Background heterogeneity and other uncertainties in estimating urban methane flux: results from the Indianapolis Flux Experiment (INFLUX)

Balashov, Nikolay V.; Davis, Kenneth J.; Miles, Natasha L.; Lauvaux, Thomas; Richardson, Scott J.; Barkley, Zachary R.; Bonin, Timothy A.

doi:https://doi.org/10.5194/acp-20-4545-2020

Articles | Volume 20, issue 7

https://doi.org/10.5194/acp-20-4545-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

The 10th International Carbon Dioxide Conference (ICDC10)...

https://doi.org/10.5194/acp-20-4545-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 7

Research article

|

17 Apr 2020

Research article |

| 17 Apr 2020

Background heterogeneity and other uncertainties in estimating urban methane flux: results from the Indianapolis Flux Experiment (INFLUX)

Nikolay V. Balashov, Kenneth J. Davis, Natasha L. Miles, Thomas Lauvaux, Scott J. Richardson, Zachary R. Barkley, and Timothy A. Bonin

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Final revised paper (published on 17 Apr 2020)
Preprint (discussion started on 13 Mar 2019)

Interactive discussion

Status: closed

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

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RC1: 'Review of Balashov et al. "Background Heterogeneity and Other Uncertainties in Estimating Urban Methane Flux: Results from the Indianapolis Flux (INFLUX) Experiment"', Anonymous Referee #2, 05 Apr 2019
RC2: 'Review of Balashov et al.', Anonymous Referee #3, 22 Jul 2019
AC1: 'Response to the Reviews', Nikolay Balashov, 09 Sep 2019

Peer-review completion

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

AR by Nikolay Balashov on behalf of the Authors (01 Oct 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Oct 2019) by Neil Harris

RR by Anonymous Referee #3 (12 Nov 2019)

Suggestions for revision or reasons for rejection

When people talk about “missing” or “unknown” NG sources in urban environments they are usually thinking of leaks that occur downstream of the customer meter. If many small leaks in homes and businesses throughout the city were each leaking a tiny amount, how would that appear on your Figures 6 and 7? Would you be able to see it? The way you describe “unknown” sources you seem to be looking for point sources rather than a broad diffusive source.

Section 2.6 in part describes how data are filtered for this analysis but it does not say how many data are remaining. I’m wondering if enough data are available to do seasonal-level analysis?

Section 2.7 – Lamb et al. 2016 gives an estimate for the SSLF based on plume dispersion modeling that would seem worth including since it an independent metric based on direct measurements. That paper says the estimates for SSLF from GHGRP, aircraft mass balance and plume dispersion modeling all yield similar estimates. Whereas in this section you give a wider range for the SSLF, citing essential the same sources as Lamb et al. 2016.

The paragraph starting at line 361 is confusing. Presumably errors were thoroughly assessed in the original work. Are you saying that you are not sure if doing the inversion for the smaller domain is valid? What kind of sensitivity analysis do you think is needed? Same comment for the sentence in the conclusions section starting at line 518. It makes it sound like you have reason to doubt this result, but you haven’t said what the reasons are other than “inversions can have large errors.”

Lines 25-26: Suggest: “…within the error range of one of the two inventories.”
Line 27: Suggest: “…higher than inventory estimates.”
Line 28: Suggest: “…spatially heterogenous and temporally variable. Variability in background mole fractions observed at any given moment and a single location could be…”
Line 31: Suggest: “from most wind directions”  “for most wind directions”
Line 35: Suggest: “No significant unknown CH4 sources are found.”  “We found no evidence for significant CH4 point sources that are otherwise missing from the inventories.”
Line 37: Suggest: “Other sources, such as leaks from the natural gas (NG) distribution system, are localized and transient, and do not appear to be a consistently large source of CH4 emissions in Indianapolis.”  “Leaks from the NG distribution system that were detected with the tower network appeared localized on non-permanent and do not appear to constitute as large of a source of CH4 as previously hypothesized by some top-down studies”
Line 39: Suggest deleting the sentence: “However some uncertainty…”
Line 51: Delete “, however, ”
Line 59: Miller et al. is not the correct citation for the EPA and EDGAR inventories.
Line 72: Suggest: “For example, two studies (McKain, Hendrick) indicate that…”
Line 76: Suggest: “due to fugitive emissions from NG infrastructure in urban environments.”
Line 96: Suggest: “estimates”  “quantification”
Line 97: Delete “estimates”
Line 99: “which are”  “specifically”
Line 109: Suggest: “from Cambaliza et al. (2015) which used”
Line 113: Delete: “This uncertainty has not yet been resolved.”
Line 118-119: Suggest for clarity: “(1) inconsistent geographic boundaries between top-down and bottom-up studies, (2) heterogeneity in the urban scale CH4 background and (3) temporal variability in urban emissions, both of which were not accounted for in top-down studies, and (4)…”
Line 138: “uggest: has been”  “was”
Line 160: CH4 scale information is irrelevant.
Line 163: Suggest: “was”  “were”
Line 168: Suggest: The accuracy of the wind speed measurements are…”
Line 181: Suggest: “…when the determined BLH is unreliable…”
Line 197: Suggest: “Aircraft mass balance studies of Indianapolis mentioned used two main…”
Line 244: Suggest: “…slow transport across the city”
Line 275: “upwind”  “downwind”?
Line 326: Suggest deleting “contigent on how much of the total city emissions are coming from NG.”
Line 333: NG distribution leaks are not mentioned as potential sources.
Line 338: Suggest: “An important question remains of whether SSLF or NG is the dominant CH4 source in Indianapolis, or whether they are approximately equal.”
Line 340: “…as described above” – not clear where you are referring to. Could just delete this last sentence.
Line 379: “enveloping”  “flowing into”
Lines 434, 439: Figure 7  5
Line 458: consistent with the larger error bar of Lamb et al. (2016) inventory calculation.
Line 475: Suggest: “Based on these observations it can be concluded that there are no other point source in Marion County comparable in size to the SSLF.
486: Suggest: “an urban”  “a specific”
Line 501: Suggest deleting: “it is imperative…”, so it reads, “However, our flux estimates…”
Line 508: Suggest: “We have examined four potential contributions to discrepancies between urban top-down and bottom-up estimates of CH4 emissions from Indianapolis: domain definition, heterogeneous background mole fractions, temporal variability in emissions, and sources missing from inventories.”
Line 523: “towers are selected”  “estimates”
Line 532: “Low sample volumes”  “small sample sizes”
Line 542: “noticeable smaller than”  “which is half the magnitude of”
Line 549: “than their individual components as”  “than several of the individual estimates”
Line 580: “since its CH4 emissions are comparable in magnitude to”  “since the enhancement signal from its CH4 emissions is comparable in magnitude to variability in”

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ED: Reconsider after major revisions (13 Nov 2019) by Neil Harris

AR by Nikolay Balashov on behalf of the Authors (08 Dec 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 Jan 2020) by Neil Harris

RR by Anonymous Referee #3 (05 Feb 2020)

Suggestions for revision or reasons for rejection

The paper has improved through the review process, but I still have a significant concern that the authors draw conclusions to eliminate the possibility of a large underestimated NG source that are not fully supported by the evidence in the paper. On the topic of whether or not there is evidence for a significant missing NG source in Indianapolis, the authors seem to be posing the wrong the question. The relevant question is not whether NG or the SSLF is the largest source in Indianapolis. No one disputes that there is a large landfill source in the middle of Indianapolis. The question which is relevant to the existing literature and society is whether NG emissions are significantly underestimated by the inventory. Can you reject the hypothesis that the 40% difference between top-down and bottom-up that remains after the domain issue is fixed is due to NG leaks? This question needs to be clearly and consistently addressed in the text. At a minimum, I would like to see the following statements in the paper revised to accurately and consistently represent the evidence given.

Abstract, Line 43: “Leaks from the natural gas distribution system that were detected with the tower network appeared localized and non-permanent and do not appear to constitute as large of a source of CH4 as previously hypothesized by some top-down studies.” It sounds like you are providing evidence to reject the findings of previous top-down studies, but you are actually only providing evidence for the absence of point sources rather than a diffuse source comprised of many small point sources. This sentence needs rephrasing to more fairly represent the evidence given in the paper.

Line 463: Please give a number for “somewhat”

Line 500: “Thus, the diffuse NG source suspected to be twice as large as the SSLF source (Lamb et al., 2016) does not appear to be supported by these data.” This is true for large point sources but not for a broad diffuse source. Please reword or add a sentence to reflect such a possibility.

Line 574: “Analysis of the INFLUX observation data suggests that inventories for Indianapolis are mostly accurate and that there is no clear evidence of a large, diffuse NG source of CH4 as implied by Lamb et al. (2016).” I take issue with the “mostly accurate” phrasing since you still have a significant difference between top-down and bottom-up (Line 369).

Other comments:

Abstract, Line 47: I do not understand the meaning of “real”. Please rephrase.

Line 27: Suggest rewording “…and (4) the presence of unknown CH4 sources.” –> “…and (4) CH4 sources that are not accounted for in the inventory” or “…and (4) CH4 source types that are absent from the inventory”

Line 30: Suggset rewording “…about 35% and thereby lessens the discrepancy by bringing total city flux within the error range of one of the two inventories.” -> “…about 35%, thereby lessening the discrepancy and bringing total city flux within the error range of one of the two inventories.”

Line 125: Not clear what “both” refers to

Line 139-140: suggest: “and boundary layer depth compared to nearby rural areas…”

Line 494: Why do you think the plumes are from the residential sector? State your evidence or else remove.

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ED: Reconsider after major revisions (10 Feb 2020) by Neil Harris

AR by Nikolay Balashov on behalf of the Authors (25 Feb 2020) Author's response Manuscript

ED: Publish subject to technical corrections (04 Mar 2020) by Neil Harris

AR by Nikolay Balashov on behalf of the Authors (12 Mar 2020) Author's response Manuscript

Short summary

An accurate independent verification methodology to estimate methane (a powerful greenhouse gas) emissions is essential for the effective implementation of policies that aim to reduce the impacts of climate change. In this paper, four uncertainties that complicate the independent estimation of urban methane emissions are identified: the definition of urban domain, background heterogeneity, emissions temporal variability, and missing sources. Ways to improve emission estimates are suggested.