This study investigated the pollution characteristics and composition of NACs during the wintertime in 11 Chinese cities. Furthermore, the formation mechanism of NACs was explored based on variations in RH, ALW, and oxidants during clean and polluted days. In fact, the composition, abundance, and sources of NACs have been frequently reported in recent years. It is recommended that the authors conduct a thorough analysis of the NAC formation mechanism to support the innovative perspective presented in this paper.

General comments:

1. Section 2: The methodology section lacks detailed descriptions, probably resulting in unreliable data.

(1) Detailed descriptions of each sampling site should be added to the main text or supplementary materials, such as the sampling site type (urban, rural, or remote site?) Are there point sources in the vicinity of the sampling sites? What is the impact of vehicle emissions? Moreover, the locations of monitoring stations used to obtain meteorological parameters and air pollutant concentrations should be described in detail, or their locations were added to Fig. S1.

(2) Were PM_2.5 samples collected using the same sampler at all sites? e.g., the high-volume air sampler (KC-1000, Laoying, China)? Do the 154 filter samples include field blank samples? How many filed blank samples were collected at each sampling site? Please provide clarification in the main text.

(3) The pretreatment processes of filters were not described in detail. For example, what is the area of filter used for NACs analysis? What is the volume of methanol used? How long should the ultrasonication last?

(4) Please add the model of UPLC-MS/MS and ion chromatography, the chromatography and mass spectrometry parameters of UPLC-MS/MS, and the method performance for analyzing NACs, including precision, accuracy, matrix effects, and limit of quantification, which are essential for ensuring data reliability.

(5) Both gas (e.g., NH3, HNO3) and particulate phase data for major inorganic components were required to conduct ISORROPIA-II thermodynamic model, how can gases data be obtained? Neglecting the gas phase may lead to significant deviations in aerosol pH and ALW, especially in southern China.

(6) How to quantify LGA? The unit of LGA in Figure 2 was not shown.

2. Section 3.1:

(1) Table S1-S4: Please double check the concentrations of 4M5NC, as they are significantly lower than the levels observed in previous winter studies. I recommend that the authors compare these findings with published data and explain the reasons for the discrepancy. In addition, how many days are classified as pollution days and clean periods, respectively? The number of days (e.g., n=5) should be added below Average, Clean period, Poll. Period in Table S1-S4.

(2) Table S1-S4: The value of ALW varied significantly across different cities. For example, XA and CD reached as high as 113 and 138 μg m-3, while KM and GZ were only around 15 μg m-3. What causes such substantial differences? It seems that the differences of PM2.5 concentrations among different cities are not so significant.

(3) Line 265-266: what components are important contributors to haze? NACs are minor constituents in PM2.5, which were not the primary cause of the heavy pollution.

3. Section 3.2: Figure 2 shows that the sampling frequency is irregular. It is recommended to list the sampling periods for all samples in Figure 2, or to add a table in the supplementary materials detailing the sampling periods for all sampling sites.

4. Section 3.3

(1) Equation (1) is not applicable for calculating secondary NACs in this study, because only around 13 samples were collected at each site, which may result in large uncertainties. In addition, the secondary NACs cannot be calculated based on data from all sites due to differences in formation mechanisms and primary sources.

(2) As the authors noted that primary emission sources such as coal combustion and biomass burning exhibited relatively high emission factors for 4NC and 4NP (lines 245–250). Yet it is difficult to believe that secondary NACs contribute over 80% of emissions, as shown in Figure 4, especially in winter. I recommend that the author rewrite this section and conduct a comparative analysis with relevant reports in the literature. In addition, I suggest providing precursor data, which may make the discussion on secondary formation of NACs more persuasive.

5. Section 3.4: In this section, I find this discussion rather confusing. Correlation analysis indicates that NAC concentrations in northern cities show negative correlations with RH, ALW, and oxidants. The authors also mentioned that secondary contributions are significant. What are the possible mechanisms for the secondary formation of NACs in northern cities? I mean that through analyzing the influencing factors, can the author propose secondary formation mechanisms for NACs in each city and provide sufficient data to support it?

Specific comments:

1. Line 49: NACs are not abundant compounds in PM2.5 compared to water-soluble ions and carbonaceous components, which typically accounted for less than 5% of PM2.5 mass.

2. Line 146: the unit of PM2.5 concentration is incorrect.

3. Figure 2: Area charts are not suitable for representing SO₂ and PM2.5 concentrations; line charts are suggested.

4. Line 337-338: ‘Symbols * and ** indicate P < 0.05 and P < 0.01, respectively’, what is meaning of ‘***’ in Figure 3? How to calculate the frequency of significant positive correlations? 5. Line 485: ‘Chins’ is a typo.

This manuscript presents a valuable large-scale synchronous field observation on nitroaromatic compounds in PM2.5 across northern and southern Chinese cities during winter, with a focused investigation on the effects of relative humidity and aerosol liquid water on nitroaromatic compound formation. The key finding of this study is that the widely reported promoting effects of relative humidity and/or aerosol liquid water on nitroaromatic compound formation are not universally observed in this large-scale field study and may be modulated or offset by other factors. Based on my understanding, some previous observational studies have also presented similar results, though they were not the primary focus of discussion. The clear north-south comparative analysis of this study underscores the complexity of real atmospheric chemical systems. Thus, the overall results may provide important insights into the necessity of considering complex field conditions in future mechanistic research on nitroaromatic compound formation. In summary, the manuscript is generally well-written, the methodology is sound, and the conclusions are supported by the data. I recommend acceptance after the authors address the following minor points to further strengthen the manuscript's clarity and impact.

Specific suggestions

1. ALW is a core variable for the conclusions. However, the method for calculating ALW is not described in either the Materials and Methods section or the SI. It is recommended that the authors provide a detailed methodology for obtaining ALW data, including thermodynamic models, key parameters, and input variables used.
2. For the 154 total filter samples across 11 cities, please supplement the number of samples per city in either the Materials and Methods section or the SI.
3. Line 127: Please confirm the abbreviation for Harbin. It is suggested that the authors change 'Haerbin (i.e., Harbin; HEB)' to 'Harbin (i.e., Haerbin; HEB)'.
4. Lines 129-133: Please refine the expression: “with average air temperatures sustained above 4°C in the southern cities generally below 2°C in the northern cities”. It is suggested that the authors revise it to “with average air temperatures sustained above 4°C in the southern cities, and generally below 2°C in the northern cities”. In addition, please refine the description of biomass/coal burning in northern and southern cities to be more precise (e.g., Biomass and coal combustion activities are prevalent in both southern and northern Chinese cities. However, as winters are typically colder in northern China, the demand for fossil fuels for heating is significantly higher there than in the south. For instance, the centralized winter heating policy in China is generally implemented only in northern regions during the cold season.), which is consistent with the study’s conclusions.
5. It is stated that “the constraining effects from O3, ∙OH, and solar radiation on NAC formation were stronger in northern China due to higher levels of light-absorbing air pollution”, but the measurement/calculation method of ∙OH concentration is not mentioned in the manuscript. Please supplement the method for obtaining ∙OH data (e.g., detailed empirical model calculation or estimation from precursor concentrations).
6. Line 215: City abbreviation error. “HW” is a typo and it should be WH (Wuhan). Please unify all city abbreviations throughout the text.
7. Line 287: “Figure 2d,d,f” is duplicated. Please correct to the correct labels (e.g., Figure 2b,d,f).
8. Lines 280-281: Please revise “the highest total NC event occurred in HEB” to “the highest total NC concentration occurred in HEB” for accuracy. When summarizing the city-level concentration order, please double-check and confirm the ranking sequence to avoid inconsistency.
9. Lines 311-316: Please strengthen the logic slightly. When interpreting weak correlations between NACs and vehicle emission tracers, please add one short sentence to clarify that this does not exclude traffic-related precursor contributions to secondary NACs. Alternatively, this does not imply that the contribution of traffic-related precursors to secondary NACs is negligible, since only particulate-phase traffic tracers were employed in the analysis.
10. Lines 321-325: When referring to Figure S5 (the fire spot map), please add further explanation to support the observation of higher biomass burning intensity in southern cities. In fact, although open fire spots are less frequent in the north, the colder climate there leads to widespread indoor use of biomass materials for heating and cooking in rural households, such as through traditional heated beds (kang).
11. Line 485: It is a typo. Please correct “in northern Chins” to “in northern China”.
12. Lines 486-487: Please change the sentence “SR was distributed in a nearly opposite direction to NCs, NPs, and NGs in the PCA plot” to “SR showed an opposite vector direction to NCs, NPs, and NGs in the PCA plot, indicating a notable inhibitory effect…”.
13. When addressing the discrepancy between chamber results (RH/ALW significantly promote NACs) and field observations (no significant promotion), please briefly emphasize both the nonlinear or multi-factor interactions present in the real atmosphere and the current limitations in understanding the underlying laboratory mechanisms.