the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2026
Synchronization of Source and Sink by Boundary Layer Evolution: A Key to New Particle Formation Under Varying Ozone Pollution
Abstract. Atmospheric new particle formation (NPF) is a vital source of aerosol and cloud condensation nuclei, regulated by complex meteorological and chemical factors. Utilizing ground aerosol particle size distributions and vertical observations, this study employs a generalized additive model (GAM) and SHapley Additive exPlanations (SHAP) to quantitatively assess the marginal contributions of factors driving NPF under the NPF scenario and three non-NPF scenarios. We found that NPF depends on the synchronization of enhanced source strength and weakened sink intensity, a process controlled by planetary boundary layer evolution. Under the NPF scenario, the breakup of the inversion layer and entrainment of cleaner air aloft promote vertical mixing. This rapidly reduces the condensation sink (CS) while transporting ozone (O3) to the surface, allowing precursor formation to coincide with a clean background, thus creating a favorable nucleation window. In contrast, nucleation is inhibited in non-NPF scenarios through distinct mechanisms: insufficient oxidation capacity (Non-O3 scenario), source–sink desynchronization caused by stable stratification suppressing vertical exchange (Low-O3 scenario), or rapid scavenging by high background particles (High-O3 scenario). Correlation analysis and SHAP method corroborate the source–sink competition mechanism. Under the NPF scenario, nucleation (Nuc) mode significantly correlates with SO2, with high temperature and sufficient precursors contributing positively to its predicted value. However, predicted Nuc values are dominated by background particles under the Low-O3 scenario and negatively influenced by T and SO2 under the Non-O3 scenario, while under the High-O3 scenario, pre-existing aerosols effectively offset precursor contributions.
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RC1: 'Comment on egusphere-2026-738', Anonymous Referee #2, 15 Apr 2026
- AC1: 'Reply on RC1', Honglei Wang, 11 May 2026
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AC2: 'Reply on RC1', Honglei Wang, 11 May 2026
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed on 12 May 2026.
Citation: https://doi.org/10.5194/egusphere-2026-738-AC2
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RC2: 'Comment on egusphere-2026-738', Anonymous Referee #1, 07 May 2026
This manuscript demonstrates that atmospheric new particle formation (NPF) is governed not by ozone (O₃) levels alone, but by the temporal synchronization between precursor production and background aerosol removal. The authors show that such synchronization is primarily controlled by the evolution of the planetary boundary layer, with NPF occurring when post‑sunrise boundary‑layer development simultaneously enhances near‑surface oxidation capacity through mixing of O3 from above and reduces the condensation sink. On the other hand, stable PBL conditions can either lead to low O3 levels or high background aerosol concentations leading to low low source rate or high sink of newly formed particles.
In my opinion, methodological strength of this work is the combination of NPF classification with interpretable machine‑learning tools. By integrating generalized additive models with SHAP analysis and vertical observational data, the authors quantitatively attribute nucleation‑mode particle variability to individual meteorological, chemical, and aerosol parameters under different pollution regimes. This approach represents an advance over traditional correlation analyses and offers a clear framework for diagnosing nonlinear source–sink competition in NPF studies.
In addition to the comments provided by Referee #2, I suggest adding a short discussion on how the authors think the identified source–sink mechanisms would be expected to generalize to other regions, seasons, or pollution regimes. Since this study is based on an intensive observation period of roughly two weeks at a single site, it would help setting this study into broader context and to prevent overinterpretation of the results obtained here.
I recommend the manuscript to be accepted for publication.
Citation: https://doi.org/10.5194/egusphere-2026-738-RC2 - AC3: 'Reply on RC2', Honglei Wang, 11 May 2026
Status: closed
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RC1: 'Comment on egusphere-2026-738', Anonymous Referee #2, 15 Apr 2026
The manuscript investigates the triggering mechanisms of new particle formation (NPF) under various O3 pollution scenarios, focusing specifically on the dynamic balance between precursor sources and the background condensation sink. The study concludes that NPF is highly dependent on the synchronicity between source enhancement and sink attenuation—a process regulated by the evolution of the planetary boundary layer. Overall, this is a highly interesting and well-structured study that provides valuable quantitative insights through the application of interpretable machine learning methods. The methodology is scientifically sound, and the data robustly support the final conclusions. I recommend publication following minor revisions. The authors are requested to consider and respond to the following specific comments:
Major comments
- Section 3.2 emphasizes the breakup of the morning inversion layer and the subsequent rapid vertical mixing as the key process leading to the synchronization of source enhancement and sink weakening. Considering that the temporal resolution of the UAV vertical observations is 3 hours, whereas this process may occur on a much shorter timescale, is this resolution sufficient to capture such rapid evolution?
- Section 3.3: In the Non-O3 scenario, RH shows a strong positive SHAP contribution, whereas T, SO2, and O3 show negative contributions. Could the authors provide more scientific interpretation for this?
Minor comments
- Line 29-33: The first paragraph of the Introduction is rather difficult to read and it is recommended to be reorganized.
- Lines 107-109: Please delete the space after the hyphen in “Low- O3” and “High- O3”.
- Line 120: “coagulation sink (Coags, S-1)” is recommended to be revised to s-1.
- Line 122: In the equation, should “Cogas” be corrected to Coags? It seems to be a spelling error.
- Section 2.4: Why was GAM chosen instead of random forest or XGBoost?
- Line 178: “As show in Table 1” should be corrected to “As shown”.
- Line 181 & Table1: “Cogas” is also a spelling error here.
- Lines 219-220: “In contrast…” is repeated from Lines 165–166, please delete it.
- Lines 226-227: The statement “According to the classification criteria in Sect. 2.2, both the NPF and Low-O3 scenarios had similarly low O3 pollution levels (160-215 μg/m3)” does not seem to be mentioned in Section 2.2. Please clarify.
- Line 254: The citation “Ail et al., 2025” is inconsistent with Ali in the References (Line 392); please revise it.
- Lines 229-300: The phrase “turning negative beyond approximately 0.085 m·s-1” is discussing CS, so the authors are advised to check the unit. Should it be s-1 instead?
- Lines 316–317: Please revise the caption of Figure 6, as “(a) Mean SHAP values across for scenario” is not grammatically smooth.
Citation: https://doi.org/10.5194/egusphere-2026-738-RC1 - AC1: 'Reply on RC1', Honglei Wang, 11 May 2026
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AC2: 'Reply on RC1', Honglei Wang, 11 May 2026
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed on 12 May 2026.
Citation: https://doi.org/10.5194/egusphere-2026-738-AC2
-
RC2: 'Comment on egusphere-2026-738', Anonymous Referee #1, 07 May 2026
This manuscript demonstrates that atmospheric new particle formation (NPF) is governed not by ozone (O₃) levels alone, but by the temporal synchronization between precursor production and background aerosol removal. The authors show that such synchronization is primarily controlled by the evolution of the planetary boundary layer, with NPF occurring when post‑sunrise boundary‑layer development simultaneously enhances near‑surface oxidation capacity through mixing of O3 from above and reduces the condensation sink. On the other hand, stable PBL conditions can either lead to low O3 levels or high background aerosol concentations leading to low low source rate or high sink of newly formed particles.
In my opinion, methodological strength of this work is the combination of NPF classification with interpretable machine‑learning tools. By integrating generalized additive models with SHAP analysis and vertical observational data, the authors quantitatively attribute nucleation‑mode particle variability to individual meteorological, chemical, and aerosol parameters under different pollution regimes. This approach represents an advance over traditional correlation analyses and offers a clear framework for diagnosing nonlinear source–sink competition in NPF studies.
In addition to the comments provided by Referee #2, I suggest adding a short discussion on how the authors think the identified source–sink mechanisms would be expected to generalize to other regions, seasons, or pollution regimes. Since this study is based on an intensive observation period of roughly two weeks at a single site, it would help setting this study into broader context and to prevent overinterpretation of the results obtained here.
I recommend the manuscript to be accepted for publication.
Citation: https://doi.org/10.5194/egusphere-2026-738-RC2 - AC3: 'Reply on RC2', Honglei Wang, 11 May 2026
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The manuscript investigates the triggering mechanisms of new particle formation (NPF) under various O3 pollution scenarios, focusing specifically on the dynamic balance between precursor sources and the background condensation sink. The study concludes that NPF is highly dependent on the synchronicity between source enhancement and sink attenuation—a process regulated by the evolution of the planetary boundary layer. Overall, this is a highly interesting and well-structured study that provides valuable quantitative insights through the application of interpretable machine learning methods. The methodology is scientifically sound, and the data robustly support the final conclusions. I recommend publication following minor revisions. The authors are requested to consider and respond to the following specific comments:
Major comments
Minor comments