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 NO₂ limited 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 γN₂O₅ values 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 N₂O₅ dominates 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 N₂O₅ heterogeneous reactions. However, the phrasing “enhancing N₂O₅ uptake” implies to me an increase in the first order N₂O₅ heterogenous 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.

This study investigates the features of pNO3- production at a typical site in southeastern China. The nighttime N2O5 uptake was found to be efficient enough and play an increasing role in pNO3- formation. Multiple methods are applied to demonstrating the dominant effect of PNO3 or precursor concentrations, in N2O5 uptake process. The manuscript is well organized. I recommend acceptance after carefully addressing the following concerns when revising this manuscript.

1. Method 2.1: I suggest more description of instrument deployment at least for I-Tof-CIMS in main text. It would be better to briefly introduce the sampling setup, calibration frequency and the variation of calibration factor, such that the reasonableness of measurements could be readily assessed.
2. Method 2.2: The algorithm proposed by Wagner 2013 should be termed as iterative box model instead of interactive box model.
3. Line 212: Suggest clearly stating what simulate parameters exhibited good agreement here.
4. Line 218-230: A direct comparison of pNO3- formation rate is also helpful to indicate the characteristics of pNO3- formation at this site.
5. Line 241: Please revise the text font of citation.
6. Line 307-314: The SHAP of TVOC exhibit minor impact on N2O5 uptake and correlate not so well with its concentration. Replacing TVOCs with specific VOC species, such as monoterpene and styrene, could provide better correlation of this feature.
7. Section 3.4: The response of pNO3- and O3 production rate to precursors is well investigated, while I figure out two confusing points in discussion part. First, the production of O3 was clearly proved to be VOC-limited, resulting in effective mitigation on O3 by reducing VOC. Meanwhile, the pNO3- production also shows larger sensitivity to VOC variation. However, the authors claim that the effect of VOC reduction is limited in mitigating both pNO3- and O3, which is confusing. Second, the title of this manuscript, a NO2-limited region, seems contradictory to the finding of O3 production limited by VOC emission.