the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jun 2024
Impact of methane and other precursor emission reductions on surface ozone in Europe: Scenario analysis using the EMEP MSC-W model
Abstract. The impacts of future methane (CH4) and other precursor emission changes are investigated for surface ozone (O3) in the United Nations Economic Commission for Europe (UNECE) region excluding North America and Israel (the "EMEP region", for European Monitoring and Evaluation Programme) for the year 2050. The analysis includes a Current Legislation (CLE) and Maximum Feasible Technical (MFR) reduction scenario, and a scenario that combines MFR reductions with an additional dietary shift that also meets the Paris Agreement objectives with respect to greenhouse gas emissions (LOW). For each scenario, background CH4 concentrations are calculated using a probabilistic Earth System model emulator, and combined with other precursor emissions in a three-dimensional Eulerian chemistry-transport model. While focus is placed on peak season maximum daily 8-hour average (MDA8) O3 concentrations, a range of other indicators for health and vegetation impacts are also discussed. Our analysis show that roughly one-thirds of the total peak season MDA8 reduction achieved between the 2050 CLE and MFR scenarios is attributable to CH4 reductions, resulting predominantly from CH4 emission reductions outside of the EMEP region. The impact of other precursor emission reductions is split nearly evenly between the reductions inside and outside of the EMEP region. However, the relative importance of CH4 and other precursor emission reductions is shown to depend on the choice of O3 indicator, though indicators sensitive to peak O3 show generally consistent results. The analysis also highlights the synergistic impacts of CH4 mitigation as reducing solely CH4 achieves, beyond air quality improvement, nearly two-thirds of the total global warming reduction calculated for the LOW scenario compared to the CLE case.
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RC1: 'Comment on egusphere-2024-1422', Anonymous Referee #1, 07 Jul 2024
General comments
An interesting study, overall clear and well written. Uses a current legislation and maximum feasible reduction scenario to study ozone changes in Europe, splitting out impacts from methane changes and precursor emissions reductions, illustrating the role of these on air quality and also how use of different impact metrics can affect their relative importance. Fig 3 is really interesting, clear and a useful summary of the main results of the study.
Specific comments
Section 2.1.2: could add comment on the timeline of introduction of emissions reduction technologies and the choice of divergence of scenarios from 2025 onwards
Section 2.1.3: A different background GHG scenario is used for LOW. To what extent does this limit being able to compare the ozone impact of methane reductions directly to the CLE and MFR scenarios? This effect is likely to be limited, due to the scenarios having the same NOx, CO, NMVOCs, but a comment on this would be helpful. Maybe related to the discussion in lines 407-411.
177/Fig 1: SSP3-7.0 is the most extreme/’pessimistic’ scenario in terms of methane, so this scenario could be added (or replace SSP5-8.5) to show the full range of methane trajectories in the SSPs
197-199: Mention that this is as expected since the methane emissions (i.e. input source terms) have not stabilized by 2050, and the system would not be expected to be in equilibrium.
248: How many stations are excluded based on the data availability requirements? And does this affect particular regions/countries? E.g. Fig S2 shows no stations in Italy, south-eastern Europe
252: Mention (perhaps in discussion) how the normalized mean bias in the model compared to obs affects the results and conclusions shown – since the results are differences/anomalies with respect to CLE or 2015 baselines this is limited but a sentence could be added to comment on this. For example, it is likely to affect the metrics based on a threshold more?
309: Would expect the spatial distribution of reduction in Fig 4c to not just be due to insolation changes but also the spatially different chemical environments? Which would affect the ozone impact of methane changes. Is this based on analysis or just a proposed explanation
348-349: I think this is an important result, and is hard to see from looking at the table with so many values (which are useful for reference). Perhaps it could be colour coded by percentage change, which would e.g. show clearly which indicators show a stronger impact, and how threshold/time/averaging makes a difference.
379: Related to climate uncertainty in CH4 projections – wetland emissions increases (and climate feedbacks) are likely to have a much larger effect than natural soil emissions, and here natural emissions are assumed to be constant (as standard in CMIP6) e.g. Kleinen et al 2021 (https://iopscience.iop.org/article/10.1088/1748-9326/ac1814/pdf), Zhang et al 2017 (https://www.pnas.org/doi/epdf/10.1073/pnas.1618765114)
Section 6.1:
391-396: To what extent are these processes included in the model? Does it have interactive OH
Also, are each of these points run for a single year? Does it require spin up for 150ppb jumps in CH4 concentration?
When concluding that Fig 5 shows that the peak season MDA8 response is linear with CH4 (411), to what extent are non-linear processes captured in these experiments? E.g. large influence of methane burden on OH concentration, affecting the oxidative capacity and therefore ozone production.
Fig 5: It would be helpful to label the dashed lines on the figure e.g. CLE 2050, MFR 2050 etc. The ordering of labels in the caption is also slightly confusing: ‘The impacts are calculated for the baseline 2015 and the 2050 CLE and LOW emission … (1574, 1834, and 2236 ppb, respectively)’, not clear which number corresponds to which scenario, if following the text it should be 1834, 1574, and 2236 ppb, respectively(?)
407-411 ‘In the analysis... concentrations are reduced’ : I found this a bit hard to understand, I think it could be rearranged/reworded to put a summary sentence to introduce what the main point being made is. E.g.: The decrease in peak season MDA8 was found to be most strongly influenced by the background CH4 concentration, rather than the precursor emissions scenario, with a 5.4 and 4.7 ug m-3 decrease with CLE and LOW precursor emissions respectively, for the same CH4 decrease (from 2236 to 1574ppb). Do the 5.4 and 4.7 numbers correspond to any rows in Table 3?
450: SSP scenarios only start in 2015, when they all branch from historical emissions? So they should all be the same before 2015 anyway. Think this sentence can be deleted or replaced with something along the lines of ‘Note that emissions/scenarios are the same before 2015.’
462-464: ‘the increase in CH4 in the CLE scenario nevertheless offsets the peak season MDA8 reductions achieved by precursor emissions reductions in the EMEP region almost entirely.’ Does this refer to +3.4% for CH4 and –5.1% for precursor emissions? Suggest to weaken/delete ‘almost entirely’. Could also add a sentence linking to impacts/policy e.g. this highlights the need for simultaneous reductions in both CH4 and precursor emissions.
489: suggest to rephrase ‘global warming reduction potential’ to avoid confusion with GWP. E.g. possible temperature reduction
Technical corrections
23: typo - ‘is also investigation’
33: please add reference and year for 1915ppb CH4 mixing ratio
119: typo - ‘construction low’
190: suggest ‘might’ -> ‘likely’
294: n.b. POD3IAMWH not defined before this, probably fine since the sentence points to sect. 5.2
327: typo ‘will discussed’
405: minor comment – maybe easier to read a % reduction in OPE rather than ‘a factor of XX lower’
Citation: https://doi.org/10.5194/egusphere-2024-1422-RC1 -
RC2: 'Comment on egusphere-2024-1422', Anonymous Referee #2, 15 Jul 2024
Atmospheric methane trends under future mitigation scenarios and their impact on climate change and air quality are currently much discussed in the scientific literature in the wake of the 2021 COP26 global methane pledge and the struggle to achieve the goals of the Paris agreement. This paper contributes to this effort by analysing the impact of methane emission reductions (increases under CLE) in the period between 2015 and 2050 on surface ozone. These impacts are put into perspective with the impact of other NTCFs/air pollutants such as NOx and CO/NMVOC. The analysis focus on the impacts on air quality, specifically on ozone exposure quantified with the MDA8 metrics, but impacts on radiative forcing are also discussed in passing. A probabilistic ES-Emulator is used to produce CH4 background concentration scenarios to assess uncertainties in the methane scenarios and as input for the methane concentration-driven EMEP-MSC-W model.
Overall, this is a solid piece of work. The text is well written but in parts a bit difficult to follow because the text becomes rather complex. Altogether, though, a well written manuscript. Figures are well prepared and support and complement the text strongly. This study is an interesting if not quite ground braking contribution to the literature in this scientific area. I think, the implications of the results for policy making could be extended a bit more in the Discussions. The question of the impact of methane mitigation and the role of co-emitted NTCFs/air pollutants is currently much debated. In conclusion, I think this manuscript is a worthy contribution to ACP. I suggest publication after minor revisions.
Specific comments can be found in the attached annotated copy of the manuscript.
- AC1: 'Comment on egusphere-2024-1422', Willem van Caspel, 09 Aug 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-1422', Anonymous Referee #1, 07 Jul 2024
General comments
An interesting study, overall clear and well written. Uses a current legislation and maximum feasible reduction scenario to study ozone changes in Europe, splitting out impacts from methane changes and precursor emissions reductions, illustrating the role of these on air quality and also how use of different impact metrics can affect their relative importance. Fig 3 is really interesting, clear and a useful summary of the main results of the study.
Specific comments
Section 2.1.2: could add comment on the timeline of introduction of emissions reduction technologies and the choice of divergence of scenarios from 2025 onwards
Section 2.1.3: A different background GHG scenario is used for LOW. To what extent does this limit being able to compare the ozone impact of methane reductions directly to the CLE and MFR scenarios? This effect is likely to be limited, due to the scenarios having the same NOx, CO, NMVOCs, but a comment on this would be helpful. Maybe related to the discussion in lines 407-411.
177/Fig 1: SSP3-7.0 is the most extreme/’pessimistic’ scenario in terms of methane, so this scenario could be added (or replace SSP5-8.5) to show the full range of methane trajectories in the SSPs
197-199: Mention that this is as expected since the methane emissions (i.e. input source terms) have not stabilized by 2050, and the system would not be expected to be in equilibrium.
248: How many stations are excluded based on the data availability requirements? And does this affect particular regions/countries? E.g. Fig S2 shows no stations in Italy, south-eastern Europe
252: Mention (perhaps in discussion) how the normalized mean bias in the model compared to obs affects the results and conclusions shown – since the results are differences/anomalies with respect to CLE or 2015 baselines this is limited but a sentence could be added to comment on this. For example, it is likely to affect the metrics based on a threshold more?
309: Would expect the spatial distribution of reduction in Fig 4c to not just be due to insolation changes but also the spatially different chemical environments? Which would affect the ozone impact of methane changes. Is this based on analysis or just a proposed explanation
348-349: I think this is an important result, and is hard to see from looking at the table with so many values (which are useful for reference). Perhaps it could be colour coded by percentage change, which would e.g. show clearly which indicators show a stronger impact, and how threshold/time/averaging makes a difference.
379: Related to climate uncertainty in CH4 projections – wetland emissions increases (and climate feedbacks) are likely to have a much larger effect than natural soil emissions, and here natural emissions are assumed to be constant (as standard in CMIP6) e.g. Kleinen et al 2021 (https://iopscience.iop.org/article/10.1088/1748-9326/ac1814/pdf), Zhang et al 2017 (https://www.pnas.org/doi/epdf/10.1073/pnas.1618765114)
Section 6.1:
391-396: To what extent are these processes included in the model? Does it have interactive OH
Also, are each of these points run for a single year? Does it require spin up for 150ppb jumps in CH4 concentration?
When concluding that Fig 5 shows that the peak season MDA8 response is linear with CH4 (411), to what extent are non-linear processes captured in these experiments? E.g. large influence of methane burden on OH concentration, affecting the oxidative capacity and therefore ozone production.
Fig 5: It would be helpful to label the dashed lines on the figure e.g. CLE 2050, MFR 2050 etc. The ordering of labels in the caption is also slightly confusing: ‘The impacts are calculated for the baseline 2015 and the 2050 CLE and LOW emission … (1574, 1834, and 2236 ppb, respectively)’, not clear which number corresponds to which scenario, if following the text it should be 1834, 1574, and 2236 ppb, respectively(?)
407-411 ‘In the analysis... concentrations are reduced’ : I found this a bit hard to understand, I think it could be rearranged/reworded to put a summary sentence to introduce what the main point being made is. E.g.: The decrease in peak season MDA8 was found to be most strongly influenced by the background CH4 concentration, rather than the precursor emissions scenario, with a 5.4 and 4.7 ug m-3 decrease with CLE and LOW precursor emissions respectively, for the same CH4 decrease (from 2236 to 1574ppb). Do the 5.4 and 4.7 numbers correspond to any rows in Table 3?
450: SSP scenarios only start in 2015, when they all branch from historical emissions? So they should all be the same before 2015 anyway. Think this sentence can be deleted or replaced with something along the lines of ‘Note that emissions/scenarios are the same before 2015.’
462-464: ‘the increase in CH4 in the CLE scenario nevertheless offsets the peak season MDA8 reductions achieved by precursor emissions reductions in the EMEP region almost entirely.’ Does this refer to +3.4% for CH4 and –5.1% for precursor emissions? Suggest to weaken/delete ‘almost entirely’. Could also add a sentence linking to impacts/policy e.g. this highlights the need for simultaneous reductions in both CH4 and precursor emissions.
489: suggest to rephrase ‘global warming reduction potential’ to avoid confusion with GWP. E.g. possible temperature reduction
Technical corrections
23: typo - ‘is also investigation’
33: please add reference and year for 1915ppb CH4 mixing ratio
119: typo - ‘construction low’
190: suggest ‘might’ -> ‘likely’
294: n.b. POD3IAMWH not defined before this, probably fine since the sentence points to sect. 5.2
327: typo ‘will discussed’
405: minor comment – maybe easier to read a % reduction in OPE rather than ‘a factor of XX lower’
Citation: https://doi.org/10.5194/egusphere-2024-1422-RC1 -
RC2: 'Comment on egusphere-2024-1422', Anonymous Referee #2, 15 Jul 2024
Atmospheric methane trends under future mitigation scenarios and their impact on climate change and air quality are currently much discussed in the scientific literature in the wake of the 2021 COP26 global methane pledge and the struggle to achieve the goals of the Paris agreement. This paper contributes to this effort by analysing the impact of methane emission reductions (increases under CLE) in the period between 2015 and 2050 on surface ozone. These impacts are put into perspective with the impact of other NTCFs/air pollutants such as NOx and CO/NMVOC. The analysis focus on the impacts on air quality, specifically on ozone exposure quantified with the MDA8 metrics, but impacts on radiative forcing are also discussed in passing. A probabilistic ES-Emulator is used to produce CH4 background concentration scenarios to assess uncertainties in the methane scenarios and as input for the methane concentration-driven EMEP-MSC-W model.
Overall, this is a solid piece of work. The text is well written but in parts a bit difficult to follow because the text becomes rather complex. Altogether, though, a well written manuscript. Figures are well prepared and support and complement the text strongly. This study is an interesting if not quite ground braking contribution to the literature in this scientific area. I think, the implications of the results for policy making could be extended a bit more in the Discussions. The question of the impact of methane mitigation and the role of co-emitted NTCFs/air pollutants is currently much debated. In conclusion, I think this manuscript is a worthy contribution to ACP. I suggest publication after minor revisions.
Specific comments can be found in the attached annotated copy of the manuscript.
- AC1: 'Comment on egusphere-2024-1422', Willem van Caspel, 09 Aug 2024
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