The authors have addressed most of the comments raised by the two anonymous reviewers and modified their manuscript accordingly. Both reviewers still have some minor comments that should be addressed before the manuscript can be published in ACP.

Reviewer #2:

I would suggest accepting contingent on the following minor corrections:

L44-45: “full source contribution” : the meaning is not altogether clear. I assume you mean zeroing-out the emissions.

L45: “to a 20%” should be “from a 20%”

L47: the meaning of O3 sensitivities is not clear here

L50-51: I would remove the list of possible differences between HTAP1 and HTAP2 results. This list is not all inclusive (for example it could be due to different models or different model designs etc). Is is sufficient, especially in the abstract to say they are different.

L177: Goal 1. This goal cannot be realized due to the long list of caveats that the authors have listed. It is probably necessary to run one model configuration from the HTAP1 focus time-period to that of HTAP2 to understand how the LRT impacts change through time. I would suggest changing this goal to comparing the different results between HTAP1 and HTAP2.

L201: “Identical” seems a bit strong here due to ambiguities in VOC speciation. The speciation is discussed to some extent further down in the manuscript. It would seem relevant to discuss the VOC speciation in this section.

L229-230: Biogenic emissions are tacked on here almost as an afterthought, but are discussed in more detail below. It would make sense to include a section on differences in non-anthropogenic emissions and to condense discussion on them. A little detail is given on the supplement for some of the models for non-anthropogenic NOx emissions. Why are STEM lightning NOx emissions offshore masked out? Why are the emissions differences not characterized over a broader domain (as the paper seems to imply much of the difference in the model sensitivities are due to domain boundary conditions)? Why are differences in isoprene emissions not given? It seems that a simple table in the supplement could encapsulate some of the differences in non-anthropogenic emissions between the different models in different source areas.

L253: What is the “NAMALL” simulation? It does not seem to follow the naming convention.

L253: Please specify explicitly what “GLO” stands for.

L269: Please make the notation in equation (2) consistent with the notation above. In (2) some terms are written as subscripts while above they are written above in parenthesis (i.e., term 2). Term 3 seems to exclude the “-20%” in some places.

L558: Are you sure the NOx emissions are overestimated? Doesn’t the paper state above that it is not straightforward to draw conclusion with respect to emissions from satellite measurements.

L657: Please revisit the Brown-Steiner and Hess paper. After looking at their paper I do not see that they claim a factor of 3 betweeen summer and other seasons.

L760-763: Please rephrase. I do not understand this point very well from what is written.

L914: “continently”

L879-883: I would change “due” to “due in part”. This list is not really complete giving all the differences why these values may differ.



Reviewer #3


The authors have addressed most of my major concerns, and I list below additional suggestions for the authors to strengthen the paper a bit more before publication.

The abstract and conclusions could better convey the main findings from the paper. For example, I was surprised that the findings from Figures 10 and 11 aren’t discussed.

L47 not clear what the sensitivities are to (Asian emissions?)

L58-60 is confusing as these events aren’t occurring at the same times in the eastern US as in the western US. How much did stratospheric and transported EAS pollution influence US ozone during these events?

L498-500 Can you say anything about what this finding implies regarding problems with the model simulations?

L598-601 is confusing as Mexico is usually considered part of North America. From Figure 1 it’s clear that it is separate, but this might be worth pointing out here.

L626 stronger-than-normal-transport from where to where? And compared to what time period (i.e., what is normal?) ?

L643-644. Given the comparison with observations, is this estimate expected to be too high or better than the other models?

L659. Why is nonlinear O3 chemistry stronger outside of summer?

L697-698, also repeated L741-742 and 915-916. How exactly will the new satellite help capture high O3 and LRT events? Does it have better sensitivity of ozone near the surface?

L734 Are the PBL depths higher during the LRT episodes? This would help to convince the reader of this interpretation.

L744-749. It seems the important message here is that EAS typically isn’t contributing to the highest days, yet this doesn’t come out til 2 paragraphs later and didn’t seem to be highlighted in the abstract or conclusions. The exception of the few sites in the southwestern U.S. in L768-772 is also important to note, and it’d be even better if these impacts are quantified in the abstract/conclusions. Are these sites at higher elevation? It might be worth to report separately high elevation sites from the rest of the western sites.

L794-799. Were these captured in the Lin et al. 2012ab studies? Wondering if this applies to all global models or just the ones used here.

L808. Unless the stratospheric contribution was diagnosed in the model, this attribution to stratospheric is speculative and should be clarified as such or deleted. Is the assimilation only assimilating stratospheric ozone or total ozone? Is it somehow tracking stratospheric ozone?

L822 should be clearer that it influenced mid-tropospheric ozone over the Northeast.

L828-831. Are the mixing depths shallower here and thus don’t entrain as much free tropospheric ozone into the surface layer?

L863-868 Why not discuss the contribution to events here?

L871 How many global models?

L874-875. It’s not clear if this bottom up NOx inventory discussion applies globally or to certain regions

L885-886. Is this the forward model GEOS-Chem or the adjoint sensitivities?

L922 and elsewhere – use of “free-running”. Does this mean the models are generating their own weather and thus not expected to match specific observations? I think the authors mean simply that they aren’t doing chemical data assimilation, but I’m not sure if this is the accepted use of this term.

Thank you for addressing the additional minor comments and questions raised by two of the reviewers and revising your manuscript accordingly. Based on these revisions, I am pleased to accept your paper for publication. Congratulations.