This study examines the formation and pollution of aerosol nitrogen-containing organic compounds (NOCs) in PM2.5 across three Chinese cities during winter. The authors systematically compare the relative abundance, precursors, and formation mechanisms of various NOCs in cities with varying energy consumption patterns. The study emphasizes the significant role of anthropogenic aromatics and aqueous-phase processes in the occurrence of urban aerosol NOC pollution, particularly during haze days. The overall results provide valuable insights into the complex interactions between emissions, meteorological conditions, and aqueous-phase chemical processes in NOC formation. In general, this study advances our understanding of urban aerosol NOCs and will be of interest to Atmospheric Chemistry and Physics readers. The work is solid and well-written, and I recommend it for publication following minor revisions.

1. Line 69: “nitrogen oxide” change to “nitrogen oxides”
2. Line 80-82: I recommend rephrasing this sentence. Here is a suggestion: Several observational studies have found a positive correlation between aerosol NOC abundance and either aerosol liquid water (ALW) or relative humidity (RH).
3. Line 87: ‘large emissions of NOC precursors…’. A reference is recommended to be added here.
4. Line 169-172: “…which included CHO−, CHON−, CHONS−, and CHOS− in ESI− mode and CHO+, CHON+, and CHN+ in ESI+ mode”. The use of “-” and “+” signs in the molecular formulas here needs clarification. It is unclear whether they refer to the detected ion forms or the molecular forms. This ambiguity should be addressed.
5. Line 222: “…proposed by Ehhalt and Rohrer (Ehhalt and Rohrer, 2000), which was reported…”, the citation format of the reference is incorrect. Please correct it.
6. Line 227-231: The authors used non-metric multidimensional scaling (NMDS) for visualizing the distribution of NOCs. Have you considered alternative data visualization methods, such as principal component analysis (PCA)? If not, could you please clarify the specific advantages of using NMDS in this context?
7. Section 2.4: Regarding the potential pathways for NOC formation, have the authors considered bimolecular reactions, such as oligomerization reactions, as potential mechanisms in the formation of NOCs?
8. Could the authors clarify whether the higher NOC abundances observed during haze periods were mainly due to secondary processes, or could there also be a contribution from primary emissions?
9. The authors focus on the sources and formation mechanisms of NOCs. Could you briefly discuss the implications of your findings for air quality management strategies?
10. Figure 6: It seems that the city label in the inset of Figure 6 is incorrect. The red circle and line should indicate Beijing (BJ), not HEB.

General comment

The study by Ma et al. deals with investigations of nitrogen-organic compounds in the aqueous phase with different emission scenarios from residential heating and measures. Individual classes of NOC were obtained from LC-qTOF analysis using ESI and complementary supporting information including ions, acids, aerosol liquid water content and meteorological data. Basics concepts from high-resolution mass spectrometry were applied to group NOC according to their elemental composition (CHN, CHON) and polarity of the detection (+/-). By using correlation analysis and literature knowledge of NOC formation, the relative importance of formation pathways of NOC in the aqueous aerosol phase were revealed.

The study is generally well-written, despite being lengthy in some sections, and of interest for the readership of Atmospheric Chemistry and Physics. After appropriately addressing few comments below, I would recommend this manuscript for publication.

Specific comments

Line 111: 9.95 millions is not a density

Line 133: What was the rationale to choose this threshold and how would a different threshold affect the outcome of this study, let’s say if 50 or 100 µg/m3 would have been used?

Line 148: 0.1% formic acid solution has a pH<3, which may potentially lead to hydrolysis or acid-catalyzed reactions. How can you be sure that you observation are not an artifact of you sample workflow?

Line 168: Were only the mass resolution exploited to calculated sum formula or additional information from retention time or fragmentation spectra? What is the mass resolution of the instrument and how can you be sure that it was sufficient for the analysis? What was the mass range of the detected compounds?

Line 325: This section 3.2. is repeated in the following one discussing correlations, could be shortened substantially or even removed for brevity.

Lien 415: The Spearman correlation coefficients potentially indicate a moving of primary NOC (of negative correlation) during ageing in the VK space, similar to the common approach by Heald et al. (2010) (doi: 10.1029/2010GL042737). Can we learn anything from the presented data using this concept? 

Line 433: Tracers have known sources and sinks, so organic compounds can only be markers (Noziére et al. (2015), doi: 10.1021/cr5003485). Furthermore, Carlton & Turpin (2013) do not write anything about oxalic acid in the referenced study. 

Line 453/458: SOA refers to particle formation by gas-to-particle conversion. If the reaction occurs between a (low-volatile) particle constituents and e.g. OH, it belongs to the class of aged-POA.

Line 516: In the supplement, the temperature induced de_N pathway (formation of nitriles by reduction of amines) is reasoned with the study by Simoneit et al. (2003) on amides and nitriles from biomass burning, so a primary source. They are proposing that e.g. unsaturated fatty acids are reacting with NH3 in the hot BB exhaust. Explicitly, they preclude that those reactions happen in the atmosphere leading to secondary aerosol formation. I understand the authors that the higher temperature in BJ and HZ is responsible for this mechanism, which I think is a false conclusion.

Line 539: Could you exclude that the low correlation for HZ is not caused by generally low intensities affected by the larger relative uncertainty of the measurement?

Line 561: You pointed out that HZ  has generally mild winters and the emission of NOC precursors is lower due to generally lower heating activities. Therefore, I do not understand the link to pollution control measures.

Line 616: Similar to line 561, I think the absent central heating policy is not a reasons for lower NOC formation as the illustration suggests but rather a consequence of a missing need for stricter measures to control emission due to a warmer climate zone.

Technical

Line 433: “Xu et al. (2022a)” does not exist in the reference list, but two times “Xu et al. (2022b)”.