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Overview

The submitted manuscript deals with ozone vertical distribution in the lower troposphere over the Chinese megacities of Beijing and Hong Kong. I think that it is an interesting study giving a further insight into the complex meteorological and chemical phenomena associated with the ozone vertical distribution, especially using the Mixing-layer-height-referenced ozone values. I find that the manuscript is generally scientifically sound and well-presented, and, in my opinion, it deserves publication after the recommendations listed below are considered.

General comments

The separation of the profiles based on Mixing-layer-height-referenced ozone into groups that are mainly influenced from the free troposphere (FTO3-dominated) or the boundary layer (MLO3-dominated) over the two examined Chinese megacities is very interesting. Since it is known that ozone pollution might affect large geographical areas through transport and photochemistry in combination to the fact that ozone is regarded as a priority pollutant in China, I think that it would be useful to examine the synoptic meteorological characteristics associated with each group of vertical ozone profiles, especially those influenced from the free troposphere and the boundary layer respectively. So, I would suggest plotting the corresponding composite charts of some meteorological parameters (geopotential height, vector wind, omega, specific humidity) in the boundary layer – free troposphere (850-700 hPa) of the highest ozone profiles (e.g. 10^th percentile) which are either FTO3-dominated or MLO3-dominated respectively at least for summer, based for example on the categorization of profiles presented on  Fig. 5 (e.g. see papers Kalabokas et al, ACP, 2013 and Kalabokas et al., Tellus, 2015, dealing with vertical ozone profiles in the Mediterranean).

The charts might cover the geographical area between the two megacities, which is densely populated and with important air pollutant emissions. This would give a good idea of the geographical extend of the large-scale tropospheric ozone subsidence observed during summer months over Beijing or respectively to the uplifting of boundary layer air towards the free troposphere observed over Hong Kong.

I think that the above supplementary work would strengthen the results of the paper by giving a further insight of the local or regional nature of the meteorological phenomena associated with FTO3-dominated or MLO3-dominated ozone profiles observed over Beijing and Hong Kong. In addition to their scientific importance, these results might be useful for the development of more elaborated ozone pollution abatement strategies in the examined areas of China.