Influence of Ammonium Sulfate Seed Particle on Optics and Compositions of Toluene Derived Organic Aerosol in Photochemistry

Round 1

Reviewer 1 Report

This manuscript has been substantially improved. I recommend to accepting the manuscript after a minor revision below.

According to the reviewer's comment about uncertainties of CRI for mono-disperse particles, the author have added that “the total uncertainty for the retrieved CRI value of poly-disperse aerosol particles was estimated to be less than …” (lines 185-189). However, the uncertainties for the retrieved real part and imaginary part values should be given separately. The relative uncertainties for imaginary part values should become larger, if the absolute value of imaginary part is very small. Therefore, it is better to give absolute uncertainties.

Author Response

Dear reviewer,

     Our revised manuscript entitled “Influence of ammonium sulfate seed particle on optics and compositions of toluene derived organic aerosol in photochemistry” by Tingting Lu et al. is submitted to Atmosphere for the consideration of publication.

     Thank you very much for the suggestions about the further improvement of it. We have carefully considered the comments and revised the manuscript accordingly. The point-by-point response to the reviewer’s comments are listed in the attachment.  Please see in the attachment. Thank you!

      Best Wishes!

                                                     Sincerely Yours

                                               Tingting Lu, Mingqiang Huang

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have answered most of the comments and questions raised in the peer-review, but the manuscript still has a few issues as listed below. The paper can be considered publication if the following comments can be carefully addressed.

Comments and corrections:

1. Following the additional information about the wavelength, please add “Unless mentioned otherwise, the optical properties of the aerosol was derived at 470 nm”. It is not necessary to mention the wavelength in each discussion of optic result.
2. Explain how the SOA mass concentration was calculated, especially in the presence of seed particles, as particle density was not measured, possible effects of wall losses, online chemical speciation for both seed and SOA mentioned or discussed in the manuscript. It is not possible to deduct the initial seed distribution from dynamic aerosol distributions without considering wall losses. Using this method the optical results concerning seed particles may have very large errors.
3. Line 26: delete “is” before decreased and increased
4. Line 28: change to “are responsible for … were detected as …”
5. Line 47: delete “experimental”
6. Line 96: add “a” before “850 L chamber”
7. Line 108: “investigating”
8. Line 117: light beam with such wide FWHM
9. Line 172: delete “is”
10. Line 239: change “absorbability” to “light absorption”, and so forth in the entire script
11. Line 258: misleading. Change to “toluene SOA formed in presence of (NH₄)₂SO₄ seed…”. Change “with … seed” to “in presence of …” in the discussion on the optical properties of toluene SOA, the optics are not the result of SOA-seed mixtures.
12. Line 260: delete “the” before “absence”
13. Line 264: confusing. Check the phrasing, no subject or verb in the sentence
14. Line 267: change “decreased” to “to decrease”
15. Line 270: “25%”
16. Line 276: change “with different” to “as a function of reaction time”
17. Line 319 and Line 342: Explain how used 100mL solvent to extract 40 or 65 μg sample, without further concentration before UV-Vis and ESI-MS test. The diluted samples (0.4 or 0.65 mg/L) cannot produce the absorption results shown in Figure 9. Assuming the optical length in the UV-Vis instrument is 1.0 cm, SOA density of 1.5 g/cm³, Figure 9(b) gives absorbance of ~0.05 at 330nm, and the corresponding mass absorption coefficient and imaginary RI were estimated to be 17.7 m²/g and 0.6, respectively, at 330nm. Taking the absorbance at 300nm in Figure 9b, the associated imaginary RI value is 8.8. Please explain such high imaginary value.
18. Line 406-408: it is meaningless to compare optical results for different BrC at different wavelengths.

Author Response

Dear reviewer,

     Our revised manuscript entitled “Influence of ammonium sulfate seed particle on optics and compositions of toluene derived organic aerosol in photochemistry” by Tingting Lu et al. is submitted to Atmosphere for the consideration of publication.

     Thank you very much for the suggestions about the further improvement of it. We have carefully considered the comments and revised the manuscript accordingly. The point-by-point response to the reviewer’s comments are listed in the attachment.  Please see in the attachment. Thank you!

      Best Wishes!

                                                     Sincerely Yours

                                               Tingting Lu, Mingqiang Huang

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Further comment on manuscript atmosphere-883834 titled “Influence of ammonium sulfate seed particle on optics and compositions of toluene derived organic aerosol in photochemistry”

The authors have answered the questions and corrected the scripts accordingly. We have no more major comments on the revised manuscript, and we agree the manuscript to be published after considering the following minor corrections.

1. Line 21: change “detected by aerosol albedometer” to “derived based on aerosol albedometer measurements”
2. Line 24-25: add imaginary CRI for toluene SOA, you cannot conclude that the absorption capacity of SOA formed in presence of seed is enhanced without mentioning imaginary CRI. Alternatively write “SSA of SOA decreased, while the imaginary part of CRI (k) of SOA increased with increasing concentration of (NH₄)₂SO₄ seed” before “demonstrating that the absorption capacity of SOA formed in presence of seed is enhanced”
3. Line 81: UV-Vis spectrometer cannot identify or measure chemical composition solely. Change to “the optical and chemical properties of SOA were further characterized by UV-Vis spectrometer and mass spectrometer”. Besides, add full name of UV-Vis.

Author Response

Dear reviewer,

    Our revised manuscript entitled “Influence of ammonium sulfate seed particle on optics and compositions of toluene derived organic aerosol in photochemistry” by Tingting Lu et al. is submitted to Atmosphere for the consideration of publication.

     Thank you very much for the suggestions about the further improvement of it. We have carefully considered the comments and revised the manuscript accordingly. The revisions we have made are listed as follows.

      Best Wishes!

                                                                  Sincerely Yours

                                                        Tingting Lu, Mingqiang Huang

Response to Reviewer:

The authors have answered the questions and corrected the scripts accordingly. We have no more major comments on the revised manuscript, and we agree the manuscript to be published after considering the following minor corrections.

Response: The authors thank the reviewer for the positive comments on the manuscript.

Line 21: change “detected by aerosol albedometer” to “derived based on aerosol albedometer measurements”

Response: We have changed “detected by aerosol albedometer” to “derived based on aerosol albedometer measurements” in Line 21.

Line 24-25: add imaginary CRI for toluene SOA, you cannot conclude that the absorption capacity of SOA formed in presence of seed is enhanced without mentioning imaginary CRI. Alternatively write “SSA of SOA decreased, while the imaginary part of CRI (k) of SOA increased with increasing concentration of (NH₄)₂SO₄ seed” before “demonstrating that the absorption capacity of SOA formed in presence of seed is enhanced”

     Response: We agreed to the reviews’ suggestion that we could not conclude that the absorption capacity of SOA formed in presence of seed was enhanced without mentioning imaginary CRI. We have changed “demonstrating that the adsorption capacity of SOA formed in presence of (NH₄)₂SO₄ seed is enhanced. SSA of SOA decreased, while the imaginary part of CRI (k) of SOA increased with increasing concentration of (NH₄)₂SO₄ seed.” to “SSA of SOA decreased, while the imaginary part of CRI (k) of SOA increased with increasing concentration of (NH₄)₂SO₄ seed, demonstrating that the adsorption capacity of SOA formed in presence of (NH₄)₂SO₄ seed is enhanced.” in Line 24-25.

Line 81: UV-Vis spectrometer cannot identify or measure chemical composition solely. Change to “the optical and chemical properties of SOA were further characterized by UV-Vis spectrometer and mass spectrometer”. Besides, add full name of UV-Vis.

Response: We have changed “the chemical compositions of SOA were measured by UV-Vis spectrometer and mass spectrometer” to “the optical and chemical properties of SOA were further characterized by ultraviolet-visible (UV-Vis) spectrometer and mass spectrometer” in Line 81. 

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Review of:
“Experimental study on the optics and compositions of toluene secondary organic aerosol with ammonium sulphate seed particle” by Lu et al.

This paper reported the optical properties of secondary organic aerosol (SOA) particles generated with or without ammonium sulfate (AS) seed particles. Determination of complex refractive index of SOA is important to evaluate the effect of aerosols on radiative balance and obtained results include new and interesting findings. However, information on the possible systematic uncertainties of the procedure to determine the CRI should be added. In addition, wavelength dependence of CRI should be taken into accounted for the comparison with the previous studies and for suggesting the influence of the obtained results to radiative balance of the atmosphere. I believe that the points below should be addressed before considering the publication.

Major points

1) Section 3.1

1-1) The PSL particles were used for the validation of the retrieval algorism of CRI. However, the response and systematic uncertainties for mono-disperse PSL particles and polydisperse SOA should be different. The author should add more information about these effect and potential uncertainties for the retrieved real and imaginary part values.

1-2) The CRI values of 1.676 + i0.015 and 1.674 +i0 are reported as values determined from scattering and extinction signals, respectively. But, the CRI for SOA seems to be determined using the both signal. The CRI values determined from both of scattering and extinction signals should be also given with estimated systematic and statistical uncertainties.

1-3) Did you determine the average CRI values for the wavelength range of 320-430 nm? The n and k values should have wavelength dependencies. It is better to determine the CRI values at several different wavelengths in the range of 320-430 nm.

1-4) The reported imaginary part value (0.015) seems to be too large as the values for PSL particles. Is this value really agreed with the previous reported values? Information on the wavelengths used for the present study and literature studies should be given for the comparison.

2) Section 3.2

2-1) It is better to add the data of mass concentrations of SOA in the text of section 3.2.

2-2) Information on the wavelengths used for the present study and literature studies should be given for the comparison (lines 173-174).

2-3) It is better to add n and k values for Figure 3 as well.

2-4) It is better to add AS concentration dependence of mass concentration of SOA and n values. It will be important to check whether the saturations are also observed for mass concentrations and n values around 200 mg/m₃ of AS or not.

3) Section 3.3

3-1) It is better to add the information on mass of SOA collected on filter and concentration of extract for SOA generated with and without AS, respectively, for Figure 7.

3-2) Information on the wavelengths used for the present study and literature studies should be given for the comparison (lines 292-295).

3-3) The wavelength dependence of SSA should be carefully considered during the comparison with the SSA for BBOA and the effect of ASOA generated with AS on the radiative balance of the atmosphere (line 297-298 and 310-314). The results of Fig.7 imply that the k value for the toluene-SOA generated with AS is smaller than that for BBOA at the wavelength above about 380 nm (which is important to determine total radiation balance). More solid discussion is needed in this part.

Other points

1) Throughout the manuscript: It may be better to use “sulfate” instead of “sulphate”.

2) Lines 16-17: It is better to add “of atmosphere” after “radiative forcing and light absorption”.

3) Lines 19: “a variety concentration of ammonium sulfate” should be better.

4) Line 52: “… was found to decrease 0.09 and 0.15”

This sentence is unclear. Do you want to say “the real part value of CRI for m-xylene SOA was found to decrease by 0.09 and 0.15 compared to those for toluene-SOA”?

5) Line 70: “CRI” should be “real part of CRI”

6) Line 72: “spectroscopic” should be “spectrometer”

Reviewer 2 Report

Comments on the manuscript titled “Experimental study on the optics and compositions of toluene secondary organic aerosols with ammonium sulfate seed particles”

Anthropogenic VOCs, such as toluene, may represent an important source of ambient secondary brown carbon aerosols (BrC). The optical properties of these BrC aerosols rely on their exact formation pathways and the atmospheric aging process. This study compares the optical and chemical properties of toluene SOA that are generated via homogeneous or heterogeneous photooxidation. It was found that heterogeneous reactions in presence of (NH₄)₂SO₄ seed particles enhance light absorption of toluene SOA, the formation of chromophoric imidazoles from aqueous phase reactions between ammonium and glyoxal was proposed to explain the light absorbing enhancement. This research is of limited innovation as this reaction was studied before. The paper needs to be better written. Thus, publication can be considered after the authors polished the manuscript and addressed the following comments.

Major comments:

1. Add more information on the chemical characterization and consider the following suggestions and questions: did you filtrate the methanol-water extraction before the UV-Vis and LC-MS measurements? how did you phase out salt influence in UV-Vis (e.g., acidity and further aqueous reaction) and subsequent MS (e.g., ionization) measurements of the toluene-SOA extraction? describe the exact extraction efficiency of toluene-SOA by 2% methanol water solvent? Provide operation parameters and brief data process/constrain method in ESI-MS measurement.
2. Optical parameters, such as SSA and CRI, are wavelength-dependent, but information on the used wavelength is missing.
3. The reaction mechanisms for heterogeneous imidazole formation are presumed to happen in the aqueous phase or at least on the surface water-layer of the particles. In this study, experiments were conducted under relatively dry humidity. Please provide evidence that 25% RH can result in hydrated condition for (NH₄)₂SO₄ According to Trainic et al. (2011, DOI: 10.5194/acp-11-9697-2011), heterogeneous reactions between glyoxal and dehydrated ammonium sulfate cannot occur at RH of 35%.
4. In figure 3, were the real-time particle extinction and scattering coefficients measured for Toluene-SOA and (NH₄)₂SO₄ mixtures or for Toluene reaction products only? In the presence of (NH₄)₂SO₄ seed particles, condensation of toluene SOA and further heterogeneous reactions on the surface of inorganic particles can possibly form core-shell mixing state at relative low humidity. Did experiments without inorganic (NH₄)₂SO₄ core influence on the optical parameters of Toluene SOA? Add more information in discussion.

Specific comments:

1. The title is misleading. “Influence of ammonium sulfate seed particle on optics and compositions of toluene derived organic aerosol in photochemistry” is suggested.
2. The wavelength information should be mentioned in describing the refractive index and single-scattering albedo in the abstract.
3. The high seed particle mass concentration is of limited atmospheric significance. Please discuss the influence of this to real environmental scenarios.
4. Line 28: add ‘that’ after chromophore of C=N
5. Line 37: change ‘leading’ to ‘leads’
6. Line 39: delete ‘radiation’
7. Line 44: ‘the CRI and SSA of aromatic SOA are still poorly characterized due to the inherent complexity in aerosol composition and difficulty in accurate measurement of particulate intrinsic optics.’
8. Line 48: misleading. Real part of SOA generated in presence or absence of NOx?
9. Line 50: add ‘real part of’ before CRI, add ‘that’ after measured, delete ‘index’.
10. Line 51: change to ‘attributed the decreasing of CRI to the gas-phase partitioning’
11. Line 51: add ‘real’ before CRI
12. Line 52: add ‘by’ before 0.09. Besides, 0.09 or 0.15 is for what? real or imaginary RI?
13. Line 53: change ‘with’ to ‘at’. This sentence describes the work by Li et al.? if true, clearly state it as ‘CRDS applied in the work by Li et al. operated at 532 nm, the gaseous absorption may bias the extinction result.’ Background measurements in particle free condition are generally conducted to calibrate the final extinction results.
14. Line 54: change ‘…providing…’ to ‘the aerosol albedometer can measure the real-time particle optical parameters, such as direct SSA and retrieved CRI.’
15. Line 63-64: the statement of “SOA is related to inorganic seed” is incorrect. The ubiquitous inorganic fine particles contribute substantially to SOA transformation by providing condensation surface in gas-particle partitioning, modifying reaction environment, involving reactions, or promoting their deposition.
16. Line 64-65: check your statement. It should be ‘Atmospheric fine particle pollution is serious in the urban areas of China’.
17. Line 68-69: check your statement. ‘use’ or ‘be used’? surface area participates in generation of SOA? Ammonium sulfate particles can act as efficient seeds to promote SOA formation mainly due to their large specific surface area. Moreover, the presence of seed particles shall change the optical and chemical results of SOA.
18. Line 69: CRI is short for complex refractive index, do not add index after CRI. Check the spelling in entire manuscript, and clearly state the real or imaginary part of CRI
19. Line 70: real or imaginary part of CRI? CRI decreases with what trend of (NH₄)₂SO₄ seeds? to decrease when seed particles were added?
20. Line 73-74: check the tense of your statement in the entire manuscript carefully. e.g., coefficients were detected, SSA are obtained
21. Line 91: ‘were delivered into the chamber’
22. Line 92: change ‘as’ to ‘at’
23. Line 98: tell the role of NO-NO2-NOx analyzer in your experiments
24. Line 106: add ‘into’ before ‘the optical fibers’
25. Line 133: how did you calculate the error from multiply charged particles?
26. Line 139: change ‘of’ to ‘from’, add comma before ‘demonstrating’
27. Line 153: is it scattering or extinction coefficient to increase to 450 Mm^-1?
28. Line 160: without turbulence, how you can make sure the uniform mixing of chemical compounds in the chamber?
29. Line 172: you’d better present and compare the CRI retrieved from both scattering and extinction coefficients, as from the above manuscript, as the CRI for PSL have large difference in the imaginary part with respect to scattering and extinction channels.
30. Line 178: you can hardly describe the toluene SOA as scattering particles without solar radiative forcing estimation. Unless the SOA related radiative balance is negative, you may claim that the particles contribute mainly to a cooling effect.
31. Line 209: ‘generates’
32. Line 211: ‘lead to’
33. Line 215: ‘be oxidized to’
34. Line 222: change caption to ‘Suggested mechanism for aldehyde and carboxylic productions in photooxidation of toluene’
35. Line 250: change ‘coagulate’ to ‘condensate’ or ‘deposit’
36. Line 253: add ‘were’ before ‘also detected by Liu et al’
37. Line 254: change ‘to’ to ‘by’
38. Line 256: imidazole products can be speculated from the ESI-MS spectra and be verified by the characteristic absorption band at 280 nm, but how can you conclude that imidazoles are the major products? You need to conduct quantitative analysis of the sample!
39. Line 268: change ‘reaction’ to ‘react’
40. Line 294: add ‘toluene derived’ before SOA
41. Line 310: add ‘be’ before ‘responsible’