Diurnal variability and controlling mechanisms of marine aerosol distributions over the South China Sea: Insights from shipborne observations

Qiao, Zhi; Cui, Shengcheng; Xu, Huiqiang; Wu, Xiaoqing; Liu, Xiaodan; Zhang, Zihan; Zhai, Mengying; Pan, Yue; Luo, Tao; Li, Xuebin

doi:https://doi.org/10.5194/egusphere-2025-1463

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https://doi.org/10.5194/egusphere-2025-1463

Preprints

28 May 2025

| 28 May 2025

Diurnal variability and controlling mechanisms of marine aerosol distributions over the South China Sea: Insights from shipborne observations

Zhi Qiao, Shengcheng Cui, Huiqiang Xu, Xiaoqing Wu, Xiaodan Liu, Zihan Zhang, Mengying Zhai, Yue Pan, Tao Luo, and Xuebin Li

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-T_2m) exhibit a stronger correlation (r = −0.82, p<0.01) with NC than the other meteorological parameters, where increased SST-T_2m 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.

Received: 27 Mar 2025 – Discussion started: 28 May 2025

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Zhi Qiao, Shengcheng Cui, Huiqiang Xu, Xiaoqing Wu, Xiaodan Liu, Zihan Zhang, Mengying Zhai, Yue Pan, Tao Luo, and Xuebin Li

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-1463', Sourita Saha, 10 Jun 2025
Comments:
Graphical abstract: make the y-axis on the right same limits for the size distributions and whats the unit? Is the right y-axis same as left y-axis?

The second panel of graphical abstract: are those correlations for pelagic or offchore? Not clear from the figure

Introduction: line 45: “ Hoppel (1979, 1985) studied the aerosol NC and the particle size distribution on the east coast of the United States, and the significant changes in the particle size distribution can be associated with the changes in meteorological parameters and oceanic air mass.” . When you say significant changes in PSD, my question is change from what? To what? Did he compare spatial distribution of aerosols? Or what you mean? Please clarify

Line 50: “In the Arctic, Leck (1996) reported that the submicrometer aerosol (Dp ≤ 1000 nm) mass concentrations during the International Arctic Ocean Expedition (IAOE-91) cruise; for instance, the average mass concentration was 0.76 μg m-3 over the ocean.:” rewrite the sentence. “reported that” doesn’t make sense if you don’t follow it with result.

Rewrite this too for clarity: “. In terms of the Antarctic, Savoie (1993) reported the submicrometer aerosol (Dp ≤ 1000 nm) concentrations, and the mean concentrations were 3.15 μg m-3 at Marsh.”

All your references measure in ug/m3, and Sakerin (2015) ’s value is in ng/m3. I suggest use same units so its easy for the reader to compare

“. For the China waters, Kim (2009) found that the average submicrometer aerosol particle (10 nm ≤ Dp ≤ 300 nm) concentrations were 4335 ± 2736 cm-3 over the East China Sea and 5972 ± 2736 cm-3 over the Yellow Sea.” This reference feels out of place specially when the previous and following references mention units in ug/m3.

“All in all, there were some discrepancies in the marine aerosol concentrations and size distributions between the different ocean areas;” what discrepancy? That they aren’t able to come to a common consensus for mass concentration? Or what you mean here?

“the major measurement data are relatively outdated and need to be updated.” What you mean by outdated? Like previous measured values have changed or science behind them changed? What exactly and why outdated?

Line 77: are generation functions and source functions same? In that case use source function. May be cite Gong et al for source function involving wind speed?

Is SCS analogous to any other place on earth in terms of meteorology? I ask this because studying marine aerosols at a single location which isn’t representative of similar areas would make little less sense but if it were to explain (or represent) marine aerosols for larger areas, it would make more sense. What I mean to ask is why is this location important to study?

Line 94. It is mentioned that diurnal variations are lacking. My question is why is it important? Add a hypothesis or reasoning why we want to learn about diurnal variations of marine aerosols and what impacts do diurnal variations have?

Line 119: “. Fig. 1 showed that the particle losses were small in the size range from 0.5 μm and 10 μm. Thereby, aerosol data within the size range of 0.5 to 10 μm were selected for future analysis in this study.” You only measured this size range and calculated loss for this range, which is understandable. But saying that this range was selected for future analysis due to a small loss raises a question of whether you know the losses of other size ranges?

I don’t think equation 1 is readable. Its all symbols and I think format needs to be changed

Line 204; avoid using words like drastically. If you use, give a quantitative measure of it’

Line 203: During the shipboard observation period, the average total marine aerosol NC was 54.01 ± 35.37 cm-3, the NC of aerosol accumulation mode was 52.35 ± 34.96 cm-3, and the NC of aerosol coarse mode was 1.66 ± 0.83 cm-3.; do the mean values include/exclude measurements during data events?

Line 205: unnecessary use of ‘meanwhile’

Rephrase this sentence for clarity: Line 207 “. The shipboard observation data recorded and showed the overall average values and standard deviations of marine aerosol NCs under different temporal and geographical conditions, which were used to compare with the marine aerosol NCs observed.”

Line 212: “This suggested that the NC of aerosol accumulation mode in the East China Sea might affected by the higher frequency of the new marine aerosol particle” can you cite some studies? Do newly formed particle have high growth rate that they can impact accumulation mode aerosols so much? Cite some studies which have shown the growth of small aerosols in the mentioned region. And studies which show npf is sparse in SCS.

Line 214: please rephrase this for clarity “Meanwhile, the total marine aerosol NC observed in this study contained the aerosol coarse mode (2 μm ≤ Dp ≤ 10 μm) and the part of aerosol accumulation mode (500 nm ≤ Dp ≤ 2000 nm), and the NC was slightly lower than the marine aerosol NC in the Atlantic by Flores et al. (2020)”. Why sudden comparison with Atlantic? Are SCS and Atlantic supposed to have similar concentrations? Or why just compare with Atlantic?

What seasons did Cai et al. (2020) and Kong et al. (2016) carry their study in? easier to mention, it leaves the reader well informed. Did you compare the rain events between their study and your? is it possible they had less rain which kept the concentrations higher?

Line 218: “Although the differences in the observation seasons, the study region, and the particle size might influenced the average NC observations, it can still show that the marine aerosol was significantly affected by the continental transport and the anthropogenic activity in the offshore areas according to the latitude and longitude.” Why did you mention differences in study region? The line before this says Cai and Kong did their study in the same region as yours. Next line says differences in study region. Its confusing.

Line 222: “However, some studies found that the aerosols might be generated on the porous surface when impinged by liquid droplets” porous surface of what? I know the next lines make it clear but it would be better if you introduce the concept when you first mention it.

Line 229: you mention only one size 4068nm. It's hard to say if ambient aerosols are just of one particular size. Please give a range.

Line 243: “. High NCs (≥ 150 cm-3) were observed almost entirely in which the WD were between NW and N that were caused by the high RH accompanied by the rainfall events, and the distributions of NCs were uniform when the wind was blowing in the other directions.” So this high NC is the artificial aerosols created by porous surface of the ship and not natural aerosols? Because rainfall is supposed to cause wet removal or deposition of aerosols and thus decreased concentrations

Table 2: when you write accumulation mode, please mention the size, I don’t think there’s any use of comparing with n10-400 because your measurement starts from 500nm.

Line 274:284: include this is methods instead of results

Figure 7b shows same shape of size distribution, ofcourse, the concentrations are different. But doesn’t same shape of distribution say same sources for the two regions? If one were affected by continental sources, wouldn’t the distributions be slightly different?

Line 298: Can you elaborate this more? “. The marine aerosols decreased slowly with the increasing particle diameters below 1.114 μm due to the transport effect.” You didn’t discuss the ‘transport effect’ before, so its difficult for the reader to associate this.

In figure 7c, where is the correlation (r=-0.87) shown between Acc NC and distance? All is see is boxplot and distance points in red. Where’s the correlation? The distance on 5/31 was higher than 5/27 but still number concentrations were higher on 5/31. How do you explain this?

Your accumulation mode starts from 500nm, how can you show that transport brings in particles which are atleast 500nm for contributing to increase in NC os accumulation mode? Can you cite someone who has shown the size of transport particles in this range? You also mentioned higher wind speed in offshore compared to pelagic areas. How can you say the higher NC at offshore was due to transport and not high WS?

In Figure 8, please add the arrows showing direction of air

Line 324: again words like significant doesn’t make much sense. Talk in numbers. Out of all components dust shows the maximum change between pelagic and offshore, the difference in the rest of the components do not look ‘siginificant’

Rephrase line 367: Fig 9a showed a clear diurnal variation emerged

During the daytime, WS increases and SST and difference in temperature between surface and 2m also decreases; so shouldn’t the daytime NC be increasing because all the factors you mentioned for the transition period aligns for daytime, yet there’s no clear trend in daytime. Why is that?

Line 433, add references of studies that showed entrained air decreased with increasing SST. Entrained where? sea surface or boundary layer? clarify

Instead of holding wither WS or SST constant for SST-T2m correlations, I suggest you perform multi linear regression or lasso regression or any suitable regression analysis to study the impact of these factors on NC variability. When you hold WS const for Fig 13, SST is still varying and when you hold SST const for Fig 14, WS is still varying.

Does the size distribution remain the same with increasing SST? What would you comment on the increase of diameter with warming SST as observed by Saliba (2019)?
Citation: https://doi.org/10.5194/egusphere-2025-1463-RC1
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

My review is attached.

Citation: https://doi.org/10.5194/egusphere-2025-1463-RC3

Zhi Qiao, Shengcheng Cui, Huiqiang Xu, Xiaoqing Wu, Xiaodan Liu, Zihan Zhang, Mengying Zhai, Yue Pan, Tao Luo, and Xuebin Li

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Short summary

We gave first insight into the diel characteristics of the marine aerosol, especially during the day night transition period, to better understand which and how meteorological elements affect the marine aerosol. Overall, wind speeds and sea surface temperature indeed play critical roles in the regulation of aerosol production and diffusion processes, while the sea-air temperature difference is found to be the most vital factor related to the variations of marine aerosol distributions.


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