the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Dec 2025
Secondary processes driven by multi-factor interactions dominate the aerosol nitroaromatic compound pollution during winter in China
Abstract. Previous observational and chamber studies have highlighted the significant promoting effects of relative humidity (RH) or aerosol liquid water (ALW) on the formation of aerosol NACs. However, the applicability of this pattern needs further validation in large-scale field observations. This study presents the simultaneous investigation of the composition, abundance, origins of nitroaromatic compounds (NACs) in PM2.5 across 11 Chinese cities during winter, with a focus on the key factors controlling NAC formation. Nitrophenols (NPs) and nitrocatechols (NCs) were identified as the main NAC groups, with their relative dominance varying by city. Higher total NAC concentrations were observed in northern cities, likely due to intensified coal and biomass burning. While secondary processes dominated wintertime NAC formation across all investigated cities, the average proportion of secondarily formed NACs was lower in the north (87 %) than in the south (93 %). This north-south disparity was more pronounced during polluted periods (82 % vs. 96 %). Furthermore, insignificant promoting effect of RH or ALW was found for most NACs except nitrosalicylic acids. 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, potentially offsetting the promoting effects of RH and ALW. These findings suggest that the RH- and ALW-promoted NAC formation may not be universally applicable in real atmospheric environments, where multi-factor interactions play a critical role. This study highlights the necessity of considering complex field conditions in future research on NAC formation mechanisms.
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Status: open (until 21 Feb 2026)
- RC1: 'Comment on egusphere-2025-5730', Anonymous Referee #1, 09 Jan 2026 reply
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- 1
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 PM2.5 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.