the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jul 2025
Measurement Report: Hygroscopicity and mixing state of submicron aerosols in the lower free troposphere over central China：local, regional and long-range transport influences
Abstract. Understanding the hygroscopicity and mixing state of atmospheric aerosol particles is crucial for improving predictions of cloud formation and climate impacts. However, measurements in the lower free troposphere - a representative atmospheric layer characterizing regional background conditions in aerosol transport and atmospheric evolution – remain sparse, especially in regions influenced by both anthropogenic emissions and long-range transported air masses. This study adds further data on size-resolved hygroscopicity and mixing state measurements of aerosols at Mt. Hua (2060 m a.s.l., central China) during October–November 2021 using a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA). Results demonstrate size-dependent hygroscopicity, with the mean hygroscopicity parameter (κmean) increasing from 0.20 (30 nm) to 0.30 (200 nm). The ambient submicron aerosols were primarily externally mixed, dominated by more-hygroscopic (MH) particles, with no significant diurnal variation, indicating minimal influence from boundary layer dynamics. Aerosols originating from Mongolia deserts tended to be less hygroscopic, associated with an enhanced number fraction of less-hygroscopic (LH) mode particles relative to those from other sources. However, during episodes of striking high relative humidity (RH > 80 %), atmospheric aerosols containing mineral dust showed unexpected hygroscopic enhancement, suggesting in situ RH-driven chemical processing that increased aerosol hygroscopicity. Atmospheric aerosols at Mt. Hua displayed distinct hygroscopic properties compared to other high-altitude sites, underscoring regional differences in aerosol sources and free tropospheric processing. These findings advance our understanding of aerosol aging and processes in the lower free troposphere over central China, and offer crucial observational constraints for modeling aerosol–cloud interaction and regional climate impacts.
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RC1: 'Comment on egusphere-2025-2643', Anonymous Referee #1, 29 Jul 2025
Shi and Zhang et al.’s manuscript presents hygroscopicity and mixing state of submicron aerosols at several mobility diameters at Mt. Han during fall 2021 using HTDMA system. This study presents a valuable and comprehensive dataset on aerosol hygroscopicity in the lower free troposphere over China, supported by concurrent aerosol chemical composition measurements. The manuscript primarily serves as a measurement report based on a robust dataset, rather than providing extensive interpretation or discussion. Given the scientific merit and relevance of the dataset, I believe this manuscript can be considered for publication in Atmospheric Chemistry and Physics after the authors adequately address the following comments and revise accordingly. Most of all, authors are encouraged to related aerosol hygroscopicity, chemical composition, and airmass origins or meteorology impacting aerosol process in the atmosphere.
- L114: The parameter “A” is defined with an equation; however, a clearer explanation of its physical meaning and implications is necessary.
- L115: “Dry” diameter appears to refer to a particle size at 20°C, as suggested in Section 3. Please clarify this here and provide a more detailed explanation.
- L173: The term “k-PDF” is introduced without definition. Please define it clearly upon first mention.
- L197: Aerosols with diameters of 100–200 nm in number concentration typically represent secondary aerosols such as sulfate and nitrate, which often show mass size distribution peaks at 400–500 nm in previously published studies of AMS measurements. Could you provide the likely chemical composition of particles in the 100–200 nm range based on your measurements or referring to previous studies?
-Figure 2: Panels (a) to (e) show time series for particles from 30 nm to 200 nm, but the text within the panels is too small to read comfortably. Please enlarge the text for improved readability. Additionally, the term “URG” in the caption (and in line 235) might be better replaced with a descriptor indicating continuous or online measurement.
- Figure 2 (continued): Starting from November 14, a decrease in hygroscopicity (k) coincides with an increase in the mass fraction of elemental carbon (EC), particularly for particles in the 60–150 nm diameter range. Was this shift possibly associated with the advection of primary particles from urban areas? This period may serve as a useful contrast case relative to periods of higher hygroscopicity.
-Figures 4 and 6: Please consider enlarging the text in both figures to improve readability.
-Figures 4 and 5: The six clusters identified in these figures appear to form two broad groups: clusters C3 and C5 versus the remaining clusters. Back-trajectory analysis in Figure 4 suggests that C3 and C5 are associated with air masses from cold and dry regions. In terms of k values, these clusters exhibit bimodal distributions, with lower values in the low hygroscopicity (LH) mode and higher values in the moderate hygroscopicity (MH) mode for particles larger than 100 nm. Could the authors discuss how aerosol composition and hygroscopicity differ across these clusters?
I am not sure how this trajectory analysis can support aerosol hygroscopicity measurements, also as 6 clusters of air mass trajectories are not clearly distinguished and related to the aerosol measurements. Do the authors consider surface emissions based on trajectory height or microscale/synoptic meteorology influencing aerosol hygroscopicity during transport? While I understand the manuscript is intended primarily as a measurement report, a brief discussion or summary connecting air mass origin, composition, and hygroscopic behavior would significantly enhance the interpretation of these results.
Citation: https://doi.org/10.5194/egusphere-2025-2643-RC1 -
RC2: 'Comment on egusphere-2025-2643', Anonymous Referee #2, 11 Aug 2025
The manuscript by Shi and Zhang et al. presents a measurement report, providing detailed description of the hygroscopicity and mixing state of the aerosols in mountain Hua in central China for few months in 2021. Despite focusing on a single location and relatively short time period, the manuscript is worth publishing due to the measurement location, and thus certain site characteristics, which have not been widely reported. The manuscript is overall well written and provides all necessary details, but I have minor concerns and suggestions for revisions that should be considered before accepting for publication.
Regarding the trajectory analysis, I wonder why the authors have selected the 24-hour long trajectories. I think reasoning for this selection is crucial and should be added to the text, especially when considering the fact that aerosols, especially the larger ones measured at the site, could originate very far. In many other studies utilizing backward trajectory analysis, longer trajectories (to my knowledge, often at least 48 hours) are usually employed (see for example, Räty et al., 2023; Khadir et al., 2023; Xu et al., 2021) to make sure long-range transport is properly captured.
Line 27: “improving predictions…climate impacts” Please reword as the current structuring is unclear
Line 34: I assume the sizes of the particles are in the brackets, please make it clear by stating it explicitly (e.g., dp = 30 nm).
Line 57: Do you mean dry deposition efficiency here? Please be explicit.
Line 120: Is this division based on the Shi et al 2022? Short explanation on why and how could be also included here in addition to the reference.
Figure 1: The color scheme for the wind direction is somewhat distracting. Please consider changing to a clear discrete color scale if you have categorized wind direction or to a continuous and perceptually uniform scale if your wind direction is actually in degrees (unclear from the figure at the moment).
Figure 2 and Figure 3: Please make sure the color scale for the k-PDF is perceptually uniform as the current rainbow scale is not and should not be used.
Sect. 3.2 title: Please avoid using abbreviations in the title, you could use “probability density functions of k” instead. Also please define what it means.
Line 235: Is the abbreviation URG defined somewhere? Even if it is, I would use the full word here too for clarity.
Figure 4: Are the colored lines/curves in this figure the cluster centroids? This should be stated in the caption. I believe adding a trajectory frequency map for each of the clusters would be very helpful. Please consider adding one to the supplementary material.
Line 245: “was not obviously observed” – what do you mean by this? If you did not measure it, you should say “was not investigated” instead or you could also just say “was not observed” if that’s is what you mean.
Sect. 3.3/trajectories: You mention trajectory clusters but provide no details on how the clusters were obtained. I am assuming a method of k-mean clustering or similar, however, this should be mentioned in the methods where you first describe your trajectory calculations. The trajectory frequency figure that I suggested to include could then be referenced in the method section already, and later noted in this 3.3 if necessary.
You show the chemical composition of each cluster nicely in Figure 4. It would be informative also to present the average size distribution for the same clusters, this could be in the supplementary material. Similarly, also the average meteorological conditions for each cluster could be reported, if possible (it is a measurement report after all, so all background information is useful for future uses of the data).
Figure 7: I would remove the lined between the points in this figure. You are comparing the hygroscopicity values for different sites and sizes, not necessary looking on how the hygroscopicity changes with size as you can also have particles with different origins (i.e., larger particle is not the smaller one that has grown larger). Please considering increasing both marker and text size in this figure too.
Supplementary figures & Figure 3a, Figure 5, Figure 7: Please avoid using red and green in the same figure to accommodate color blind readers.
References
Räty, M., Sogacheva, L., Keskinen, H.-M., Kerminen, V.-M., Nieminen, T., Petäjä, T., Ezhova, E., and Kulmala, M.: Dynamics of aerosol, humidity, and clouds in air masses travelling over Fennoscandian boreal forests, Atmos. Chem. Phys., 23, 3779–3798, https://doi.org/10.5194/acp-23-3779-2023, 2023.
Khadir, T., Riipinen, I., Talvinen, S., Heslin-Rees, D., Pöhlker, C., Rizzo, L., et al. (2023). Sink, source or something in-between? Net effects of precipitation on aerosol particle populations. Geophysical Research Letters, 50, e2023GL104325. https://doi.org/10.1029/2023GL104325
Xu, L., Liu, X., Gao, H., Yao, X., Zhang, D., Bi, L., Liu, L., Zhang, J., Zhang, Y., Wang, Y., Yuan, Q., and Li, W.: Long-range transport of anthropogenic air pollutants into the marine air: insight into fine particle transport and chloride depletion on sea salts, Atmos. Chem. Phys., 21, 17715–17726, https://doi.org/10.5194/acp-21-17715-2021, 2021.
Citation: https://doi.org/10.5194/egusphere-2025-2643-RC2
Data sets
Hygroscopicity and mixing state of submicron aerosols in the lower free troposphere over central China:local, regional and long-range transport influences Jingnan Shi and Juan Hong https://doi.org/10.5281/zenodo.15589884
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