Low-NO atmospheric oxidation pathways in a polluted megacity

Newland, Mike J.; Bryant, Daniel J.; Dunmore, Rachel E.; Bannan, Thomas J.; Acton, W. Joe F.; Langford, Ben; Hopkins, James R.; Squires, Freya A.; Dixon, William; Drysdale, William S.; Ivatt, Peter D.; Evans, Mathew J.; Edwards, Peter M.; Whalley, Lisa K.; Heard, Dwayne E.; Slater, Eloise J.; Woodward-Massey, Robert; Ye, Chunxiang; Mehra, Archit; Worrall, Stephen D.; Bacak, Asan; Coe, Hugh; Percival, Carl J.; Hewitt, C. Nicholas; Lee, James D.; Cui, Tianqu; Surratt, Jason D.; Wang, Xinming; Lewis, Alastair C.; Rickard, Andrew R.; Hamilton, Jacqueline F.

doi:https://doi.org/10.5194/acp-21-1613-2021

Articles | Volume 21, issue 3

https://doi.org/10.5194/acp-21-1613-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

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https://doi.org/10.5194/acp-21-1613-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 21, issue 3

Research article

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08 Feb 2021

Research article | Highlight paper |

| 08 Feb 2021

Low-NO atmospheric oxidation pathways in a polluted megacity

Mike J. Newland, Daniel J. Bryant, Rachel E. Dunmore, Thomas J. Bannan, W. Joe F. Acton, Ben Langford, James R. Hopkins, Freya A. Squires, William Dixon, William S. Drysdale, Peter D. Ivatt, Mathew J. Evans, Peter M. Edwards, Lisa K. Whalley, Dwayne E. Heard, Eloise J. Slater, Robert Woodward-Massey, Chunxiang Ye, Archit Mehra, Stephen D. Worrall, Asan Bacak, Hugh Coe, Carl J. Percival, C. Nicholas Hewitt, James D. Lee, Tianqu Cui, Jason D. Surratt, Xinming Wang, Alastair C. Lewis, Andrew R. Rickard, and Jacqueline F. Hamilton

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Final revised paper (published on 08 Feb 2021)
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Preprint (discussion started on 13 Feb 2020)
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Status: closed

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

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RC1: 'Review1', Anonymous Referee #2, 12 Mar 2020
RC2: 'review comment', Anonymous Referee #1, 26 Mar 2020
AC1: 'Response to Reviewers of: Rainforest-like Atmospheric Chemistry in a Polluted Megacity by Newland et al., 2020, submitted to ACP', Mike Newland, 22 May 2020

Peer-review completion

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

AR by Mike Newland on behalf of the Authors (01 Jun 2020) Author's response Manuscript

ED: Reconsider after major revisions (23 Jun 2020) by Frank Keutsch

ED: Referee Nomination & Report Request started (24 Jun 2020) by Frank Keutsch

RR by Anonymous Referee #2 (08 Jul 2020)

RR by Anonymous Referee #1 (20 Jul 2020)

Suggestions for revision or reasons for rejection

The aim of the revised manuscript by Newland et al. is now tightly focused on the key, high impact result:
The authors use isoprene oxidation products as photochemical markers of the changing chemical pathways throughout the daytime and demonstrate that the model used to capture this diurnal change fail.

Figure 4 shows that measured NO is high in the morning and low in the afternoon.
It is not surprising that isoprene oxidation forms typically high NO products in the morning and low NO products in the afternoon.

At low NO, RO2 radicals especially ISOPOO are reacting with HO2 and RO2 rather than with NO. In Beijing the complex VOC composition leads to a complex mixture of RO2 radicals. The reaction between these RO2 radicals might be the key to get a better match with measurements when included in models.

The authors explicitly deny to describe the VOC composition in Beijing, talk about the role of isoprene chemistry in Beijing, or talk about ozone production in Beijing.

Therefore I question the key, high impact of the result presented here.
As explained in the next paragraph measured PTR-MS signal at 71.05 m/z cannot be assigned quantitatively to MVK+MACR. The new Figure S5, thought to support MVK+MACR assignment of signal at 71.05 m/z, is misinterpreted.

Fig. S5 demonstrates that there is agreement between the PTR-MS signal at 71.05 m/z measured in the morning from 7:00 to 11:00 comparing different inlet lines. But there is a striking difference in the afternoon. From 16:00 to 18:00 both lines taking air at 102 m show 0.8 ppb (flux line, PFA, transport time 68 s) vs. < 0.6 ppb (gradient line, sample being drawn into stainless steel containers).
In the morning when high NO isoprene chemistry produces prevailingly MVK + MACR that is detected at 71.05 m/z the agreement between the two inlet lines from the same intake point is reasonable.
In the afternoon under low NO, MVK, MACR, ISOPOOH and IEPOX is formed contributing with different sensitivities to the 71.05 m/z PTR signal.
All PTR-MS data and discussion termed MVK+MACR have to be taken out from the manuscript.

587 The PTR-TOF-MS signal at 71.05 m/z is not MVK+MACR. Fig. S5 demonstrates that different inlet lines lead to strongly different signal intensities erroneously termed MVK+MACR.
590 1,2-ISOPOOH, which is mainly produced in isoprene OH reactions converts at stainless steel surfaces to MVK.

Figure 2f: The assignment of the signal at 71.05 m/z as MVK+MACR is not correct.
Figure 2f has to be taken out.

The new title
“Rainforest-like Atmospheric Oxidation Pathways in a Polluted Megacity”
is misleading because:
The key result here is connected with changing chemical pathways throughout the daytime due to NO availability.
There is not such a NO change in the rainforest!
The VOC composition and reactivity is also very much different in the rainforest.

Therefore I suggest to take out the word “rainforest” from the title, which is misleading.

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ED: Publish subject to minor revisions (review by editor) (26 Sep 2020) by Frank Keutsch

Dear authors,

Thank you for your detailed responses to the reviewer comments. The manuscript is greatly improved. In addition to the new comments of the reviewers I would like to add some additional comments:

1. Like reviewer 2, I am not convinced that these are "rainforest-like" conditions. The work describes a missing sink of NO that does not appear to produce ozone (similar to the Hofzumahaus publication referenced in the manuscript and as you point out). This means that the low NO has an unknown cause. This unknown chemistry puts in question whether the chemical regime is the same as in rainforests, or are the authors implying that the same unknown chemistry to keep NO low is active there. Figure 3 also is not very convincing for the conditions in Beijing from this work to be similar to those of Borneo and the rural SE-USA, although they are clearly different than London or NY. I think a clarification would be useful.

2. Unless I missed it, the manuscript does not comment on uncertainty or variability of measurements, with the exception of figure 4. Therefore, it is not possible to draw conclusions on whether there are statistically significant differences. Calibrations are only mentioned very briefly. For example, in figure S5 that one of the reviewer 2 comments one, it is not clear whether there are statistically significant differences or not. Uncertainty and variability should be considered in comparisons.

3. Similarly, the abstract has the statement "significant formation of gas and aerosol phase oxidation products associated with the low-NO ‘rainforest like’ regime". The word significance should be replaced with a quantitative number as it is not clear what significant means here. Are the amounts of ISOPOOH and IEPOX that are observed significant and what are they significant for? Similarly, are 20 ng/m3 of 2-MT-OS significant? The manuscript does not discuss how much overall SOA was observed and how much of it was from low-NO pathways, so the statement that there is significant formation of aerosol phase oxidation products is hard to evaluate.

4. According to Nguyen et al. (PCCP 17, 17914-17926, 2015) MPAN produces SOA via oxidation by OH to form HMML without any participation NO or HO2, so it would be useful if the authors could explain how SOA from MPAN indicates low NO conditions.

5. Lastly, it would be useful to comment on HO2 measurements, which LIF FAGE instruments often measure, as a comparison could help evaluate the fNO number as it in large part depends on competition between NO and HO2.

The results themselves are clearly interesting and important, but I encourage the authors to address these comments.

Best Regards,

Frank Keutsch

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AR by Mike Newland on behalf of the Authors (26 Oct 2020) Author's response Manuscript

ED: Publish subject to technical corrections (30 Nov 2020) by Frank Keutsch

ED: Publish subject to technical corrections (01 Dec 2020) by Ulrich Pöschl (Executive editor)

AR by Mike Newland on behalf of the Authors (07 Dec 2020) Author's response Manuscript

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Short summary

We report the formation of secondary pollutants in the urban megacity of Beijing that are typically associated with remote regions such as rainforests. This is caused by extremely low levels of nitric oxide (NO), typically expected to be high in urban areas, observed in the afternoon. This work has significant implications for how we understand atmospheric chemistry in the urban environment and thus for how to implement effective policies to improve urban air quality.