Identifying and quantifying source contributions of air quality contaminants during unconventional shale gas extraction

Orak, Nur H.; Reeder, Matthew; Pekney, Natalie J.

doi:https://doi.org/10.5194/acp-21-4729-2021

Articles | Volume 21, issue 6

https://doi.org/10.5194/acp-21-4729-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-21-4729-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 21, issue 6

Research article

|

26 Mar 2021

Research article |

| 26 Mar 2021

Identifying and quantifying source contributions of air quality contaminants during unconventional shale gas extraction

Nur H. Orak, Matthew Reeder, and Natalie J. Pekney

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Final revised paper (published on 26 Mar 2021)
Supplement to the final revised paper
Preprint (discussion started on 08 Sep 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

AC1: 'Response to Reviewers', Nur Orak, 20 Oct 2020
- EC1: 'Quick review', Thomas Karl, 17 Nov 2020
RC1: 'Solicited Referee Comments', Anonymous Referee #2, 23 Nov 2020
RC2: 'Review of Orak et al.', Anonymous Referee #1, 26 Nov 2020

Peer-review completion

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

AR by Nur Orak on behalf of the Authors (07 Dec 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (10 Dec 2020) by Thomas Karl

RR by Anonymous Referee #1 (21 Dec 2020)

Suggestions for revision or reasons for rejection

The author’s made improvements to the article but many of the reviewer questions remain unanswered/unclear. This is a valuable dataset but the analysis and manuscript could still use improvement in my opinion. I completely understand that there will be future papers coming from this unique dataset, but it would be very helpful to the readers to know what is slated for future work to help put this analysis into better context.

Line 99-102: Hecobian specifically compares the emissions of distinct operation phases of natural gas extraction. The analysis presented here would be greatly improved by doing a more direct comparison of the emissions at each stage of well production with Hecobian (e.g., for lines 276-281). Both datasets are incredibly rare and valuable, so it would be really helpful to see if they compare for gas fields in different regions of the U.S. and would further help to put Orak measurements into context.

The addition of the trailer location to Figure 1 is very helpful. This combined with Figure 2 highlights the fact that the wind was rarely from the SE sector where the majority of the drilling equipment, and presumably the drilling activity, was occurring. It would be really helpful to show what the air composition was as a function of wind direction. Was methane, ethane, etc. higher when wind was coming from the SE sector? Does Factor 1 of the PMF results have the highest contribution to the ambient measurements when wind is from this direction? This was one of the unaddressed questions (Reviewer 1, Q1). You should have a timeseries of each factor of your final PMF model that can then be analyzed as a function of any other variable that was NOT included in the PMF such as wind direction. If the Natural Gas factor is most prevalent in the SE sector, then it further adds confidence in the analysis.

Figure 3 is the key figure in this manuscript, in my opinion. This clearly shows the different chemical composition of the air measured during each of the important drilling/hydraulic fracturing/production steps. Why doesn’t PMF pick up these differences and lump all into a single generic factor? How do these results compare to other studies?

Lines 288-292: Be sure to specify the units as you are comparing the ethane (ppb) to methane (ppm) ratioor else add 10^-3 to the ratios. How does the ethane to methane ratio compare to other ONG emissions (i.e., Yakovitch et al. and may other papers)?

Lines 328: Source Profiles. Another useful reference regarding PMF analysis in an oil and gas field to add to the discussion could be:
Source characterization of volatile organic compounds in the Colorado Northern Front Range Metropolitan Area during spring and summer 2015. A. Abeleira, I. B. Pollack, B. Sive, Y. Zhou, E. V. Fischer and D. K. Farmer. 122(6), 3595-3613, doi:https://doi.org/10.1002/2016JD026227, 2017

Figure 6: I’m still trying to figure out why CO2 and methane have virtually the same PMF factor fingerprints. Clearly, the natural gas factor isn’t just natural gas as raw natural gas does NOT contain NOx and I would not expect it to be composed primarily of CO2. Also, the engine emissions factor doesn’t contain an appreciable amount of CO2. Why? It seems to me that the PMF factors aren’t fully resolving in a meaningful way. Also, why do hexane and benzene not have any attribution to “regional transport” as these two species are sufficiently long-lived in the atmosphere to have a significant background, much more so than toluene that has ~25% attributed to transport?

The last, “big picture”, piece of this analysis that is missing is how the natural gas drilling activities actually affected air quality, which is stated as being the motivation for this paper. Since you have the PMF factors for each species, then you should be able to answer the question of how the air quality would be different if the “natural gas” factor was removed or how it compared to the other factors by calculating an Air Quality Index or OH-reactivity, or some other metric.

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ED: Publish subject to minor revisions (review by editor) (15 Feb 2021) by Thomas Karl

AR by Nur Orak on behalf of the Authors (17 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (19 Feb 2021) by Thomas Karl

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Short summary

In this paper, we investigate the effect of unconventional natural gas development activities on local air quality. This is the first study, to our knowledge, to collect high-time-resolution ambient concentrations of compounds emitted from well pad activity on Marcellus Shale during various phases of operation such that the relative air quality effect of each phase of development can be investigated.