Net ozone production and its relationship to nitrogen oxides and volatile organic compounds in the marine boundary layer around the Arabian Peninsula

Tadic, Ivan; Crowley, John N.; Dienhart, Dirk; Eger, Philipp; Harder, Hartwig; Hottmann, Bettina; Martinez, Monica; Parchatka, Uwe; Paris, Jean-Daniel; Pozzer, Andrea; Rohloff, Roland; Schuladen, Jan; Shenolikar, Justin; Tauer, Sebastian; Lelieveld, Jos; Fischer, Horst

doi:https://doi.org/10.5194/acp-20-6769-2020

Articles | Volume 20, issue 11

Atmos. Chem. Phys., 20, 6769–6787, 2020

https://doi.org/10.5194/acp-20-6769-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Atmos. Chem. Phys., 20, 6769–6787, 2020

https://doi.org/10.5194/acp-20-6769-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 11

Research article 09 Jun 2020

Research article | 09 Jun 2020

Net ozone production and its relationship to nitrogen oxides and volatile organic compounds in the marine boundary layer around the Arabian Peninsula

Ivan Tadic et al.

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Final revised paper (published on 09 Jun 2020)
Supplement to the final revised paper
Preprint (discussion started on 03 Dec 2019)
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', Anonymous Referee #1, 30 Jan 2020
- AC1: 'Reply to RC1', Ivan Tadic, 05 Mar 2020
RC2: 'Referee report', Anonymous Referee #2, 14 Feb 2020
- AC2: 'Reply to RC2', Ivan Tadic, 05 Mar 2020

Peer-review completion

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

AR by Ivan Tadic on behalf of the Authors (06 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Mar 2020) by Andreas Hofzumahaus

RR by Anonymous Referee #2 (30 Mar 2020)

Suggestions for revision or reasons for rejection

Review of Tadic et al revision of “Net ozone production and its relationship to NOx and VOCs in the marine boundary layer around the Arabian Peninsula” for ACP

Many of my comments concern uncertainty estimates. The authors add errors in quadrature, as appropriate for determining the variance of a quantity that depends on several variables. However, it is not the variance that is used in the paper; derived variables are presented with relative uncertainties as a percent of the variable value.

I will use the error analysis for NO2 as an example. I have written this text with the expectation that Greek characters, partial derivative symbols, and exponents will not survive cut and paste: d’s are partial derivative, ^2 means squared, and sigma is written out. The same analysis applies to other quantities:

x = f(u, v, ...) (1)

If errors are normally distributed,

sigma(x)^2 = sigma(u)^2 (dx/du)^2 + sigma(v)^2 (dx/dv)^2 + ... (2)

NO and NO2 are determined from the chemiluminescent reaction NO+O3 -> NO2; NO is measured directly; in a second channel, called NOc, a light source converts a fraction K of NO2 into NO. As noted in the response the concentration of NO2 is given by

NO2 = (NOc-NO)/K (3)

The relative uncertainties of NO. NOc, and K are given as 6%, 6% and 3% respectively. Applying Eq 2 to NO2, gives

sigma(NO2)^2 = sigma(NOc)^2 (dNO2/dNOc)^2 + sigma(NO)^2 (dNO2/dNO)^2 + sigma(K)^2 (dNO2/dK)^2

Evaluating the partial derivatives, gives

sigma(NO2)^2 = sigma(NOc)^2 *(1/K)^2 + sigma(NO)^2 *(1/K)^2 + sigma(K)^2 *((NOc-NO)/K)^2

Define the relative uncertainty in x as R(x) = sigma(x)/x. Then,

R(NO2)^2 = R(NOc)^2 *(NOc/NO2)^2 *(1/K)^2 + R(NO)^2 *(NO/NO2)^2 * (1/K)^2 + R(K)^2 * (K/NO2)^2 * ((NOc-NO)/K^2)^2

The uncertainty of NO2 depends on the concentration of NO and NO2. As an example, NO2 = 4ppb, NO = 1 ppb, giving NOc = 2.2 ppb. For convenience, I have rounded K to 0.3. With these concentrations, the above formula based on adding errors in quadrature gives R(NO2) = 12.4%. Equation (2), properly applied can give uncertainties that are greater or less than that obtained by the addition-in-quadrature formulas of the revised paper. In this case, the uncertainty is greater than given in the paper. I would hope that the input uncertainties, R(NO), R(NOc), and R(K) were determined as in Eqs 1-2, rather than by the formulas in the paper, or worse still by manufacturers recommendations.

Similar considerations apply to the formulas in the manuscript for Delta(HO2), Delta(RO2), and Delta(NOPR).

Regarding the measurement of HO2, my understanding is that HO2 is converted to OH and OH is measured by LIF. The point that I was trying to make is that if the uncertainty in OH is 20% (independent of concentration), then the uncertainty of HO2 has to be greater, as there are uncertainties associated with the conversion.

Comment 7. Your response was confusing. NOPR is expected to vary between noon and noon – 3 hours. In order to calculate NOPR, jNO2, O3, NO, and NO2 should be reasonably constant over a 5-minute period. One should look at the change in jNO2 over 5 minutes relative to its mean value over that 5-minute period (not relative to its mean value at noon).

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RR by Anonymous Referee #1 (31 Mar 2020)

Suggestions for revision or reasons for rejection

The authors have responded adequately to most of my concerns. However, there are still some issues which have not been properly addressed. I list my initial concern for those sections which the authors have not responded properly and then include my comment [New remark] to the authors' reply:

Page 12, Eq. 7 and Fig 9:
It would be nice to see a break-down of the different terms in Eq. 7 for different legs as shown in Fig. 9 to evaluate what processes might be most relevant/different in those different legs.
[New remark] I appreciate the new figures S10-S13. It would be good to include some discussion associated with these figures, as at times some notable deviation of "Estimated based on measured data" from "Estimated based on simulated data" occurs (e.g. in S12 and S13).

Page 12, L276-277:
The loss mechanism through H2O is important. Also, it seems to vary a lot. Some parts of the ship cruising legs might have already been exposed high humidity due to the Indian monsoon system. It would be good to see the absolute humidity variation along the legs similar to Figs 3 and S4.
[New remark] I appreciate the new figures S5, but I am missing some discussion related to S5.

Page 13, L310:
Authors mention NOx values of several hundred ppbs. Where do they show up in Figs. 3, S2, and S3? What were the megacities along the cruising legs? I could think about Cairo, but according to Fig 3 NOx values do not show extremely high values.
[New remark] I did not see changes in the text. (1) The authors still mention several hundred of ppbv NOx, although in their response they say they have removed those from the final data set, as contamination from the ship exhaust could not be excluded. (2) I am not sure what Megacity definition the authors are thinking about, but I am used to a definition of a Megacity as having at least 10 million inhabitants. I assume this only applies to Cairo. Keeping these statements the way the authors wrote is misleading.

Page 16, L334-355:
This section should include some more explanations: it seems there is a huge variation in NOx and O3 in AG (also a huge variation in NOPR as shown in Fig 9). What is the major driver of this: point sources from ships? Why are the highest NOx values in OG and why are some of the lowest O3 values found in OG? Why would you consider air masses over the Mediterranean as photochemically aged air masses due to the small whisker-interval, while the whisker-plots for AS and OG show pretty much the same with, but at much lower absolute O3 ranges. There are no emission sources in that area of the Mediterranean?
[New remark] Still, it is not clear, why there are highest NOx values in OG. It should be spelled out explicitly what sources those might have been, even if this information might have already been given in other papers. Here it is critical to mention/repeat this information, as it obviously has a major impact on O3.

Page 25, L516-517:
Actually, Figure 10 shows that in almost all areas O3 formation is NOx limited. However, the authors say that this is typical for photochemically aged air masses over the Mediterranean. As already mentioned further above, why do the authors explicitly consider the Mediterranean area having aged air masses? It is even more surprising as the results for the Mediterranean area in Figure 10 indicate that the Box-Whisker plot stretches into the transition between NOx and VOC limitation.
[New remark] I think these lengthy discussions about local differences of NOx-VOC limitations vs conditions of the larger Mediterranean area do not provide new insights. It pretty much resembles studies at any other location, i.e. the closer to a fossil fuel combustion emission source the fresher and least photochemically processed the pollution plume is. Here, it is about ship point sources. So what?

Page 25. L517-519:
Why would higher NOx lead to higher O3 pollution? For instance, according to Figure 4, OG has the highest NOx values, but also pretty low O3 values. With regard to NOPR, the Box-Whisker plot for OG shows positive, but also large negative values. In any case NOPR values are significantly lower than for AG, for instance.
[New remark] This somehow ties into the above question about where the high NOx in OG comes from. According to the authors it is from "increased shipping in the Arabian Gulf". A few follow-up questions: Why is shipping "increased" in the Arabian Gulf (is it higher than during any other time)? Why is there a further increase of NOx in OG? Are there more ships than in the Arabian Gulf? I think it should be the same number of ships, assuming that (1) most of them are oil-tankers and (2) the number of those ships entering and leaving the Strait of Hormuz would be the same.

Figure 7:
The legend mentions "Measurements", the figure captions says "estimated". From Eq 3 I understand that RO2 was neither measured nor estimated, but calculated. Also, what would be the interpretation of the negative RO2 concentrations (blue Box-Whisker plots) when calculated from Eq 3?
[New remark] Still, it is not clear. Why is it "estimated", when it is either measured or simulated?

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ED: Reconsider after major revisions (01 Apr 2020) by Andreas Hofzumahaus

AR by Ivan Tadic on behalf of the Authors (06 Apr 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (20 Apr 2020) by Andreas Hofzumahaus

AR by Ivan Tadic on behalf of the Authors (22 Apr 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (27 Apr 2020) by Andreas Hofzumahaus

AR by Ivan Tadic on behalf of the Authors (27 Apr 2020) Author's response Manuscript

ED: Publish as is (11 May 2020) by Andreas Hofzumahaus

Short summary

We present shipborne observations of NO, NO₂, O₃, HCHO, OH, HO₂, H₂O and the actinic flux obtained in the marine boundary layer (MBL) around the Arabian Peninsula during the summer 2017 AQABA ship campaign. NO_x (NO+NO₂) and O₃ observations clearly showed anthropogenic influence in the MBL around the Arabian Peninsula. The observations were also used to calculate net O₃ production in the MBL around the Arabian Peninsula, which was greatest over the northern Red Sea, Oman Gulf and Arabian Gulf.