Overall evaluations:

Zhang et al. compared field measurements of N2O5 and related species during and after Beijing winter Olympics. Time series and diurnal patterns of N2O5-related species were reported. Furthermore, key kinetic parameters were investigated, such as NO3 reactivity (kNO3), N2O5 uptake, and N2O5 lifetime. Regarding kNO3, the contribution of NO and VOCs were discussed. As for N2O5 uptake, the steady-state method was applied to calculate the uptake coefficient. The influencing factors of N2O5 lifetime were also examined.

  The investigated topic, i.e., reactive nitrogen chemistry, is important within the scope of ACP journal. The presented contents are suitable and align with previous studies. However, as a measurement report, some essential details of measurement methods are lacking. Uncertainty analysis should also be provided. In terms of writing, the authors are suggested to further polish the language with particular attention to some contradictory expressions. Other major issues as listed below concern data quality and the reliability of measurement interpretations. Overall, major revision is needed, and potential publication depends on the quality of revision.

Major comments:

1. In section 2.2, the statement of NO3 and N2O5 measurement should be significantly enhanced. The data quality is in doubt without enough information provided, especially when considering that the instrument was in a malfunction state (line 99). At a minimum, the authors should make use of SI to record more technical details. Detailed comments regarding this issue are shown as follows.

(1) In lines 96-97, it looks like the authors can separately measure NO3 and N2O5. However, in lines 99-100, the authors said only the sum of NO3 + N2O5 can be measured. The above two statements are inconsistent.

(2) Lines 100-101, how was the limit of detection determined? What factors contributed to the overall uncertainty of 13.7%? Also, what was the background level of the instrument?

(3) Lines 101-104, only the inlet issue was mentioned, while the calibration factor, or in other words, the sensitivity of the instrument is still not clearly stated.

1. In line 155, the aerosol surface area (Sa) was calculated by an empirical parameterization using PM2.5. This calculation could bias Sa, which influences the results presented in the figure 4, figure 6, and figure 7. Considering the impact of Sa on the calculation of N2O5 uptake, an evaluation of the accuracy of this empirical formula should be provided.

1. Line 283-291: Further to major comment 2, the accuracy of the empirical Sa can affect the discussion here. Also, when looking at the raw datapoints in figure 4b, a clear trend between N2O5 lifetime and Sa could not be identified.

1. The k(NO3) calculated from VOCs oxidation (on the order of 1e-4 shown in figure 5) and steady-state analysis (up to 0.3, table S2) are totally different. What is the reason behind, and what is the influence of this issue on the calculated N2O5 uptake coefficient by the steady-state method? In addition, the k(NO3) stated in lines 364 to 365 (1.14 to 3.06) is even higher than those in figure 5 and table S2, which is also inconsistent.

1. Line 374: in Table 4, k(NO3) due to NO was too high during POP. Under this condition, it looks like N2O5 should not exist at all. Is this consistent with N2O5 observations during POP? The analysis in Table 4 depends heavily on the data quality of NO. However, NO sometimes displayed negative values, bringing big concern of its data quality (see minor comments No. 8).

1. The conclusion part should be reorganized. The first two paragraphs repeated some observations which have already been presented in the results part. The report of observational results in the conclusion part should be synthesized and condensed. Real insights and implications from this study need to be highlighted more.

Minor comments:

1. Lines 34-36: The authors stated that N2O5 uptake is crucial for NO3 removal at night, while N2O5 uptake only accounted for 20% of NO3 removal. This expression is somehow inconsistent, which means that more important contributors of NO3 removal should also be mentioned here.

1. Line 43: The expression “considered in tandem” is not accurate if the authors would like to say considered simultaneously.

1. Lines 86-94: The site description is too brief. More information could be added, e.g., the emission sources nearby.

1. Line 91: In figure 1, the sources of the two maps should be mentioned. Also, pay attention to the improper usage of capital letters in the figure caption.

1. Lines 105-109: How were the NOx, O3, and VOCs instruments calibrated? A brief statement should be provided.

1. Line 125-127: it is good to note that the N2O5 lifetime calculated here refers specifically to nocturnal N2O5 lifetime.

1. Line 167: “at Beijing” should be changed to “in Beijing”. Please check other places for grammar issues. Overall, the language of this manuscript could be further improved.

1. Line 236, figure 3: the font size in panel a and b is different. The range of NO2 and O3 mixing ratios could be made consistent to facilitate a comparison of their levels. NO levels were sometimes below zero, which should be explained or eliminated. Also, NO3 levels in panel b were below zero occasionally.

1. Line 274-275: Why N2O5 lifetime increased with RH when RH was below 35%? Could other factors influence N2O5 lifetime more significantly during these relatively dryer periods?

1. Line 276-277: RH > 60% does not necessarily mean rain or snow conditions. Please check the meteorological record in Beijing during the observation period.

1. Line 307: “biogenic” should not be capitalized.

1. Line 308: reaction rate coefficients should be discussed here rather than reaction rate. To convince the readers more clearly, the authors are encouraged to compare the rate constant of the NO3 + isoprene reaction with that of the NO3 + other VOCs reactions.

1. Line 312: here, the authors stated that AVOCs dominated NO3 reactivity. However, in lines 305-306, the authors mentioned that AVOCs were negligible for NO3 loss. These two statements are contradictory.

1. Line 315: what does “landscape” mean here? It is difficult to comprehend this expression.

1. Line 326: Besides the VOCs, how much did NO contribute to NO3 reactivity in figure 5?

Reactive nitrogen species (RNS), particularly NO₃ and N₂O₅, play critical roles in nighttime atmospheric chemistry and pollution processes. Zhang et al. present field observations conducted during and after the 2022 Winter Olympics, examining the influence of precursor levels on nocturnal NO3 and N2O5 chemistry in urban Beijing. While the study addresses an important research topic and falls within the journal's scope, the manuscript requires significant improvements in organization, clarity, and scientific rigor before it can be considered for publication. Below are my major concerns:

1. As the article type of “Measurement reports”, this work is expected to present substantial new results from measurements with high quality. However, the study only presents one month's worth of observational data. Although these winter observations are somewhat valuable due to data scarcity, the paper shows particularly inadequate attention to data quality assessment and presentation.

1) In 2.2 section, the authors describe that ambient NO3 were determined by CRDS analyzer, whereas N2O5 was quantified through its thermal decomposition reaction. If I would understand correctly that NO3 and N2O5 was measured directly and indirectly, respectively (Zhang et al., 2024). If NO3 measurement chamber becomes non-operational, how are simultaneous measurement of both species maintained? Section 2.3 suggests that NO3 concentration was determined by the dividing the N2O5 by equilibrium constant and NO2. Could the authors clarify the primary data sources and detailed derivation process for both NO3 and N2O5? A more explicit description of the measurement hierarchy (direct vs. indirect) and any data reconciliation methods would strengthen the methodology.

2) Another methodological question is raised that how do measurement uncertainties of NO3 and N2O5 affect the accuracy of the derived NO3 concentrations?

3) Table 1, Considering the limit of detection of N2O5 and working status of instrument, what is the expected LOD for NO3? Fig. 2 appears to include the full dataset. Were measurements below the LOD excluded from statistical analysis and subsequent interpretation? If not, how were these low-signal data points handled to avoid bias? Please address this in the Methods or Supplementary.

4) The role of VOCs in modulating NO₃ lifetime and reactivity is a critical aspect of this study. However, the current manuscript lacks visualization of VOC time series. At minimum, please include: A supplementary figure showing temporal trends of key VOC species (e.g., alkenes, isoprene) that dominate NO₃ A brief discussion of how VOC variability might influence the observed NO3/N2O5 behavior, particularly during periods of high reactivity.

2. Structural and Writing Issues

The manuscript lacks a clear and logical flow, making it difficult to follow the scientific narrative.

1) In Sect. 3.1 and 3.2, the authors extensively compare their observations with previous studies. However, these comparisons lack meaningful insights as the cited observations were conducted at different locations, times, and under distinct atmospheric chemistry conditions. This approach not only fails to highlight significant scientific value but also renders the manuscript unnecessarily verbose.

2) Line 168 to Line 184 frequently cited the numbers of the mean concentrations of these species. Please include another column for the statistic of total average in Table 2. Also VOCs data should be included in Table 2.

3) In Sect 4, key findings are not sufficiently highlighted, and the discussion often lacks depth in connecting observations to broader atmospheric implications. How the results extend and compare with current knowledge of nocturnal NO3/N2O5 chemistry. 

4) The unique atmospheric conditions during and after the Winter Olympics (e.g., emission controls) should be discussed in relation to the findings. The influence of precursor levels (e.g., NO₂, O₃) on NO3/N2O5 chemistry is not thoroughly explored.

5) how to determine the photolysis rate of NO3?

6) How do meteorological conditions (e.g., temperature, humidity, boundary layer height) affect the observed trends and chemical behavior?

Minors:

1. Line 138, E.q. (4) should change the items of reaction between NO3 and VOCs as E.q. (5) to show the different species i of VOCs.
2. Terminology should be used more precisely (e.g., distinguish between "reaction activity" and "reactivity" where appropriate, “Photolytic decomposition” and “photolysis”).
3. Line 153, the empirical formula for Sa should specify their applicable range of PM2.5 condition.
4. Figure 6, homogeneous uptake?
5. Line 369-373, Please provide the scatter plot between nighttime N2O5 uptake (y-axis) and RH (x-axis) for individual day to support the conclusion.
6. Fig. S1, what is the red dot line? Please show the related parameters if it is the regression line.
7. Please correct the wrong citations, e.g. Hu et al., 2023, Tham et al., 2018, etc.