Review of “Causes of growing middle-upper tropospheric ozone over the Northwest Pacific region” by Xiaodan Ma et al.

Summary and General Comments:

This paper presents an analysis of ozone trends from ozonesonde observations and EMAC model simulations to demonstrate positive ozone trends in the mid-to-upper troposphere over the past ~30 years. The authors analyze the seasonal and spatial ozone distribution from ozonesonde profiles at Hong Kong, Naha, Tsukuba, and Sapporo and leverage model ozone tracers to attribute tropospheric ozone growth primarily to production in the troposphere, with some input from increased stratosphere-to-troposphere transport.

General Comment: The CI to ECC ozonesonde transition at the Japanese stations in ~2009 is mentioned, and the authors correctly remove the normalization factor from the ozone profile data. However, given the emphasis on ozone trends calculations, it would be prudent to demonstrate that these data do not contain a notable step-change in the time series in the Supplemental Information of this manuscript.

This paper is extremely well written, and I have only a few mostly minor suggestions, edits, and comments for the authors to address. One note: I couldn’t tell if this paper has been submitted to the Tropospheric Ozone Assessment Report 2 Special Issue: https://bg.copernicus.org/articles/special_issue10_1256.html. This paper would be an excellent candidate for the issue given its topic and quality, and it may receive more attention if it is submitted to that Special Issue.

Recommendation:

I recommend this paper’s publication once the authors address the mostly minor and technical comments listed below.

Specific and Line-by-Line Comments:

Line 27 (Abstract): By “hotspots” do you mean the largest trends? Reconsider the language here.

Line 88: Because of the time response delay of the ozonesonde sensor, the profiles have at best 100 meter vertical resolution, even if the data are reported every ~5 to 10 meters.

Line 117: I think you mean “underestimation” of the uncertainties rather than “overestimation.”

Line 129: This may qualify as more than a minor comment. Both the ascent and descent data are being used here? I do have concerns about using descent profiles given the uncertainty of solution evaporation and loss from the balloon burst/tumbling through the atmosphere.

Line 216: Change “like” to “such as”

Line 318: “, while not so clear for the seasonal difference in the middle-upper troposphere.” I don’t quite follow this part. Please rewrite for more clarity.

Line 344: This is Figure 6b3, not 7c2

Line 345: This is Figure 6a2, not 7b1. Please also correct the Figure 6 caption. For example, Hong Kong is (a1-a3), not (a1-c1).

Line 356: “Figure .7”, remove the period

Line 362: Change “than” to “compared to”

Figure 7: Not sure if I missed whether all model output for a station is used, or only output coincident with the ozonesonde profiles.

Figure 7: It would be helpful to also include some of the trend values from Table 2 on the figure itself.

Line 382: I think you mean Figure 4, not Figure 3 here.

Line 521: There is an extra “u” in this sentence.

Data Availability: Are the model output available publicly?