Substantial organic impurities at the surface of synthetic ammonium sulfate particles
- 1Aix Marseille Univ, CNRS, LCE, Marseille, France
- 2Aix Marseille Univ, CNRS, ICR, Marseille, France
- 1Aix Marseille Univ, CNRS, LCE, Marseille, France
- 2Aix Marseille Univ, CNRS, ICR, Marseille, France
Abstract. Ammonium sulfate (AS) particles are widely used for studying the physical-chemistry processes of aerosols and for instrument calibrations. Small quantities of organic matter can greatly influence the studied properties, as observed by many laboratory studies. In this work, monodisperse particles (from 200 nm to 500 nm) were generated by nebulizing various AS solutions and organic impurities were quantified relative to sulfate using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). The organic content found in AS solutions was also tentatively identified using a Liquid Chromatography–tandem Mass Spectrometry (LC-MS). The results from both analytical techniques were consistent and demonstrated that the organic impurities contained oxygen, nitrogen and/or sulfur, their molecular masses ranged from m/z 69 to 420, they likely originate from the commercial AS crystals. For AS particle sizes ranging from 200 nm to 500 nm, the total mass fraction of organic (relative to sulfate) ranged from 3.8 % to 1.5 % respectively. An inorganic-organic mixture model suggested that the organic impurities were coated on the AS particle surface with a density of 1.1 × 10−3 g m−2. A series of tests were performed to remove the organic content (using pure N2 in the flow, ultrapure water in the solutions, and very high AS quality), showing that at least 40 % of the organic impurities could be removed. In conclusion, it is recommended to use AS seeds with caution, especially when small particles are used, in terms of AS purity and water purity when aqueous solutions are used for atomization.
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Junteng Wu et al.
Status: final response (author comments only)
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RC1: 'Comment on amt-2021-327', Anonymous Referee #1, 20 Dec 2021
Please find my detailed report in the attached pdf.
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AC1: 'Reply on RC1 and RC2', Junteng Wu, 06 Mar 2022
Object – Answers to the Reviewers for the paper: “Substantial organic impurities at the surface of synthetic ammonium sulfate particles”, Ref. amt-2021-327
Dear Editor, dear Reviewers,
We would like to thank the Editorial Board for considering our paper “Substantial organic impurities at the surface of synthetic ammonium sulfate particles” for publication in AMT. We would also like to thank the reviewers for the constructive comments, suggestions and corrections in English writing which clearly help us to improve this article. We have carefully studied the comments, and the original paper has been revised accordingly. In particular, as suggested by the reviewers, more arguments have been added in the introduction to highlight the case of why the amount of organic impurities on AS aerosol particles is so important. We have also completed some details of the experiments to illustrate the thoroughness of the results.
With our best regards,
Dr. Junteng Wu on behalf of all authors
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AC1: 'Reply on RC1 and RC2', Junteng Wu, 06 Mar 2022
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RC2: 'Comment on amt-2021-327', Anonymous Referee #2, 05 Jan 2022
This paper describes a series of experiments looking at the amount of organic impurities in ammonium sulfate solutions and also across different particle sizes when those solutions have been aerosolised. The authors present a convincing argument for a significant size dependence on the ratio of [Org]/[sulfate] and have suggested the results indicate that the organic is concentrated at the surface of the particles rather than forming an internally mixed particle. This is a highly relevant and interesting result, which has important implications for laboratory simulations and measurements of physiochemical properties. I recommend publication after some minor corrections.
Line 82: Organosulfates can be formed via a range of routes, not only by isoprene epoxides. I would suggest adding a few more examples and references
Line 88: It would be helpful for the reader to know a bit more about how ammonium sulfate is manufactured. This would allow some indication of the potential sources of organic contaminants.
Line 90: is the 0.8 % in the ammonium sulfate without humic acid? Its is a bit confusing what is meant here.
Line 115: I would like some more details about the air/N2 difference. Zeroi air is also likely to contain organics and it is important to determine whether the air is also a source of absorbing material.
Line 159: Need to explain that the second two are relative to the NO3.
Line 200: What is the difference in the chromatograms using the two different eluents – you say its been optimised but I don’t see any discussion about why one set was used over another. Also, why is only +ve mode used for optimisation? This may result in you discounting negative mode as unimportant, when it hasn’t actually been optimised specifically.
Line 221: “slightly higher (respectively lower)” – this is unclear and I don’t follow the meaning.
Line 237: what is the source of the nitrate ions?
Line 240: “large organic molecules”? The AMS doesn’t provide justification for this statement. The LC-MS may, but I don’t think this statement is justified here. Also, what does “large” mean?
Figure 4: While not outside your uncertainties, there certainly seems to be a pattern in the effect of concentration on the ratio. Can this data be included in the SI?
Line 255: what does “multi-characterization” mean here?
Figure 6: I would change the colour scheme here – I got confused with the standard AMS colour scheme i.e. red = sulfate.
Line 322: Give the DBE for the neutral masses not the ions.
Line 325 – 327: This sentence doesn’t make sense. Needs reworded.
Figure 7: Is this the TIC or base peak chromatogram? I assume the later due to the quality of the chromatogram.
Line 334 and earlier – To me direct injection means injection of a solution directly into the MS source without prior chromatography. This needs to clarified.
Line 340: I think “mark” should be “make”
Line 339: no data is presented to back up the “factor or 20 lower” – can you include a chromatogram in the SI?
Line 355: Is the mass accuracy really good enough to assign a C24 peak?
Line 379 – add the particle size to this final statement.
Line 394-396: This needs to be reworded – it seems like one sentence split into 3
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AC1: 'Reply on RC1 and RC2', Junteng Wu, 06 Mar 2022
This paper describes a series of experiments looking at the amount of organic impurities in ammonium sulfate solutions and also across different particle sizes when those solutions have been aerosolised. The authors present a convincing argument for a significant size dependence on the ratio of [Org]/[sulfate] and have suggested the results indicate that the organic is concentrated at the surface of the particles rather than forming an internally mixed particle. This is a highly relevant and interesting result, which has important implications for laboratory simulations and measurements of physiochemical properties. I recommend publication after some minor corrections.
Line 82: Organosulfates can be formed via a range of routes, not only by isoprene epoxides. I would suggest adding a few more examples and references
Line 88: It would be helpful for the reader to know a bit more about how ammonium sulfate is manufactured. This would allow some indication of the potential sources of organic contaminants.
Line 90: is the 0.8 % in the ammonium sulfate without humic acid? Its is a bit confusing what is meant here.
Line 115: I would like some more details about the air/N2 difference. Zeroi air is also likely to contain organics and it is important to determine whether the air is also a source of absorbing material.
Line 159: Need to explain that the second two are relative to the NO3.
Line 200: What is the difference in the chromatograms using the two different eluents – you say its been optimised but I don’t see any discussion about why one set was used over another. Also, why is only +ve mode used for optimisation? This may result in you discounting negative mode as unimportant, when it hasn’t actually been optimised specifically.
Line 221: “slightly higher (respectively lower)” – this is unclear and I don’t follow the meaning.
Line 237: what is the source of the nitrate ions?
Line 240: “large organic molecules”? The AMS doesn’t provide justification for this statement. The LC-MS may, but I don’t think this statement is justified here. Also, what does “large” mean?
Figure 4: While not outside your uncertainties, there certainly seems to be a pattern in the effect of concentration on the ratio. Can this data be included in the SI?
Line 255: what does “multi-characterization” mean here?
Figure 6: I would change the colour scheme here – I got confused with the standard AMS colour scheme i.e. red = sulfate.
Line 322: Give the DBE for the neutral masses not the ions.
Line 325 – 327: This sentence doesn’t make sense. Needs reworded.
Figure 7: Is this the TIC or base peak chromatogram? I assume the later due to the quality of the chromatogram.
Line 334 and earlier – To me direct injection means injection of a solution directly into the MS source without prior chromatography. This needs to clarified.
Line 340: I think “mark” should be “make”
Line 339: no data is presented to back up the “factor or 20 lower” – can you include a chromatogram in the SI?
Line 355: Is the mass accuracy really good enough to assign a C24 peak?
Line 379 – add the particle size to this final statement.
Line 394-396: This needs to be reworded – it seems like one sentence split into 3
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AC1: 'Reply on RC1 and RC2', Junteng Wu, 06 Mar 2022
Junteng Wu et al.
Junteng Wu et al.
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