the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Nov 2024
Seasonal trends in the wintertime photochemical regime of the Uinta Basin, Utah, USA
Abstract. Several lines of evidence indicate that the photochemical regime, i.e., the degree to which ozone production is either VOC- or NOx-limited, varies with season in the Northern Hemisphere. VOC-sensitivity seems to be more likely in winter and NOx-sensitivity in summer. For most regions, the question is patently academic, since excessive ozone occurs only in summer. However, the Uinta Basin in Utah, USA exhibits ozone in excess of regulatory standards in both winter and summer. We have performed extensive F0AM box modelling to better understand these trends. The models indicate that in late December the Basin’s ozone system is VOC-sensitive, and either NOx-insensitive or NOx-saturated. Sensitivity to NOx grows throughout the winter, and in early March, the system is about equally sensitive to VOC and NOx. The main driver for this trend is the increase in available solar energy as indicated by the noontime solar zenith angle. A secondary driver is a decrease in precursor concentrations throughout the winter, which decrease because of, first, a dilution effect as thermal inversions weaken, and second, an emission effect because certain emission sources are stronger at colder temperatures. On the other hand, temperature and absolute humidity are not important direct drivers of the trend.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Supplement
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2024-3114', Gail Tonnesen, 07 Nov 2024
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AC3: 'Reply on CC1', Marc Mansfield, 13 Mar 2025
At times, the reviewers have requested clarifications that could best be answered by performing additional model runs, but we believe such requests are beyond the scope of the study. Moreover, because of personnel changes at our Institute, running such models would place an undue burden on our staff and cannot be accomplished.
- Apologies to Dr. Tonnesen for missing her reference during the first go-round. It has now been added to the manuscript.
- Figure 3 does include satellite data for all days in the indicated periods. We agree that limiting the analysis to high ozone days would be useful, but it would also intensify the concerns you mentioned about a shallow boundary layer. Figures 2 and 3 are not the central crux of the paper – they only indicate trends that we evaluate exhaustively in the remainder of the text. The possibility of bias in column data when we are only interested in a shallow boundary layer is a significant, open question. And one that is beyond the scope of the current work.
- All the axis labels have been converted to ppb or ppm units. For reasons mentioned above, we cannot do any runs at higher NOx.
Citation: https://doi.org/10.5194/egusphere-2024-3114-AC3
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AC3: 'Reply on CC1', Marc Mansfield, 13 Mar 2025
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RC1: 'Comment on egusphere-2024-3114', Anonymous Referee #1, 11 Nov 2024
Review of Mansfield & Lyman “Seasonal trends in the wintertime photochemical regime of the Uinta Basin, Utah, USA” 2024
This study uses observational data and a chemical box model to explore the changing sensitivities of wintertime ozone production in the Uinta Basin. Wintertime ozone pollution events have been a significant air quality issue in the Uinta Basin for at least the past decade and this analysis provides highly relevant conclusions for potential mitigation strategies. As such this work represents a valuable contribution to the field and will be of interest to a wide range of readers from academic atmospheric chemists through to applied air quality practitioners. The paper is well written and easy to follow, and I recommend publication once the following comments have been addressed.
Comments:
- A more detail justification is required on the approach used for constraining the VOCs in the model simulations. In particular, where did the VOC scaling factors used (Table 2) come from and how sensitive are the conclusions of this work to the assumption made that VOC speciation does not vary? Is there any observational data that supports this assumption?
- The paper would be greatly improved if the authors discussed the chemistry that explains the sensitivities they see in their model simulations. This would not only help further support and justify the conclusions, but also help the work be extrapolated to other scenarios / seasons. Ultimately NOx and VOC sensitive regimes are explained by the fates of the hydroxyl (OH) radical and the peroxy (HO2 and RO2) radicals that are involved in the catalytic cycles that oxidise VOCs and convert NO to NO2. NOx saturated regimes are also determined by the fate of the radicals but also the increased rate of removal of ozone through reaction with NO. The radical source strength also plays a significant role in the amount of ozone produced in a particular chemical regime and is likely the reason for the observed sensitivity to solar zenith angle and also the impact of snow cover, and potentially the small sensitivity to temperature. This has implications for my first comment, as different VOCs regenerate varying quantities of secondary radicals during their photo-oxidation (e.g. Edwards et al. 2014) which will change the effectiveness of ozone production and in extreme cases can alter the chemical sensitivities, especially when the system is in a transitional regime like many of the datapoints in this work seem to be. By explaining the competition between NOx and VOCs for reaction with OH and the competition between NOx and self-reaction for the peroxy radicals in the various scenarios explored the authors will support their findings through chemical understanding rather than just presenting them. All the information required for this should be readily extractable from the F0AM box model used.
Citation: https://doi.org/10.5194/egusphere-2024-3114-RC1 -
AC2: 'Reply on RC1', Marc Mansfield, 13 Mar 2025
At times, the reviewers have requested clarifications that could best be answered by performing additional model runs, but we believe such requests are beyond the scope of the study. Moreover, because of personnel changes at our Institute, running such models would place an undue burden on our staff and cannot be accomplished.
- He/she requests a more detailed explanation of the approach used to constrain VOC concentrations. Two sentences beginning with “The VOC speciation profile … ,” line 197, have been added. The reviewer’s additional request, to examine sensitivity to variations in VOC speciation, would require additional modeling, and we feel this is beyond the scope of the current study.
- He/she requests additional discussion of the chemistry. In response, we have added a paragraph to the introduction, lines 33 - 44.
Citation: https://doi.org/10.5194/egusphere-2024-3114-AC2
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RC2: 'Comment on egusphere-2024-3114', Anonymous Referee #2, 04 Dec 2024
Mansfield and Lyman present modeling results aimed at understanding the seasonal trends in wintertime ozone formation sensitivity to NOx and VOCs, and what dictates this change in Utah’s Uinta Basin. Using F0AM box modeling, they determine that the basin is always sensitive to VOC reductions, but that the NOx sensitivity changes during the winter, starting at NOx-saturated in December and moving to NOx-sensitive towards March. They demonstrate through additional modeling that this change is largely driven by the changing actinic flux, somewhat by changing emissions, and not strongly by temperature or relative humidity.
This is an interesting analysis that sheds new light on how NOx and VOC sensitivity can change over a given season, and provides a scientific bases for recommending emissions reductions. I would recommend publication, but first would like to see some of the modeling and experimental details described further.
Line 7 – 8: The authors state that NOx-sensitivity is more likely in the summer. However, there are many papers showing that this really depends where you are, relative to the sources. So I recommend not making this generalization, since it’s not needed for this analysis anyway.
Line 24: I recommend stating more clearly that these are ozone events due to multi-day thermal inversions, to differentiate from the typical nighttime inversions.
Line 35: Suggest stating your definition of Ox and NOz here.
Line 52: Isn’t the Basin both VOC-sensitive and NOx-saturated? Why state this as “or”?
Line 62: The authors mention the difference between Edwards et al and this study as a difference in how VOC speciation is employed, but do not describe this any further. What is different about the speciation?
Section 2.1, paragraph 1: More details should be provided about the measurements used. What is measured at Horsepool? How was NOz determined? The Ox vs NOz trend line can often been fairly noisy. Were there any filters used to remove days when Ox and NOz were not well correlated? What hours of the day were used in the “daily” slope?
Line 70: One major question I have is whether the results from this paper are generalizable to the whole basin, or whether they are specific to the Horsepool region and its nearby gas wells. Do you expect the results to be different in, say, Duchesne, which is more influenced by oil wells with a different VOC speciation profile? One way to address this would be to examine the column HCHO to NO2 from OMI at different pixels within the Uinta Basin, to observe whether there are significant differences in the seasonal trends.
Figure 1: It would be helpful to have the relevant OMI pixel used in Figure 3 marked in this figure to show its scale.
Figure 3: I suggest putting a band between 1 and 2 to show the nominal cutoff region between VOC and NOx sensitivity.
Line 119: What are these sites from which you retrieve daily temperature? Is this a citable reference?
Line 125: As before, I suggest changing “persistent” to “multi-day” inversions, as persistent can be vague.
Figure 7 and 8: These could be move to the SI, since they are quite minor results.
Line 164: What is the reasoning behind assuming that drilling rigs behave similarly to diesel-powered vehicles?
Line 174: It’s not clear what the relevance is of the ratio of means in Figure 5 (mislabeled here as Figure 4). Since the authors say nothing should be read into this difference, I recommend deleting this entire paragraph.
Table 1: More detail is need about how this VOC profile was constructed and how it differs from Edwards 2014. Furthermore, since CH4 and non-methane hydrocarbons have different sources, what is the justifications for scaling them together?
Line 219: Change “S” to “Sx”
Line 222: How and why were these 24 days selected. Presumably there were many more than 24 wintertime ozone exceedances over 8 years?
Line 226: How was “agreement with measurements” determined?
Line 250: The statement about replacing a variable with an earlier or later model is confusing. Why link to a different model rather than just scaling the variable within a certain range?
Line 253: I disagree that this makes the interpretation easier. I would suggest just leaving the x-axis in its normal orientation and allowing the readers to interpret the trends.
Figure 10: The y-axis labels should have “VOC” and “NOx” subscripted, rather than in brackets
Table 4: It seems that the ΔSVOC and ΔSNOx vertical displacement range is only relevant for this particular range in the variables SZA, NOx, etc. I think this table isn’t adding much and could be deleted.
Section 3.3: It isn’t clear in the figure what data is the model result and what is the kriging interpolation between values, so it’s hard to understand how much of this isopleth is based on true results and how much is an estimate.
Citation: https://doi.org/10.5194/egusphere-2024-3114-RC2 -
AC1: 'Reply on RC2', Marc Mansfield, 13 Mar 2025
Responses to Reviewer 2
At times, the reviewers have requested clarifications that could best be answered by performing additional model runs, but we believe such requests are beyond the scope of the study. Moreover, because of personnel changes at our Institute, running such models would place an undue burden on our staff and cannot be accomplished.
- The line about summertime NOx sensitivity has been struck from the abstract.
- The modifier “multi-day” has been added at line 24.
- Definitions of Ox and NOz are now supplied at line 47.
- We are using more restrictive definitions of these terms. So we will keep the word “or,” but we have also added the parenthetical phrase “(in the more restrictive …)” at line 65.
- We chose to use our own VOC speciation data since it was more recent and more extensive. We have added the phrase “based on a more recent, extensive measurement set …” at line 75.
- A new section, Section 2.1, lines 79-87, has been added to describe the measurement protocols. No filtering of data to improve the correlation between Ox and NOz was applied.
7, 8. It is true that we only used data from the Horsepool station. Emission sources do vary throughout the Basin (e.g., population centers versus isolated areas, different ratios of oil versus gas production). So it is conceivable that the photochemical regime may be different in those areas. However, addressing seasonal changes in photochemical regime at other locations in the Basin would require additional modeling and is beyond the scope of this manuscript. As it is, it was a major undertaking for us to generate the 24 isopleth plots presented here. We have high confidence the same trend (i.e., moving towards more NOx sensitivity later in the winter) occurs throughout the Basin, for reasons described in the text. Because of the size of the Basin relative to the resolution of the OMI instrument, we are not confident the suggested method would be effective. While we agree that a study of other Uinta Basin locations would be useful, we believe that the points made in the paper are well supported using just the Horsepool location.
- Figures 2 and 3 have been modified as suggested.
- The selection of sites is explained completely in the two Mansfield & Hall papers cited at line 140. We don’t feel there is a need to add to the discussion here.
- The modifier “multi-day” has been inserted at line 149.
- The two figures have been moved to the Supplemental Information.
- We see no need for any revision here. After all, we did say “may behave similarly” at line 181.
- We have deleted those sentences.
- We addressed this question in paragraph 5 above.
- As explained in Section 2.5, we are using “S” to represent a generic sensitivity. We only add a subscript when necessary to indicate sensitivity to something.
- We assume confusion has arisen because we said we analyzed 24 high-ozone “days.” We have reworded the first paragraph of section 3.1 and Table 3 to indicate that we selected 24 multi-day inversion episodes. That includes a significant majority of the ozone events during the indicated period. Figure 10 shows us that there were 160 exceedance days total. If each episode is 5 days long on average, that only gives 32 episodes. 24 episodes do not constitute the complete list, but they come close.
- We modified the sentence to read “agreed with the peak-ozone measurement,” line 237.
- We clearly explain at line 261 that we are probing the effect of changing just one variable. And frankly, it’s just a different way, in the words of the reviewer, to “scale a variable within a certain range.” We applied a different, yet still typical, value of that one variable.
- We disagree and choose to keep the figure as is. The slope of the line tells the reader whether the variable drives an increase or a decrease in sensitivity. (See also paragraph 21 below.)
- We feel that Figure 8 y-axis labels are adequately descriptive.
- Perhaps, but we believe the Table is useful and we prefer to keep it. It indicates at a glance the important driving variables.
- We believe that modifying the diagrams to indicate which pixels were calculated and which were krigged would make them extremely difficult to read. To clarify the question, we have indicated in Section 3.3 the resolution (one-tenth) at which pixels were calculated. And let me finish with a positive plug for krigging. It is an excellent two-dimensional interpolation procedure, especially at 10% resolution and with all pixels calculated at the boundaries. Those pixels may be estimated, but they are good estimates.
Citation: https://doi.org/10.5194/egusphere-2024-3114-AC1
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AC1: 'Reply on RC2', Marc Mansfield, 13 Mar 2025
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2024-3114', Gail Tonnesen, 07 Nov 2024
This is a useful and important study with interesting results that are highly relevant for managing air quality in the Uinta Basin.
At line 40, I suggest also citing Tonnesen and Dennis 2000 (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999JD900372) who first proposed HCHO/NO2 as an indicator of O3 sensitivity based on an analysis of radical propagation efficiency, with high HCHO associated with more OH reacting with VOC, and high NO2 associated more with OH reacting with NO2. Maximum values of OH production and O3 production occur at intermediate ratios of HCHO/NO2 that maximize radical propagation efficiency.
Does Figure 3 include satellite data for all days or only for O3 greater than 60 ppb? It should be limited to the high O3 days because we are interested in the HCHO/NO2 ratio for high O3 days. More description of the OMI data is also needed to determine if it is useful for this application - high ozone days have very shallow inversion layers on the order of 100 to 300 meters. How sensitive is OMI to HCHO and NO2 in a shallow surface layer? Is it mostly detecting HCHO and NO2 in the column above the inversion layer? It would be interesting to use the model sensitivity results to evaluate the ratio of HCHO/NO2 associated with the transition from NOx-saturated to NOx-limited conditions. It is possible that the transition ratio is different for winter O3 chemistry compared to summer.
Fig 11. It would be useful to show actual VOC and NOx concentration in the isopleth plots, instead of arbitrary units, to get a sense of the VOC/NOx ratios associated with peak O3 and NOx-saturated vs NOx limited conditions. Previous studies fond maximum O3 production on the ridgeline of the isopleth plot at about 10 VOC/NOx in summer and 100 VOC/NOx in winter (as ppbC/ppb). Also, I’m surprised that the plots do not show a more prominent peak O3 ridgeline with a more rapid decrease in O3 at higher NOx saturated conditions. I suggest including higher NOx levels in the isopleth so that we can see the bending of contour lines to lower O3 concentrations in the NOx saturated region of the isopleth, for example as in Supplement Figure 5.
Citation: https://doi.org/10.5194/egusphere-2024-3114-CC1 -
AC3: 'Reply on CC1', Marc Mansfield, 13 Mar 2025
At times, the reviewers have requested clarifications that could best be answered by performing additional model runs, but we believe such requests are beyond the scope of the study. Moreover, because of personnel changes at our Institute, running such models would place an undue burden on our staff and cannot be accomplished.
- Apologies to Dr. Tonnesen for missing her reference during the first go-round. It has now been added to the manuscript.
- Figure 3 does include satellite data for all days in the indicated periods. We agree that limiting the analysis to high ozone days would be useful, but it would also intensify the concerns you mentioned about a shallow boundary layer. Figures 2 and 3 are not the central crux of the paper – they only indicate trends that we evaluate exhaustively in the remainder of the text. The possibility of bias in column data when we are only interested in a shallow boundary layer is a significant, open question. And one that is beyond the scope of the current work.
- All the axis labels have been converted to ppb or ppm units. For reasons mentioned above, we cannot do any runs at higher NOx.
Citation: https://doi.org/10.5194/egusphere-2024-3114-AC3
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AC3: 'Reply on CC1', Marc Mansfield, 13 Mar 2025
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RC1: 'Comment on egusphere-2024-3114', Anonymous Referee #1, 11 Nov 2024
Review of Mansfield & Lyman “Seasonal trends in the wintertime photochemical regime of the Uinta Basin, Utah, USA” 2024
This study uses observational data and a chemical box model to explore the changing sensitivities of wintertime ozone production in the Uinta Basin. Wintertime ozone pollution events have been a significant air quality issue in the Uinta Basin for at least the past decade and this analysis provides highly relevant conclusions for potential mitigation strategies. As such this work represents a valuable contribution to the field and will be of interest to a wide range of readers from academic atmospheric chemists through to applied air quality practitioners. The paper is well written and easy to follow, and I recommend publication once the following comments have been addressed.
Comments:
- A more detail justification is required on the approach used for constraining the VOCs in the model simulations. In particular, where did the VOC scaling factors used (Table 2) come from and how sensitive are the conclusions of this work to the assumption made that VOC speciation does not vary? Is there any observational data that supports this assumption?
- The paper would be greatly improved if the authors discussed the chemistry that explains the sensitivities they see in their model simulations. This would not only help further support and justify the conclusions, but also help the work be extrapolated to other scenarios / seasons. Ultimately NOx and VOC sensitive regimes are explained by the fates of the hydroxyl (OH) radical and the peroxy (HO2 and RO2) radicals that are involved in the catalytic cycles that oxidise VOCs and convert NO to NO2. NOx saturated regimes are also determined by the fate of the radicals but also the increased rate of removal of ozone through reaction with NO. The radical source strength also plays a significant role in the amount of ozone produced in a particular chemical regime and is likely the reason for the observed sensitivity to solar zenith angle and also the impact of snow cover, and potentially the small sensitivity to temperature. This has implications for my first comment, as different VOCs regenerate varying quantities of secondary radicals during their photo-oxidation (e.g. Edwards et al. 2014) which will change the effectiveness of ozone production and in extreme cases can alter the chemical sensitivities, especially when the system is in a transitional regime like many of the datapoints in this work seem to be. By explaining the competition between NOx and VOCs for reaction with OH and the competition between NOx and self-reaction for the peroxy radicals in the various scenarios explored the authors will support their findings through chemical understanding rather than just presenting them. All the information required for this should be readily extractable from the F0AM box model used.
Citation: https://doi.org/10.5194/egusphere-2024-3114-RC1 -
AC2: 'Reply on RC1', Marc Mansfield, 13 Mar 2025
At times, the reviewers have requested clarifications that could best be answered by performing additional model runs, but we believe such requests are beyond the scope of the study. Moreover, because of personnel changes at our Institute, running such models would place an undue burden on our staff and cannot be accomplished.
- He/she requests a more detailed explanation of the approach used to constrain VOC concentrations. Two sentences beginning with “The VOC speciation profile … ,” line 197, have been added. The reviewer’s additional request, to examine sensitivity to variations in VOC speciation, would require additional modeling, and we feel this is beyond the scope of the current study.
- He/she requests additional discussion of the chemistry. In response, we have added a paragraph to the introduction, lines 33 - 44.
Citation: https://doi.org/10.5194/egusphere-2024-3114-AC2
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RC2: 'Comment on egusphere-2024-3114', Anonymous Referee #2, 04 Dec 2024
Mansfield and Lyman present modeling results aimed at understanding the seasonal trends in wintertime ozone formation sensitivity to NOx and VOCs, and what dictates this change in Utah’s Uinta Basin. Using F0AM box modeling, they determine that the basin is always sensitive to VOC reductions, but that the NOx sensitivity changes during the winter, starting at NOx-saturated in December and moving to NOx-sensitive towards March. They demonstrate through additional modeling that this change is largely driven by the changing actinic flux, somewhat by changing emissions, and not strongly by temperature or relative humidity.
This is an interesting analysis that sheds new light on how NOx and VOC sensitivity can change over a given season, and provides a scientific bases for recommending emissions reductions. I would recommend publication, but first would like to see some of the modeling and experimental details described further.
Line 7 – 8: The authors state that NOx-sensitivity is more likely in the summer. However, there are many papers showing that this really depends where you are, relative to the sources. So I recommend not making this generalization, since it’s not needed for this analysis anyway.
Line 24: I recommend stating more clearly that these are ozone events due to multi-day thermal inversions, to differentiate from the typical nighttime inversions.
Line 35: Suggest stating your definition of Ox and NOz here.
Line 52: Isn’t the Basin both VOC-sensitive and NOx-saturated? Why state this as “or”?
Line 62: The authors mention the difference between Edwards et al and this study as a difference in how VOC speciation is employed, but do not describe this any further. What is different about the speciation?
Section 2.1, paragraph 1: More details should be provided about the measurements used. What is measured at Horsepool? How was NOz determined? The Ox vs NOz trend line can often been fairly noisy. Were there any filters used to remove days when Ox and NOz were not well correlated? What hours of the day were used in the “daily” slope?
Line 70: One major question I have is whether the results from this paper are generalizable to the whole basin, or whether they are specific to the Horsepool region and its nearby gas wells. Do you expect the results to be different in, say, Duchesne, which is more influenced by oil wells with a different VOC speciation profile? One way to address this would be to examine the column HCHO to NO2 from OMI at different pixels within the Uinta Basin, to observe whether there are significant differences in the seasonal trends.
Figure 1: It would be helpful to have the relevant OMI pixel used in Figure 3 marked in this figure to show its scale.
Figure 3: I suggest putting a band between 1 and 2 to show the nominal cutoff region between VOC and NOx sensitivity.
Line 119: What are these sites from which you retrieve daily temperature? Is this a citable reference?
Line 125: As before, I suggest changing “persistent” to “multi-day” inversions, as persistent can be vague.
Figure 7 and 8: These could be move to the SI, since they are quite minor results.
Line 164: What is the reasoning behind assuming that drilling rigs behave similarly to diesel-powered vehicles?
Line 174: It’s not clear what the relevance is of the ratio of means in Figure 5 (mislabeled here as Figure 4). Since the authors say nothing should be read into this difference, I recommend deleting this entire paragraph.
Table 1: More detail is need about how this VOC profile was constructed and how it differs from Edwards 2014. Furthermore, since CH4 and non-methane hydrocarbons have different sources, what is the justifications for scaling them together?
Line 219: Change “S” to “Sx”
Line 222: How and why were these 24 days selected. Presumably there were many more than 24 wintertime ozone exceedances over 8 years?
Line 226: How was “agreement with measurements” determined?
Line 250: The statement about replacing a variable with an earlier or later model is confusing. Why link to a different model rather than just scaling the variable within a certain range?
Line 253: I disagree that this makes the interpretation easier. I would suggest just leaving the x-axis in its normal orientation and allowing the readers to interpret the trends.
Figure 10: The y-axis labels should have “VOC” and “NOx” subscripted, rather than in brackets
Table 4: It seems that the ΔSVOC and ΔSNOx vertical displacement range is only relevant for this particular range in the variables SZA, NOx, etc. I think this table isn’t adding much and could be deleted.
Section 3.3: It isn’t clear in the figure what data is the model result and what is the kriging interpolation between values, so it’s hard to understand how much of this isopleth is based on true results and how much is an estimate.
Citation: https://doi.org/10.5194/egusphere-2024-3114-RC2 -
AC1: 'Reply on RC2', Marc Mansfield, 13 Mar 2025
Responses to Reviewer 2
At times, the reviewers have requested clarifications that could best be answered by performing additional model runs, but we believe such requests are beyond the scope of the study. Moreover, because of personnel changes at our Institute, running such models would place an undue burden on our staff and cannot be accomplished.
- The line about summertime NOx sensitivity has been struck from the abstract.
- The modifier “multi-day” has been added at line 24.
- Definitions of Ox and NOz are now supplied at line 47.
- We are using more restrictive definitions of these terms. So we will keep the word “or,” but we have also added the parenthetical phrase “(in the more restrictive …)” at line 65.
- We chose to use our own VOC speciation data since it was more recent and more extensive. We have added the phrase “based on a more recent, extensive measurement set …” at line 75.
- A new section, Section 2.1, lines 79-87, has been added to describe the measurement protocols. No filtering of data to improve the correlation between Ox and NOz was applied.
7, 8. It is true that we only used data from the Horsepool station. Emission sources do vary throughout the Basin (e.g., population centers versus isolated areas, different ratios of oil versus gas production). So it is conceivable that the photochemical regime may be different in those areas. However, addressing seasonal changes in photochemical regime at other locations in the Basin would require additional modeling and is beyond the scope of this manuscript. As it is, it was a major undertaking for us to generate the 24 isopleth plots presented here. We have high confidence the same trend (i.e., moving towards more NOx sensitivity later in the winter) occurs throughout the Basin, for reasons described in the text. Because of the size of the Basin relative to the resolution of the OMI instrument, we are not confident the suggested method would be effective. While we agree that a study of other Uinta Basin locations would be useful, we believe that the points made in the paper are well supported using just the Horsepool location.
- Figures 2 and 3 have been modified as suggested.
- The selection of sites is explained completely in the two Mansfield & Hall papers cited at line 140. We don’t feel there is a need to add to the discussion here.
- The modifier “multi-day” has been inserted at line 149.
- The two figures have been moved to the Supplemental Information.
- We see no need for any revision here. After all, we did say “may behave similarly” at line 181.
- We have deleted those sentences.
- We addressed this question in paragraph 5 above.
- As explained in Section 2.5, we are using “S” to represent a generic sensitivity. We only add a subscript when necessary to indicate sensitivity to something.
- We assume confusion has arisen because we said we analyzed 24 high-ozone “days.” We have reworded the first paragraph of section 3.1 and Table 3 to indicate that we selected 24 multi-day inversion episodes. That includes a significant majority of the ozone events during the indicated period. Figure 10 shows us that there were 160 exceedance days total. If each episode is 5 days long on average, that only gives 32 episodes. 24 episodes do not constitute the complete list, but they come close.
- We modified the sentence to read “agreed with the peak-ozone measurement,” line 237.
- We clearly explain at line 261 that we are probing the effect of changing just one variable. And frankly, it’s just a different way, in the words of the reviewer, to “scale a variable within a certain range.” We applied a different, yet still typical, value of that one variable.
- We disagree and choose to keep the figure as is. The slope of the line tells the reader whether the variable drives an increase or a decrease in sensitivity. (See also paragraph 21 below.)
- We feel that Figure 8 y-axis labels are adequately descriptive.
- Perhaps, but we believe the Table is useful and we prefer to keep it. It indicates at a glance the important driving variables.
- We believe that modifying the diagrams to indicate which pixels were calculated and which were krigged would make them extremely difficult to read. To clarify the question, we have indicated in Section 3.3 the resolution (one-tenth) at which pixels were calculated. And let me finish with a positive plug for krigging. It is an excellent two-dimensional interpolation procedure, especially at 10% resolution and with all pixels calculated at the boundaries. Those pixels may be estimated, but they are good estimates.
Citation: https://doi.org/10.5194/egusphere-2024-3114-AC1
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AC1: 'Reply on RC2', Marc Mansfield, 13 Mar 2025
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Seth N. Lyman
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Supplement
(27009 KB) - BibTeX
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- Final revised paper
This is a useful and important study with interesting results that are highly relevant for managing air quality in the Uinta Basin.
At line 40, I suggest also citing Tonnesen and Dennis 2000 (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999JD900372) who first proposed HCHO/NO2 as an indicator of O3 sensitivity based on an analysis of radical propagation efficiency, with high HCHO associated with more OH reacting with VOC, and high NO2 associated more with OH reacting with NO2. Maximum values of OH production and O3 production occur at intermediate ratios of HCHO/NO2 that maximize radical propagation efficiency.
Does Figure 3 include satellite data for all days or only for O3 greater than 60 ppb? It should be limited to the high O3 days because we are interested in the HCHO/NO2 ratio for high O3 days. More description of the OMI data is also needed to determine if it is useful for this application - high ozone days have very shallow inversion layers on the order of 100 to 300 meters. How sensitive is OMI to HCHO and NO2 in a shallow surface layer? Is it mostly detecting HCHO and NO2 in the column above the inversion layer? It would be interesting to use the model sensitivity results to evaluate the ratio of HCHO/NO2 associated with the transition from NOx-saturated to NOx-limited conditions. It is possible that the transition ratio is different for winter O3 chemistry compared to summer.
Fig 11. It would be useful to show actual VOC and NOx concentration in the isopleth plots, instead of arbitrary units, to get a sense of the VOC/NOx ratios associated with peak O3 and NOx-saturated vs NOx limited conditions. Previous studies fond maximum O3 production on the ridgeline of the isopleth plot at about 10 VOC/NOx in summer and 100 VOC/NOx in winter (as ppbC/ppb). Also, I’m surprised that the plots do not show a more prominent peak O3 ridgeline with a more rapid decrease in O3 at higher NOx saturated conditions. I suggest including higher NOx levels in the isopleth so that we can see the bending of contour lines to lower O3 concentrations in the NOx saturated region of the isopleth, for example as in Supplement Figure 5.