Opinion: Beyond Global Means: Novel Space-Based Approaches to Indirectly Constrain the Concentrations, Trends, and Variations of Tropospheric Hydroxyl Radical (OH)

Duncan, Bryan; Anderson, Daniel; Fiore, Arlene; Joiner, Joanna; Krotkov, Nickolay; Li, Can; Millet, Dylan; Nicely, Julie; Oman, Luke; St. Clair, Jason; Shutter, Joshua; Souri, Amir; Strode, Sarah; Weir, Brad; Wolfe, Glenn; Worden, Helen; Zhu, Qindan

doi:https://doi.org/10.5194/egusphere-2024-2331

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https://doi.org/10.5194/egusphere-2024-2331

Preprints

30 Jul 2024

| 30 Jul 2024

Opinion: Beyond Global Means: Novel Space-Based Approaches to Indirectly Constrain the Concentrations, Trends, and Variations of Tropospheric Hydroxyl Radical (OH)

Bryan Duncan, Daniel Anderson, Arlene Fiore, Joanna Joiner, Nickolay Krotkov, Can Li, Dylan Millet, Julie Nicely, Luke Oman, Jason St. Clair, Joshua Shutter, Amir Souri, Sarah Strode, Brad Weir, Glenn Wolfe, Helen Worden, and Qindan Zhu

Abstract. The hydroxyl radical (OH) plays a central role in tropospheric chemistry as well as influencing the lifetimes of some climate gases, such as methane. Because of limitations in our ability to observe OH, we have historically relied on indirect methods to constrain its concentrations, trends, and variations, but only as annual global or semi-hemispheric averages. Recent methods demonstrated the feasibility of indirectly constraining tropospheric OH on finer spatio-temporal scales (e.g., seasonal, 1° latitude x 1° longitude), using satellite observations as proxies of the photochemical drivers of OH (e.g., nitrogen dioxide, formaldehyde, isoprene, water vapor, ozone). We found that there are currently reasonable satellite proxies to constrain about 75 % of the global source of tropospheric OH and about 50 % of the global sink. With additional research and investment in observing various volatile organic compounds, there is the potential to constrain an additional 10 % of the global source and 30 % of the global sink. In addition, these novel methods could be refined and made more robust by improvements in the capabilities of satellite instruments (e.g., signal-to-noise, spatial resolution) and retrieval algorithms that are used to develop data products. Another benefit of more robust data products is that they may be used to better constrain the chemical and dynamical processes in atmospheric chemical transport models that simulate the spatio-temporal variations of OH and OH drivers. Therefore, we propose steps forward for the development of a strategic and comprehensive space-based and suborbital observing strategy, which will improve our ability to indirectly constrain OH on much finer spatio-temporal scales than previously achieved. We discuss the strengths and limitations of such an observing strategy and potential improvements to current satellite instrument observing capabilities that would enable better constraint of OH. These improvements include ones that are obtainable with current technologies (e.g., more observations, co-located observations) as well as ones requiring additional technology development (e.g., to obtain vertically-resolved observations). Suborbital observations, which are required for information difficult to obtain from space and for validation of satellite-based OH estimates, will be an integral part of a comprehensive observing strategy.

Received: 23 Jul 2024 – Discussion started: 30 Jul 2024

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Journal article(s) based on this preprint

26 Nov 2024

| Highlight paper

Opinion: Beyond global means – novel space-based approaches to indirectly constrain the concentrations of and trends and variations in the tropospheric hydroxyl radical (OH)

Bryan N. Duncan, Daniel C. Anderson, Arlene M. Fiore, Joanna Joiner, Nickolay A. Krotkov, Can Li, Dylan B. Millet, Julie M. Nicely, Luke D. Oman, Jason M. St. Clair, Joshua D. Shutter, Amir H. Souri, Sarah A. Strode, Brad Weir, Glenn M. Wolfe, Helen M. Worden, and Qindan Zhu

Atmos. Chem. Phys., 24, 13001–13023, https://doi.org/10.5194/acp-24-13001-2024,https://doi.org/10.5194/acp-24-13001-2024, 2024

Short summary Executive editor

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2331', Maarten Krol, 08 Aug 2024

This opinion paper communicates views on an important subject: the use of current and future satellite data to constrain global troposphere OH. Given the central role of OH in removing pollution (including CH4), this is an important opinion paper that aims to advise the upcoming 2027-2037 Earth Science Decadal Survey (ESDS) for the National Aeronautics and Space Administration (NASA).
The paper contains valuable views. However, the paper is rather wordy (saying sensible things), and it would help to provide some more structural elements like tables listing available and future missions. The figures presented in the paper seem to merely advertise personal work, rather than helping the reader to grasp the research agenda concerning OH-proxies.
Scattered around the paper, different observing strategies are mentioned (Polar orbiting, Geostationary, UV/vis, NIR, IR, lighting observations, etc.). Here it would really help to provide a table of existing strategies to monitor OH proxies, right at the beginning of the paper.
In that sense, the order is rather odd. I would expect first an overview of the role of OH in tropospheric chemistry, and historic developments e.g (Lelieveld et al., 2004, 2006). Currently, some of this information is provided later in the paper (in the “thought experiment”). This order might be OK for an opinion paper, but I was distracted by the lack of proper introduction of the subject.
What is also a missing element in the paper is a thorough discussion about the quality of the products. Observing isoprene, H2O2, and formaldehyde is exiting, but using these quantitively is a different game. The same holds for tropospheric ozone. They author mention the 2006 Ziemke approach, and indeed since then not much happened to reliably determine tropospheric ozone. The reason is that it is simply a difficult problem. Although mentioned at places, the authors should be more specific what approaches are feasible to improve on the quality of the OH proxies. For tropospheric ozone, for instance, multi-wavelength satellite observations (IR, vis/Uv) could bring the scientific community further (there are existing studies in this field). As written now, the paper seems to argue that “all” mentioned proxies are of the same quality. Maybe adding a column in table one about the current accuracy would help to guide future needs for scientific research.
I understand the US-central approach, since this paper aims to inform the 2027-2037 Earth Science Decadal Survey (yet, to be published in a European journal?). However, some developments in Europe could help the US developments, and have played a vital role. As an example of the US-central approach of the paper, I would like to point to: As another example, discussion surrounding continuity of the NO2 VCD (Section 5.1.1) started with OMI (launched in 2004), though such observations actually began in 1996 with ESA’s Global Ozone Monitoring Experiment.
Other European elements that are missing are the ESA efforts in the AQ4ECV program (e.g. for NO2, CH2O). Focus should be on synergy, e.g. with the developments of satellites in geostationary orbits (TEMPO, GEMS, S4).
In the final recommendations I miss the further development of techniques (ML, data-assimilation, …) that help the interpretation of the data. Models are needed because they act as an integrating operator that moves around the longer-lived gases towards the “next” satellite observation. Moreover, models provide first guess profiles for retrievals, and may fill in unobserved parts of the atmosphere. Observing system development should be paired with model developments and the developments of techniques to integrate satellite data in models. In that sense, again, no mention is made of the European Copernicus program, which in my opinion is a missed opportunity to guide the US developments.
Minor comments (see also annotated PDF).
Figure1: Although interesting, this paper should not be used to present new results. At most, it could be used as an uncertain exploration of newly developed techniques. Now there is no uncertainty quantification, the legend fails to mention “tropical ocean”.
Line 30: suborbital observing strategy: unclear (in an abstract).
Line 36: “will be an integral part of a comprehensive observing strategy”
Maybe “should be an integral part of a comprehensive observing strategy.” Is better?
Line 70: contributes significantly to the overall uncertainty in the budget, interannual variability, and trends of CH4 (Saunois et al., 2020).

I agree here with uncertainties in the budget, but the interannual variability and trend in methane are well constrained by observations. Of course, the impact of OH on trend and variability in methane is large (what you intend to write), but this phrasing suggests that trend in methane itself is uncertain.

Line 164: “In contrast, decreases in CO can lead to decreased OH and longer CH4 lifetimes.”
I guess this should be “increased OH” and “shorter CH4 lifetimes”.

Lelieveld, J., Brenninkmeijer, C., Joeckel, P., Isaksen, I., Krol, M., Mak, J., Dlugokencky, E., Montzka, S., Novelli, P., & Peters, W. (2006). New Directions: Watching over tropospheric hydroxyl (OH). Atmospheric Environment, 40(29), 5741–5743. https://doi.org/10.1016/j.atmosenv.2006.04.008
Lelieveld, J., Dentener, F. J., Peters, W., & Krol, M. C. (2004). On the role of hydroxyl radicals in the self-cleansing capacity of the troposphere. Atmospheric Chemistry and Physics, 4, 2337–2344.

Citation: https://doi.org/10.5194/egusphere-2024-2331-RC1
- AC1: 'Reply on RC1', Bryan N. Duncan, 25 Aug 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2331/egusphere-2024-2331-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2331-AC1
RC2:
'Comment on egusphere-2024-2331', Anonymous Referee #2, 24 Sep 2024

Summary
The following manuscript provides a review of the current state of retrieving OH from space, and provides recommendations for going forward. OH is the most critical chemical species in the atmosphere, as it dictates the oxidative capacity, i.e., the atmospheric lifetime before chemical removal of the majority of air pollutants and reactive greenhouse gases. I find no issues with the scientific content or accuracy, and agree with the recommendations, so I recommend publication following addressing the following general comment.

General Comment
In Line 72, the manuscript states that it is not possible to retrieve OH directly via remote sensing, but then later in Line 228 states that such technologies could be developed. As a review paper, I would appreciate a greater summary of the physical reasons that OH spectrometry is unable to be retrieved from passive methods, and how active methods like LIDAR may be developed. Also, is it really hopeless that there are no OH absorption lines that might be used in light of the recent work with IASI for VOC retrievals as discussed in section 2.2?

Citation: https://doi.org/10.5194/egusphere-2024-2331-RC2
- AC2: 'Reply on RC2', Bryan N. Duncan, 26 Sep 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2331/egusphere-2024-2331-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2331-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2331', Maarten Krol, 08 Aug 2024

This opinion paper communicates views on an important subject: the use of current and future satellite data to constrain global troposphere OH. Given the central role of OH in removing pollution (including CH4), this is an important opinion paper that aims to advise the upcoming 2027-2037 Earth Science Decadal Survey (ESDS) for the National Aeronautics and Space Administration (NASA).
The paper contains valuable views. However, the paper is rather wordy (saying sensible things), and it would help to provide some more structural elements like tables listing available and future missions. The figures presented in the paper seem to merely advertise personal work, rather than helping the reader to grasp the research agenda concerning OH-proxies.
Scattered around the paper, different observing strategies are mentioned (Polar orbiting, Geostationary, UV/vis, NIR, IR, lighting observations, etc.). Here it would really help to provide a table of existing strategies to monitor OH proxies, right at the beginning of the paper.
In that sense, the order is rather odd. I would expect first an overview of the role of OH in tropospheric chemistry, and historic developments e.g (Lelieveld et al., 2004, 2006). Currently, some of this information is provided later in the paper (in the “thought experiment”). This order might be OK for an opinion paper, but I was distracted by the lack of proper introduction of the subject.
What is also a missing element in the paper is a thorough discussion about the quality of the products. Observing isoprene, H2O2, and formaldehyde is exiting, but using these quantitively is a different game. The same holds for tropospheric ozone. They author mention the 2006 Ziemke approach, and indeed since then not much happened to reliably determine tropospheric ozone. The reason is that it is simply a difficult problem. Although mentioned at places, the authors should be more specific what approaches are feasible to improve on the quality of the OH proxies. For tropospheric ozone, for instance, multi-wavelength satellite observations (IR, vis/Uv) could bring the scientific community further (there are existing studies in this field). As written now, the paper seems to argue that “all” mentioned proxies are of the same quality. Maybe adding a column in table one about the current accuracy would help to guide future needs for scientific research.
I understand the US-central approach, since this paper aims to inform the 2027-2037 Earth Science Decadal Survey (yet, to be published in a European journal?). However, some developments in Europe could help the US developments, and have played a vital role. As an example of the US-central approach of the paper, I would like to point to: As another example, discussion surrounding continuity of the NO2 VCD (Section 5.1.1) started with OMI (launched in 2004), though such observations actually began in 1996 with ESA’s Global Ozone Monitoring Experiment.
Other European elements that are missing are the ESA efforts in the AQ4ECV program (e.g. for NO2, CH2O). Focus should be on synergy, e.g. with the developments of satellites in geostationary orbits (TEMPO, GEMS, S4).
In the final recommendations I miss the further development of techniques (ML, data-assimilation, …) that help the interpretation of the data. Models are needed because they act as an integrating operator that moves around the longer-lived gases towards the “next” satellite observation. Moreover, models provide first guess profiles for retrievals, and may fill in unobserved parts of the atmosphere. Observing system development should be paired with model developments and the developments of techniques to integrate satellite data in models. In that sense, again, no mention is made of the European Copernicus program, which in my opinion is a missed opportunity to guide the US developments.
Minor comments (see also annotated PDF).
Figure1: Although interesting, this paper should not be used to present new results. At most, it could be used as an uncertain exploration of newly developed techniques. Now there is no uncertainty quantification, the legend fails to mention “tropical ocean”.
Line 30: suborbital observing strategy: unclear (in an abstract).
Line 36: “will be an integral part of a comprehensive observing strategy”
Maybe “should be an integral part of a comprehensive observing strategy.” Is better?
Line 70: contributes significantly to the overall uncertainty in the budget, interannual variability, and trends of CH4 (Saunois et al., 2020).

I agree here with uncertainties in the budget, but the interannual variability and trend in methane are well constrained by observations. Of course, the impact of OH on trend and variability in methane is large (what you intend to write), but this phrasing suggests that trend in methane itself is uncertain.

Line 164: “In contrast, decreases in CO can lead to decreased OH and longer CH4 lifetimes.”
I guess this should be “increased OH” and “shorter CH4 lifetimes”.

Lelieveld, J., Brenninkmeijer, C., Joeckel, P., Isaksen, I., Krol, M., Mak, J., Dlugokencky, E., Montzka, S., Novelli, P., & Peters, W. (2006). New Directions: Watching over tropospheric hydroxyl (OH). Atmospheric Environment, 40(29), 5741–5743. https://doi.org/10.1016/j.atmosenv.2006.04.008
Lelieveld, J., Dentener, F. J., Peters, W., & Krol, M. C. (2004). On the role of hydroxyl radicals in the self-cleansing capacity of the troposphere. Atmospheric Chemistry and Physics, 4, 2337–2344.

Citation: https://doi.org/10.5194/egusphere-2024-2331-RC1
- AC1: 'Reply on RC1', Bryan N. Duncan, 25 Aug 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2331/egusphere-2024-2331-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2331-AC1
RC2:
'Comment on egusphere-2024-2331', Anonymous Referee #2, 24 Sep 2024

Summary
The following manuscript provides a review of the current state of retrieving OH from space, and provides recommendations for going forward. OH is the most critical chemical species in the atmosphere, as it dictates the oxidative capacity, i.e., the atmospheric lifetime before chemical removal of the majority of air pollutants and reactive greenhouse gases. I find no issues with the scientific content or accuracy, and agree with the recommendations, so I recommend publication following addressing the following general comment.

General Comment
In Line 72, the manuscript states that it is not possible to retrieve OH directly via remote sensing, but then later in Line 228 states that such technologies could be developed. As a review paper, I would appreciate a greater summary of the physical reasons that OH spectrometry is unable to be retrieved from passive methods, and how active methods like LIDAR may be developed. Also, is it really hopeless that there are no OH absorption lines that might be used in light of the recent work with IASI for VOC retrievals as discussed in section 2.2?

Citation: https://doi.org/10.5194/egusphere-2024-2331-RC2
- AC2: 'Reply on RC2', Bryan N. Duncan, 26 Sep 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2331/egusphere-2024-2331-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2331-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Bryan N. Duncan on behalf of the Authors (26 Sep 2024) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (02 Oct 2024) by Barbara Ervens

AR by Bryan N. Duncan on behalf of the Authors (03 Oct 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (07 Oct 2024) by Barbara Ervens

ED: Publish as is (10 Oct 2024) by Andreas Hofzumahaus (Executive editor)

AR by Bryan N. Duncan on behalf of the Authors (11 Oct 2024)

Journal article(s) based on this preprint

26 Nov 2024

| Highlight paper

Opinion: Beyond global means – novel space-based approaches to indirectly constrain the concentrations of and trends and variations in the tropospheric hydroxyl radical (OH)

Atmos. Chem. Phys., 24, 13001–13023, https://doi.org/10.5194/acp-24-13001-2024,https://doi.org/10.5194/acp-24-13001-2024, 2024

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Tropospheric OH plays a central role in the chemical degradation and formation of air pollutants and climate-relevant trace gases. Traditional direct measurements of the extremely short-lived OH radicals provide localised information on the OH budget. Due to the high technical complexity and scarcity of such measurements, they are practically unsuitable for mapping global spatio-temporal OH distributions. Budget analyses of long-lived tracers (e.g. methyl chloroform) that react exclusively with OH provide estimates of the global mean OH and its temporal variability, but lack spatial resolution. This forward-looking opinion paper discusses new ideas and proposals to constrain global chemical sources and sinks of tropospheric OH using satellite and suborbital observations. The proposed approaches promise a better understanding of the global OH budget at higher spatial and temporal resolution than has been possible to date.

Short summary

Trace gases emitted to or formed within the atmosphere may be chemically or physically removed from the atmosphere. One trace gas, the hydroxyl radical (OH), is responsible for initiating the chemical removal of many trace gases, including some climate gases. Despite its importance, scientists have not been able to adequately measure OH. In this opinion piece, we discuss promising new methods to indirectly constrain OH using satellite data of trace gases that control the abundance of OH.


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