|This is a second round review. This manuscript is substantially improved from the previous version and the combination of the two manuscripts was definitely warranted. There are number of errors and a need for some better organization for the manuscript to be publishable, as these do change the scientific meaning of the results in several places. Of key importance is that authors do a better job of distinguishing NOx versus NO versus NO2. This needs to be made very clear upfront in the manuscript and if the authors are going to choose to use NOx as a shorthand when referring to NO or NO2 then that needs to be spelled out specifically. NO is the primary emission from all sources in this work, except potentially diesel emissions. This is never mentioned nor discussed and has an important context here since there are no direct observations of d15NO alone and what the model is actually simulating is the release of NO, not the release of NOx. This should be rectified if the authors want the atmospheric chemistry community to be interested in this work. |
It is easiest to document suggested changes in order of the manuscript. Some are very minor typos, others are more significant and should be revisited.
It should be made clear in the abstract the difference between SMOKE and CMAQ (i.e. that the latter includes mixing and transport). This will help setup the reader for why these two models are used and what differences can be expected.
Spell out NADP since this is the first use and also make clear that these are rainwater nitrate measurements.
Page 3, Line 3: The very first word of the introduction should not be a symbol – define nitrogen oxides first, then use NOx subsequently.
Line 16: It seems strange to give a single percent (15%) and then give a range of values for soil NOx emissions. It would be better to introduce the range in total NOx emissions from global models and then point out the uncertainty range associated with individual sources such biogenic soil emissions. Also, it should be explained what soil emissions represent (ie. Nitrification and denitrification.
Line 17-18: Satellite observations should be mentioned as reference cited in this sentence include these.
Line 24: change “while acting” to “versus acting” or similar, otherwise the sentence is incomplete
Line 40-41: Natural versus anthropogenic sources is not correct here as anthropogenic sources themselves has distinct isotopic range. I suggest you remove “natural and anthropogenic” from the sentence
Page 4, line 11: change “implement” to “implementation”
Line 20: add “species” after d15NOy
Figure 1 caption: CFPP needs to be defined in the figure caption.
Page 5, line 4-7: might make sense to have this sentence be past tense since that paper is already published.
Throughout section 2 and its subsections there needs to be consistent use of n values, consistent use of ranges versus average values, consistent discussion of methodologies and there needs to appear a discussion of NOx versus NO2 versus NO. Many of the measurements are not able to be compared to one another as some measure NOx and some measure NO2.
Page 7, line 1-4: Miller et al and Yu & Elliott both use active sampling techniques for collecting soil emissions. Not clear what “using a similar methodology at the end of Line 1 means, sinc the previous study mentioned used a passive sampling technique. Further, the passive measurements should report an n value and not a range since I believe these two numbers represent two discrete measurements that represent month long averages.
Section 2.1.2 – it needs to be justified why Miller et al (2017) is not included. This was brought up in the previous review both reviewers. This study is an actual on-road study that captures the mix of tailpipe emissions. The Walter’s study is more representative of tailpipe type measurements, not the signal that is found on-road when vehicles are moving under real traffic conditions with a mix of vehicle types. The Miller study seems more representative in this case. If it is not, it should be reasoned why other values are being used.
Line 22: remove “s” in this sentence
Page 8, section 2.1.3: this section does not make mention of the methods used, nor does it distinguish what was measured in terms of NOx vs NO2.
Line 19-20: how can a range of 9 to 26 lead to an average of 4.51? Please double check.
Page 9, line 36: change the terminology of “d15NOx effect” as this is not an accurate statement. It’s really the effect on d15NOx
Line 37: unbold “not”
Page 10, line 6: change “the” to “a”
Page 11, line 3: change “to” to “in”
Page 14, line 2-3: please also define the anthropogenic fraction since that is used several times in the text below
Line 21: change “the clear” to “a clear”
Line 20-24: justify to the reader why your are selecting April to June to show here (I assume b/c this is the season when soil NOx emissions should be maximum?)
Page 15, Figure 5 caption: explain that the white box is defined in the Figure 6 caption.
Page 18, line 8: change here since April to October is not a “month”
Page 20, line 20: change to “a seasonal”
Line 22: change to “NOx was emitted”
Line 31: this line does not make sense, please rephrase.
Page 20, Figure 8: draw a line to make clear that the insert is Indiana (ie draw a line to the state to connect it) or label the smaller box just above as Indiana to make clear to the reader.
Page 21, line 18: This should start a new section as the emphasis here is no longer on seasonality and rather on the impact of different meteorological fields.
I would argue that Figure 9 and 10 could be combined with a different coloring or dashed lines to represent the different years of met fields.
Page 22, line 4: add “the” before simulation
Figure 11 is unnecessary as the text described this very well and the changes are smaller than even the analytical reproducibility of measurements so there is no way to even test where this distribution is realistic (and the boundary lines are clearly not realistic/an artifact of the model domain).
Page 24, Section 3.5: the phrasing here would be better as NOx loss rather than NOx deposition. The increase in NOx deposition is purely artificial – i.e. this is not, for instance, testing a range in the literature to look at sensitivity. It is a change in the amount of NOx loss to simulate what happens to the isotopic composition given NOx loss from the atmosphere.
Page 25, line 7: remove “of atmospheric”
Line 9: there is no Figure 12a. Figure 12 is not really utilized in this section and it is never explained why a particular time point in the model is chosen for this comparison. Either robustly explain the purpose of the figure and the conclusion being drawn from it or remove it.
Line 10: change “as a similar fashion” to “in a similar fashion”
Line 17: change to “lead to lower”
Line 19-20: change to “atmospheric NOx on a seasonal basis”
Line 20: the range here says a maximum of 5.34 per mil but the figure range only goes to 4 per mil?
Line 25-27: the sentence beginning with “The enhanced deposition simulation somehow presents the isotope effects….” This sentence is not accurate. This cannot represent the fractionations associated with chemistry. What it does show should be the focus here. And it should be explained here that this is used as a proxy for NOx loss from the atmosphere as would be the case in conversion to NOy species. But this is not at all representative of how and why chemical processes would lead to fractionation of NOy species.
Page 26, line 4: change “comparing” to “compared”
Page 27-28, Section 3.6: the two paragraphs of this section are repetitive. This section should be re-organized. I would suggest making the section on comparison to observations its own as Section 4, etc. And the beginning of the section should explain what observations will be used for comparison purposes with the model. It should also be explained what the observations represent and how NOx was computed from d15NO2 measurements. It should also be explained by isotopic measurements of nitrate in rainwater from NADP sites is used as a comparison here as well. Then move onto the actual comparisons.
Page 28, line 20: change to “based on 2002”
Page 29, line 1: “collect” should be “collected”
Line 3: change to “show similar monthly variations”
Line 10: chage to “NO3- are about”
Page 30: The paragraph below Figure 16 belongs before the previous paragraph and these two paragraph are repetitive and should be edited further.
Figure 16: It’s not clear what is being gained from the Figure 16 comparison. The text focuses on discussion of how the seasonal variability is captured. Clearly the ranges are not always captured and while in some cases the variability is decently captured (i.e. R2>0.4) we are not really learning anything by comparing the data in this way. And it is not discussed as to why some comparisons are good and others are not at all. How can we expect the model to ever get d15NO3- values exactly right when chemistry is not even included in the model? It would make a lot more sense to compare seasonal averages or time series from the model versus time series of the observations and focus on the relative changes from season to season rather than leading the reader to expect that there should be a 1:1 relationship for the model-observation comparison. Further the last line of this section (page 31, line 10) is not appropriate speculation. There’s nothing to justify this statement and again the model is not simulating nitrate fields.
Page 31, Conclusion:
The discussion of “uncertainty” needs to be clarified. This is not addressed earlier in the text, so it is not clear how an uncertainty of less than 5 per mil (line 18) was determined. The word uncertainty does not make sense in the context here. And discussion of uncertainty is also unwarranted since the model does not consider/test the full ranges of the observed NOx source signatures for different emission sources. In fact the word uncertainty is used again to describe the uncertainty in NOx emission, which is documented in the literature. Really what is being discussed is the narrow range predicted by the model – or so it appears to me.
Line 28: this sentence makes no sense and it is never discussed in the text that there is such a large uncertainty in NOx power plant emissions.
Line 35 and 37: change the phrase “are possible to be”
Line 37: change “estimated” to “predicted” and add “to be” following NOx
Line 44: what constitutes a “better” simulation? Ie. Based on what is it deemed better?