Measurement report: High contribution of N2O5 uptake to particulate nitrate formation in NO2-limited urban areas

Lin, Ziyi; Ying, Chuanyou; Xu, Lingling; Ji, Xiaoting; Zhang, Keran; Zhang, Feng; Chen, Gaojie; Li, Lingjun; Yang, Chen; Chen, Yuping; Chen, Ziying; Chen, Jinsheng

doi:https://doi.org/10.5194/egusphere-2025-3697

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https://doi.org/10.5194/egusphere-2025-3697

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16 Sep 2025

| 16 Sep 2025

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

Measurement report: High contribution of N₂O₅ uptake to particulate nitrate formation in NO₂-limited urban areas

Ziyi Lin, Chuanyou Ying, Lingling Xu, Xiaoting Ji, Keran Zhang, Feng Zhang, Gaojie Chen, Lingjun Li, Chen Yang, Yuping Chen, Ziying Chen, and Jinsheng Chen

Abstract. Particulate nitrate (pNO₃^-) is a major component of fine particle in Chinese urban areas. However, the relative contributions of pNO₃^-formation pathways in NO₂-limited urban areas remain poorly quantified, hindering further particulate pollution control. In this study, comprehensive winter field observations were conducted in urban Xiamen, Southeast China. We observed significantly elevated nighttime pNO₃^- levels concurrent with increased N₂O₅concentrations. Quantification using an observation-constrained model revealed that N₂O₅ uptake contributed 51.2 % to total pNO₃^- formation, which was comparable to that of the OH + NO₂ reaction. The N₂O₅ uptake was found to be mainly driven by nocturnal NO₃ oxidation capacity (modulated by NO₂ and O₃ levels) rather than by heterogeneous reaction conditions. Sensitivity simulations further demonstrated that pNO₃^- formation rate was more sensitive to NOx variations than to VOCs variations. Implementing NOx control measures at nighttime was shown to effectively reduce pNO₃^- by abating N₂O₅ uptake while simultaneously preventing daytime O₃increase. Our findings enhance the understanding of pNO₃^- formation in NO₂-limited urban areas and provide valuable insights for developing joint PM_2.5 and O₃ mitigation strategies.

Received: 30 Jul 2025 – Discussion started: 16 Sep 2025

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Ziyi Lin, Chuanyou Ying, Lingling Xu, Xiaoting Ji, Keran Zhang, Feng Zhang, Gaojie Chen, Lingjun Li, Chen Yang, Yuping Chen, Ziying Chen, and Jinsheng Chen

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RC1:
'Comment on egusphere-2025-3697', Anonymous Referee #2, 13 Oct 2025 reply
Lin et al. present an analysis of the controlling factors for particulate nitrate (pNO3^-) production in Xiamen, Southeast China. Xiamen is notable compared to many other Chinese urban areas because N₂O₅ production there is NO₂ limited, in contrast to the O₃ limited conditions of other regions such as Beijing. They show that under these NO₂limited conditions, N₂O₅ heterogeneous uptake contributes significantly to pNO3^-. These findings are significant as the conditions in the study region may be increasingly relevant to other urban areas in China, especially as emissions controls continue to change NO_x, O₃, and VOC loadings. Relatedly optimal emissions control strategies to reduce pNO3^-and O₃ can be in conflict as elucidated in box model sensitivity simulations. Overall, this work provides useful new insights into pNO3^-in the NO₂ limited regime for N₂O₅ production. The analysis is of a high quality, and conclusions are well supported. I believe this work will be a useful addition to the literature and will likely be well suited for publication in ACP following revision and response to the comments below.
Main Comments
Was aerosol surface area density measured? If so, I would encourage the authors to also present values for the N₂O₅ heterogeneous uptake coefficient (γN₂O₅) derived from the iterative box model. γN₂O₅ is known to depend on pNO3^- concentrations and it could be quite interesting to see if that feedback impacts overall pNO3^-formation from N₂O₅ γN₂O₅values would also help with interpretation of the analytical results and iterative model skill (e.g. why is kN₂O₅ so much higher in this work than in other urban areas as noted in Line 298, is this due to differences in surface area or γN₂O₅)

Some additional details on the VOC measurements and the fraction of NO3 reactivity captured by the measured VOCs would be useful in the main text. Isoprene, styrene, and 2-butene have were shown to dominate VOC nitrate reactivity during winter in Beijing (Hu et al. 2023). Were those same species found to dominate NO3 reactivity here, and are any unmeasured VOC expected to matter for NO3 reactivity. More generally how do the specific VOC measured impact the discussion of pNO3- response to NOx and VOCs.

Minor Comments:
L19 and 29: The meaning of NO₂-limited in the abstract may not be clear to the reader as these regimes have not yet been introduced or defined.

L63: The meaning of this sentence isn’t clear. Are you saying that when N₂O₅dominates pNO3^-, N₂O₅ production is typically NO₂ limited or aerosol surface area is large.

L78 and elsewhere: I would encourage making sure the terminology distinguishing various effects is clear throughout the manuscript. I understand the that the intended meaning is that VOC reduction will decrease the removal of NO₃ by VOCs, leading to higher N₂O₅ production rates and therefore more pNO3^-production from N₂O₅heterogeneous reactions. However, the phrasing “enhancing N₂O₅ uptake” implies to me an increase in the first order N₂O₅heterogenous rate (kN₂O₅) which is independent of VOC. (also lines 279, 281)

Line 131: while the R² is good the slopes seem like they are far from 1. Please give values for these slopes and discuss implications.

Line 135: NO3^- from N₂O₅ can also partition to the gas phase as HNO₃. I don’t think this is an important point for this analysis, but it is not clear that this effect would lead to an overestimation of the OH + NO2 pathway.

Fig 3: Panel A. Doesn’t the right y-axis show the percent contribution not the ratio?

Supplement L56 and L65: Were N2O5 and ClNO2 calibrated through the full 2 meter stainless steel inlet used for the ambient observations? If not, was an inlet loss rate determined. N2O5 loss on that length of stainless steel could be substantial.

Supplement L62: IClNO2- is at m/z 208

Supplement L82: At what averaging time?

Figure S2: These sensitivities are notably quite low compared to typical Iodide CIMS instruments. Also, the LODs quoted in line L82 seem very good given the poor sensitivity. Can you expand further on how these values were derived.

Supplement L85: What time resolution data was used for the iterative box model

References:
Hu, H., Wang, H., Lu, K., Wang, J., Zheng, Z., Xu, X., Zhai, T., Chen, X., Lu, X., Fu, W., Li, X., Zeng, L., Hu, M., Zhang, Y., and Fan, S.: Variation and trend of nitrate radical reactivity towards volatile organic compounds in Beijing, China, Atmos. Chem. Phys., 23, 8211–8223, https://doi.org/10.5194/acp-23-8211-2023, 2023.

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Citation: https://doi.org/10.5194/egusphere-2025-3697-RC1

Ziyi Lin, Chuanyou Ying, Lingling Xu, Xiaoting Ji, Keran Zhang, Feng Zhang, Gaojie Chen, Lingjun Li, Chen Yang, Yuping Chen, Ziying Chen, and Jinsheng Chen

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https://doi.org/10.5194/egusphere-2025-3697-supplement

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Data availability about the measurement report titled "Measurement report: High contribution of N2O5 uptake to particulate nitrate formation in NO2-limited urban areas" Z. Lin https://doi.org/10.6084/m9.figshare.29670629

Ziyi Lin, Chuanyou Ying, Lingling Xu, Xiaoting Ji, Keran Zhang, Feng Zhang, Gaojie Chen, Lingjun Li, Chen Yang, Yuping Chen, Ziying Chen, and Jinsheng Chen

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Short summary

Based on field observations of N₂O₅, we found extremely high nighttime concentrations of N₂O₅, with a maximum value of 2.52 ppb. Further multiphase box model analysis revealed that the heterogeneous uptake of N₂O₅ is the most significant nitrate formation pathway in NO₂-limited urban areas. Additionally, we further analyzed the reasons for this high N₂O₅ uptake contribution, and discussed the synergistic reduction of nitrates and O₃.


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