Atmospheric bromoform at Cape Point, South Africa: an initial fixed-point data set on the African continent

Kuyper, Brett; Palmer, Carl J.; Labuschagne, Casper; Reason, Chris J. C.

doi:https://doi.org/10.5194/acp-18-5785-2018

Articles | Volume 18, issue 8

https://doi.org/10.5194/acp-18-5785-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-18-5785-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 18, issue 8

Research article

|

25 Apr 2018

Research article |

| 25 Apr 2018

Atmospheric bromoform at Cape Point, South Africa: an initial fixed-point data set on the African continent

Brett Kuyper, Carl J. Palmer, Casper Labuschagne, and Chris J. C. Reason

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Final revised paper (published on 25 Apr 2018)
Supplement to the final revised paper
Preprint (discussion started on 16 May 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review', Anonymous Referee #1, 14 Jun 2017
- AC1: 'Author response to Reviewer 1', Brett Kuyper, 23 Aug 2017
RC2: 'Review of ACP 2017 44', Anonymous Referee #2, 07 Jul 2017
- AC2: 'Author response to Reviewer 2', Brett Kuyper, 23 Aug 2017

Peer-review completion

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

AR by Brett Kuyper on behalf of the Authors (20 Sep 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 Sep 2017) by Andreas Hofzumahaus

RR by Anonymous Referee #2 (15 Oct 2017)

Suggestions for revision or reasons for rejection

The revised manuscript of Kuyper et al. has addressed many of the issues raised in the reviews. However, there are still some points that need clarification or correction.

1) General comment. Be sure to clearly identify all components of tables and figures in the associated captions.

2) Significance of Cape Point site to tropical convection….The authors make a reasonable claim that it is important to understand the sources and budget of bromoform in the global and regional atmospheres. However, if they claim that the emissions near Cape Point are entrained in tropical convection, I’d appreciate some reference that shows the significance of this transport path from the S. African MBL to the tropical UTLS.

3) Calibrations. While the authors have expanded on their discussion of analytical protocols and calibration, I still remain confused on some basic operations. First, the authors describe using sample loops for calibrating the system, as is a common practice. However, they then also describe an error associated with timing of the standard addition to the system, e.g., 30 sec instead of 1 minute for standards. If the standards are injected via single or multiple fixed volume loop injections of the output of the permeation tube, how does timing make any difference? I am confused. Second, I do not understand the relationship between the calibration curve shown in the figure and the use of intermittent standards to track drift in system response. Presumably, response factors for bromoform are calculated regularly with runs of different amounts of standard. How these different standards (a choice of from 1 – 3 standards) are used is not at all clear. How is drift calculated, and how does that relate to the calibration curve, which I have to repeat, is not correct as shown (no matter if it was used by Wevill and Carpenter). Please refer to most any textbook on analytical chemistry. Based on the calibration curve shown, I find it hard to believe that the error bars represent the 7% uncertainty as claimed in the author response.

4) Diurnal variation..p13. The authors suggest that increased photolysis of bromoform during the day is a major factor in the observed diurnal variability. However, I think that photolysis would be only a minor factor given the approximately 3 week lifetime of bromoform, which suggests that perhaps 5%/day is lost by photolysis. Thus even a constant source of bromoform would show only a minor diurnal cycle due to photolysis. Variation in source location, emission, and mixing offer much more reasonable hypotheses for the observed changes. Unfortunately, the authors seem to want to test every known factor that can influence bromoform concentrations, but they don’t have sufficient data to adequately rule out or confirm any particular factor. This is, unfortunately, the weakness of the paper. Still, the data are useful and the discussion, while it could be improved, is not unreasonable.

5) Anthropogenic sources. It is clear from the Rn and CO measurements that the Cape Point site samples air with continental and anthropogenic influence. I think the authors still claim that anthropogenic sources can enhance the bromoform mixing ratios seen at Cape Point. I don’t believe that the authors make any credible case for this influence, and perhaps the entire sections on “events” might be removed since no clear conclusions can be found to explain the variation in the data. For example, the event on the 18th of October is suggested to be influenced by emissions from the nuclear power plant cooling waters. This is due to the trajectory analysis and the observation of the highest CHBr3 level seen. However, as I look at the data, I note that: 1) The maximum on the 18th is not so different from that on the 17th or during other periods of marine biogenic influence only. 2) The trajectories on the 17th are very different from some on the 18th, with concentrations very comparable. 3) The authors suggestion implies that emissions from the Koeberg reactor would produce atmospheric CHBr3 levels much higher than the high peaks observed from marine only influence. It would be helpful to have some reasonable calculations, at least, to show that this might be possible.4) Based on the time series, it seems that the Rn/CO peak actually precedes the CHBr3 peak.

6) Additional comments…1) While kelp are known sources of bromoform, there may certainly be other sources near coastlines. I say this from observations near my institution which contain very high levels of dissolved bromoform and high atmospheric levels (around 15 to 25 pptv) with no evidence of kelp in the area. We haven’t yet investigated the specific sources yet. 2) If the authors continue with this work, it would be helpful to their analyses of different sources if they could measure a wider range of trace gases from their samples. The technology to do this sort of analysis is well established.

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RR by Anonymous Referee #1 (23 Oct 2017)

ED: Reconsider after major revisions (26 Oct 2017) by Andreas Hofzumahaus

AR by Brett Kuyper on behalf of the Authors (07 Dec 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Dec 2017) by Andreas Hofzumahaus

RR by Anonymous Referee #2 (23 Dec 2017)

Suggestions for revision or reasons for rejection

The manuscript by Kuyper et al. is greatly improved in its organization and presentation of the results. However, I have found some issues that I think need additional clarification. These are related to the analytical methods and calibration. I thank the authors for providing an actual spreadsheet file of their standards and air analysis. This definitely helped me understand the procedure much better, but also brought up some new issues. In my view, though the authors show a good fit to the bromoform peak area vs mixing ratio calibration, the random errors are very large. Furthermore, there are a few statements in the description that are not accurate. First, a linear regression is not a measure of accuracy, so the claim of 99.6% accuracy is not appropriate. In terms of regression analysis, I put the authors’ data, not including the measurements from August, into a statistical program (SYSTAT) and found an R^2 of 0.86 and a regression line (similar to the authors’) of Area = 129 (-+168) + 227 (-+7.5) * Std Mixing Ratio, where the -+ is the standard error of the estimate. The regression analysis was used by the authors to calculate all of the air measurements. The authors apparently did not use the standards interspersed with the samples to calculate any drift as claimed in the manuscript. The same area/mixing ratio relationship was used for all calculations. If the standard responses were used for drift calculation, then the calculated data would be much different. One point regarding this method is that a zero area produces a mixing ratio of 0.83 pptv. It may be worth pointing that out.

There are a number of troubling aspects to the calibration that suggest problems with their technique. The variability of each standard level is remarkably high, with ranges over a factor of 2 or more in the response factor at each level. Another issue is the variability of the blank. As indicated on the spreadsheet about 25% of the blanks produced measurable peak areas, and these range from 1.36 to 8.85 pptv bromoform. Clearly there was some issue with carryover or some other problem that needs to be examined. The next major issue is that the response variability at the 21 ppt level (near the mean ambient concentration) is about 55% for one standard deviation. Since the authors have no data on replicate ambient analysis, one must assume that the variability of the ambient data is also in this range. An overall variation in the response factor considering all standards is 43% (1 SD), still remarkably high, and this variability must be considered for the ambient samples. Or, if the authors think the analytical system for ambient samples somehow has a better precision than that demonstrated by the loop injections of the standards, there should be some explanation of why. Or they should assume that the response variation in the standard injections reflects actual response variations of the detector and correct the ambient data for that response change. For the future, my suggestion would be to work with one or more secondary ambient air samples that are calibrated against a primary standard. (And to determine the source of the variability in the loop injections!).

The authors note that the drift was measured from day to day using the 1 – 3 loop injections. However, this day to day “drift”, sometimes calculated from one standard only and sometimes from multiple standards, varies over a factor of 3. Apparently, though this daily “drift” was not taken into account in the calculations. This should be discussed in the methods section.

Another discrepancy, based on the spreadsheet information, is the frequency of the standard injections. The manuscript describes standard injections every five samples. However, this does not seem to be the case for each day. Some days have no standard injections, others multiple, and others with 7 – 8 samples run with no standard injection. Typically, no standard injection ended the daily runs. Please correct this description of standardization frequency in the text.

Finally, I am curious about data that appeared in the spreadsheet, but was not included in the manuscript. The spreadsheet shows several values of bromoform > 60 ppt that are not shown in the manuscript, and apparently were also not included in calculating sample average. It is not uncommon to have “flyers” sometimes in the data, but it would be helpful to mention if some data are excluded.

I thank the authors again for sharing their raw data with the reviewers. Based on this data, I request a more thorough and accurate description of the analytical procedures, especially related to calibration and calculation, and how the variability observed impacts the uncertainties associated with the reported measurements. At this stage, I can’t be sure that the lack of drift correction hasn’t masked features in the data, and I am also unsure about the absolute calibration, which makes it difficult to compare this data with others. If these measurements are to be published, there needs to a much clearer description of the various uncertainties and limitations.

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ED: Reconsider after major revisions (08 Jan 2018) by Andreas Hofzumahaus

AR by Brett Kuyper on behalf of the Authors (26 Feb 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (28 Feb 2018) by Andreas Hofzumahaus

RR by Anonymous Referee #2 (16 Mar 2018)

ED: Publish subject to minor revisions (review by editor) (19 Mar 2018) by Andreas Hofzumahaus

AR by Brett Kuyper on behalf of the Authors (23 Mar 2018) Author's response Manuscript

ED: Publish as is (03 Apr 2018) by Andreas Hofzumahaus

AR by Brett Kuyper on behalf of the Authors (12 Apr 2018)

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Short summary

Atmospheric measurements of a single biogenic compound, bromoform, released from phytoplankton and kelp were made at Cape Point, South Africa. These measurements are the first long-term, fixed-point measurements in southern Africa. This compound is the largest transport of bromine to the atmosphere, and this plays an important role in climate change. The short time series presented here shows large quantities of bromoform in the atmosphere.