the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Nov 2025
Aerosol organic nitrogen across the global marine boundary layer: distribution patterns and controlling factors
Abstract. Organic nitrogen (ON) is an important yet poorly constrained component of aerosol total nitrogen (TN), particularly over remote oceans. We quantified aerosol ON in 92 total suspended particulate samples collected across approximately 160° of latitude in the marine atmospheric boundary layer (MABL) during Chinese Antarctic and Arctic expeditions (2019–2024), using a newly developed method that simultaneously determines ON and inorganic nitrogen. A significant latitudinal gradient was observed, with significantly higher ON concentrations in the Northern Hemisphere (83.3±141.4 ng m-3) than in the Southern Hemisphere (15.4±12.4 ng mm-3). Regionally, coastal East Asia recorded the highest ON levels (164.6±179.1 ng mm-3) but a lower ON/TN ratio (21.1 %), indicating strong terrestrial and anthropogenic influence. In contrast, the Arctic Ocean had lower ON concentrations (19.1±19.0 ng mm-3) but the highest ON/TN ratio (38.6 %), suggesting dominant marine biogenic sources. The Southern Ocean showed the lowest ON concentration (12.0±7.1 ng m-3) yet a relatively high ON/TN ratio (27.8 %), also pointing to oceanic origins. Near Antarctica, samples influenced by sea-ice air masses displayed markedly elevated ON and ON/TN ratios. These increases were strongly correlated with sea ice concentration and chlorophyll-a exposure, indicating enhanced biogenic emissions from sea-ice-associated ecosystems. This study offers the first direct ON measurements along a global MABL transect, revealing distinct latitudinal and regional patterns, and emphasizing the combined roles of continental inputs and marine sources. It also identifies sea-ice dynamics as a key factor influencing ON in Antarctic regions, providing crucial data for improving atmospheric and climate models.
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Status: open (until 10 Jan 2026)
- RC1: 'Comment on egusphere-2025-5458', Anonymous Referee #1, 04 Jan 2026 reply
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RC2: 'Comment on egusphere-2025-5458', Anonymous Referee #2, 04 Jan 2026
reply
Sun et al. reported measurements of organic nitrogen (ON) across the global marine atmospheric boundary layer. They observed that ON/TN ratios in polar regions are significantly higher than in other regions, attributing this difference to marine biogenic emissions. Furthermore, in sea ice-covered areas of Antarctica, elevated ON concentrations were detected, with sea-ice-associated ecosystems proposed as the primary driver for enhanced ON production. Overall, this manuscript presents the first global-scale measurements of ON and provides measurement-based ON/TN ratios for the marine boundary layer, thereby offering a very valuable dataset and novel insights into atmospheric ON.
One major concern is the potential lack of methodological details. Specifically, how did the authors calculate sea ice concentrations for individual samples? While remote sensing data are mentioned for illustrating spatiotemporal distributions of sea ice in the Southern Ocean, the types of remote sensing data employed are not specified, indicating a need for further clarification. Also see my comments below.
L54-55: It may be more appropriate to attribute the elevated ON to sea-ice-associated ecosystems rather than sea ice dynamics.
L98-99: In open oceans, do previous studies suggest a strong correlation between ocean primary productivity and ON or WSON? In polar regions, sea ice variability directly affects primary productivity and consequently ON levels, implying a potential linkage to sea ice dynamics.
L104: Perhaps both Antarctic and Arctic campaigns should be referenced, as indicated in Section 2.1 on sample collection.
L194: Should the title of Section 2.5 be revised to "Potential Source Contribution Function analysis" for consistency with the main text?
L222: Why was a 20 km radius selected for the AEC index calculation? Additionally, why were pressures below 850 hPa assigned a Chl-a value of zero? These aspects require clarification.
L282-284: This raises an important point: could the previously underestimated percentage result from the exclusion of WION? If so, can atmospheric ON fractions, WION and WSON, be quantified or estimated?
L299-304: Might secondary marine sources involve organic species from biological activities that undergo atmospheric oxidation to form ON? Beyond reactions with acidic species, are there alternative production pathways in the marine boundary layer?
Regarding "continental sources," does this refer to ON formed over continents and subsequently transported to oceanic regions?
L306: How was it determined that air masses spent 22.6% of their 5-day trajectories over continental areas? Is a single trajectory track used per sample? Methodological details appear insufficient, aligning with general concerns.
Figure 4: Note that EC is not an ionic species.
L364-368: Employing a longer time interval in backward trajectory analysis might reveal air masses traversing continents. As demonstrated, EC detected in remote oceans confirms long-range transport from continental sources. Similarly, fine ON particles could be transported to the remote Southern Ocean. The absence of correlation between ON and EC or nssCa2+ may indicate that ON is not predominantly of continental origin.
L377-382: Previous studies have reported high primary productivity at sea ice edges.
L389: "sea-ice–linked" should be corrected to "sea-ice–associated" for consistency.
Figure 5 caption: The statement "owing to missing satellite data and the methodologies in Sections 2.6 and 2.7" is ambiguous.
L456-460: If WION was substantially underestimated in prior studies, the current observations hold significant climate relevance, as WION may function as cloud condensation nuclei.
Please double-check the Supplementary material. In particular, the full name instead of abbreviation may be better in the Figure captions.
Citation: https://doi.org/10.5194/egusphere-2025-5458-RC2
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- 1
This manuscript presents the first direct measurements of aerosol organic nitrogen (ON) along a global marine atmospheric boundary layer transect, revealing clear latitudinal and regional patterns. The study is based on substantial and carefully conducted field campaigns and addresses an important knowledge gap in marine atmospheric nitrogen cycling. The results are potentially impactful for understanding ON sources and for improving atmospheric and climate model representations. I therefore recommend publication of this manuscript after the issues and questions raised below are adequately addressed.
Special comments:
(1) Line 33. Please check the unit “ng mm-3” is right?
(2) Line 35. The variance of ON/TN ratio should also be mentioned
(3) Line 56-102. The narrative flow of the "Introduction" could be improved. For example, the first paragraph highlights the limitations of traditional ON analytical approaches but does not immediately introduce your solutions; and the discussion of ON sources in the second and fourth paragraphs is interrupted by the third paragraph on the geochemical significance of ON, which makes the overall structure somewhat difficult to follow.
(4) Line 169-184. Given the very low ON concentrations in marine aerosol samples, it would be helpful for the authors to clarify the detection limit of the new method and whether it offers a clear advantage over traditional IC-based approaches? Additionally, more information on the separation and discrimination between IN and ON would strengthen confidence in the measurements.
(5) Line 191. I didn’t get it. As the ship was continuously moving and each sample integrates air masses over approximately 2–4 degrees of latitude, it is not entirely clear how the backward air-mass trajectories were constructed for an individual sample. The authors should clarify how the temporal and spatial variability during sampling was accounted for in the trajectory analysis.
(6) Line 222. Why 20-km radius was chosen?
(7) Line 245 and Figure5. Given the large variability in the dataset and limited data numbers, it would be advisable to assess the normality of the data in the Supporting Information to confirm whether the use of t-tests is statistically appropriate (or other statistical methos such as Mann-Whitney U test).
(8). Line 276. I am confused about how total nitrogen (TN) was determined using ion chromatography in previous studies (Table 1); could the authors clarify whether the reported WSON/TN ratios in the literature actually refer to WSON/WSTN instead?
(9) Table1: I suggest to add your own dataset in Table1
(10) Line 295 "4.1 Source identification of ON". As correlation alone does not allow one to distinguish between common emission sources and shared atmospheric transport or processing, I suggest that the authors either temper causal language (e.g., “dominated by”, “primarily controlled by”) or provide additional independent evidence to strengthen the source attribution.
(11) Line 327-335. TThe author said that the correlation between ON and nss-Ca2+ is good, but there is no correlation between ON and nss-K+ or EC. Is this a contradiction? I suggest the authors further explain this inconsistency. From the perspective of air masses, the backward trajectories of SATO samples have few intersections with the continental region. Therefore, can nss-Ca²⁺ be regarded as a reliable indicator of continental transport?