the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Aug 2025
Measurement Report: New insights into the boundary layer revolution impact on new particle formation characteristics in three megacities of China
Abstract. New particle formation (NPF) events contribute more than 60 % of ultrafine particles particularly in the boundary layer. This study retrieved the particle number size distribution and the NPF parameters and their relationship with planetary boundary layer height (PBLH) evolution, as well as the air mass back trajectories during NPF events in three Chinese cities: Beijing, Guangzhou, and Shanghai. Furthermore, all NPF events has been classified into three types: new particles grow rapidly during the initial rise of the boundary layer in Type Ⅰ events, while they grow after the boundary layer reaches a certain height (above 800 m) in Type Ⅱ events, and the shrinkage cases are the Type III. The results show that particle growth dynamics categorized into distinct types demonstrate that sustained particle growth predominantly occurred under conditions of stable and elevated PBLH. Survival parameters ranged from 13.1 to 115.9 in Beijing, 9.0 to 110.2 in Guangzhou, and 8.4 to 25.6 in Shanghai. Specifically, Type I events were associated with survival parameters between 14.0 and 45.2. A significant negative correlation is observed between survival parameters and PBLH (R2 = 0.2 in Beijing, R2 = 0.02 in Guangzhou, and R2 = 0.99 in Shanghai, respectively). The main source of Aitken mode transport to Beijing is from Mongolia region. In Guangzhou, the contribution mainly comes from Jiangxi and Fujian provinces located in the northeast, while in Shanghai, the source lies to the northwest. This research provides valuable insights into developing strategies to manage the atmospheric environment.
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- RC1: 'Comment on egusphere-2025-3637', Anonymous Referee #1, 02 Oct 2025
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CC1: 'Comment on egusphere-2025-3637', Jianfei Peng, 07 Oct 2025
This study reports on the influence of planetary boundary layer height (PBLH) on new particle formation (NPF) in megacities. Long-term observations were carried out in Beijing, Guangzhou, and Shanghai, measuring both particle number size distributions and PBLH. NPF events were classified according to PBLH, and the survival parameter was used to illustrate directly how PBLH affects NPF. The results indicate that higher PBLH conditions promote the occurrence of NPF events.
The topic of this paper is certainly of interest to this journal. This measurement report contains high-quality data and presents novel findings with broad scientific impact. The draft is well written and includes well-prepared figures and tables.
- Please clarify whether the same model of instruments was deployed simultaneously at the three sites, or whether a set of instruments was rotated sequentially among the sites. How do you ensure the accuracy and comparability of the dataset under the chosen deployment scheme?
- In Section 3.6 "The backward trajectories of particles during NPF events", the HYSPLIT model was applied. However, the starting height was not specified, which needs to be clarified. The sources of air masses in Shanghai are not limited to the northwest direction; a substantial proportion also originates from the southwest and southern regions.
- Many sentences are overly long with multiple clauses. For readability, break them into shorter sentences.
Minor Comments:
- Line 12 “all NPF events has been classified”
- Line 61: “High relative humility”
- 3 Please standardize the placement of (a), (b), and (c).
- In Fig. 6, the P values for Beijing exceed 160. Please verify whether these values are reasonable. Furthermore, would it be more appropriate to apply a more relevant fitting curve to examine the relationship between boundary layer height and P values?
- Line305: “Fig. 4c and 4d depict the temporal correlation between average PBLH and Vehicular emissions”. Is Fig. 4c and 4d intended to show the relationship between PBLH and vehicular emissions? It should instead reflect the relationship between PBLH and relative time. Please also include the relevant references about vehicular emissions and greenhouse effects to support this point.
- Line324: “NPF may be primarily driven by low-volatility organics or H2SO4.” How do you define cases where aerosol formation is primarily driven by H2SO4? Provide appropriate references to support this definition.
- Line373: The dots with black borders represent Type Ⅰ. However, the meaning of the other data points is not explained. Please provide the missing information.
- In conclusion, “During the observation period, March and May in BJ exhibited the highest frequencies of NPF occurrence, accounting for 25.9% and 23.8%, respectively.” The data for May are mentioned, but they are not presented or discussed in the main text.
- In summary, the classification is described as two types, but in fact, three types are presented. Please clarify this inconsistency.
Citation: https://doi.org/10.5194/egusphere-2025-3637-CC1 -
RC2: 'Comment on egusphere-2025-3637', Anonymous Referee #2, 23 Oct 2025
The manuscript by Hu et al., investigate the plausible connection between PBLH and occurrence of NPF based on observations at three megacities in China. This topic can be potentially interesting and important. Unfortunately, the analyses presented in the manuscript are rather hand-waving, and the demonstration of the connection between PBLH and NPF have many alternative explanations. Overall, I cannot recommend this article to be published in ACP.
Major comments:
- Data utilized.
In Line 96, the authors claimed that they utilized long-term observational data sets, which is totally overstated. The data set in Beijing merely contains 408 days, which cannot be called “long-term”, and let alone the data sets in other cities.
In addition, the data included in the analyses are too simple (just PBLH and PNSD), leaving no room for real in-depth analysis. As far as I know, there should be data (at least trace gases) from Chinese government that are readily accessible.
- Unclear statistics methods.
The PNSD shown in Fig.1 looks abnormal, please double check and specify if the contour plot is made from mean or median average data.
Line 258-259. “with frequencies ranging from 3.4% to 20.0%”. It is not clear what such frequencies are calculated upon? (monthly?)
In Fig.2, how was PBLH calculated for each city in different months. Are they day-time average or full-day average? Are they median or mean average? What are the variation range in PBLH?
Relevant to the previous comment, they authors stated that the PBLH between NPF and non-NPF days in Beijing shows significant difference. However, in Fig.3, the pattern and daily maxima of PBLH in Beijing do not very much. Is this 100 meter difference considered significant?
- Problematic interpretations and vague statements. There are too many, and I just list few here:
Line 57. The impact of PBLH on NPF is complex and significant, involving … Please specify in which way PBLH can influence particle growth mechanisms.
Line 100. HYSPLIT model does not consider emissions.
Line 136. I don’t understand why primary particles can facilitate new particle growth?
Fig.4, Fig.5, and relevant discussion. The classification of Type I and Type II merely based on their starting time. But this can be easily explained by the dial pattern of nucleating precursors (H2SO4 and others). This can be more relevant to the seasonal change in solar radiation, which just coincide with the evolution of PBLH.
Line 383-384. I question the interpretation of PSCF results. It is well understood that in Beijing, high nucleation-mode particles were often associated with northerly wind, because the clean air with low CS favors NPF. Mongolia should not be considered as a pollution source of nucleation-mode particles.
- This manuscript is not well written. There is enormous amount of grammatic errors and improper references. If the authors decide to submit this article to other journals, the writing MUST be improved substantially. To list a few places:
Line 42, the citation of Chan et al., 2020 is not appropriate, as this paper is mostly about data inversion of PSM
Line 51, the citation of Shengjie et al., 2001 seems to be problematic.
Line 53, “it found that a sluggish GR and the presence of pristine background aerosols…” I guess the author meant to say “pre-existing background aerosols”
Line 61, “relative humidity”
…
Line 107-110. There is no Chapter in the manuscript.
Line 312. “catalyst” is not properly used in this context.
Line 380. Fig.8 should be Fig.7
Line 420-421. Grammatic error
Line 424-425. Grammatic error
Citation: https://doi.org/10.5194/egusphere-2025-3637-RC2
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New insights into the boundary layer revolution correlation with new particle formation characteristics in megacities of China H. Hu https://doi.org/10.17632/zpwjj5ymmp.1
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Hu et al. investigate NPF events observed in three Chinese megacities. The main focus of the analysis is on the influence of planetary boundary layer height (PBLH) on the occurrence and type of observed NPF events. They also investigate shortly the air mass trajectories related to NPF events. Both boundary layer and air mass transport are crucial aspects for better understanding NPF, but there are several interlinked factors that make the quantification of their influence difficult. Unfortunately, the analysis by Hu et al. has several significant shortcomings, discussed below, which leaves no room for other suggestion than to reject the manuscript from publishing in ACP. With improvements to the analysis, interpretations of the results and the representation of the observations and results, the authors could consider submitting to a journal with lesser requirements on the novelty of results, but I cannot see that the presented data could include enough novelty for publishing in ACP.
General comments:
The authors mainly discuss the connection between PBLH on NPF via the decrease in condensation and coagulation sink in the morning due to the extending PBLH. This effect has been studied for over 20 years (Nilsson et al., 2001) by several groups and is widely accepted as one of the main reasons why NPF events are observed to take place on clear sky days, starting some hours after the sunrise, when the PBLH development starts. The other main reason is the important role of sulphuric acid, produced via photo-oxidation of SO2, in NPF. The possible roles of decreasing RH and mixing of compounds in the mixing layer and the residual layer are also investigated. Many of these are not caused by increase of PBLH, but because of increase in solar radiation on the surface and the resulting increase in temperature. In this manuscript, the authors study the relations between BBLH and NPF occurrence only via the connection between PBLH and dimensionless parameter P (ratio of condensation sink and particle growth rate). If the aim is to use this parameter as the ratio between factors inhibiting and enhancing NPF, the vapours responsible for the growth should be assumed to be the same that form the initial clusters in NPF. This is not likely, as the GR already in < 3nm is typically observed to be higher than sulphuric acid would produce. The authors do not present the connections between PBLH and CS or CS and NPF occurrence, even though they discuss these connections repeatedly. The determined GRs, or the size ranges they represent, are not shown and their reliability not discussed.
The results on the connection between PBLH and NPF occurrence are first based on the mentioned positive correlation between monthly mean PBLH and NPF frequencies (lines 251-253). However, the highest values of PBLH are between April and July (Table 1), but the lowest NPF occurrence is during summer months (lines 245-246 and figure 2). This correlation should be represented and investigated more in detail.
The difference in PBLH between event and non-event days (Fig. 3) is surprisingly small. This might be related to the apparently applied linear concentration scale in the heat-map plots (e.g. Fig. 1d-1i, Figs 4 and 5), with which the strongest events are observed, but often events producing lower concentrations are not. The shaded areas in Fig. 3 should be explained. E.g., in panel (a), it seems that the PBLH patterns on non-NPF days were in practise identical, which is not plausible.
The classification of events to Type 1 and Type 2 is done based on the PBLH at the time when the NPF event is determined to start. Based on the example figures in Fig. 4, the start time of the event may be when the PNC_nuc is less than 50 % of the maximum (Fig. 4a) or when it is almost 90 % of the maximum (Fig. 4b). The classification criteria should be considered carefully and presented clearly to justify the interpretation of the classification results. Additionally, the PNC_nuc seems to be calculated for particles larger than 10 nm. If so, the concentration PNC_nuc would elevate one or several hours after the start time of the NPF.
The air mass back trajectories are calculated for 48 hours backwards. While on line 377 the nucleation mode concentrations are mentioned, on line 391 particles smaller than 100 nm are discussed. Whichever the size range considered for NPF event duration for which the trajectories are calculated, it should be noticed that if the NPF mode has diameters of several tens or even close to hundred nanometres, the original formation of particles has happened up to 20 or 30 hours before the air mass arrives to the station, whereas for particles in sizes close to 10 nm the NPF has taken place only up to few hours before observation. This should be considered when discussing the information of the depicted trajectories. Additionally, using expressions like “pollution sources” for the main air mass origins several tens of hours prior to the NPF events is misleading, since the NPF events typically occur in Beijing under relatively clean Northern air masses, when the CS is low. It is likely that the most intense events occur when the clean air mass with low CS arrives to areas with high emissions of NPF precursors and thus the “pollution sources” are near the observation site, not several days away in the upwind direction.
To further analyse the connections between the air mass transportation, the observed PNSDs and NPF events I suggest reading the works by Hakala et al. (2022, 2023).
In addition, the manuscript includes various expressions that are difficult to understand or that seem counterintuitive:
Lines 67-68: NPF can change boundary layer structure directly or indirectly (reference to article discussing aerosol optical depth that is influenced by particles with several hundreds of nanometres in diameter)’
Lines 80-83 and in general: what are the variables that correlate? Concentrations, diameters, occurrences, intensities?
Lines 94-95: the extensive data observations conducted in Nanjing by Nanjing University are not mentioned.
Lines 134-135: PNC_nuc remained below 1000 cm^-3 mentioned as the reason for NPF being lower. I would assume it is the outcome.
Lines 136-137: UFPs are typically primary particles that facilitate the growth of new particles in the atmosphere.
Several figures are too small and the text in them as well.
Hakala, S., Vakkari, V., Bianchi, F., Dada, L., Deng, C., Dällenbach, K. R., Fu, Y., Jiang, J., Kangasluoma, J., Kujansuu, J., Liu, Y., Petäjä, T., Wang, L., Yan, C., Kulmala, M. and Paasonen, P.: Observed coupling between air mass history, secondary growth of nucleation mode particles and aerosol pollution levels in Beijing, Environ. Sci.: Atmos., http://dx.doi.org/10.1039/D1EA00089F, 2022.
Hakala, S., Vakkari, V., Lihavainen, H., Hyvärinen, A.-P., Neitola, K., Kontkanen, J., Kerminen, V.-M., Kulmala, M., Petäjä, T., Hussein, T., Khoder, M. I., Alghamdi, M. A., and Paasonen, P.: Explaining apparent particle shrinkage related to new particle formation events in western Saudi Arabia does not require evaporation, Atmos. Chem. Phys., 23, 9287–9321, https://doi.org/10.5194/acp-23-9287-2023, 2023.
Nilsson, E. D., Rannik, U., Kulmala, M., Buzorius, G., and ¨ O’Dowd, C. D.: Effects of continental boundary layer evolution, convection, turbulence and entrainment, on aerosol formation, Tellus, 53B, 441–461, 2001b