the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Aug 2025
Measurement report: Insights into seasonal dynamics and planetary boundary layer influences on aerosol chemical components in suburban Nanjing from a long-term observation
Abstract. Understanding the seasonal behavior of fine particles (PM2.5) and its chemical components is critical for improving air quality in the Yangtze River Delta (YRD), a densely populated and polluted region in China. While previous studies have addressed PM2.5 mass trends, the role of planetary boundary layer height (PBLH) in modulating chemical composition remains insufficiently explored. This study investigates seasonal variations and PBLH effects on PM2.5 chemical components based on year-round field measurements (December 2020–November 2021) at Nanjing University of Information Science and Technology. Annual mean PM2.5 mass concentration is 30.0 ± 18.5 μg m-3, with winter peaks (48.3 μg m⁻3) and summer lows (20.4 μg m⁻3). Organic aerosol dominates PM2.5, followed by sulfate in warmer seasons and nitrate in winter. PBLH strongly influences component dynamics: low PBLH in winter enhances nitrate and primary aerosol accumulation, while high PBLH in summer promotes secondary organic aerosol and sulfate formation via photochemistry. Nitrate is most sensitive to PBLH changes, showing rapid buildup under stable conditions. The potential source contribution function analyses identify seasonal source regions: southern combustion for primary organic aerosol in warm seasons, northern industrial and rural areas in winter, and biogenic and coal combustion sources for secondary organic aerosol. Sulfate and nitrate exhibit shifts between local and regional origins. These findings highlight the need for season-specific emission control strategies, such as targeting volatile organic compounds in summer and reducing industrial nitrogen oxides in winter, to effectively mitigate PM2.5 pollution in the YRD.
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Status: open (until 29 Sep 2025)
- RC1: 'Comment on egusphere-2025-3184', Anonymous Referee #1, 25 Aug 2025 reply
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RC2: 'Comment on egusphere-2025-3184', Anonymous Referee #2, 26 Aug 2025
reply
This manuscript presents a comprehensive one-year field observation in Nanjing to investigate the seasonal dynamics of PM2.5 chemical components and the modulation effects of planetary boundary layer height (PBLH). The work addresses an important knowledge gap in understanding how boundary layer processes regulate aerosol composition in the Yangtze River Delta (YRD). The dataset is valuable, the research objectives are interesting and the study provides insights with both scientific and policy relevance. However, the results are largely enumerative, lacking conciseness, and the analysis of PBLH effects remains insufficiently in-depth. Therefore, several aspects of the manuscript require further clarification and strengthening before it can be considered for publication.
Major:
1. Since PBLH exerts strong quantitative influence on the results, and lidar-based retrievals of PBLH are subject to substantial uncertainties. The manuscript should explicitly discuss these uncertainties. If necessary, cross-validation with reanalysis datasets (e.g., ECMWF PBLH) could be added to enhance the credibility of the findings.
2. The analysis of PBLH influences remains insufficiently in-depth. For example, Section 3.5 on potential source contributions appears somewhat detached from the central theme of boundary-layer effects. This section neeeds improvement.Specifics：
1. In the Introduction, the narrative could be restructured. It is better to first present the broad scientific problem, and then narrow down to the atmospheric situation in Nanjing, rather than the current order.
2. Lines 69-70: The statement “Comprehensive, long-term observational studies … are still lacking in the YRD” is somewhat overstated. Relevant studies exist, so this should be rephrased more cautiously.
3. Lines 90-93: The Introduction should not present conclusions, it should focus on research objectives and study plan.
4. Lines 94-99: Introducing each section by restating its content is meaningless.
5. Lines 217-223: The correlation analysis with temperature and PBLH could also include primary species, such as BC, not only POA.
6. In Fig. 4b, one conspicuously high PBLH data point appears to drive much of the correlation. If this point were excluded, would the correlation still remain as strong? Would the influence of the PBLH not be as significant？
7. The use of the [concentration] × PBLH method to eliminate dilution effects requires justification, has this approach been adopted in previous studies? If so, relevant references should be cited. If not, how reliable is it in this context?
8. In Section 3.4, simply describing higher or lower pollutant concentrations may not adequately represent “sensitivity.” It should be clarified whether sensitivity refers to the magnitude of concentration change with PBLH variations, rather than absolute concentration levels.
9. Lines 354-355: The statement “As PBLH decreases, the average mass concentration of POA … exceeds that of BC, suggesting POA is more sensitive …” is unclear. POA concentrations are higher than BC in Figure 8. Therefore, what is meant by “exceeds” here? Is it referring to the rate of change with PBLH? This needs clarification.Citation: https://doi.org/10.5194/egusphere-2025-3184-RC2 -
RC3: 'Comment on egusphere-2025-3184', Anonymous Referee #3, 26 Aug 2025
reply
This article analyzed the year-long observations of chemical components of fine particulate matter and their association with PBLH across seasons. The study offered valuable insight from the perspective of observational data. My general concern is the representativeness of single site for the discussion of suburban Nanjing and the generalization to the regional and spatial patterns. I am also concerned about the novelty of the paper mainly showing the effects of PBLH as a vertical dispersive term on PM2.5 concentrations, which was shown by various prior studies.
General Comments
- The descriptive statements from this study are from single observational site. How representative is it for the suburban Nanjing and the pollution pattern in the YRD region? Are there any other or prior site observations can be gathered to the plot for representativeness?
- A long-term observation usually refers to multi-year observation. I would suggest changing the title to be “year-long” or something equivalent to avoid overstatement.
- For the potential source contributions, how can the regional receptor with the grid cells at 1 by 1 degree resolution inform the source contributions of the single site?
- The effects of PBLH on the concentrations of PM2.5 is overinterpreted. PBLH is only a vertical dispersive term, while the concentrations of primary aerosols are governed by their emissions, and the concentrations of secondary aerosols are governed by their chemical mechanisms. PBLH would only modulate the surface concentrations temporarily without changing the aerosol vertical burden.
Specific Comments
- Line 166-167, what is the vertical height of the receptor grid cells? How would it compare with the height of the observation site?
- Figure 2b, there are large uncertainties for the PBLH height. How would that affect the correlation between PBLH and PM2.5 concentrations?
- Figure 4, the mass fraction of a certain component would be affected by the concentrations of other components. What would be the corresponding results for the correlation for absolute mass concentrations?
Citation: https://doi.org/10.5194/egusphere-2025-3184-RC3
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acsm_gas_meteo J. Xu et al. http://gofile.me/5JhP4/arwG2CvGf
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- 1
This manuscript presents a comprehensive 1-year dataset on the seasonal dynamics of PM2.5 chemical components in suburban Nanjing, with particular emphasis on the role of planetary boundary layer height (PBLH). The study is well-structured, and the results provide valuable insights into aerosol–PBL interactions in the Yangtze River Delta. Overall, the paper is suitable for publication after addressing the following comments.
Major Comments
Sensitivity of nitrate to PBLH: One of the central conclusions is that nitrate is the most sensitive component to PBLH changes (Section 3.4), supported by the sharp concentration increase under low-PBLH conditions. However, the mechanism remains unclear. It is not evident whether this increase is primarily driven by suppressed thermodynamic volatilization, enhanced chemical production (e.g., N2O5 hydrolysis), or both. The authors are encouraged to analyze the joint distribution of nitrate concentration with temperature and relative humidity across different PBLH intervals. Such an analysis would clarify the dominant drivers of nitrate buildup under low PBLH and strengthen the mechanistic interpretation beyond statistical correlations.
Conceptual figure: Consider adding a schematic (conceptual diagram) to synthesize the key findings. A figure linking seasonal variations, PBLH effects, and dominant aerosol components would improve accessibility and provide readers with an integrated overview.
Regional transport vs. local formation: The PSCF and pollution rose analyses provide strong evidence of seasonal source regions. However, the discussion could better disentangle the relative roles of regional transport and local photochemical/heterogeneous formation under different PBLH regimes. For example, the paper notes sulfate’s regional transport characteristics but does not explicitly compare this with in-situ secondary formation under varying boundary layer conditions. Expanding this discussion would clarify how transport and local chemistry interact.
Link to air quality management: The policy implications in the conclusions are valuable but could be made more explicit. For instance, the recommendation to reduce NOₓ in winter and VOCs in summer could be directly tied to the mechanistic findings (e.g., nitrate sensitivity to PBLH vs. SOA formation under strong photochemistry). Strengthening these connections would enhance the paper’s impact for both scientific and regulatory audiences.
Minor Concerns
The Summary and Conclusions section is slightly lengthy. Please condense it to highlight the most important findings and implications.
In Section 2.2, abbreviations (e.g., ACSM, OA, NO3-, SO42-, NH4+, Chl, BC) are somewhat repetitive. Please streamline and ensure consistency throughout the manuscript.
In several figures (e.g., Fig. 2 and Fig. 5), labels and legends are too small. Please enlarge font size or bold key elements to improve readability.
At line 239 (beginning of Section 3.2: “In order to investigate…”), the text could be shortened to directly state the research objective.
Please clarify the calculation basis of the “Relative Change Rate” in Figure S7 to ensure reproducibility.