the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Aug 2025
Measurement report: Size-Resolved and Seasonal Variations in Aerosol Hygroscopicity Dominated by Organic Formation and Aging: Insights from a Year-Long Observation in Nanjing
Abstract. Aerosol hygroscopicity plays a significant role in atmospheric chemistry, radiation, and climate effects. While previous studies have investigated regional differences in aerosol hygroscopicity, long-term observational studies focusing on seasonal variations in specific regions remain scarce. This study explores size-resolved and seasonal variations in aerosol hygroscopicity in northern Nanjing, using one-year hygroscopicity-tandem differential mobility analyser (H-TDMA) measurements in 2021. Aerosols in the region show relatively low hygroscopicity due to a high organic mass fraction (annual average mass fraction: 42.92 % in PM2.5) in fine particles. The mean hygroscopicity parameter (κmean) increases with particle size across all seasons, with more pronounced size dependence in nucleation-mode particles. Particles (40–200 nm) show seasonal κmean variations: winter (0.12–0.24) and spring (0.14–0.25) display higher values attributable to secondary inorganic aerosols, while summer (0.12–0.21) and autumn (0.10–0.20) exhibit weaker hygroscopicity due to enhanced contributions from less hygroscopic components. Diurnal patterns are shaped by photochemical aging and aqueous-phase reactions, leading to κmean peaks for larger particles in the afternoon and evening. New particle formation events are most frequent in spring, producing initially less hygroscopic particles that become more hygroscopic with aging. Regional transport analysis reveals distinct controlling factors: hygroscopicity of nucleation-mode particles is mainly controlled by local sources, while accumulation-mode particles are more influenced by seasonal air mass transport. These results improve understanding of aerosol–cloud interactions and support regional climate modeling and air quality management in urbanizing areas.
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Status: open (until 27 Sep 2025)
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AC1: 'Correction to Figure 7: Seasonal Back Trajectories', Yuying Wang, 09 Aug 2025
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During our recent review of this manuscript, we identified an error in the presentation of Figure 7 in the uploaded version, where back trajectories for all seasons were incorrectly shown as those for autumn. This error has now been corrected in the supplementary material, where the revised figure properly displays season-specific back trajectories. This correction does not affect any of the original data, analytical results, or scientific conclusions of the study. The key findings presented in Section 3.3 (“Impact of regional transport on aerosol hygroscopicity”) remain fully valid and supported.
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RC1: 'Comment on egusphere-2025-3186', Anonymous Referee #1, 30 Aug 2025
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This manuscript investigates aerosol hygroscopicity in an urban environment based on year-long measurements. The paper is not particularly novel but, the length of the data set combined with a careful scientific analysis makes the paper certainly worthy of publication. I have a few issues that should be addressed before that final acceptance.
Scientific issues
The authors call particles smaller than 100 nm in diameter a nucleation mode. This is inconsistent with current scientific literature which usually separates ultrafine (<100 nm) particles into nucleation and Aitken modes, the border between these two being in the range 20-30 nm. Considering that the smallest particles considered in this work have a diameter of 40 nm, I strongly recommend that the authors call <100 nm particles as Aitken mode particles, not nucleation mode particles.
Related to the previous comment, the authors call 40 nm particles as newly formed nucleation mode particles. Such particle cannot really be called newly formed, not even recently formed, since for typical growth rates of newly formed (a few nm in diameter) particles, its takes in minimum a few hours to reach 40 nm. Again, I would recommend calling 40 nm particles something else than newly formed particles. For example, small particles originating from NPF (e.g. line 322) might be an appropriate term.
One should be careful when discussing size-dependent behavior of different quantities, as the natural measured of particle size scale logarithmically rather than linearly. For example, kappa parameter may appear having a larger size-dependency in a linear scale (Fig. 1a, lines 205-208), but this may not be the case when comparing the change in kappa from 40 to 80 nm to that from 100 nm to 200 nm (log diameter scale).
The authors state that traffic emission reduce kappa across all particles sizes during rush hours (lines 271-272). I am not convinced about this statement: when looking at fig. 5, there are clearly seasons and size ranges for which no clear decrease in kappa is visible during rush hours. Please modify the statement.
The decrease in kappa due to traffic is observed to be the largest in summer (lines 272-273). What might be the reason for this? Based on particle number size distributions (fig. 4a), the influence of traffic during rush hours is not particularly strong in summer, and appears even weaker than in winter or spring.
The statement on lines 335-339 is strange: what is meant by a size dependency of 110 nm particles, as having a size dependency necessarily involves multiple sizes?
Saying that the NPF event enhance the hygroscopicity of accumulation mode particles gives an impression of a cause-effect relationship between these two. I do not think such a conclusion can be drawn. It is at least equally possible that there are certain condition that simultaneously favor NPF and high hygroscopicity of accumulation mode particles.
line 409: saying that lower than in other areas gives an impression that this location has the lowest kappa of all urban areas. I suppose this is not what the authors mean. Please modify.
Minor/technical issues
line 231: more pronounced compared to what? Compared to Aitken or ultrafine particles?
line 296: The stated accuracy (20.65%) is overly high. 21% would be more reasonable. Please check out throughout the paper.
line 331: similar to …
line 416: … a high … content …
lines 425-426: The sentence "As particles … by late afternoon" does not work in its present form. Please modify.
Citation: https://doi.org/10.5194/egusphere-2025-3186-RC1 -
RC2: 'Comment on egusphere-2025-3186', Anonymous Referee #2, 12 Sep 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3186/egusphere-2025-3186-RC2-supplement.pdf
Data sets
NUIST_TDMA_GF_PDF_timalined J. Zhang et al. http://gofile.me/5JhP4/wZoKYiAJn
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