Review on the manuscript titled “Extreme ozone episodes in a major Mediterranean urban area” by Jordi Massagué et al.

This paper analyzed three O3 episodes that occurred in Barcelona, Spain during the summers of 2015, 2018 and 2019. During the three episodes, the EU O3 threshold was exceeded. The paper aims at investigating the complex factors affecting the episodes by using observational data as well as simulations. Although the topic is interesting and the 3 episodes have been described in detail with comprehensive data, the paper is not well structured and the main scientific research findings are not clearly presented.

1. Abstract, there are too many factors presented (7 in total), but there is a lacking of the logistics among these factors. How do these factors influence the 3 extreme ozone episodes? What are commonalities and differences of these factors among the 3 episodes? It is strange that there are even no quantitative results presented in the abstract. What do we better understand now about the formation, transport and concentrations of the extreme ozone than we did before what you did? What new insights can you provide? How would that help inform potential control measures or predictions? These are not well presented in the paper and in the abstract.
2. The Introduction section is not well organized. Some contents are not directly related to this research topic. The main targets of the study are not well presented.
3. Line 30, the major O3 precursors are NOx and VOCs; while the reactions of CH4 with radicals to generate O3 are very slow, suggest removing CH4 here. Additionally, the production of O3 is not only enhanced by high temperatures and low relative humidity, while solar radiation is one of the most important factors.
4. Line 45, delete “characteristic”.
5. Line 71, “orography” should be a typo.
6. Line 130-131, need to give references or websites of the RAMS and HYPACT models.
7. The anthropogenic emissions data used in this study are derived from EDGAR, which is published in Dec.2017. The emissions inventory is not UpToDate, which will surely introduce uncertainties.
8. What kind of mechanisms are adopted in the CAMx model, and how do VOCs emissions speciated? This should be clearly given in the methodology section. Additionally, what about the model performance? The model needs to be evaluated before it can be used for further analysis.
9. How do those factors (initial regional O3 accumulation; Tramontana winds; accumulation via vertical recirculation and horizontal circulation; convergence of polluted air masses from multiple sources; upper layer winds; weekend effect; abnormally high temperatures, etc) contribute to one specific O3 episode? Can the authors conclude with integrated systematic results? In fact, some of the factors are mixed and it is difficult to distinguish.
10. It is hard to distinguish the commonalities and differences among all the influencing factors that cause the extreme O3 episodes. Most content are qualitative and descriptive, without further quantitative analysis.

The study entitled "Extreme ozone episodes in a major Mediterranean urban area" describes in detail 3 ozone pollution events in Barcelona that took place in 2015, 2018, and 2019, where the Europe information threshold (and alarm threshold on one occasion) where exceeded at several of the AQMSs in the metropolitain area. 

The article goes into great detail through all three events, listing the onset conditions in terms of synoptic conditions, and compares meteorology and atmospheric composition observations with several model outputs and trajectory analysis. The authors conclude with a set of common conditions under which the three extreme ozone episodes happened.

I have found the study very informative and somehow suited to a diverse range of audiences with interests in meteorology, air quality, modeling, and observations. One limiting aspect is the repetitiveness in the description of the three episodes, though it does provide a systematic means of presenting all parameters that make up the analysis of each event.

I would have liked to see a quantitative comparison of the model predictions - in relation to ozone levels in particular - with the observations. The article clearly states that the study provides a summary of conditions that permit such levels of ozone to build up in Barcelona as a way to better predict ozone pollution events, but the photochemical model systematically modeled elevated ozone for all three events - so I am currently not clear as to where the improved understanding needs to be inputted. 

As mentioned above, I struggle to understand what aspects of the conclusion need to be accounted by whom and what. The future work to follow on this study is also not clear. For example, the authors mention the weekend effects, which suggest the BMA is NOx limited - this has wide-ranging implications as anthropogenic emissions are expected to be further controlled/reduced.

A number of figures in the supplements are of poor to very poor resolution (S1.2a/b/c, S1.3, S2.2a), some captions are on a different page, and S2.2 is missing its (C). Please check all figures in the main document and supplements and improve the resolution or clarity of dates, axis, and colorbars where necessary.

A few specific issues are listed below:

L51: "Other basins in Spain". Spain had yet been mentioned thus far in the intro.

L86: typo with reference

L132: "Centred on Barcelona" maybe?

L179: This is the only mention of ppb in the study, while everything else is in ug/m3.

L228: I think you meant S2.2b. Also, I cannot see a reduction in solar radiation in the figures (maybe a minimal reduction in S2.2d)



Disclaimer: I am an experimental scientist, with some but limited knowledge and experience in CTM and trajectory analysis.