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the Creative Commons Attribution 4.0 License.
| 25 Aug 2026
Measurement report: Distinct urban and rural organonitrate formation regimes in a mixed anthropogenic-biogenic subtropical atmosphere
Abstract. Particulate organonitrates (pON) are important components of secondary organic aerosols and key reservoir of reactive nitrogen, yet their sources and formation pathways in different atmospheric environments remain poorly understood. Here, we present pON measured in field observations at urban and rural sites in Chinese subtropical region utilizing a soot-particle aerosol mass spectrometer (SP-AMS) combined with positive matrix factorization for source- resolved quantification. With comparable fractions of pON (in total nitrates and organics) and similar average concentrations at both sites, contrasting formation regimes for pON were revealed: at the urban site, daytime photochemistry (16 %) and regional transport (33 %) dominated, whereas nighttime formation was more pronounced (29 %) in rural areas. Elevated volatile organic compounds (VOCs) availability may enhance photochemical organonitrate formation, while nighttime formation was favored under NOx‑rich conditions, highlighting the distinct sensitivities of the OH- and NO3-initiated pathways. Evaluation of organonitrate production potentials showed monoterpenes, isoprene, alkanes, and aromatics dominated the OH-initiated pathway, while styrene and phenols were important NO3-initiated precursors. Despite overall dominance of biogenic VOCs, anthropogenic VOCs contributed remarkably (59 %) to rural organonitrate formation. Refractory pON associated with biomass burning and nighttime formation appeared at the rural site, representing a fraction largely missed by conventional AMS. Our results demonstrate a pronounced urban-rural shift in pON formation regimes, where anthropogenic precursors unexpectedly influence nocturnal chemistry at the rural site, highlighting the complexity of organonitrate formation in mixed anthropogenic–biogenic subtropical environments and the need to consider refractory pON in future studies.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Status: open (until 06 Oct 2026)
- RC1: 'Comment on egusphere-2026-4552', Anonymous Referee #1, 09 Sep 2026 reply
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RC2: 'Comment on egusphere-2026-4552', Anonymous Referee #2, 11 Sep 2026
reply
Review of Huang et al. “Measurement report: Distinct urban and rural organonitrate formation regimes in a mixed anthropogenic-biogenic subtropical atmosphere”
Summary:
The study by Huang et al. presents mass spectrometric measurements of organic nitrates in urban and rural areas of Pearl River Delta, China. By using PMF for source apportionment, they explain the formation pathways of organic nitrates in these different environments and differentiating day- and nighttime processes. In addition, the usage of a SP-AMS gives them the ability to measure refractory components, which is seldom presented in these kinds of measurements.
Generally, the work is suitable as a Measurement Report, if the authors can clarify a few major and minor subjects, as listed below.
Major comments:
The use of FIGAERO-CIMS:
P5 L143: you estimated the molecular weight of pON to be 250 g/mol based on previous studies. But you had a FIGAER-CIMS at your site, so why not use it to determine the molecular weight of pON. This is also the recommended method according to Takeuchi et al. (2024) , and has been done at least in Chen et al. (2020); Graeffe et al. (2023). Using measured molecular weight of pON, the quantitative calculations are more reliable.
P6 L172: “independent CIMS measurement”. Please write more about the FIGAERO-CIMS measurements, now you only mention that there is this instrument and this is the results. Write more about the measurements and data processing, what compounds did you detect etc.
P7 L184: “CIMS captured only a fraction of total organonitrate”, related to previous comment.
NOx ratio calculations and AMS calibration:
P7 L174-176: The NOx-ratio vs CIMS or PMF method shows very low correlation. Speculate why this is the case. What could be the reason that your NOx ratio calculation doesn’t correlate with the other methods, as this should be the case.
Supplement, Text S2: why is R_AN so different between the two sites? I assume that the AMS is the same, and if you haven’t done any major maintenance, the AN calibration shouldn’t change that much. This affects directly the pON calculations. Calibration issues were mentioned for the rural site, but this is still a large difference.
P4 L110-113: Why not use the urban RIE values at the rural site? These values should be quite stable if no major maintenance has been done.
Minor comments:
P3 L92: Some reference to the supersite would be good.
P4 L100: “local authorities” doesn’t sound right here, just write down the institute or authority.
P4 L102: You have the table with detailed info of the instruments but add some reference to the instruments if possible.
P5 L140: “same as below” What is this referring to? Please clarify
P7 L183: add “in China” after “comparable with previous studies”.
P9 Section 3.1.1: what are the differences between aBBOA, BBOA and BBSOA? Is it some specific marker fragment or just the oxidation?
P16 L420: Add country for “Bakersfield” to guide the reader to the right country.
Supplement, Table S2-S3: What does “TS” and “MS” stand for in the table? Time series and mass spectra?
Supplement, Fig S5b: add year on x-axis (like in Fig S4)
Supplement, many figures: When using shaded area around some data points, please write in the caption what the shaded area stands for.
Chen, Y., Takeuchi, M., Nah, T., Xu, L., Canagaratna, M. R., Stark, H., Baumann, K., Canonaco, F., Prévôt, A. S. H., Huey, L. G., Weber, R. J., and Ng, N. L.: Chemical characterization of secondary organic aerosol at a rural site in the southeastern US: insights from simultaneous high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and FIGAERO chemical ionization mass spectrometer (CIMS) measurements, Atmos. Chem. Phys., 20, 8421-8440, 10.5194/acp-20-8421-2020, 2020.
Graeffe, F., Heikkinen, L., Garmash, O., Äijälä, M., Allan, J., Feron, A., Cirtog, M., Petit, J. E., Bonnaire, N., Lambe, A., Favez, O., Albinet, A., Williams, L. R., and Ehn, M.: Detecting and Characterizing Particulate Organic Nitrates with an Aerodyne Long-ToF Aerosol Mass Spectrometer, Acs Earth and Space Chemistry, 7, 230-242, 10.1021/acsearthspacechem.2c00314, 2023.
Takeuchi, M., Wang, Y., Nault, B. A., Chen, Y., Canagaratna, M. R., and Ng, N. L.: Evaluating the response of the Aerodyne aerosol mass spectrometer to monoterpene- and isoprene-derived organic nitrate standards, Aerosol Science and Technology, 58, 1371-1388, 10.1080/02786826.2024.2389183, 2024.
Citation: https://doi.org/10.5194/egusphere-2026-4552-RC2
Data sets
Urban and rural organonitrate in PM1 in PRD Shan Huang https://doi.org/10.17632/b57cffj2kf.1
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- 1
Huang et al. present a very comprehensive characterization of particulate organonitrates (pON) in rural and urban areas in the Pearl River Delta, combining SP-AMS measurements, FIGAERO-CIMS measurements, as well as supporting VOC measurements. Through source apportionment, they discovered the difference in pON formation mechanisms in the two environments. In particular, daytime photochemical and aged/regional components are more important during the urban campaign, whereas nighttime chemistry and biomass-burning-related processes make larger contributions during the rural campaign. The characterization of nonrefractory organic nitrates is novel. Overall, I believe the methodology is sound. The dataset is valuable and the topic is suitable for ACP as a Measurement Report. I have the following minor comments for the authors to consider.
Specific comments
Page 5 Table 1 What are the differences between aBBOA and BBSOA? How did the authors believe they originate from biomass burning? I feel more justifications to these factors can be provided.
Page 7 Figure 1 The authors stated that because PMF-pON correlates better with pON-CIMS compared with NOx-ratio ON, they chosse the PMF method. I am wondering how are the FIGAERO-CIMS signals converted to pON-CIMS?
Page 12 Lines 315-320: When discussing MOOA in the urban area, the authors suggest daytime photochemistry may have contributed to it. However, stronger photochemistry is also expected in daytime at the rural site, yet the afternoon peak was not observed. The authors may want to briefly explain the difference.
Page 13 Line 342-343: I agree that because of their peak at morning commuting hour likely reflect their primary origins. But how do amines contribute to nitrate then?
Page 14 Line 364: I feel the VOC/NOx ratio (5.5) that is useful for diagnosing ozone sensitivity is not necessarily directly transferable to organic-nitrate formation. Mentioning this specific ratio may imply that O3 and pON are formed through the same mechanism.
Page 15 Figure 5: Has the effect of time of day been controlled in this analysis? I recommend that the authors clarify the temporal selection used in Figure 5 and test whether the relationships remain after controlling for time of day (e.g., by restricting the analysis to a fixed daytime/nighttime window).
Minor comments
Page 11 Figure 3 In the diurnal plots, are the data points average values or median values?
Page 14 Line 353: Is this ratio ppb VOCs/ppb NOx?
Page 18 Figure 7 caption: “branching ratio”