This is a very nice analysis that provides a lot of useful information and insight regarding the production of ozone associated with the reduction of anthropogenic emissions during the COVID-19 pandemic, as well as the changes in the chemical regime associated with it.

Specific comments

Line 22: Add more recent references as Fleming et al (2018), Sillman et al (2021)

Line 70: remove 70

Section 2: some of the discussion belongs to Introduction.

Lines 174-185:  the discussion could be part of the supplementary material.

Line 193: Fig 2 is refereed  first time after Figs. 3 and 4

Section 3.1:  Mar et al (2016), Im et al (2016) showed that RADM2 underestimates the O3 concentration when compared to other chemical mechanisms. A discussion about the choice of chemical mechanism would be beneficial since it looks like the Authors obtained the right answers for the wrong reasons.  

Section 3.3: Please check the numbers in the Tables, not always the MB=MM-OM

Lines 301-314: A lot of this information should go to the Figures caption (e.g. “The dots in the lower row represent the land use for each grid cell, which is the key to understanding how industrial, open urban, compact urban, water, agriculture, natural open and forestland uses influenced the O3 regimes”)

Line 315: please specify the land-use categories that belong to “green areas”.

Figures 3-4: Increase the size of the cross and explain what it represents.

Figure 5 Sectors A and G, B and H, as well as the pollutants CO and NOx and NH3 and PM10 have similar colors and it is difficult to distinguish between different lines.

Figures 6-8 As before, we can’t really distinguish the colors. I would suggest using a discrete color scale.

Figures 12-14 There is no reference to these Figures in the text.

Table 1 define F0, F1, F2, F3

Fleming, Z., Doherty, R., Von Schneidemesser, E., Malley, C., Cooper, O., Pinto, J., Colette, A., Xu, X., Simpson, D., Schultz, M., Lefohn, A., Hamad, S., Moolla, R., Solberg, S., and Feng, Z.: Tropospheric Ozone Assessment Report: Present-day ozone distribution and trends relevant to human health, Elementa, 6, 12, https://doi.org/10.1525/elementa.273, 2018

Sillmann, J., Aunan, K., Emberson, L., Büker, P., Van Oort, B., O'Neill, C., Otero, N., Pandey, D., and Brisebois, A.: Combined impacts of climate and air pollution on human health and agricultural productivity, Environ. Res. Lett., 16, 093004, https://doi.org/10.1088/1748-9326/ac1df8, 2021. 

Mar, K. A., Ojha, N., Pozzer, A., and Butler, T. M.: Ozone air quality simulations with WRF-Chem (v3.5.1) over Europe: model evaluation and chemical mechanism comparison, Geosci. Model Dev., 9, 3699–3728, https://doi.org/10.5194/gmd-9-3699-2016, 2016.

Im, U., Bianconi, R., Solazzo, E., Kioutsioukis, I., Badia, A., Balzarini, A., Baro, R., Bellasio, R., Brunner, D., Chemel, C., Curci, G., Flemming, J., Forkel, R.,  Giordano, L., Jimenez-Guerrero, P., Hirtl, M., Hodzic, A., Honzak, L., Jorba, O., Knote, C., Kuenen, J.J.P., Makar, P.A., Manders-Groot, A., Neal, L., Perez, J.L., Pirovano, G., Pouliot, G., San Jose, R., Savage, N., Schroder, W., Sokhi, R.S., Syrakov, D., Torian, A., Tuccella, P., Werhahn, K., Wolke, R., Yahya, K., Zabkar, R., Zhang, Y., Zhang, J., Hogrefe, C., Galmarini, S., 2015. Evaluation of operational online-coupled regional air quality models over Europe and North America in the context of AQMEII phase 2. Part I: Ozone. Atmos. Environ. 115, 404e420.

This is an interesting paper looking at the impact of emissions reductions on atmospheric chemistry.  It takes the area in and around the Barcelona metropolitan area as a natural laboratory, and studies two periods in 2020 as exemplar systems to understand the effect of emissions reductions on ozone and NO2.

The paper describes a model study using WRF-Chem coupled to an urban canopy model to look at atmospheric processes over the AMB region, and FLEXPART-WRF to do some trajectory analysis to study the chemistry occurring as air flows inland.

This is an ambitious study which aims to use the connection between the natural experiment of emissions reductions in the months of April/May 2020, and through analysis of idealised counterfactual experiments perform attribution of the effect of 

emissions reductions.  

The experimental design and analysis appear sound and this manuscript fits well within the scope of ACP.  I feel the structure of the manuscript could be improved, and the discussion should be improved in places before publication.

The title is the first area to address - I felt it was perhaps a bit too general, as the main focus is the impact of lockdown.  The abstract can also be a bit more explicit eg L8/9 'response of ozone chemistry to changes' could make more explicit what reduction is under discussion.  AOC needs to be defined in the abstract, and for the sake of clarity that it excludes O3 oxidant.

Abstract Conclusion # 3 is not clear - what is the mechanism?  Conclusion #4 could the authors explain why May is important?  Conclusion 5 - not sure what is meant by a change contributing to a plume.  Perhaps re-word?

S2 describes the region selected for study, geography, Barcelona's air quality with respect to guidelines, the Vic Plain and the ozone situation in 20202. Two periods are identified for closer study, and also days of even closer study.  I feel the manuscript would be more readable if it would it be possible to decide on a single consistent nomenclature for the two periods, eg P1 and P2 and so avoid changing between Mar-Apr/lockdown/first period and May/relaxation through the text

L174-185 Some of this section could be grouped with the discussion of the trajectory experiments,  as it mixes model description with some analysis that probably belongs with the discussion in S5.3.  It would be interesting to better justify why these days were chosen for further study - what aspects do these days/analysis bring out? 

S3 describes the WRF-Chem experiments performs some model evaluation against observations.  The model is shown to be more skillful in meteorology than chemistry, with ozone biases around 20-30% shown.  The authors do not discuss if the bias in the model means that the model correctly simulates the difference in ozone/NO2 from a change in emissions.  Given the chemistry is non-linear, would the response be greater/smaller in a less biased model?  Would it make sense to compare ozone changes in the model with differences in climatology/COVID period at the observation stations of interest to assess if the model gets delta_O3 correct?

S4 describes the results of the experiments.  This section in the MS has the most potential for improvement, I feel.  Firstly, the changes in O3, NO2 and Ox are given in the supplementary.  Is it worth moving figures S3 and S4 into the main text?  I appreciate the changes are given in absolute (LH) and relative (RH) terms for both period 1 adn period 2.  Could the labelling of these figures be improved to indicate what data are plotted in each of the four rows are?  There are no labels on each of the second and fourth rows.  The text in S4.1 could do with a further polish, e.g.

L27 is 1% significant?

L285 the 'two simulations' of what?

L287 constant between what?

SS5.1 is interesting.  I presume the graphs shown are for model results, and it would help to have this stated.  Did the authors consider performing a similar analysis for observational data for this period?  Is it difficult due to a lack of VOC data? The captions of Figure 6-8 needs to state explicitly that these are 'Changes...' between BAU and COVID

Colouring data by ozone change and land use/land cover is interesting, and the broken lines make the analysis goal clear. I would like to see the analysis better justified, though. I assume it is correct to use the lines which are derived from an analysis of transition regimes based on NOx and VOC emissions in Sillman (rather than changes in NOx/VOC levels used here) but I'd like the paper to discuss somewhere how these regimes apply when discussing a _change_ in O3 and a change in NOx or VOC levels, particularly in identifying regions of the diagrams here with NOx- or VOC-limited regimes, which seems key.  I've not seen an analysis like this before, so would like to see this expanded upon.

L315 and on, could the authors explain how the figures can be used to support this statement?  L326, L330 the discussion reverts to ozone levels, not differences between scenarios.  Could this discussion be made more consistent?

L319 grid points not grid.

SS5.2 discusses the oxidising capacity in terms of OH and NO3.  Is there any impact on ozone budgets seen from changing HO (and presumably OH:HO2 ratio), particularly in HO2 + NO vs OH + NO2 vs OH + O3?

SS5.3 is very nice, and might be improved in consistency with a discussion of Ox (and maybe formation of Ox/NOx reservoirs and sinks) as in previous sections.  

S6 summarises nicely. The sentence in L438 'this was consistent...' needs to be expanded.

L454 'data used in this study...' add 'are'

Overall