A study of the morphology and effective density of externally mixed black carbon aerosols in ambient air using a size-resolved single-particle soot photometer (SP2)

Wu, Yunfei; Xia, Yunjie; Huang, Rujin; Deng, Zhaoze; Tian, Ping; Xia, Xiangao; Zhang, Renjian

doi:https://doi.org/10.5194/amt-12-4347-2019

Articles | Volume 12, issue 8

https://doi.org/10.5194/amt-12-4347-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-12-4347-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 8

Research article

|

12 Aug 2019

Research article |

| 12 Aug 2019

A study of the morphology and effective density of externally mixed black carbon aerosols in ambient air using a size-resolved single-particle soot photometer (SP2)

Yunfei Wu, Yunjie Xia, Rujin Huang, Zhaoze Deng, Ping Tian, Xiangao Xia, and Renjian Zhang

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Final revised paper (published on 12 Aug 2019)
Supplement to the final revised paper
Preprint (discussion started on 10 Dec 2018)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of Wu et al. amt-2018-408', Anonymous Referee #1, 07 Jan 2019
- AC1: 'Reply to comments of anonymous Referee#1', Yunfei Wu, 05 Mar 2019
RC2: 'Referee comment', Anonymous Referee #2, 07 Jan 2019
- AC2: 'Reply to comments of anonymous Referee#2', Yunfei Wu, 05 Mar 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Yunfei Wu on behalf of the Authors (05 Mar 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Apr 2019) by Paolo Laj

RR by Anonymous Referee #2 (07 Jun 2019)

Suggestions for revision or reasons for rejection

The authors have responded comprehensively to the first round of reviews. The manuscript is greatly improved and I believe that it could be published in ACP once the following comments have been addressed.

The level of English language writing is still an issue. There are a large number of grammatical errors. I recognize that the authors have made efforts to improve this, including the hiring of professional language editing help. However, the level is simply still too low. There are still far too many grammatical errors to list in a scientific review. I recommend that another round of professional copy editing is required.

The authors have scaled their Aqauadag calibrations by a size-independent factor to make them more similar to fullerene calibrations, which are thought to be more representative of ambient BC. The use of a size-independent scaling factor may introduce error since the ratio of Aquadag to fullerene peak heights in the SP2 is size dependent (Laborde et al., 2012). To avoid this problem, Baumgardner et al., (2012) introduced a single-point scaling procedure. I would recommend the authors also use this approach to make their results consistent with other SP2 studies.

The authors have done a good job of including extra information in the revised supplementary information that increases confidence in their measurements.

The campaign was conducted over 19 days but only very limited time-resolved measurements are presented (one comparison between a clean period and a polluted period, which is very interesting). This seems like a missed opportunity. Were there any interesting variations in Dfm or rho_eff over time? Even if not this would still be interesting to know. Is it possible to add a Figure that shows the time series of Dfm and rho_eff at given mobility diameters? Perhaps also with the PM2.5 time series shown in Fig. S7 to see if there is any correlation. One of the advantages of the tandem DMA-SP2 technique is that it can perform measurements at relatively high time resolution, it seems a shame not to use this advantage.

The focus of this manuscript is on externally mixed BC particles (the revised manuscript now contains good, solid arguments for how the authors have isolated these particles). I think it would also be interesting to present the campaign-averaged Dfm and rho_eff of internally mixed BC particles (i.e. those displaying delay time greater than 2 microseconds). This will require extra calculations and I know it is outside the main focus of the manuscript. But I think its important to put the extBC results in context, which is the main focus of the manuscript. For example, is it actually true that the extBC particles display low Dfm because they are uncoated and more aggregate-like? This could be partially answered by checking if the coated particles that were present at the same time displayed higher Dfm.

Specific comments:

L 46: Higher rho_eff values than what? This is an ambiguous statement. I guess it is meant higher than one might expect based on the trend observed at lower mobility diameters. But this is a poorly defined reference point.

L 104: What is meant by the term 'virtual Df'? Please clarify.

L 108: This sentence needs rewriting to clarify that it is the parameter effective density that is difficult to characterize by TEM, not the tandem measurements. (At least I think this is the intended meaning of the sentence.)

L 113: Change to 'system detection limit'

L 121: 'Extrapolation' is a better and more specific word to use than 'applicability' here.

L 421: Are these extBC masses averaged over the full campaign? If so this needs to be explicitly stated.

L 535: Did any of the DMA-(APM, ELPI) studies use a thermodenuder to remove volatile components? This would not necessarily successfully remove all volatile material (as indicated on L 564 for the Rissler study). But it would be good to indicate if one was used or not here (as is done for the Rissler study), to indicate how much volatile material one might expect to be present in these previous measurements.

L 567: My interpretation of this observation (that the discrepancy between DMA-APM and DMA-SP2 measurements is smaller at larger diameters) is that the particles at larger diameters are actually less coated (hence SP2 mass would agree more closely with APM mass). The ~300 nm diameter at which the discrepancy starts to decrease is consistent with the shift from a line with Dfm of 2.51 to one with Dfm of 2.07 shown in Fig. 3 (the kinks in the curves in Figs. 3 and 4 both occur ~300 nm). I'm not sure which interpretation is correct but perhaps this alternative one should at least be mentioned.

References:
Baumgardner, D., Popovicheva, O., Allan, J., Bernardoni, V., Cao, J., Cavalli, F., Cozic, J., Diapouli, E., Eleftheriadis, K., Genberg, P. J., Gonzalez, C., Gysel, M., John, A., Kirchstetter, T. W., Kuhlbusch, T. A. J., Laborde, M., Lack, D., Müller, T., Niessner, R., Petzold, A., Piazzalunga, A., Putaud, J. P., Schwarz, J., Sheridan, P., Subramanian, R., Swietlicki, E., Valli, G., Vecchi, R. and Viana, M.: Soot reference materials for instrument calibration and intercomparisons: a workshop summary with recommendations, Atmos. Meas. Tech., 5(8), 1869–1887, doi:10.5194/amt-5-1869-2012, 2012.

Laborde, M., Schnaiter, M., Linke, C., Saathoff, H., Naumann, K.-H., Möhler, O., Berlenz, S., Wagner, U., Taylor, J. W., Liu, D., Flynn, M., Allan, J. D., Coe, H., Heimerl, K., Dahlkötter, F., Weinzierl, B., Wollny, A. G., Zanatta, M., Cozic, J., Laj, P., Hitzenberger, R., Schwarz, J. P. and Gysel, M.: Single Particle Soot Photometer intercomparison at the AIDA chamber, Atmos. Meas. Tech., 5(12), 3077–3097, doi:10.5194/amt-5-3077-2012, 2012.

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ED: Publish subject to minor revisions (review by editor) (01 Jul 2019) by Paolo Laj

AR by Yunfei Wu on behalf of the Authors (11 Jul 2019) Author's response Manuscript

ED: Publish as is (23 Jul 2019) by Paolo Laj

AR by Yunfei Wu on behalf of the Authors (24 Jul 2019) Manuscript

Short summary

The morphology and effective density of externally mixed black carbon (extBC) aerosols were studied using a tandem technique coupling a DMA with a SP2. The study extended the mass–mobility relationship to large extBC with a mobility diameter larger than 350 nm, a size range seldom included in previous tandem measurements of BC aggregates. On this basis, quantities such as the mass–mobility scaling exponent were revealed for extBC in urban Beijing.