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:

1. 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?
2. 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.

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 “S_x”

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 ΔS_VOC and ΔS_NOx 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.