the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Diurnal variability and controlling mechanisms of marine aerosol distributions over the South China Sea: Insights from shipborne observations
Abstract. Marine aerosols critically influence Earth's radiation budget and climate dynamics through their spatial distributions and components due to their generation and transport processes. However, in-situ observational datasets remain limited, particularly in the South China Sea (SCS). Based on our comprehensive shipborne measurements, this study presents a quantitative analysis of marine aerosol distributions and compositional variations between the offshore and pelagic environments over the SCS. Our data demonstrate a 120 % elevation in offshore aerosol number concentrations (NCs, 0.5–10 μm) relative to pelagic baselines, featuring 120 % higher accumulation-mode particles (0.5–2 μm) and 70 % higher coarse-mode particles (2–10 μm), quantitatively confirming continental transport affects offshore aerosol signatures. In contrast, in the pelagic areas, marine aerosols are virtually unaffected by continental transport and distinctly represent characteristics of the local generation. Meteorological analyses identified wind speed (WS) and sea surface temperature (SST) as primary regulators of NC. However, observed NC variations at fixed WS and SST values suggest additional controlling factors. We demonstrate that sea-air temperature differences (SST-T2m) exhibit a stronger correlation (r = −0.82, p<0.01) with NC than the other meteorological parameters, where increased SST-T2m corresponded to decreased marine aerosol production. This temperature gradient effect drives pronounced diurnal NC variations, with maximum differences of 35 % observed between daytime, nighttime, and transition periods. These results prove the key explanations for the variations of spatial and diurnal distributions of marine aerosols to understand marine aerosol generation and transport better.
- Preprint
(5284 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1463', Sourita Saha, 10 Jun 2025
-
RC2: 'Comment on egusphere-2025-1463', Georges Saliba, 24 Jun 2025
This manuscript present APS measurements conducted in the South China Sea. The authors use APS measurement and apportion them to "accumulation" and "coarse" mode particles. The authors conclude that there exist significant differences in aerosol number concentrations and size distributions of the aerosol in offshore regions and pelagic regions. Furthermore, the authors find an inverse relationship between sea-air temperature differences and aerosol number concentrations. Overall, the authors need to address the below points before this manuscript can be published.
Major comments:
I have two major concerns for this study. First is the use of a single instrument (APS) to quantify the marine size distribution over the South China Sea. The APS measures aerosol concentrations in the size range of 0.5 - 20 μm with high uncertainty in the first channel which aggregates all aerosol with aerodynamic diameter <=0.5 μm. I am not convinced that the authors can achieve a defensible case for separating accumulation mode and coarse particle mode using the APS as the only measuring instrument.
My second concern is how the authors define pelagic regions as being 50 km or more from the coast. Even at moderate wind speeds of 6 m/s, it would only take about two and a half hours to transport the aerosol from continents to the ship's location. This period is significantly shorter than the time it would take to remove continental aerosol through wet or dry deposition. Even at 300 km from the shore, it would take about 14 hours for the air originating over the continent to reach the ship's location. As a result, I don't find the author's claim that the aerosols sampled 50 km away from the coast are free from continental influence to be defensible.
Specific comments
below are line-by-line comments
- Line 47: define NC
- line 70: define SST
- line 79: "surface tension" and not "tension"
- Line 90: not sure what is meant by : "the subsequent updates simultaneously were lacking." reword
- Line 120: "were selected for future analysis". I think the work "future" is misleading here. I suggest removing
- Figure 1 can be moved to the SI
- Line 185: "The new aerosol generation events often accompanied the increased nucleation events." This sentence needs rewording
- Line 202: "Fig. 4 a-b presented the trends of the aerosol size distribution and the comparison of the accumulation and coarse mode particle NCs" The authors do not describe how the two aerosol modes were separated? This should be explicitly mentioned in the method section. Was there a threshold diameter above which the authors consider particles to be coarse? (based on the next paragraph it seems that the threshold is 2 μm which seems arbitrary). Also accumulation mode particles are generally thought to include 0.1 - 0.5 μm particles which are unfortunately not measured by the APS. Please refer to my general comment above.
- Line 204: "We found that the marine aerosol NC changed drastically with the temporal differences during the shipboard observation period." by how much? This is not described in the text
- Line 222: "However, some studies found that the aerosols might be generated on the porous surface when impinged by liquid droplets
(Bird et al., 2010; Joung & Buie, 2015; Zhou et al., 2020)." I fail to understand the meaning of this sentence and how it logically connects with the previous one - Lin 232: "The correlation coefficient between the two aerosol particle modes was R = 0.71." The fact that the authors observed a good correlation between the "accumulation" and "coarse" particle modes is further evidence that the two sources are dependent and likely sea-spray. However, from observation, we know that sulfates and organics also contribute significantly to the accumulation mode particle size distribution and number (e.g., Saliba et al. 2020 https://doi.org/10.1029/2020JD033145) . As a result, if the entire accumulation mode was included, we should not expect such high correlations between the two.
- Line 238: "he observed air temperature was in excellent agreement with the reanalyzed air temperature from Merra-2,..." Provide R2 of relation
- Line 240. I assume that VIS refers to visibility. How was this calculated?
- Line 248: "For example, the NCs changed little in the region of 0-6 m s-1 WS because the WS was low for activation of the spume droplets and the marine aerosol generations." This is misleading, you can still generate sea-spray particles at low wind speeds. Spume represents one generation mechanism of sea-spray, usually at high wind speed. Bubble bursting process can occur at much lower speeds
- Figure 4: the time axis for (a) is not aligned with the rest of the figures (b - e). Align all subplots to make it easier to interpret the figure.
- Figure 6: Again, the fact that both "accumulation" and "coarse" particle modes exhibited similar dependence with wind speed suggests to me that you are only capturing a part of the accumulation mode particles that are dominantly sea-spray particles. Other accumulation mode particles, mainly those formed through secondary processes, are not included in your definition of accumulation mode.
- Line 292: "with little differences in the offshore and pelagic regions, where the NCs of the coarse mode in the offshore areas were 2.68 cm-3 and 1.57 cm-3 in the pelagic areas" Looking at Fig 7, it seems that the differences (although small) might be statistically significant. Why is that?
- Figure 7: Why are coarse mode particle concentrations higher in the off-shore region compared to the pelagic region? Are the differences statistically significant?
- Line 300: "However, in the 5.0-10 μm particle size range, the number size distributions in the offshore areas were in excellent
agreement with those in the pelagic areas." However, at concentrations of ~1 cm-3 the uncertainty of the instrument can be as large which would make any comparison meaningless. I suggest the author add a discussion of measurement uncertainty - Line 309: "After the ship entered the pelagic area, the influence of air mass transport almost disappeared" I don't think this statement is true. At 50 km away from the shore and under a moderate wind speed of 6 m/s, it takes about 2.5hr for air originating over the continents to reach the ship's location which is significantly shorter than the time it takes to remove anthropogenic aerosol by wet and dry deposition. This sentence should be removed.
- Line 323: "Therefore, in the pelagic environments, the marine aerosol was not significantly affected by aerosol transport and anthropogenic activity" Same comment as above. The authors do not have detailed in-situ chemical speciation measurements to back these claims
- Line 409: remove the word "obvious" as some correlations are not that obvious (see next comment)
- Line 415: "Therefore, under the WS increased accompanied by synergistic influences of the gas-to-particle conversion and sea surface wind physical friction, the NCs increased in the pelagic region." I don't understand this sentence. Suggest the authors re-write it
- Figure 11-15, I suggest the authors make the legend fonts bigger. These are hard to see. Also, a lot of information on these figures is redundant. I suggest the authors pick the most relevant figure and keep it in the main manuscript and move the remaining to the SI. This will make the manuscript more readable.
Citation: https://doi.org/10.5194/egusphere-2025-1463-RC2 - RC3: 'Comment on egusphere-2025-1463', Anonymous Referee #3, 24 Jun 2025
Viewed
HTML | XML | Total | BibTeX | EndNote | |
---|---|---|---|---|---|
664 | 58 | 16 | 738 | 10 | 18 |
- HTML: 664
- PDF: 58
- XML: 16
- Total: 738
- BibTeX: 10
- EndNote: 18
Viewed (geographical distribution)
Country | # | Views | % |
---|
Total: | 0 |
HTML: | 0 |
PDF: | 0 |
XML: | 0 |
- 1
Comments: