High Frequency of Urban New Particle Formation on the Tibetan Plateau: Quantifying Formation Rates, Growth Rates, and CCN Production

Mo, Qiqi; Wen, Longjie; Zhao, Gang; Ye, Chunxiang; Sun, Jie; Lin, Weili; Shen, Fengjun; Liu, Tong; Gao, Yicheng; Chen, Yi; Zhang, Tiantian; Zhu, Tong

doi:10.5194/egusphere-2026-1628

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https://doi.org/10.5194/egusphere-2026-1628

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29 Apr 2026

| 29 Apr 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

High Frequency of Urban New Particle Formation on the Tibetan Plateau: Quantifying Formation Rates, Growth Rates, and CCN Production

Qiqi Mo, Longjie Wen, Gang Zhao, Chunxiang Ye, Jie Sun, Weili Lin, Fengjun Shen, Tong Liu, Yicheng Gao, Yi Chen, Tiantian Zhang, and Tong Zhu

Abstract. New particle formation (NPF) strongly influences aerosol number size distributions and cloud condensation nuclei (CCN), yet its characteristics and climatic relevance in high-altitude urban environments over the Tibetan Plateau (TP) remain poorly constrained. We conducted in situ observations at Tibet University in Lhasa during the monsoon and pre-monsoon seasons. NPF event frequencies were 86.2 % during the monsoon season and 55.1 % during the pre-monsoon period. Observation days were classified into four types (Class I, Class II, Undefined, and Non-Event) based on the evolution of particle number size distribution (PNSD) and modal development. Seasonal comparisons indicate clear differences in the factors associated with NPF: monsoon season NPF was mainly associated with sulfur precursor availability, whereas no single clear controlling factor was evident during the pre-monsoon period. NPF substantially enhanced CCN at TU, with mean concentrations of 3351 ± 1919 cm⁻³ on Class I days, 1540.2 ± 872 cm⁻³ on Class II days, 880.5 ± 374 cm⁻³ on Undefined days, and 1278.1 ± 667 cm⁻³ on Non-Event days. The impact of NPF on CCN depends not only on nucleation, but also on the continued growth and survival of newly formed particles. This study provides new insights into urban NPF characteristics and the contribution of NPF to CCN on the TP, improving our understanding of aerosol–climate interactions in high-altitude urban environments.

Received: 24 Mar 2026 – Discussion started: 29 Apr 2026

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Qiqi Mo, Longjie Wen, Gang Zhao, Chunxiang Ye, Jie Sun, Weili Lin, Fengjun Shen, Tong Liu, Yicheng Gao, Yi Chen, Tiantian Zhang, and Tong Zhu

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CC1:
'Comment on egusphere-2026-1628', Nima Zafarmomen, 14 May 2026 reply
The paper investigates new particle formation (NPF) in Lhasa, Tibet, a high-altitude urban environment on the Tibetan Plateau. The authors use in situ particle number size distribution measurements collected at Tibet University during the monsoon and pre-monsoon periods. The main objectives are to quantify the frequency of NPF events, determine particle formation and growth rates, identify the factors controlling NPF occurrence and development, and evaluate the potential contribution of NPF to cloud condensation nuclei (CCN). Overall, the manuscript is scientifically relevant and timely. Its strengths include its focus on an understudied urban Tibetan Plateau environment, the use of high-time-resolution SMPS observations, and its attempt to link NPF events with potential CCN production.
Minor comment:
The study is based on two relatively short observational campaigns: 9 October–10 November 2024 and 25 March–22 April 2025. While these measurements provide valuable insight into NPF characteristics during the selected monsoon and pre-monsoon periods, the limited duration may not fully represent the broader seasonal variability. The authors should add a short discussion on the representativeness of these campaign periods and clarify whether the reported seasonal differences can be generalized to typical monsoon and pre-monsoon conditions over Lhasa.

The NPF event classification relies mainly on visual inspection of particle size distribution evolution. The authors should briefly clarify how classification uncertainty was handled, especially for weak or discontinuous events.

Since CCN concentrations were estimated rather than directly measured, the authors should mention the uncertainty associated with the assumed κ value and supersaturation.

The possible role of VOCs in pre-monsoon particle growth is discussed, but VOCs were not measured. This point should be stated more cautiously.

The site is influenced by local urban sources such as traffic, combustion, construction, and dust. The authors should briefly discuss how these emissions may affect NPF interpretation.

I strongly recommend the authors should consider citing the study “Comprehensive spatiotemporal analysis of long-term mobile monitoring for traffic-related particles in a complex urban environment” in the Introduction or site-description discussion, as it provides relevant context on traffic-related particle variability in complex urban environments.

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Citation: https://doi.org/10.5194/egusphere-2026-1628-CC1
RC1: 'Comment on egusphere-2026-1628', Anonymous Referee #2, 23 May 2026 reply

First, thanks for considering me as a reviewer for this nice work. I have read the manuscript, and I believe that it presents very good observations of frequent NPF in a high-altitude urban environment on the Tibetan Plateau. The dataset is interesting and potentially important for understanding aerosol formation in unique mountain urban regions. However, several central conclusions depend on subjective event classification (that’s a well-known general problem) and some limitation on NPF mechanistic understanding. I like the study and I would like to see it published in ACP but it would need some revisions.
Here my comments:
Introduction:
The introduction is overall well structured; However, it might be that the literature review is somehow narrow. I do understand that the focus of the paper is the Tibetan Plateau however there are several other studies with a focus on high altitude measurements and location very close to the one done in this study. This introduction focuses strongly on recent Tibetan Plateau background studies while giving limited attention to the broader high-altitude and free-tropospheric NPF literature. I can imagine that a comparison done with measurements presented in Himalaya such as Bianchi et al. (2021, Nature Geoscience) but also many other studies could be good for this work. Also, this comparison can been done when discussing the possible precursors. In this way the authors can also benefit by distinguish their urban Lhasa case from other mountain NPF types.
Classification event:
It is well known that the actual classification method is subjective and probably quite qualitative. However, I have the feeling that in this case can be improved. Also, because many central results depend on the classification, like NPF frequency, J/GR statistics, and CCN formation. For example, in figure 2, I would even think that type II and even undefined can be considered as a class I event. Mostly, it seems that, the separation between Class II and Undefined events is not really clear. Something useful could be a day-by-day classification table, including event start/end times, growth-rate time windows, and so on. An additional approach could be to use a ranking method that classified that days in function of particles that have been produced.
Monsoon NPF mechanism and NPF mechanism in general:
The study says that monsoon NPF is mainly associated with SO₂ availability and therefore H₂SO₄ formation. This is can also be reasonable, but since the authors couldn’t measure all the parameters such as sulfuric acid, or bases such as ammonia or amines, or organic molecules and their oxidation products. SO2 is clearly quite high and therefore can be a reason for the NPF. One possibility could be to calculate Sulphuric Acid proxy from SO2 and see how it compare to the formation rate. Because of all of that I would say that the interpretation that monsoon NPF is sulfur-precursor-driven should be softened unless additional analysis such as at least Sulphuric Acid proxy Vs Formation rate. Also, the paper discusses possible VOC-related oxidation in the pre-monsoon season but cannot constrain it. We understand that these measurements are very complicated, especially in such location, however, considering recent literature on HOMs/OOMs in mountain and TP environments, the author should be more critical in highlighting these limitations. They have a beautiful data set also without going too much in details on the NPF mechanism.
Urban primary emissions vs NPF:
I think it would be good if the authors could try to better separate regional NPF from local primary ultrafine particle emissions. This could include analysis of wind direction, BC/NO₂/SO₂ spikes, rush-hour timing, and whether events occur simultaneously across broader size ranges. Considering the unique location this would add strength and very interesting new information to the study.
Other minor comments:
Line 25 – Define TU. I guess you mean TP.
Figure 1 – “Tibetan Plateau” is not visible. Also, define TU in the caption.
Figure 2 – Fix the Panel a Y-axes description. Caption is missing explanation.
Figure 3 – fixe axes labels here as well. Too many overlaps.
Caption in general could contain more information related to the figures.

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Citation: https://doi.org/10.5194/egusphere-2026-1628-RC1

Qiqi Mo, Longjie Wen, Gang Zhao, Chunxiang Ye, Jie Sun, Weili Lin, Fengjun Shen, Tong Liu, Yicheng Gao, Yi Chen, Tiantian Zhang, and Tong Zhu

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https://doi.org/10.5194/egusphere-2026-1628-supplement

Qiqi Mo, Longjie Wen, Gang Zhao, Chunxiang Ye, Jie Sun, Weili Lin, Fengjun Shen, Tong Liu, Yicheng Gao, Yi Chen, Tiantian Zhang, and Tong Zhu

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Short summary

Using field observations in Lhasa, a high-altitude city on the Tibetan Plateau, we show that new particle formation occurs frequently and contributes substantially to cloud-forming particles. Event frequency was higher during the monsoon season than during the pre-monsoon period, indicating clear seasonal differences in the processes controlling particle formation and growth. These findings improve understanding of climate effects in urban plateau environments.


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