Contrasting physical properties of black carbon in urban Beijing between winter and summer

Liu, Dantong; Joshi, Rutambhara; Wang, Junfeng; Yu, Chenjie; Allan, James D.; Coe, Hugh; Flynn, Michael J.; Xie, Conghui; Lee, James; Squires, Freya; Kotthaus, Simone; Grimmond, Sue; Ge, Xinlei; Sun, Yele; Fu, Pingqing

doi:https://doi.org/10.5194/acp-19-6749-2019

Articles | Volume 19, issue 10

https://doi.org/10.5194/acp-19-6749-2019

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

Special issue:

In-depth study of air pollution sources and processes within...

https://doi.org/10.5194/acp-19-6749-2019

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

Articles | Volume 19, issue 10

Research article

|

21 May 2019

Research article |

| 21 May 2019

Contrasting physical properties of black carbon in urban Beijing between winter and summer

Dantong Liu, Rutambhara Joshi, Junfeng Wang, Chenjie Yu, James D. Allan, Hugh Coe, Michael J. Flynn, Conghui Xie, James Lee, Freya Squires, Simone Kotthaus, Sue Grimmond, Xinlei Ge, Yele Sun, and Pingqing Fu

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Final revised paper (published on 21 May 2019)
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Preprint (discussion started on 15 Nov 2018)
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Interactive discussion

Status: closed

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

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RC1: 'Review of ACP-2018-1142', Anonymous Referee #1, 10 Dec 2018
RC2: 'Review of Liu et al.', Anonymous Referee #2, 18 Dec 2018
AC1: 'comments reply', Dantong Liu, 20 Feb 2019

Peer-review completion

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

AR by Dantong Liu on behalf of the Authors (20 Feb 2019) Author's response Manuscript

ED: Reconsider after major revisions (05 Mar 2019) by Frank Keutsch

AR by Dantong Liu on behalf of the Authors (06 Mar 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Mar 2019) by Frank Keutsch

RR by Anonymous Referee #1 (14 Mar 2019)

RR by Anonymous Referee #2 (09 Apr 2019)

Suggestions for revision or reasons for rejection

The authors have done a generally good job responding to the reviewer comments. I do still find that the paper could be easier to follow, but as the other reviewer did not seem to have the same difficulty I do not wish to belabour my suggestion about organizational structure. I would ask the authors to address the below additional concerns.

L320: I guess I don’t see the need to state “While none of our other analyses indicate the presence of such a source…”. I actually don’t think it’s consistent with the rest of the paper. The authors discuss explicitly that there are likely increased emissions in the winter when concentrations are high. This undoubtedly results in a shift in the relative contributions from different source types (e.g. vehicle versus home heating versus energy production, etc.). This is evident in the supplementary figure S1. The residential is substantially larger in winter. There is absolutely a shift in sources between summer and winter. And within a season there is absolutely a dependence on which regions contribute with BC concentration (Fig. 5), and the relative balance of sources differs between regions. These issues are even discussed in the previous section of the paper. I suggest this is removed.

Fig. S2: While I find the revised Fig. S2 improved, it is still evident that in many cases the distributions are not log normal, in contrast to the statements in the manuscript. There are not three “log normal modes”. The sm+lg,uncoat distribution itself has at least two modes in nearly every case. Is there a need to argue for log normality? Or am I missing something?

Previous Reviewer Comment: OOA: Per the complementary Wang et al. paper, the OOA2/BC ratio is notably larger than the OOA1/BC ratio. Yet the OOA2_BC factor is more correlated with “moderately” coated BC while the OOA1_BC is more correlated with “thickly” coated BC. Can these be reconciled?

Clarified comment: The authors indicate they are unsure what I am asking for with this comment. Basically, I am saying that if OOA1/BC is larger than OOA2/BC, then why does “thickly” coated BC correlate well with OOA2_BC while the “moderately” coated BC correlates with OOA1_BC. Wouldn’t one expect the more “thickly” coated BC to correlate better with the factor where the coating/BC is larger?

L492: The comma after “material” should be a period.

L494: How do high concentrations of precursors “exacerbate” coagulation? What matters is not the precursors, but the particles. Are the authors trying to say that there may be more particles when the precursor concentrations are high? If this is the case they should just say this. Also, implicit in this is the idea that this will lead to nucleation and formation of new particles. Without nucleation, the precursors will form new secondary material on particles, but not influence the particle number concentration, which is what matters for coagulation. They might also consider including data (if they have it) or a reference indicating that higher mass concentrations are also correlated with higher number concentrations in the region. It is not a given that they will (although I suspect this is the case).

L504: I have some difficulty understanding the argument that the different BC types have differing contributions due to their varying absorption efficiency alone, which is what is implied as stated. I think the authors should also indicate that variations in absolute contributions of these types will also matter and, likely, dominate the behavior here.

L507: It should be noted that there is no strong experimental evidence (or at least none that I’m aware of) that the absorptivity of atmospheric BC falls off at large sizes. This is something that happens in Mie theory, to be sure, but it’s not clear that this actually occurs, or if it does that it occurs to nearly the same extent as Mie theory predicts. (This has to do with atmospheric BC not actually being spherical.)

L505: It should be stated explicitly here that these are calculations, and not measurements, of the MAC. Also, given the shift to 870 nm in Fig. 13 (the wavelength of the measurements), it is not clear why the authors have retained using 550 nm here. They should also state explicitly that they assume non-absorbing coatings. I realize these points are made in the “methods” section but they should be repeated here.

L511: The authors should state these not as ~, but with the mean and a meaningful metric such as the standard deviation.

L514: An estimated uncertainty for the 1.4 and 1.6 should be provided. As noted previously, the calculated MAC values may be low, since it is known that Mie theory often underestimates MAC values relative to measurements. Consequently, the Eabs values here, determined as a ratio between a measurement and a calculation, have a decent chance of being biased high.

L524: It is not “particularly” when the coating is absorbing. It is only when the coating is absorbing.

Table A1 is a welcome addition, but missing many of the abbreviations used throughout.

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ED: Publish subject to minor revisions (review by editor) (22 Apr 2019) by Frank Keutsch

AR by Dantong Liu on behalf of the Authors (29 Apr 2019)

ED: Publish as is (02 May 2019) by Frank Keutsch

AR by Dantong Liu on behalf of the Authors (02 May 2019)

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Short summary

This study provides source attribution and characterization of BC in the Beijing urban environment in both winter and summer. For the first time, the physically and chemically based source apportionments are compared to evaluate the primary source contribution and secondary processing of BC-containing particles. A method is proposed to isolate the BC from the transportation sector and coal combustion sources.