This short letter describes an analysis that combines MODIS satellite estimates of cloud droplet concentration in liquid-dominated marine low clouds with trajectory analysis over the Southern Ocean. The findings indicate that high concentrations of cloud droplets (Nd) tend to occur to the south (poleward) of a boundary previously identified as a “compositional front” that rings Antarctica. South of the “atmosphere compositional front of Antarctica (ACFA)” at roughly 60S comprises extremely biologically rich ocean waters that are copious sources of aerosol precursor gases (in particular dimethyl sulfide). Air mass back trajectories from high Nd clouds tend originate more frequently south of the ACFA. The high Nd south of 60S are associated with smaller effective radii and higher cloud optical thickness, but only marginally higher LWP, indicating that the cloud optical depth increase is largely driven by higher Nd, i.e., Twomey brightening.

The results presented here are interesting and important and I think very relevant to the ACP readership. I recommend publication subject to some minor revisions.

The main question I would like to raise is that I believe that the latitudinal gradient of light precipitation may also play an important role in setting the Nd latitudinal gradient through coalescence scavenging, in addition to the consideration of aerosol sources. We know from spaceborne 94 GHz radar that light precipitation maximizes at around 55S and decreases southward of this (see e.g., McCoy et al., 2020), so the reducing precipitation south of the ACFA may also be partly responsible for high Nd there. Another paper by Kang et al. (2022) illustrates the significant role that precipitation sinks may play. I wonder if the authors have tried to use any of the ship or aircraft measurements associated with CAPRICORN/MARCUS/SOCRATES to explore how precipitation sinks may change across the ACFA.

Other points

1. Line 35. Albedo increases with solar zenith angle, so how is this accounted for? Also, I didn’t see any albedo measurements in the paper.
2. Line 47-50: Why does a lack of precipitation make clouds more sensitive to CCN? Shiptracks in precipitating boundary layers tend to more visually apparent than those forming in non-precipitating clouds.
3. Figure 1b does not seem important. Can’t the essence of this simply be stated in the text?
4. Line 95 and several studies point out the importance of air masses moving from interior Antarctica over the ocean as being the source of new particles. I do not understand why the Antarctic continent would be a good source of aerosol or aerosol precursor gases. It seems as though the highly productive ocean waters south of the ACFA are the main sources of aerosol. Can the authors comment on this?
5. Figure 1a: why not provide the correlation coefficients between cloud variables to make the points quantitatively?
6. Line 116: LWP can remove aerosol, suppressing Nd (Wood et al., 2012). Nd can suppress precipitation, but the LWP response to this is bidirectional, and depends upon whether the background clouds were precipitating and up the dryness of the free troposphere. I don't think you can necessarily conclude that the seasonal cycle of LWP is dominated by meteorology (i.e. is NOT driven by aerosol, at least in part).
7. Line 122: Provide evidence of the one month lag between Chl-a and Nd. Is this at all locations across the SO?
8. Fig 2/Line 134: This Nd gradient is documented and discussed in McCoy et al. (2020).
9. Line 156: Cite Korhonen et al. (2008), who established the pathway through the free troposphere. I would have expected the need for transport to the FT and nucleation of new particles to effectively reduce the sharpness of the Nd gradient driven by the gradient in surface-emitted precursor gases. Sources will lose their identity through the mid-deep tropospheric mixing and latitudinal displacement related to cyclonic systems. I would appreciate if the authors can comment on this issue. Line 179 seems to partly challenge the Korhonen transport pathway being primarily through the free troposphere.
10. Line 169-171: Are these 3D trajectories, or 2D? What method was used to determine the vertical ascent (model vertical velocity, isentropic....)?
11. 4: The differences between the latitudes crossed by high Nd and low Nd trajectories shown here are quite modest yet are described as “overwhelming” (line 179). Does this statement pertain to clouds only south of the ACFA? It certainly does not pertain to high Nd cloud north of 60S since the majority of trajectories ending north of 60S never go below 60S.
12. Line 245: No shortwave measurements are presented in the paper, so I’m not sure that the term “brightening” is appropriate unless said measurements are presented.

References

Kang, L., Marchand, R. T., Wood, R., & McCoy, I. L. (2022). Coalescence scavenging drives droplet number concentration in Southern Ocean low clouds. Geophysical Research Letters, 49, e2022GL097819.

Korhonen, H., Carslaw, K. S., Spracklen, D. V., Mann, G. W., & Woodhouse, M. T. (2008). Influence of oceanic dimethyl sulfide emissions on cloud condensation nuclei concentrations and seasonality over the remote Southern Hemisphere oceans: A global model study. Journal of Geophysical Research, 113(D15). https://doi.org/10.1029/2007JD009718

McCoy, I. L., McCoy, D. T., Wood, R., Regayre, L., Watson-Parris, D., Grosvenor, D. P., Mulcahy, J. P., Hu, Y., Bender, F. A.-M., Field, P. R., Carslaw, K. S., & Gordon, H. (2020). The hemispheric contrast in cloud microphysical properties constrains aerosol forcing. Proceedings of the National Academy of Sciences, 117(32), 18998–19006. https://doi.org/10.1073/pnas.1922502117

Find my review attached below.