the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2023
Diverse mixing state and ice nucleation properties of aerosol particles over the Western Pacific and the Southern Ocean
Abstract. Atmospheric particles from different sources can impact cloud formation and play a critical role in regulating cloud properties. However, particle characteristics at single particle level and their abilities to serve as ice nucleating particles (INPs) over different marine atmosphere are poorly understood. Here, we present characterizations and ice nucleation properties of particles collected during a northern and southern hemisphere cruise from South Korea to Antarctica. Micro-spectroscopic analysis was used to obtain composition of individual particles and mixing state of particle populations. Major particle classes were identified and have different contributions over the Western Pacific and the Southern Ocean, including fresh and aged sea salt, sea salt mixed with sulfate, carbonaceous, sulfur-containing, and dust particles. Increasing contribution of fresh sea salt particles, the dominate particle class in the samples, tended toward an increasing mixing state index indicating the population becoming more internally mixed. Aging processes and new particle sources introduce particles with new compositions resulting in external mixtures. We found that the investigated particles demonstrated a variety of ice nucleation abilities at cirrus conditions. The identified INPs are all major particle classes present in the population, and the sea salt mixed sulfate particle is enriched in INPs. Aging processes affected both particle mixing state and their ice nucleation abilities. We tested different ice nucleation parameterizations of marine atmospheric particles for their applicability. Finally, we discuss how the mixing state of particle populations impacts ice nucleation in the atmosphere.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-2646', Anonymous Referee #1, 22 Dec 2023
- AC2: 'Reply on RC1', Bingbing Wang, 22 Mar 2024
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RC2: 'Comment on egusphere-2023-2646', Anonymous Referee #2, 24 Dec 2023
This paper presents an analysis of individual aerosol particles that were collected on a cruise that extended from South Korea to Antarctica, altogether 29 samples. The authors use micro-spectroscopic techniques to determine the particle composition and morphology. The analysis has two themes, the first is to determine quantitatively the chemical mixing state using diversity metrics, and the second is to quantify the particles’ ice nucleation properties at cirrus conditions.
This is a rich dataset from an area of the world where particle samples are scarce, and I want to commend the authors for the work they have done in their analysis. The paper fits within the scope of ACP, and merits publication. As it stands, the paper is quite long and reads more like a report of “what is out there” (with some additional pieces of information such as the ice nucleation rate coefficients/INAS densities). Given that we don’t have much data of this detailed level from this region of the earth, this may be fine, however I think the impact of the paper as it’s presented is limited and could be strengthened by emphasizing the new contributions.
Major comments:
- Currently the mixing state analysis and the ice nucleation analysis are disconnected. When I started reading the paper, I expected that the paper would elucidate the connection of the two, i.e., how does mixing state impact ice nucleation properties (which would be a very exciting topic). However, the connection is very weak, basically remaining at a level of speculation (line 753) “These variations are likely contributed to not only the complex compositions but also the physical and chemical mixing state of these particles.” There are several studies out there that already show this. Is there any way to connect the two topics more closely? This may not be easily possible because I’m not sure if the chi metric is suitable to predict errors in ice nucleation abilities when assuming some idealized mixing state (this has been shown to be possible to some extent for CCN activity (Ching et al., 2017, 10.5194/acp-17-7445-2017.) and optical properties (Yao et al., 2022, 10.5194/acp-22-9265-2022.)). Considering this, I wonder if it would be better to separate the two topics into two separate (but shorter) papers.
- Even when considering the mixing state topic and the ice nucleation topic separately, I’m left with the question “so what”? For the mixing state topic, the authors could strengthen their discussion if they connected their observations to what models currently assume. Is this consistent or not with their findings? What could models learn from these observations? For example, it is interesting that the authors find partially externally mixed populations in these regions, and the fact that they identified aging mechanisms that lead to a more external mixture is also very interesting. For the ice nucleation topic, I’m not sure what to do with the analysis in section 3.6.2-3.6.4. These parameters could be useful for modelers, but other studies have been putting forward parameterizations like these for similar particle types – how are the results presented here similar or different from previous studies? Or, if this is the first time such parameterizations are derived for a given particle type, say so.
Minor comments:
- The English language needs some polishing throughout the paper (typos and grammar).
- Looks like Figure 4 and 5A display the same information? Omit Figure 4?
- Line 512: the difference in mixing state of super-micron and submicron particles is interesting. When discussing this, it would be helpful to state what the species/elements are that are internally (or not internally) mixed. Information like this would be helpful in other places of the paper as well.
- What is the duration of each ice nucleation experiment?
Citation: https://doi.org/10.5194/egusphere-2023-2646-RC2 - AC3: 'Reply on RC2', Bingbing Wang, 22 Mar 2024
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RC3: 'Comment on egusphere-2023-2646', Anonymous Referee #3, 01 Jan 2024
In the manuscript submitted by Xue et al., the authors conducted a comprehensive investigation into the mixing state and ice nucleation abilities of ambient particles collected over the Western Pacific and the Southern Ocean. Considering that the aerosol samples were collected on a broad spatial scale and there were limited studies on ice nucleating particles (INP) over the covered regions, I suggest that this paper fits well within the scope of ACP. Nevertheless, there is a need for overall improvement in the English language expression, and a major revision is recommended before publication.
Major comments:
- The manuscript needs overall improvement in English language expression.
- In the introduction section, there is a lack of discussion on the state-of-the-art information regarding how the mixing state influences the ice nucleation of particles. The limited studies in the literature could serve as a significant motivation for the present study.
- The method section lacks details concerning the sampling time, cut-off size of particles used in different analyses, and operational procedures.
- One of the major conclusions on coating thickness or coating compositions may influence the ice nucleation efficiency of collected particles is not sufficiently supported by the present work. The discussion focuses on one sample (S14) with a low ice nucleation efficiency.
- The author has developed several parameterizations to predict the heterogeneous ice nucleation of marine aerosols. However, the motivation behind deriving these parameterizations is not clearly presented. The derived parameterizations have different assumptions, and their forms vary among different particle classes. How to apply these parameterizations and to which aspect can they contribute to the prediction of particle ice nucleation are not clearly explained. I would suggest focusing on one or two parameterizations that are atmospheric-relevant and feasible.
Specific comments:
- Quantified results need to be included in the abstract. For example, the percentage of different aerosol sources that contribute to the collected particles (L27-L28).
- Some statements in the abstract are unclear. For example, the statement “We tested different ice nucleation parameterizations of marine atmospheric particles for their applicability. Finally, we discuss how the mixing state of particle populations impacts ice nucleation in the atmosphere” lack specific outcomes. Could you provide more details on the results of these tests and discussions?
- L85-L87 are repeating with L73-L75.
- L96-L97: Consider deleting L96-L97, as the main focus in this paragraph is on chemical composition and mixing state.
- L138-L140 should be moved to the following paragraph.
- The collection time for each sample exhibits large variation (Table S1). Could the authors provide an explanation for this and include these details in the SI? This information will help indicate the atmospheric representativeness of the aerosol samples.
- L161-L163: Were both the TEM grid and silicon wafer chips collected using the SKC sampler? It is not clear whether they were collected in parallel or not. This should be clarified for better understanding.
- L167-L168: Are only samples in the fourth state used for further measurements? Could the authors explain the rationale behind applying such a small cut-off size?
- L196: What kind of particle size do you mean here. Is it referring to the aerodynamic size (AD) determined by the SKC sampler, or is it the electrical mobility diameter (ECD)? This should be clarified in the main text.
- L222: Any citations for using 0.8 as a specific threshold value for fresh and aged SS?
- Line 229: There is a missing period in this sentence.
- L244: Why was a hemispherical shape of particles assumed rather than a sphere?
- L308: Which particle sample, TEM or silicon wafer?
- L316: Ice formation of one particle or all particles on the grid/chip? I assumed you have many particles in one sample?
- Figure 3: Is the scale unit on TEM images missing?
- L431: Citations are needed here regarding particle formation from biogenic emissions in the Rose Sea.
- L488-L489: Not sure if I followed this sentence.
- Table 1: BBA exhibits relatively larger particle size compared to particles from other sources. Could the author provide an explanation for this?
- L540-L541: The negative correlation between the number percentages of AgedSS and SS/Sulf particles and χ is not supported by the low R2 (<0.11) in Figure 6B. Therefore, the statement on “The negative correlation between the number percentages of AgedSS and SS/Sulf particles and χ indicates that aging resulted in a more externally mixed particle population.” needs justification.
- Figure 6C: The negative correlation between the number percentages of CNOS and χ needs justification, as it is likely caused by a few extreme points. I would suggest repositioning these extreme points and conducting the fitting again.
- Equation 12. The legend used here is confusing. If A represents a certain class of particles and 𝑁 is the number of particles in that class, it may make more sense for to be represented as ?
- L723-L724. I would refrain from asserting that S14 is ice-active given the large uncertainties in its onset conditions (Figure 11). This is also contrary to the author’s earlier statement (L578) that “The RHice onsets were only about 3% lower than the homogeneous nucleation limits between 228 K to 220 K, and thus samples dominated by BBA may not have been efficient heterogeneous ice nuclei.”
- AC4: 'Reply on RC3', Bingbing Wang, 22 Mar 2024
- AC1: 'Comment on egusphere-2023-2646', Bingbing Wang, 22 Mar 2024
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