the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jul 2022
Marine nitrogen fixation as a possible source of atmospheric water-soluble organic nitrogen aerosols in the subtropical North Pacific
Tsukasa Dobashi
Eri Tachibana
Kazutaka Takahashi
Sachiko Horii
Yoko Iwamoto
Shu-Kuan Wong
Koji Hamasaki
Abstract. Water-soluble organic nitrogen (WSON) in marine atmospheric aerosols affect the water-solubility, acidity, and light-absorbing properties of aerosol particles, which are important parameters in assessing both the climate impact and the biogeochemical cycling of bioelements. Size-segregated aerosol and surface seawater (SSW) samples were simultaneously collected over the subtropical North Pacific to investigate the origin of WSON in the marine atmosphere. The fine-mode WSON concentration (7.5±6.6 ngN m−3) at 200–240º E along 23º N defined as the eastern North Pacific (ENP) was significantly higher than that (2.4±1.9 ngN m−3) at 135–200º E, defined as the western North Pacific (WNP). Analysis of the stable carbon isotope ratio of water-soluble organic carbon (WSOC) (δ13CWSOC) together with backward trajectory indicated that most of the observed WSON in the fine particles in the ENP originated from the ocean surface. We found a significant positive correlation between the WSON concentrations and nitrogen fixation rate in SSW. The result indicates that reactive nitrogen (dissolved organic nitrogen and ammonium), produced and exuded by nitrogen-fixing microorganisms in SSW, contributed to the formation of WSON aerosols. This study provides new insights into the role of ocean-derived reactive nitrogen aerosols associated with marine microbial activity.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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- Final revised paper
Journal article(s) based on this preprint
Tsukasa Dobashi et al.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-561', Anonymous Referee #1, 08 Sep 2022
This paper attempts to address the uncertainty around sources of aerosol WSON in the marine atmosphere of the subtropical North Pacific. They report data from an east to west cruise transect across the N. Pacific including WSON aerosol concentration and surface ocean chl, primary productivity and N2 fixation rates. Their main approach is to compare east to west trends in the measured parameters. They conclude that since N2 fixation and aerosol WSON are both higher in the eastern N. Pacific than the western N. Pacific, then N2 fixation must be the source of aerosol WSON. The mechanism the authors invoke is that N2 fixation increases ammonium and DON concentrations in the surface ocean which then flux to the atmosphere and lead to secondary WSON. This paper is an example of “correlation does not equal causation.” It is undeniable that the east to west trends are similar in N2 fixation and WSON aerosol concentrations, as in, they are both higher in the east than the west. But that could be due to multiple factors, and in no way suggests that one is causing the other. The authors proposed mechanism is completely untestable as they do not present ammonium or surface ocean DON concentrations. They suggest the WSON must be secondary as it does not correlate with sodium, but it also does not correlate with MSA, a classic indicator of secondary processing. I have chosen not to present a detailed review of the manuscript as the general framework presented is not supported in the literature, nor do the authors present a mechanism that can be tested by the existing data. The conclusions drawn are therefore based on a single correlation and are not supported in any way by what is presented in the paper.
Citation: https://doi.org/10.5194/egusphere-2022-561-RC1 -
AC1: 'Reply on RC1', Yuzo Miyazaki, 30 Nov 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-561/egusphere-2022-561-AC1-supplement.pdf
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AC1: 'Reply on RC1', Yuzo Miyazaki, 30 Nov 2022
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RC2: 'Comment on egusphere-2022-561', Anonymous Referee #2, 14 Oct 2022
The paper by Dobashi et al. explores an important subject of secondary aerosol formation by nitrogen containing precursors over the open North Pacific. These type of studies are rare and difficult to come by and thus deserve to be published. The difficulty with the marine atmosphere research, especially in the Northern Hemisphere, is that it is often impacted by anthropogenic emissions via direct continental outflow or remnant terrestrial background. Multiple lines of evidence are, therefore, needed to rule out such impact and link atmospheric measurements to marine biological processes. The authors made a large effort in analysing multiple tracers or species, but have not discussed the results in coherent and comprehensive manner. At the moment the paper is more like a measurement report without an in-depth analysis of underlying mechanisms or processes and often overselling correlation or similarity for a causal link. I encourage the authors to play devil’s advocate and take an opposite side to assess if their argument can be falsified using same data. Therefore, a discussion where all evidence would be weighted pro and against is badly lacking. It may well be that the authors can compile compelling argument for their proposed link between nitrogen assisted secondary aerosol formation and nitrogen fixation in biologically active sea water, but a fair account should be given for the so called “null hypothesis” that observations were coincidental. It is also unclear how further studies should be geared in answering the remaining questions as a simple repetition will deliver same result.
Comments as they appear in the text:
Abstract, line 22 "The result suggests..." (at best) and does not indicate, because the suggested link is only a proposition.
Page 2, line 31. The manual operation of sampling clearly helped to avoid own ship emissions, but what about transcontinental/continental outflow pollution? Air mass trajectories suggest no contact with land, but they typically do not rule out entrainment of pollution from decoupled boundary layer or free troposphere. Were there any tracers like black carbon measured to rule out anthropogenic impact and confirm marine origin of the sampled air mass? Note that higher WSON concentration observed over ENP where trajectories originated close to the continental boundary.
Page 2, line 37. Size segregated samples were collected every 48hours, but there were 51 fine and 9 coarse particles samples. More confusing are similar percentages of total samples collected. Please clarify why many coarse samples were missing in which case their percentage could not be as high.
Page 3, line 23. Were SSW samples collected overboard with an acid cleaned bucket or using onboard plumbing system into acid cleaned bucket? In any case what was the approximate depth of SSW samples?
Page 4, line 14. NH4 and NO3-N point to anthropogenic origin of nitrogen. NO3 in coarse particles results from chloride depletion by HNO3, while ammonia reduced ammonium ion is reacting with sulphate of whatever origin. Despite relatively low concentration of all inorganic species, their origin is pointing towards anthropogenic sources. It should also be noted that natural ammonia source over the oceans is almost negligible (unless the authors have evidence it is otherwise). I think that such discussion is essential, otherwise paper would look like a report instead of a scientific paper.
Page 4, line 22. Is the similar longitudinal distribution of chlorophyll coincidental or causal? Similarity or regression cannot prove causal relationship unless more substantial support is provided and thoroughly discussed. One way to look at it is to use some specific anthropogenic pollution tracers showing no connection or opposite relationship with WSON and inorganic species.
Figure 3 is as useful as can be misleading as it contains only 17 of 51 fine aerosol samples and contains only 3 points with WSON >10ngN/m3. Most of the marine origin deltaC13 points (20-24 permille) probably belong to WNP region, so the highest value is of those 8 samples where WSON was >10ngN/m3. If those 8 points were eliminated, ENP and WNP regions become indistinguishable.
Figure 4. Absence of the correlation between MSA and WSON could be due to the temperature impact which favours SO4 formation in subtropical latitudes instead of MSA. Have the authors looked at SO4 to WSON correlation. If the source of sulphur and nitrogen species is biogenic, they should correlate to some extent, because 48hour sampling has a footprint of ~1000-1500km and biogenic processes overlap even if underlying mechanisms differ somewhat.
Page 5, line 11. The authors correctly discuss primary versus secondary origin of WSON species, but secondary formation does not necessarily is of marine origin. The real challenge is to rule out anthropogenic sources of gaseous precursors leading to secondary WSON.
Page 5, line 24. Why would N2 fixation result primarily in inorganic dissolved nitrogen and not organic like amines?
Page 6, line 18. Different ratio can arise from preferential production as well as from remnant anthropogenic source. Despite coming back to the same argument of anthropogenic origin I do not object authors argument about biogenic origin of WSON, but rather encourage the authors to look for more direct evidence in discounting anthropogenic origin.
Page 6, line 27. What about amines as a WSON species?
Page 6, line 40. Discussion section is recommended to discuss all information presented in previous chapters and discussing them simultaneously in a comprehensive manner. I encourage the authors of playing devil's advocate for their own advantage in taking critical view to their own data.
Citation: https://doi.org/10.5194/egusphere-2022-561-RC2 -
AC2: 'Reply on RC2', Yuzo Miyazaki, 30 Nov 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-561/egusphere-2022-561-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Yuzo Miyazaki, 30 Nov 2022
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-561', Anonymous Referee #1, 08 Sep 2022
This paper attempts to address the uncertainty around sources of aerosol WSON in the marine atmosphere of the subtropical North Pacific. They report data from an east to west cruise transect across the N. Pacific including WSON aerosol concentration and surface ocean chl, primary productivity and N2 fixation rates. Their main approach is to compare east to west trends in the measured parameters. They conclude that since N2 fixation and aerosol WSON are both higher in the eastern N. Pacific than the western N. Pacific, then N2 fixation must be the source of aerosol WSON. The mechanism the authors invoke is that N2 fixation increases ammonium and DON concentrations in the surface ocean which then flux to the atmosphere and lead to secondary WSON. This paper is an example of “correlation does not equal causation.” It is undeniable that the east to west trends are similar in N2 fixation and WSON aerosol concentrations, as in, they are both higher in the east than the west. But that could be due to multiple factors, and in no way suggests that one is causing the other. The authors proposed mechanism is completely untestable as they do not present ammonium or surface ocean DON concentrations. They suggest the WSON must be secondary as it does not correlate with sodium, but it also does not correlate with MSA, a classic indicator of secondary processing. I have chosen not to present a detailed review of the manuscript as the general framework presented is not supported in the literature, nor do the authors present a mechanism that can be tested by the existing data. The conclusions drawn are therefore based on a single correlation and are not supported in any way by what is presented in the paper.
Citation: https://doi.org/10.5194/egusphere-2022-561-RC1 -
AC1: 'Reply on RC1', Yuzo Miyazaki, 30 Nov 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-561/egusphere-2022-561-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Yuzo Miyazaki, 30 Nov 2022
-
RC2: 'Comment on egusphere-2022-561', Anonymous Referee #2, 14 Oct 2022
The paper by Dobashi et al. explores an important subject of secondary aerosol formation by nitrogen containing precursors over the open North Pacific. These type of studies are rare and difficult to come by and thus deserve to be published. The difficulty with the marine atmosphere research, especially in the Northern Hemisphere, is that it is often impacted by anthropogenic emissions via direct continental outflow or remnant terrestrial background. Multiple lines of evidence are, therefore, needed to rule out such impact and link atmospheric measurements to marine biological processes. The authors made a large effort in analysing multiple tracers or species, but have not discussed the results in coherent and comprehensive manner. At the moment the paper is more like a measurement report without an in-depth analysis of underlying mechanisms or processes and often overselling correlation or similarity for a causal link. I encourage the authors to play devil’s advocate and take an opposite side to assess if their argument can be falsified using same data. Therefore, a discussion where all evidence would be weighted pro and against is badly lacking. It may well be that the authors can compile compelling argument for their proposed link between nitrogen assisted secondary aerosol formation and nitrogen fixation in biologically active sea water, but a fair account should be given for the so called “null hypothesis” that observations were coincidental. It is also unclear how further studies should be geared in answering the remaining questions as a simple repetition will deliver same result.
Comments as they appear in the text:
Abstract, line 22 "The result suggests..." (at best) and does not indicate, because the suggested link is only a proposition.
Page 2, line 31. The manual operation of sampling clearly helped to avoid own ship emissions, but what about transcontinental/continental outflow pollution? Air mass trajectories suggest no contact with land, but they typically do not rule out entrainment of pollution from decoupled boundary layer or free troposphere. Were there any tracers like black carbon measured to rule out anthropogenic impact and confirm marine origin of the sampled air mass? Note that higher WSON concentration observed over ENP where trajectories originated close to the continental boundary.
Page 2, line 37. Size segregated samples were collected every 48hours, but there were 51 fine and 9 coarse particles samples. More confusing are similar percentages of total samples collected. Please clarify why many coarse samples were missing in which case their percentage could not be as high.
Page 3, line 23. Were SSW samples collected overboard with an acid cleaned bucket or using onboard plumbing system into acid cleaned bucket? In any case what was the approximate depth of SSW samples?
Page 4, line 14. NH4 and NO3-N point to anthropogenic origin of nitrogen. NO3 in coarse particles results from chloride depletion by HNO3, while ammonia reduced ammonium ion is reacting with sulphate of whatever origin. Despite relatively low concentration of all inorganic species, their origin is pointing towards anthropogenic sources. It should also be noted that natural ammonia source over the oceans is almost negligible (unless the authors have evidence it is otherwise). I think that such discussion is essential, otherwise paper would look like a report instead of a scientific paper.
Page 4, line 22. Is the similar longitudinal distribution of chlorophyll coincidental or causal? Similarity or regression cannot prove causal relationship unless more substantial support is provided and thoroughly discussed. One way to look at it is to use some specific anthropogenic pollution tracers showing no connection or opposite relationship with WSON and inorganic species.
Figure 3 is as useful as can be misleading as it contains only 17 of 51 fine aerosol samples and contains only 3 points with WSON >10ngN/m3. Most of the marine origin deltaC13 points (20-24 permille) probably belong to WNP region, so the highest value is of those 8 samples where WSON was >10ngN/m3. If those 8 points were eliminated, ENP and WNP regions become indistinguishable.
Figure 4. Absence of the correlation between MSA and WSON could be due to the temperature impact which favours SO4 formation in subtropical latitudes instead of MSA. Have the authors looked at SO4 to WSON correlation. If the source of sulphur and nitrogen species is biogenic, they should correlate to some extent, because 48hour sampling has a footprint of ~1000-1500km and biogenic processes overlap even if underlying mechanisms differ somewhat.
Page 5, line 11. The authors correctly discuss primary versus secondary origin of WSON species, but secondary formation does not necessarily is of marine origin. The real challenge is to rule out anthropogenic sources of gaseous precursors leading to secondary WSON.
Page 5, line 24. Why would N2 fixation result primarily in inorganic dissolved nitrogen and not organic like amines?
Page 6, line 18. Different ratio can arise from preferential production as well as from remnant anthropogenic source. Despite coming back to the same argument of anthropogenic origin I do not object authors argument about biogenic origin of WSON, but rather encourage the authors to look for more direct evidence in discounting anthropogenic origin.
Page 6, line 27. What about amines as a WSON species?
Page 6, line 40. Discussion section is recommended to discuss all information presented in previous chapters and discussing them simultaneously in a comprehensive manner. I encourage the authors of playing devil's advocate for their own advantage in taking critical view to their own data.
Citation: https://doi.org/10.5194/egusphere-2022-561-RC2 -
AC2: 'Reply on RC2', Yuzo Miyazaki, 30 Nov 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-561/egusphere-2022-561-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Yuzo Miyazaki, 30 Nov 2022
Peer review completion
Journal article(s) based on this preprint
Tsukasa Dobashi et al.
Tsukasa Dobashi et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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(1868 KB) - Metadata XML
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Supplement
(1034 KB) - BibTeX
- EndNote
- Final revised paper