General Comments

• Understanding the processes controlling the O3 concentration in a specific area is important to design emission reduction strategies to reduce the harmful effects of tropospheric O3. This paper focuses on two processes that take place in the O3 cycle: transport and photochemistry.
• The paper discusses two methodological approaches to understand two O3 processes (transport and photochemistry) which are the O3 budget and the O3 source apportionment. Authors claims that there is a contradictory view on the role of transport and photochemistry in O3 pollution between the budget calculation studies and O3 source apportionment studies, because both studies provide different information. In my point of view, they are two different approaches difficult to compare, so it is normal they provide different results. However, as the authors show in the paper it is possible to learn from the both of them.
• I think the paper is organized in a way that it does not help to understand its objective and methodology, even it shows a hard work behind. So, in my opinion, this manuscript was hard to follow and understand, and consequently to review. Furthermore, it could help if authors improve the readability of the text. Overall, there are too many pronouns and missing nouns that make difficult to follow the main idea of some sentences. Authors should review the text carefully and provide a more accurate reference to key concepts, also being consistent in the way they do it along the manuscript.

I would suggest few clarifications and corrections:

Specific comments

• Abstract: difficult to get the important of the problem from the four first lines.
• The abstract does not help to understands the objective and the methodology approach. Ozone budget calculation and O3 source apportionment studies seems two different type of approaches difficult to compare, so it is normal the provide different results.
• Line 29: you mention two budgets, but you have not introduced them in the abstract. Is that related with the two type of studies?
• Line 74: the subject of that sentence “O3 source” does not make sence. Could your elaborate more the idea in that sentence.
• Line 88: “O3 source studies”. Use the same set of words to mention these studies. I guess in this case you want to say “O3 source apportionment studies”. The same comment in lines 90-91, “source apportionment studies” and “O3 budget studies”.
• Line 93: “CTM are capable of reproducing O3 processes”. In this sentence, you are attributing too much credibility to CTM, but models are not perfect and not always reproduce all the processes. I would be more realistic with what CTM can do, so I would suggest to rewrite this sentence.
• Line 220: “acceptable” from which point of view?
• Line 221: “reasonable” from which point of view?
• Lines 93-103 is specifically to CMAQ, it does not apply to any Eulerian CTMS (i.e. not CTM has a PA module).
• Line 121: “Horizontal transport through the borders of the PRD in four directions”. Is that correct? I guess you have two horizontal direction (x and y).
• Section 2.5 Model setup and validation. Even the model setup is described in the Qu et al. (2021) some basic details should be provided in the text, for example CMAQ and WRF version. Furthermore, the section is named “validation”. You mainly referenced Qu et al. (2021) but readers would appreciate a paragraph describing “why” we can trust on your modeling system’s results. The evaluation of ABL height with IAGOS measurements is very interesting. Could you elaborate more on the problems with CMAQ during the night?
• Line 236-237: Is that sentence well written?
• Source apportion method: Could you comment on the brute force disadvantages for O3 source apportionment calculation? Could CMAQ-ISAM source apportionment method improve your results?
• Conclusions: “This study concluded that transport and gas-phase chemistry play the main role in the O3 concentration and mass budgets”. Is it not new, right? Could you elaborate more this sentence as the main conclusion of this work.
• Conclusions: Could you elaborate more in the biases in your modelling results? For example, discussing the uncertainties in your emission data, meteorological fields, chemical and meteorological boundary conditions, chemistry in the models.

Technical corrections

• Line 93: CTM not defined
• Line 95: PA module not defined.
• I would suggest used “tropospheric ozone” instead of “ambient O3” when possible.

Review Qu et al., 2022

Qu et al. present an analysis of the O3 budget in the ABL in two different ways: a concentration budget and a mass budget. They apply the budget calculations to the O3 budget over the Pearl River Delta based on simulations with WRF-CMAQ. The 2 different ways of calculating the O3 budget lead to opposing views on the main contributions to the O3 budget: while photochemistry dominates in the concentration budget, (vertical) transport dominates the mass budget. A tool is developed to calculate the budget contributions. A control simulation is performed, and in addition 3 brute force emission reduction scenarios are carried out. Budget calculation following the 2 methods are performed and the differences discussed.

Unfortunately, the way the paper is written makes it hard to judge its scientific merits, and I cannot recommend acceptation in its current form.

Major comments

• This is a dense paper without much guidance for the reader as to where you are going, which makes it hard to follow, and hard to judge the scientific merits of the work you describe. I had to reread it 3 times and still I am getting lost in the details. Please rewrite it in a more structured way, and indicate the purpose of each section in its first sentence. For instance, in section 2.6 a number of scenario runs seems to appear out of the blue. Where are the results of these runs used/discussed?
• What is actually lacking is an explanation of why 2 different budget methods give such different results. Is it mainly a boundary conditions problem? A change in mass does not lead to a change in concentration when the background concentration is similar over larger regions? Maybe it is discussed in L445-448?

Minor comments

L50 (and throughout MS): O3 processes --> O3-related processes

L74: pls rephrase sentence

L416: “High contributions of …” Unclear sentence. Please rephrase.

L461: what do you mean by ‘a longer time’?