This manuscript presents a comprehensive multi-site wintertime dataset of water-soluble brown carbon (BrC) across the North China Plain (NCP). By combining optical measurements, molecular markers, PMF source apportionment, and statistical (random forest) analysis, the authors argue that nitrogen-containing organic compounds (NOCs), largely formed via ammonia-driven aqueous chemistry, play a dominant role in BrC light absorption during haze events. The dataset is extensive, spatially representative, and timely. The topic is of high relevance to atmospheric chemistry, particularly given increasing attention to ammonia-rich environments and nitrogen-dominated BrC absorption. The manuscript is generally well written and technically competent, and many results are consistent with recent literature. However, several key conclusions are currently overstated, and important methodological assumptions require stronger justification or sensitivity analysis. I therefore recommend major revision before the manuscript can be considered for publication.

 Major comments

1. A central conclusion of the manuscript is that “ammonia-driven aqueous reactions were identified as the predominant pathway governing the secondary formation of NOCs.” While the presented evidence is suggestive, it remains primarily correlational. The RF/SHAP analysis identifies NH₄⁺ and ALWC as the most influential predictors of WSONsec; however, variable-importance metrics alone do not establish causality. During winter haze in the NCP, NH₄⁺ is tightly covariant with PM₂.₅ mass, nitrate, RH/ALWC, and stagnation-driven accumulation, making it difficult to disentangle whether NH₄⁺ acts as a true chemical driver, a co-product of the same processes, or simply an indicator of the broader haze/aerosol-water regime.
2. Figure 5 combines EC-tracer estimates, RF/SHAP analysis, and correlation analyses to support ammonia-driven aqueous formation of WSONsec. However, NH₄⁺, ALWC, PM₂.₅, and pH are strongly covariant under winter haze conditions. The authors should clarify to what extent the relationships shown in Fig. 5b-d demonstrate a causal role of NH₄⁺, rather than reflecting co-variation with aerosol water content and stagnation-enhanced secondary processing. In particular, it would be important to demonstrate whether NH₄⁺ retains explanatory power for WSONsec when controlling for PM₂.₅ and/or ALWC.
3. The EC-based tracer approach is extended from SOC to WSON and further to NACs. This is a non-trivial assumption. EC is an imperfect tracer in regions with mixed combustion sources (coal, biomass burning, traffic), and secondary nitrogen-containing species may not scale with EC in the same manner as SOC. Moreover, the choice of the “lowest 15%” percentile to define primary ratios is somewhat arbitrary, and its sensitivity is not explored. A sensitivity analysis using alternative cutoffs (e.g., 10% and 20%) would help assess the robustness of the inferred secondary fractions.
4. The use of offline AMS analysis on filter extracts to infer imidazole-related species is innovative but raises concerns. Nebulization and drying of extracts may alter chemical speciation, and fragment ions such as C₃H₃N₂⁺ are not unique to imidazoles. These measurements should therefore be framed more cautiously as supporting rather than definitive evidence for specific N-heterocyclic compounds, with clearer discussion of potential artifacts and uncertainties.
5. The manuscript concludes that gas-phase photochemical oxidation dominates NAC formation, while aqueous pathways are limited. However, the weak correlation with ALWC alone does not rule out aqueous-phase nitration, particularly given the known importance of aerosol pH. In addition, the interpretation of temperature dependence focuses primarily on gas-particle partitioning, while potential impacts on reaction kinetics are not discussed.
6. The conclusion that NH₃ emission control is required to mitigate BrC pollution is directionally reasonable but currently overgeneralized. The study is limited to winter conditions in the NCP, where ALWC and secondary processing are enhanced. The relative importance of NH₃ may differ under summer conditions, lower ALWC, or in cleaner environments. This limitation should be more explicitly acknowledged when discussing broader implications.

Review: Light Absorption Properties and Composition of Brown carbon in North China

This study investigates the composition and formation pathways of secondary brown carbon (BrC) and their contribution to aerosol light absorption during winter in the North China Plain (NCP). The authors show that particulate nitrogen-containing organic compounds (NOCs) are important contributors to BrC in this region. They report that combustion emissions, particularly biomass burning, represent important BrC sources at the rural site. At the four urban sites, BrC particles exhibit lower absorption at 365 nm, which the authors attribute to the stronger influence of traffic emissions and/or the atmospheric aging of light-absorbing aerosols. In addition, the light absorption at 365 nm is enhanced during haze conditions and coincides with elevated NOC concentrations.

The authors quantify the contribution of secondary water-soluble organic nitrogen (WSON_sec) using a tracer-based approach, suggesting that secondary formation dominates across all sites. Using Random Forest analysis, they further show that WSON_sec formation is associated with elevated levels of NH₄⁺ and aerosol liquid water content (ALWC), suggesting that NOCs may form through ammonia-related aqueous-phase processing. In addition, the authors identify an alternative formation pathway during a haze episode in which secondary nitro-aromatic compounds (NACs) are produced through gas-phase oxidation of aromatic precursors followed by partitioning into the particle phase.

Based on these results, the authors emphasize the importance of controlling ammonia emissions in order to mitigate wintertime haze and elevated BrC concentrations in the NCP.

Overall, the manuscript presents a comprehensive dataset and combines several analytical approaches to investigate the formation pathways of NOCs and their relationship with water-soluble aerosol light absorption. The methodology is generally sound and the figures are clear and informative. The study provides valuable insights into the potential role of aqueous-phase chemistry in BrC formation in the NCP. However, some interpretations appear somewhat stronger than what can be directly supported by the presented analyses, and some aspects would benefit from further clarification or discussion.

Major comments

1. The manuscript attributes an important role to ammonia-driven aqueous chemistry in the formation of secondary WSON and BrC based on the observed relationships between WSON_sec ​, NH₄⁺, and aerosol liquid water content (ALWC). While these correlations indeed suggest that aqueous-phase processing plays a significant role, they do not necessarily demonstrate that ammonia-induced reactions dominate the formation of BrC. ALWC represents a key parameter controlling multiphase chemistry in atmospheric particles, and elevated ALWC can promote various aqueous reactions. Therefore, the observed correlations may more generally indicate enhanced aqueous processing rather than specifically ammonia-driven chemistry. Additional discussion acknowledging this limitation would help avoid overinterpretation of the results. Furthermore, the manuscript itself identifies alternative pathways for WSON_sec formation, such as the formation of secondary NACs through gas-phase oxidation of aromatic precursors followed by gas-to-particle partitioning. This pathway highlights that gas-phase photochemistry may also contribute significantly to secondary BrC formation during haze episodes. The relative importance of these competing mechanisms would benefit from further discussion.

1. The study focuses primarily on the water-soluble fraction of organic nitrogen and its relationship with BrC absorption. However, brown carbon is composed of a broad range of compounds, including both water-soluble and water-insoluble components. For example, Huang et al. (2020) reported that insoluble organics dominate BrC absorption in Wintertime in Beijing. If the water-soluble fraction represents only part of the total BrC absorption, this may influence the interpretation of the dominant formation mechanisms discussed in the manuscript. To less extent, other light-absorbing aerosol components could also contribute to the observed absorption, including sulfur-containing organic aerosols and mineral dust, acknowledging their potential influence would provide a more complete picture.

Minor comments and technical corrections:

Main text:

The authors occasionally alternate between the terms NOCs, WSON, and NACs. A clearer distinction between these terms would improve readability.

Methodology: In section 2.2 it is mentioned that WSOC and WSTN are estimated using a total organic carbon analyser. Then, in section 2.4 the term WSON is introduced without mentioning how it is estimated. A brief sentence would help and having the contribution of WSON to WSOC in a pie chart or histogram in the SI would be beneficial.

In some figures it could be useful to indicate in the captions what the markers and the bars represent (average/median, sites, standard deviation at all sites). Also, are similar trends also observed when considering individual sites or urban/rural sites?

Line 50: “while”

Line 55: Replace “were the important” with “were important”

Lines 70, 76, 99, 112 … recurrent throughout the manuscript: Replace “;” with “.”

Line 109: Replace “the stringent […]” with “stringent […]”, I would also suggest to name the specific emissions control measures that were expected to reduce BrC primary emissions.

Line 117: It would be beneficial to the reader to know the sources that lead to the increase in oxidation capacity and NH3 levels, not just quoting the papers referring to it.

Line 148: remove « the » after 40 mL.

Line 195: Maybe the authors could provide some explanation regarding o the principle of SHAP analysis.

Line 222: repeat of “f denotes the fraction of organic matter in particle”

Line 234-235: the reference should be to Figure S2 and “3-flod” needs to be corrected.

Line 251: Could the authors introduce what BkF stands for before using the abbreviation (same comment for other PAHs)?

Line 267: “filed observations”

Line 292: Correct the word “countries”.

Line 306: Introduce CWT term before using the abbreviation.

Line 317: Spell out all PAHs before using abbreviations and maybe explain why the ratios of BeP/(BaP+BeP) supports the idea that the lower MAE measured in BJ is due to BrC aging ?

Lines 328-331: The correlation between abs365 and N:C shows that chromophores and N-containing organic compounds might be co-emitted or co-transported. “(NOCs) were the pivotal chromophores governing BrC optical properties in the NCP,” seems a strong statement.

Line 331: to which extent are abs and N:C correlated? Also, it is true of the average of all data but the points at some sites are quite scattered.

Line 375: “corrected” change to “correlated”

Lines 392-393: “through carbonyl-to-NH3 reactions”

Lines 408-410, “correlation between secondary NACs and NO2 was indicative of gas-phase oxidation being a significant formation pathway for NACs”, need to be careful here as both species could be co-emitted by combustion sources.

Supplements:

Test S2: Maybe add a reference to the original approach.  The tracer-based approach here assume that EC is a stable tracer of primary emissions in order to estimate secondary WSON and NAC. Nonetheless, organic to EC ratios can vary depending on the sources (e.g. traffic, biomass burning or coal combustion) and perhaps to less extent during atmospheric processing. Would a fix ratio of WSON/EC be representative of all sources for both urban and rural sites? Also, EC is generally associated with water-insoluble primary organic aerosols, whereas WSOC is largely composed of secondary / processed organic compounds. Using EC as a tracer to estimate primary contributions therefore imply that primary emissions contribute proportionally to both the water-soluble and water-insoluble fractions, which may not always be the case. Could this potentially lead to an overestimation of the secondary WSON/NAC fraction. A brief discussion of the uncertainties regarding this approach would help contextualize the conclusion that secondary WSON dominates across all sites.

Text S3: Are the references to equation 5 and 6 correct or is it equation S2 and S3?

Text S4 line 92-93: “HNH3*is equilibrium constant calculated by Eq S4”, isn’t it S6?

Fig S3: Typo in caption “BbF”. Also, the levoglucosan+BfK levels exceed 200ug/m3 at LC and TJ, when your hourly concentrations of PM2.5 rarely exceed 200 ug/m3 and OM only representing up to 25% of PM2.5. Could levoglucosan concentrations be overestimated?

Reference

Huang, R.-J., Yang, L., Shen, J., Yuan, W., Gong, Y., Guo, J., Cao, W., Duan, J., Ni, H., Zhu, C., Dai, W., Li, Y., Chen, Y., Chen, Q., Wu, Y., Zhang, R., Dusek, U., O’Dowd, C., Hoffmann, T., 2020. Water-Insoluble Organics Dominate Brown Carbon in Wintertime Urban Aerosol of China: Chemical Characteristics and Optical Properties. Environ. Sci. Technol. 54, 7836–7847. https://doi.org/10.1021/acs.est.0c01149