Characterizing the evolution of physical properties and mixing state of black carbon particles: from near a major highway to the broader urban plume in Los Angeles

Krasowsky, Trevor S.; McMeeking, Gavin R.; Sioutas, Constantinos; Ban-Weiss, George

doi:https://doi.org/10.5194/acp-18-11991-2018

Articles | Volume 18, issue 16

https://doi.org/10.5194/acp-18-11991-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-18-11991-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 16

Research article

|

22 Aug 2018

Research article |

| 22 Aug 2018

Characterizing the evolution of physical properties and mixing state of black carbon particles: from near a major highway to the broader urban plume in Los Angeles

Trevor S. Krasowsky, Gavin R. McMeeking, Constantinos Sioutas, and George Ban-Weiss

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Final revised paper (published on 22 Aug 2018)
Supplement to the final revised paper
Preprint (discussion started on 09 Nov 2017)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Figures', Anonymous Referee #1, 13 Nov 2017
- SC1: 'Response to Anonymous Referee #1', George Ban-Weiss, 13 Nov 2017
- SC2: 'Figures', George Ban-Weiss, 20 Nov 2017
RC2: 'poor experimental setup', Anonymous Referee #2, 17 Feb 2018
- AC1: 'reply to reviewer #2', George Ban-Weiss, 22 Feb 2018
RC3: 'Referee Comment', Anonymous Referee #3, 18 Feb 2018
- SC3: 'reviewer #3', George Ban-Weiss, 22 Feb 2018

Peer-review completion

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

AR by George Ban-Weiss on behalf of the Authors (05 Apr 2018) Manuscript

ED: Referee Nomination & Report Request started (09 Apr 2018) by Radovan Krejci

RR by Anonymous Referee #1 (20 Apr 2018)

Suggestions for revision or reasons for rejection

The authors have done a generally good job of responding to all reviewer comments. There are a few minor lingering issues that should be dealt with prior to publication.

Figure 2: It should be noted that the 7th order polynomial is completely arbitrary and without physical meaning. I could just as easily draw a line by hand. Really, this is just a guide for the eye as clearly a different polynomial would yield a different curve. The authors should consider adding binned values (mean/median or even better, box and whisker) to better illustrate the actual behavior, rather than relying on an arbitrarily chosen function alone.

Figure 3 and Section 3.1.2: It is evident that a single log-normal fit is insufficient, especially at 30 m. This should be noted. It should also be clarified whether the fit curves shown in the two graphs are the same (i.e. either the mass or number distribution was fit, and the fit used to calculate the other) or whether they were fit independently. If the fits are independent, some additional details regarding how the peak of the number distribution was constrained during fitting, if at all.

Figure 8 and Figure 9: Please indicate the number of data points comprising each bin.

Figure 11: I am skeptical that once this figure is made to fit into the ACP format that it will be readable.

The authors have added the sentence:

“We note that the trends reported in Figure 1 can have important implications on both human
exposures to BC downwind of highways, and airway deposition probabilities for BC, which are
expected to depend on particle hygroscopicity and thus coatings.”

I find this needs a bit more context. What type of implications? And, do the authors really think that a shift from 0.03 to 0.05 in the “thickly coated fraction” is meaningful in terms of having an impact? How important would this be relative to the decline in concentration? A little more context would be welcome.

Until section 3.4, the authors use fractions not percents. It is recommended that they use consistent notation.

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RR by Anonymous Referee #3 (21 Apr 2018)

RR by Anonymous Referee #2 (02 May 2018)

ED: Reconsider after major revisions (07 May 2018) by Radovan Krejci

AR by George Ban-Weiss on behalf of the Authors (30 May 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 Jun 2018) by Radovan Krejci

RR by Anonymous Referee #4 (27 Jun 2018)

Suggestions for revision or reasons for rejection

I write this as a reviewer who is late into the process. This paper presents observations of changes in BC mixing state as measured by an SP2 at varying distances from pollution source and uses a photochemical age metric to try to contextualise the changes. A weekend effect is observed, which is a good indicator that diesel emissions are the dominant source.

While the observations are certainly interesting, the authors seem to be interpreting the data primarily from the perspective that it is photochemical processing that is mainly responsible for the changes. I regard this as fundamentally unsafe; other processes will almost certainly be at work, which will include mixing with background air and coagulation. While the authors do mention these, I would contend that they do not go far enough because the observations presented here are incapable of discriminating which of the process is responsible for the changes. It should be noted that while correlation with a NOx based photochemical age metric is reported, diesel is also a major source of NOx as well as BC, so mixing alone will cause a suppression of this metric close to source. Similarly, because coagulation progresses with time and is most significant for the smallest (uncoated) particles, this too will exhibit a positive correlation with airmass age. As such, any relationship here does not favour one process over another.

All this said, the observations are still interesting and add to previous measurements conducted in this region, in particular as part of CalNex. I would recommend that this paper be published but restructured (in terms of objectives, conclusions and general interpretations) as a phenomenological paper rather than observations of photochemical ageing. Possible mechanisms for the evolution of the particle population can certainly be discussed, but without any good evidence to support one hypothesis over another, this should be done in a strictly speculative manner.

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ED: Reconsider after major revisions (28 Jun 2018) by Radovan Krejci

AR by George Ban-Weiss on behalf of the Authors (09 Jul 2018) Author's response Manuscript

ED: Publish as is (13 Jul 2018) by Radovan Krejci

AR by George Ban-Weiss on behalf of the Authors (23 Jul 2018) Manuscript

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Short summary

Black carbon (BC) particles can have deleterious health consequences and impact regional and global climate. We aim to observe how processing near highways and longer timescale aging in an urban plume affect BC physical properties including coatings. Measurements in distinct regions of the Los Angeles Basin indicate that health-relevant physical properties can change within 100 m of the freeway. Climate-relevant properties downwind of Los Angeles depend on day of week and overall meteorology.