the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Dec 2023
Opinion: Exploring potential atmospheric methane removal approaches: an example research roadmap for chlorine radical enhancement
Abstract. The escalating climate crisis requires rapid action to reduce the concentrations of atmospheric greenhouse gases and lower global surface temperatures. Methane will play a critical role in near-term warming due to its high radiative forcing and short atmospheric lifetime. Methane emissions have accelerated in recent years and there is significant risk and uncertainty associated with the future growth in natural emissions. The largest natural sink of methane occurs through oxidation reactions with atmospheric hydroxyl and chlorine radicals. Enhanced atmospheric oxidation could be a potential approach to remove atmospheric methane. One method proposes the addition of iron salt aerosols (ISA) to the atmosphere, mimicking a natural process proposed to occur when mineral dust mixes with chloride from sea spray to form iron chlorides, which are photolyzed by sunlight to produce chlorine radicals. Under the right conditions, lofting ISA into the atmosphere could potentially reduce atmospheric methane concentrations and lower global surface temperatures. Recognizing that potential atmospheric methane removal must only be considered as an additive measure – in addition to, not replacing, crucial anthropogenic greenhouse gas emission reductions and carbon dioxide removal – roadmaps can be a valuable tool to organize and streamline interdisciplinary and multifaceted research to efficiently move towards understanding whether an approach may be viable and socially acceptable, or if it is nonviable and further research should be deprioritized. Here we present an example five-year research roadmap to explore whether ISA enhancement of the chlorine radical sink could be a viable and socially acceptable atmospheric methane removal approach.
Status: closed
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RC1: 'Comment on egusphere-2023-2959', Anonymous Referee #1, 30 Dec 2023
This ‘opinion’ paper lays out a road map for exploring the feasibility of injecting iron salt aerosol (ISA) to increase Cl atom concentrations in the lower troposphere and thus increase the removal rate of methane. There has been a lot of murmur in the community recently about proposals to accelerate methane removal by increasing tropospheric chlorine, and it is nice to see this paper provide a review of where things stand and outline a research program to study the possibility further. I find the paper to be well informed and generally well written though with some platitudes and repetitions. The abstract is a fair representation of the paper. I recommend publication but have several comments for the authors’ consideration.
- This is effectively a white paper for a research program. The first authors are from Spark Climate Solutions, which funds research, so the paper probably describes a current or planned Spark research program rather than being a grass-roots opinion piece. It would be good to have transparency by stating upfront the role of Spark and its plans toward this research program.
- In the same vein, I don’t see the point of the word ‘example’ in the Title.
- Figure 1 does not add anything to Table 1 and could be deleted.
- Please scour the paper for opportunities to make the text more concise.
- Although I enjoyed reading the paper and getting up to speed on ISA ideas, I’m still not sold at all on the idea that we need accelerated removal of methane. It’s a very different matter for CO2 which has a much longer lifetime – even if we go to carbon zero we will still need to remove CO2 from the atmosphere to achieve near-term climate stabilization. There is no such need for methane, except perhaps if feedback from natural sources leads to a situation where increasing methane is outside our control. That’s a reasonable position and the current intro hints at it but I would like to see a clearer statement of why the authors think that accelerating the methane sink is an ‘essential’ part of the climate solution as stated in lines 341-342.
- Line 46: ‘Methane (CH4) has contributed 0.5 °C to the current 1.1 °C temperature increase…’ is misleading because of the offsetting effect of aerosols. It would be fairer to say that methane has contributed one third of the greenhouse warming since preindustrial time.
- Line 49: Methane is now triple preindustrial levels.
- Line 59: also cite Qu, Z., D. J. Jacob, Y. Zhang, L. Shen, D. J. Varon, X. Lu, T. Scarpelli, A. Bloom, J. Worden, and R. J. Parker , Attribution of the 2020 surge in atmospheric methane by inverse analysis of GOSAT observations, Environ. Res. Lett., 17, 094003, 2022.
- Line 64: the 11 Tg a-1 loss is for tropospheric chlorine, which is of course the focus of this paper, but is important to clarify because the loss from stratospheric chlorine is of comparable magnitude.
- Lines 200-201: Reaction of Cl with ozone conserves chlorine radicals (produces ClO). ClO will go back to Cl very quickly and so it doesn’t represent an effective sink for Cl – the cited Wang et al. 2019 paper certainly does not argue that it would be. A more effective sink (from the Wang paper) would be deposition of HCl to the oceans.
- Line 260: another negative impact would be for Cl to drift into urban areas and produce ClNO2 to stimulate urban ozone formation. Wang, X., D.J. Jacob, X. Fu, T. Wang, M. Le Breton, M. Hallquist, Z. Liu, E.E. McDuffie, and H. Liao, Effects of anthropogenic chlorine on PM2.5 and ozone air quality in China, Sci. Technol., 54, 9908-9916, 2020.
- Lines 272-273: ‘reduced hydroxyl radical production may increase lifetimes of volatile organic compounds that are detrimental to human health’. In principle yes, but I am hard pressed to find an example. Carcinogenic VOCs tend to have short lifetimes so would be oxidized within urban areas where this ISA chemistry would only have faint effect. Benzene, maybe? But for such a long-lived VOC the sink in urban air is effectively ventilation, not reaction with OH.
- Section 3.2.3: seems to me that this section is not really saying anything beyond vague generalities. Make it more substantive?
Citation: https://doi.org/10.5194/egusphere-2023-2959-RC1 -
RC2: 'Comment on egusphere-2023-2959', Anonymous Referee #2, 10 Jan 2024
This opinion article presents a case for considering broad research goals and timelines coordinated across disciplines for the topic of atmospheric methane removal via Iron Salt Aerosols (ISA). The natural analogue of ISA, mineral dust sea salt aerosol (MDSA) interaction, is proposed to produce chlorine radicals that can react with methane based on lab studies and isotope measurements at ground-based stations. The article includes discussion of the effectiveness, side effects, scalability, and ethical governance needs for ISA to be considered a viable method for methane removal. By outlining science, engineering, and social science priorities, this manuscript does indeed propose a roadmap for this potential technology.
I find the article to be compelling and of thorough scope, well-researched and appropriately cautious while aiming to push science toward technological solutions to curb climate change. The article should find interest from the audience reached by ACP, and could likely stoke discussion and research on the outlined priorities as intended. Figures and text are presented clearly and with sufficient detail. I include below one suggestion for a non-critical addition as well as a few points for clarification; otherwise, I consider this opinion piece to be a solid candidate for publication in ACP.
Major comment:
I would be interested to understand the role of roadmapping in atmospheric science, historically and/or presently. The authors implicitly make the case that this practice will accelerate society’s ability to translate science into technological solutions for climate change. Have other organizations or entities taken such a targeted approach to strategizing about a research portfolio in support of a technological development in a geophysics field? The approach certainly makes sense to me; I just wonder if there are examples that point to increased success, accelerated timelines, etc. associated with this practice. I don’t consider this addition to be critical, just an opportunity, perhaps, to bolster the justification for this methodology.
Minor comments:
L77: Could cite Holmes, 2018 for a thorough discussion of the methane feedback phenomenon: https://doi.org/10.1002/2017MS001196
L119: Perhaps it is worth naming the four ground-based stations in the North Atlantic, for interested readers.
L172: Some language in this section is repetitive, could be streamlined. For instance, I first latched onto these three assessment criteria: feasibility, scalability, and social license to operate. These don't exactly marry up to the questions/subsections below. Similarly, some of the sections below touch on topics from the other sections (e.g., ~L240, social license issues brought up in the “effectiveness” section).
L286: Could the authors clarify what is meant by this sentence, “To be cost-plausible…,” for those who are not familiar with socioeconomic terms? Does this mean one must compare the cost of preventing emission of methane in the first place against the cost to remove it via ISA?
Table 1: The use of "ongoing" in the "Anticipated duration" column is a bit confusing to me, suggests the same as "underway" used in the prior column. Does this just mean the task is of indeterminate length?
I see now from Fig. 1 that the “ongoing” items in Table 1 are meant to continue throughout the duration of the timeline; perhaps "continuous" or some other wording would be better? Or can the use of "+" elsewhere in this column be adapted for this (i.e., wouldn't "5+ years" convey the same thing as “ongoing” in most of these cases)?
Citation: https://doi.org/10.5194/egusphere-2023-2959-RC2 -
AC1: 'Comment on egusphere-2023-2959', Katrine Gorham, 09 Feb 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2959/egusphere-2023-2959-AC1-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2023-2959', Anonymous Referee #1, 30 Dec 2023
This ‘opinion’ paper lays out a road map for exploring the feasibility of injecting iron salt aerosol (ISA) to increase Cl atom concentrations in the lower troposphere and thus increase the removal rate of methane. There has been a lot of murmur in the community recently about proposals to accelerate methane removal by increasing tropospheric chlorine, and it is nice to see this paper provide a review of where things stand and outline a research program to study the possibility further. I find the paper to be well informed and generally well written though with some platitudes and repetitions. The abstract is a fair representation of the paper. I recommend publication but have several comments for the authors’ consideration.
- This is effectively a white paper for a research program. The first authors are from Spark Climate Solutions, which funds research, so the paper probably describes a current or planned Spark research program rather than being a grass-roots opinion piece. It would be good to have transparency by stating upfront the role of Spark and its plans toward this research program.
- In the same vein, I don’t see the point of the word ‘example’ in the Title.
- Figure 1 does not add anything to Table 1 and could be deleted.
- Please scour the paper for opportunities to make the text more concise.
- Although I enjoyed reading the paper and getting up to speed on ISA ideas, I’m still not sold at all on the idea that we need accelerated removal of methane. It’s a very different matter for CO2 which has a much longer lifetime – even if we go to carbon zero we will still need to remove CO2 from the atmosphere to achieve near-term climate stabilization. There is no such need for methane, except perhaps if feedback from natural sources leads to a situation where increasing methane is outside our control. That’s a reasonable position and the current intro hints at it but I would like to see a clearer statement of why the authors think that accelerating the methane sink is an ‘essential’ part of the climate solution as stated in lines 341-342.
- Line 46: ‘Methane (CH4) has contributed 0.5 °C to the current 1.1 °C temperature increase…’ is misleading because of the offsetting effect of aerosols. It would be fairer to say that methane has contributed one third of the greenhouse warming since preindustrial time.
- Line 49: Methane is now triple preindustrial levels.
- Line 59: also cite Qu, Z., D. J. Jacob, Y. Zhang, L. Shen, D. J. Varon, X. Lu, T. Scarpelli, A. Bloom, J. Worden, and R. J. Parker , Attribution of the 2020 surge in atmospheric methane by inverse analysis of GOSAT observations, Environ. Res. Lett., 17, 094003, 2022.
- Line 64: the 11 Tg a-1 loss is for tropospheric chlorine, which is of course the focus of this paper, but is important to clarify because the loss from stratospheric chlorine is of comparable magnitude.
- Lines 200-201: Reaction of Cl with ozone conserves chlorine radicals (produces ClO). ClO will go back to Cl very quickly and so it doesn’t represent an effective sink for Cl – the cited Wang et al. 2019 paper certainly does not argue that it would be. A more effective sink (from the Wang paper) would be deposition of HCl to the oceans.
- Line 260: another negative impact would be for Cl to drift into urban areas and produce ClNO2 to stimulate urban ozone formation. Wang, X., D.J. Jacob, X. Fu, T. Wang, M. Le Breton, M. Hallquist, Z. Liu, E.E. McDuffie, and H. Liao, Effects of anthropogenic chlorine on PM2.5 and ozone air quality in China, Sci. Technol., 54, 9908-9916, 2020.
- Lines 272-273: ‘reduced hydroxyl radical production may increase lifetimes of volatile organic compounds that are detrimental to human health’. In principle yes, but I am hard pressed to find an example. Carcinogenic VOCs tend to have short lifetimes so would be oxidized within urban areas where this ISA chemistry would only have faint effect. Benzene, maybe? But for such a long-lived VOC the sink in urban air is effectively ventilation, not reaction with OH.
- Section 3.2.3: seems to me that this section is not really saying anything beyond vague generalities. Make it more substantive?
Citation: https://doi.org/10.5194/egusphere-2023-2959-RC1 -
RC2: 'Comment on egusphere-2023-2959', Anonymous Referee #2, 10 Jan 2024
This opinion article presents a case for considering broad research goals and timelines coordinated across disciplines for the topic of atmospheric methane removal via Iron Salt Aerosols (ISA). The natural analogue of ISA, mineral dust sea salt aerosol (MDSA) interaction, is proposed to produce chlorine radicals that can react with methane based on lab studies and isotope measurements at ground-based stations. The article includes discussion of the effectiveness, side effects, scalability, and ethical governance needs for ISA to be considered a viable method for methane removal. By outlining science, engineering, and social science priorities, this manuscript does indeed propose a roadmap for this potential technology.
I find the article to be compelling and of thorough scope, well-researched and appropriately cautious while aiming to push science toward technological solutions to curb climate change. The article should find interest from the audience reached by ACP, and could likely stoke discussion and research on the outlined priorities as intended. Figures and text are presented clearly and with sufficient detail. I include below one suggestion for a non-critical addition as well as a few points for clarification; otherwise, I consider this opinion piece to be a solid candidate for publication in ACP.
Major comment:
I would be interested to understand the role of roadmapping in atmospheric science, historically and/or presently. The authors implicitly make the case that this practice will accelerate society’s ability to translate science into technological solutions for climate change. Have other organizations or entities taken such a targeted approach to strategizing about a research portfolio in support of a technological development in a geophysics field? The approach certainly makes sense to me; I just wonder if there are examples that point to increased success, accelerated timelines, etc. associated with this practice. I don’t consider this addition to be critical, just an opportunity, perhaps, to bolster the justification for this methodology.
Minor comments:
L77: Could cite Holmes, 2018 for a thorough discussion of the methane feedback phenomenon: https://doi.org/10.1002/2017MS001196
L119: Perhaps it is worth naming the four ground-based stations in the North Atlantic, for interested readers.
L172: Some language in this section is repetitive, could be streamlined. For instance, I first latched onto these three assessment criteria: feasibility, scalability, and social license to operate. These don't exactly marry up to the questions/subsections below. Similarly, some of the sections below touch on topics from the other sections (e.g., ~L240, social license issues brought up in the “effectiveness” section).
L286: Could the authors clarify what is meant by this sentence, “To be cost-plausible…,” for those who are not familiar with socioeconomic terms? Does this mean one must compare the cost of preventing emission of methane in the first place against the cost to remove it via ISA?
Table 1: The use of "ongoing" in the "Anticipated duration" column is a bit confusing to me, suggests the same as "underway" used in the prior column. Does this just mean the task is of indeterminate length?
I see now from Fig. 1 that the “ongoing” items in Table 1 are meant to continue throughout the duration of the timeline; perhaps "continuous" or some other wording would be better? Or can the use of "+" elsewhere in this column be adapted for this (i.e., wouldn't "5+ years" convey the same thing as “ongoing” in most of these cases)?
Citation: https://doi.org/10.5194/egusphere-2023-2959-RC2 -
AC1: 'Comment on egusphere-2023-2959', Katrine Gorham, 09 Feb 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2959/egusphere-2023-2959-AC1-supplement.pdf
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