

Additionally, the relative isolation of the GBR from major continental sources provides a unique opportunity to investigate the contribution of primary marine sources and marine biochemistry to marine aerosols.

General Comments

The manuscript compares CCN concentrations and particle size distributions across six field campaigns. While this comparative approach is useful, I have several general comments regarding the experimental design and data interpretation, followed by more specific comments on the manuscript.

1. Instrument consistency and uncertainty

The study combines datasets obtained using a range of instruments across different campaigns. This is a great approach to improve both the spatial and temporal significance of observations. However, given that a substantial part of this study relies on comparing measurements across campaigns, it is essential that the uncertainties amongst all instruments should be carefully addressed. The operation and calibration procedures should also be explained or referenced if already explained elsewhere. Most of these instruments have an acceptable variation of 20% amongst the instruments (CAIS-ECAC).

Without this, it is difficult to assess whether observed differences between campaigns are physically meaningful or within instrumental uncertainty.

1. Heterogeneity of datasets and air mass variability

The datasets originate from different time periods and locations, meaning they are influenced by different meteorological conditions and air mass histories.

It may be beneficial to:

• Group measurements based on air mass origin or meteorological conditions, rather than campaign alone
• Identify air mass analogues, like the approach of Petit et al. (2021)

This would help disentangle the effects of source regions from campaign-to-campaign variability and strengthen the interpretation.

In addition, given that this study is in some way to provide information on background aerosol concentration for the RRAP program and to assess how the weather (radiation, long range transported airmasses) impact aerosol concentrations this would be relevant.

1. Quantitative relationship between aerosol modes and CCN

One of the stated objectives is to understand how aerosol properties influence CCN concentrations. While the manuscript presents a detailed statistical framework, the quantitative relationship between accumulation mode particles and CCN is not clearly expressed.

Specifically:

• How much of the variability in CCN can be explained by the accumulation mode alone? Can you provide a parametrization? Does this change depending on open ocean vs local airmasses?
• What fraction of the remaining variability is explained by Aitken mode particles or other factors?

Providing a more explicit parameterization or quantitative breakdown would significantly strengthen the conclusions.

1. Conclusions and future perspectives

The conclusions would benefit from a short section outlining:

Implications for climate modeling (how do the relationships between CCN and Accumulation mode aerosols compare with previous studies? ). Comparison of these measurements with these global studies would be immensely valuable.

• Andrews, E., Zabala, I., Carrillo-Cardenas, G. et al. Harmonized aerosol size distribution, cloud condensation nuclei, chemistry and optical properties at 10 sites. Sci Data 12, 937 (2025). https://doi.org/10.1038/s41597-025-04931-y4
• Schmale, J. et al. Collocated observations of cloud condensation nuclei, particle size distributions, and chemical composition. Sci. Data 4, https://doi.org/10.1038/sdata.2017.3 (2017).

Recommendations for future measurements (e.g., long-term monitoring, chemical composition)

Specific Comments

Introduction

• The structure of the introduction could be improved for clarity. Some sections (e.g., around line 50) appear out of place within the current paragraph.
• The objectives of the study should be clearly stated at the end of the introduction.
• It would also be helpful to explicitly state that this work represents a reanalysis/synthesis of multiple previous campaigns.

Methodology

• Several GBR-related studies are discussed in methodology but not in the introduction. Their main findings and gaps in the findings should be summarized earlier to provide context.
• In Table 1, please include references to the original publications for each campaign.

CCN supersaturation

• Explain the limitations of using a single supersaturation, and also of such a high value 0.5% ?
• Under what atmospheric conditions is this supersaturation expected over the GBR?
• Are particles realistically transported to altitudes where such supersaturation occurs? What bias can be implied from using such high SS.

Instrument uncertainties

As mentioned in the general comments, multiple instruments (SMPS, APS) are used for size distribution measurements. Please include typical uncertainties for each instrument and a brief discussion of inter-instrument comparability

In the manuscript a discussion is already provided for absorption instruments; a comparable statement should be included for particle sizing instruments.

Ship measurements

• How were pollution events (e.g., ship exhaust) filtered? What % of data was removed?

Mixing state

• Page 6, line 155: Is there any estimate of internal vs. external mixing based on the size distribution data (number of modes etc)?

Results

Kappa values : The reported hygroscopicity (κ) values should be compared to large-scale CCN datasets (e.g., long-term measurements such as those reported by Schmale, Andrews et al.). This would provide context for whether the observed values are typical or unusual.

Figure 1: Does it make sense to directly compare all campaigns in this way? Ship-based measurements (open ocean) vs. land-based (reef/cay) measurements may not be directly comparable. Are there observable gradients between coastal, reef, and open-ocean environments?

Interpretation of aerosol modes

Line 272: The statement regarding accumulation mode particles should be rephrased to reflect that this is consistent with previous studies.

I maybe did not fully appreciate the method used in this section but the role of sub-100 nm particles in CCN formation requires clarification. Is their contribution evaluated independently of accumulation mode particles? Could the observed relationship simply reflect correlation between modes?

In relation to my general comments, Is it possible to quantify CCN variability explained by accumulation mode and then analyze the residual variability in relation to Aitken mode particles ?

Statistical significance

Page 13, line 290: Differences between campaigns appear small and may fall within instrumental uncertainty. Have you performed some significant tests on the different presented in the different figures (e.g. Fig 2)

Nucleation and Aitken mode

It was stated that the nucleation mode measurements were not available. Based on previous studies can you estimate the contribution from new particle formation events and under what conditions they are likely to occur.

Line 304: Do you suspect that the Aitken mode primarily due to primary emissions, or Secondary formation processes? Can these processes be distinquished?

Discussion and conclusions

Line 319 The comparison with Dall’Osto et al. is interesting. Are Southern Ocean and North Atlantic marine environments directly comparable? Given the undersampling of the Southern Hemisphere, this point could be expanded.

In the conclusion it would also be useful to state what the implications of this work are with respect to the Reef restoration and adaptation program ?

Summary

This is a valuable study addressing an important gap in Southern Hemisphere aerosol observations. However, the manuscript would benefit from clearer treatment of instrument uncertainties. A different approach to handing the different datasets, comparing them based on meteorological and airmass signatures rather than from different periods.