the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Aug 2026
Exploring drivers of Cloud Condensation Nuclei in the Indian Sector of the Southern Ocean
Abstract. The Southern Ocean plays a critical role in Earth's climate system, acting as a major sink for heat and carbon dioxide. Clouds over this region strongly influence the regional radiation budget, yet their formation remains poorly understood and represented in climate models. This is partially driven by the lack of in-situ observations, particularly over the undersampled Indian sector. In this study, we report on the latitudinal variability of cloud condensation nuclei (CCN) and examine the meteorological and biological factors that can influence their distribution across this region. CCN concentrations were measured during the 12th Indian Scientific Expedition to the Southern Ocean during February and March 2025 aboard a research vessel traversing from 20° S to 68° S. CCN concentrations varied across supersaturation levels, with the highest values approximately 500 cm-3 observed at S = 1.0 % in the polar band (60–70° S) and lower concentrations at lower S levels. CCN concentrations showed a poor correlation with coarse-mode aerosols, suggesting that wind-driven sea spray does not play a major role in controlling the CCN variability in this region. Chlorophyll a, used as a proxy for biological productivity, showed a positive relationship with CCN concentrations. Air masses passing over biologically productive waters during the preceding 24–48 h were associated with higher CCN concentrations, with the strongest relationship observed within the 30–50° S latitude band encompassing the subtropical and subantarctic fronts. This suggests a closer association between marine biological activity and CCN variability in this region. In contrast, the weaker relationship outside this latitude band indicates that physical processes and non-biological sources may exert a greater influence on CCN concentrations in other regions.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 18 Sep 2026)
- RC1: 'Comment on egusphere-2026-3546', Anonymous Referee #2, 27 Aug 2026 reply
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- 1
This manuscript presents in-situ cloud condensation nuclei (CCN) measurements across a broad latitudinal transect (20°S–68°S) in the Indian Sector of the Southern Ocean (ISSO) from the 12th Indian Scientific Expedition. The methodological approach, combining in-situ CCN measurements at multiple supersaturation levels, aerosol size distribution data, black carbon filtering for contamination control, back-trajectory analysis, and satellite-derived chlorophyll-a exposure calculations, is comprehensive and generally well-executed. The quality control procedures for removing ship-stack contamination are appropriate. This valuable dataset provides a useful baseline for future model evaluation.
However, the manuscript currently has notable weaknesses: the incremental novelty is not clearly articulated, several mechanistic interpretations rely on untested speculation, and uncertainties in satellite retrievals and data analysis are insufficiently discussed. Therefore, I recommend a major revision is required before further consideration.
Major comments:
1. In the abstract, the phrase "CCN concentrations peaked at ~500 cm⁻³ across five supersaturation levels" is misleading. Table 2 shows ~500 cm⁻³ is the maximum observed value at the highest supersaturation (SS=1.0%), not a peak value common to all SS levels. This should be rephrased to avoid ambiguity. In addition, the claim that biological activity "strongly controls CCN formation over most of the Indian sector" is not supported: the strongest biological signal is confined to 30–50°S, while polar and subtropical bands show much weaker or non-significant correlations.
2. Black carbon (BC) is used to filter ship exhaust contamination, but no quantitative threshold for "sudden increases" in BC is provided. The criteria used to flag and remove contaminated periods should be specified.
3. The justification for discarding the first 3.5 minutes and last 30 seconds of each supersaturation cycle is not explained. Information on full cycle duration and temperature stabilization for each SS should be supplied.
4. The APS only covers coarse-mode particles (>0.5 μm), leaving the dominant CCN size range uncharacterized. The APS vs. CCN correlation (Figure 6) shows that coarse-mode particles contribute only a small fraction to CCN, but the manuscript does not quantify what fraction of CCN is associated with particles >0.5 µm vs. <0.5 µm.
5. Section 3.6 discusses the CCN-Nd relationship and finds that the correlation is significant only in the 30-50°S band. The authors attribute the lack of correlation at higher latitudes to mixed-phase cloud processes, precipitation scavenging, and riming. However, this discussion is brief and somewhat speculative without supporting microphysical observations. Only warm clouds (liquid phase) should be used to discuss the relationships between the retrieved Nd and CCN concentration. Were mixed-phase or ice-phase clouds excluded?
6. Improving the clarity of figures and tables, particularly by including statistical information directly on the figures.
7. The manuscript interprets low power‑law exponent k values in the polar band as direct evidence for abundant freshly nucleated, low‑hygroscopicity aerosol particles. However, exponent k is jointly controlled by both aerosol size distribution and aerosol hygroscopic properties. Without supporting aerosol chemical measurements, it is difficult to fully differentiate these two effects.