Long-term total OH reactivity measurements in a boreal forest

Praplan, Arnaud P.; Tykkä, Toni; Chen, Dean; Boy, Michael; Taipale, Ditte; Vakkari, Ville; Zhou, Putian; Petäjä, Tuukka; Hellén, Heidi

doi:https://doi.org/10.5194/acp-19-14431-2019

Articles | Volume 19, issue 23

https://doi.org/10.5194/acp-19-14431-2019

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

https://doi.org/10.5194/acp-19-14431-2019

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

Articles | Volume 19, issue 23

Research article

|

29 Nov 2019

Research article |

| 29 Nov 2019

Long-term total OH reactivity measurements in a boreal forest

Arnaud P. Praplan, Toni Tykkä, Dean Chen, Michael Boy, Ditte Taipale, Ville Vakkari, Putian Zhou, Tuukka Petäjä, and Heidi Hellén

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Final revised paper (published on 29 Nov 2019)
Preprint (discussion started on 20 Feb 2019)

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, 18 Mar 2019
RC2: 'Review', Anonymous Referee #2, 29 Apr 2019
AC1: 'Answers to the referees', Arnaud P. Praplan, 07 Jun 2019

Peer-review completion

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

AR by Arnaud P. Praplan on behalf of the Authors (07 Jun 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (18 Jun 2019) by Yugo Kanaya

RR by Anonymous Referee #2 (27 Jun 2019)

Suggestions for revision or reasons for rejection

I acknowledge the efforts put in by the authors in revising the paper. However I still have very major concerns that have arisen with some of the revised plots made in the new version to address concerns raised in the original review. These concerns cast serious doubt on the quality and comparability of the OH reactivity measurements during the different seasons.
I have listed these below:
1) Fig 2: Although the authors claim that the GC-PID dectector has no dependence on humidity, the calibration plots they provide in Fig 2 are not at all convincing. There are several major issues with it.
Looking at the right panel, one would surmise that the detector signal in June at 25 ppb of pyrrole was close to zero. The fits were arrived at using just three points and making the lines pass through zero, not even allowing for the fact there could be different residual noise in the detector under wet and dry conditions....the wet and dry calibration points were not even done at the same introduced pyrrole mixing ratios, which could have been helpful if the information were to be inferred using just 2-3 points to get a fit!
Coming to the left hand figure, it does have many more points but the lowest point even for 10 ppb pyrrole when the instrument was more sensitive in April at the start of their seasonal measurements for the long term deployment, shows a detector signal close to zero. The delta C3 (that is C3-C2) values would be typically less than 10 ppb for the ambient OH reactivity values measured during their study. How then can one distinguish the signal from noise beyond reasonable doubt and just trust the values blindly by applying statistics on the measured signals ?
2) The NO2 correction is still beyond my comprehension….first of all the plot in Fig3 shows change in C3 signal to be appreciable only starting at 20 ppb in the reactor. Considering there is large dilution of ambient air before it enters the reactor this would imply that much higher ambient NO2 of even 30 ppb would not change C3 by much. In this relatively clean forest site how much of the time period that the authors measured was characterized by 20-30 ppb NO2? How many data points? Even the reasoning they give for NO2 interference being important is not unclear. They assert NO2 photolysis produces NO which reacts with HO2 to give OH. But why woudn’t the reaction of NO also occur with O3 which is more abundant in the reactor and competitive reaction wise, not give back NO2 and compensate this effect?
It does not seem as though they quantified reaction rates of both reactions for the range of conditions inside their reactor... I cannot understand why the authors wish to pursue with such experiments and analysis in the present work, and try to draw conclusion attributing corrections to a single factors despite the occurrence of several possible confounders during these experiments?
I am regret very much that this version is still lacking as this is a very good research group which is well known for its VOC measurement expertise in the world. Just this work in my opinion has serious shortcomings for me to have confidence in the results and conclusions of the study.

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ED: Reconsider after major revisions (07 Jul 2019) by Yugo Kanaya

AR by Arnaud P. Praplan on behalf of the Authors (09 Sep 2019) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (25 Sep 2019) by Yugo Kanaya

AR by Arnaud P. Praplan on behalf of the Authors (01 Oct 2019) Author's response Manuscript

ED: Publish subject to technical corrections (16 Oct 2019) by Yugo Kanaya

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Short summary

Our study shows that, despite our best efforts and recent progress, our knowledge of the chemical composition of the air under the canopy of a boreal forest still cannot be fully characterized. The discrepancy between the measured total reactivity of the air and the reactivity derived from the known chemical composition highlights the need to better understand the emissions from vegetation, but also other sources, such as the forest soil.