General Comments

This study describes a new database of tropospheric ozone measurements over the oceans, coastal, and polar regions from buoys, ships, aircraft, ozonesondes, and surface sites. The creation of this database is important since measurements of tropospheric ozone are lacking over these sink regions, and this study is an important component of TOAR-II. The authors present a comprehensive description of data sources, flagging methods, and backward trajectory methods. They provide a preliminary assessment of the data statistics and diurnal patterns, in preparation for the TOAR-II assessment report on this topic. I recommend that this paper be accepted after minor revisions.

Specific Comments

• In the first paragraph of the Introduction, the authors state that “The overall budget of tropospheric ozone is dominated by the photochemical production and loss terms” and then important loss terms over the ocean and in remote regions are discussed, but none of the production terms are discussed. To better tie in this sentence and fully explain how global ozone is controlled, it could be helpful to add a sentence somewhere in this paragraph about the photochemical production of ozone and whether it’s location dependent.
• Lines 149-151 state that no data are assigned to the Mediterranean Sea and Black Sea region because of the predominant continental influences. Was there data available over those bodies of water? If data was available but then excluded because the air masses would’ve had a too-high influence from the land, this should be stated.
• In the paragraph beginning on line 203, the authors should consider adding a statement describing why Radon is used as a tracer for land influence.
• Why is IAGOS data not included in the aircraft data set?
• When selecting a sonde data point every 200 m (line 240), did you ensure that the data point had high quality? For example, some sonde profiles are noisy, so you may select a data point that is erroneously low or high. This issue may be resolved by first smoothing the profiles.
• Line 290: Do you mean “longitudinal distribution”?
• Tables 1-5 are very detailed and long. Consider moving them to the SI.
• Consider adding a reference to Chang et al. (2024, https://doi.org/10.5194/acp-24-6197-2024) in the last paragraph of the Conclusions when you mention the lack of dense/homogeneous data.
• This paper does not mention satellites which provide data over the oceans and coasts. The authors should mention that this data exists but is being managed by another TOAR working group and is therefore not within the scope of this paper/database.

Technical Corrections

• The legend and region labels (R1, R2, etc.) in Figure 1 should be larger.
• Line 170: “e.g.” should be “i.e.” if you list all cruises with 1-minute based data.
• Section 2.2 should reference Figure 1.
• To the Figure S2 caption, add a comment about why the top row is red and the bottom row is blue.
• Many acronyms are undefined. Rather than defining them in-text, consider adding a table of acronyms to the SI.
• Please replace the in-text URLs with proper references:
  • Line 144
  • Line 190
  • Line 242 (can reference Van Malderen et al., 2025 instead)
  • Line 257
  • Line 263

This manuscript presents the development of a new data set on tropospheric ozone concentration, focusing on the lowest troposphere over the oceans and polar regions. It is being developed as part of the TOAR-II project, and is based on observations from ship campaigns, drifting buoys, aircraft and balloon soundings, as well as surface measurements on islands and at coastal stations. A selection of observations weakly influenced by continental emissions uses HYSPLIT trajectories and a land mask

The authors state that this paper does not constitute an exhaustive analysis of the processes controlling tropospheric ozone for oceanic or polar regions. The objective is limited to a presentation of data sources, their consistency and the main characteristics of ozone measurements in 10 regions presented in figure 1. I recommend publication of this work after clarification of the methodology for selecting the last land contact at 72 hours (LCL72) and after improving the readability of figure 1 and tables 1, 2 and 3.

Major comments :

line 96 to 103. Please provide the order of magnitude of loss terms (photochemical destruction and dry deposition) and photochemical production terms. Long-range transport of ozone in the free troposphere is also an important source in the ozone budget in remote regions. As the document provides a global assessment of ozone observations, expected differences between tropical and mid-latitude ocean regions could be mentioned.

line 110 to 113. Cite the contribution of the 2007-2009 International Polar Year project, e.g. see the POLARCAT ACP special issue (https://acp.copernicus.org/articles/special_issue182.html)

line 134 Add a reference to the long term trend study in the marine boudary layer carried out by Parrish et al. (ACP 2009)

Figure 1 is useful, but its quality is not very good: the font size is too small in the top panel (especially for region names R1 to R11), and the grid lines between regions are not sufficiently marked. The ozone scale is quite nice in the middle and bottom panels, but an upper altitude of 2000 m is more useful than an upper limit at 5000 m in the bottom panel. This will improve the comparability of surface and aircraft/ozonesonde data. The 2000m-5000m altitude range is strongly controlled by long-range transport and is sensitive to stratosphere-troposphere exchange (STE), so processes specific to global oceans and polar regions cannot be discussed very well.

line 156 The total number of hourly observations is not very significant for illustrating the temporal coverage of the data. Tables 1, 2 and 3 should show the number of measurement days. It is also difficult to assess the contribution of each season to the data variability included in the 10 selected regions. An additional 10x4 region/season table with the number of observations might be useful to assess the representativeness of the database.

Line 191 : I am confused by the criteria to define the land mask. Do you really consider air mass over land above 2500 m as being not controlled by continental emission ? This is not true for areas with biomass burning, fast uplifting of polluted air masses or large aircraft NOx emissions. For example grey trajectories are visible in Fig. 2a above Siberia. Will they remain grey if you still consider land mask when the air mass is above 2500m ?

line 224 : The total number of data records is not very useful as said hereabove. The number of fligth days is really the key parameter.

Line 231 and 244 : It is odd to find more LCL72 air masses in the 0-5000 m layer than in the 0-2000m layer. Is it still the same with a land mask extended above 2500 m ?

Line 237 : Geopotential heigths can be easily calculated with the ozonesonde pressure and temperature data. It is a pity to drop historical data based on this

criteria.

Line 240 : Undersampling the sonde data at point every 200 m is not a very good option and vertical filtering is a better way to keep fine scale features.

Line 290 : What do you mean by flat ? Neither the latitudinal plot nor the longitudinal plot show constant ozone concentrations. The interhemispheric gradient and the proximity of continental emissions are clearly seen in these two very nice plots.

Line 302 : Could you also provide the number ratio winter/summer of data at mid-latitudes or the number ratio dry-wet season of data in the tropical regions ?

Line 312 : R9 also shows and interesting east/west gradient. For the elevated values in R8 it is worth considering african biomass burning emissions transported by the trade winds. I guess it is not always filter out by the LCL72 criteria.

Line 319-321 : The boundary layer diurnal growth is also weak above the ocean and less ozone is transported from the free troposphere as it is often observed over land.

Line 339 : The Izana case should be removed as it is difficult to compare a moutain station with the ozonesonde value at the mountain altitude. Ozonesonde observations below the moutain site altitude would take into account upslope transport and would reduce the bias between the two datasets.

Fig. 2a Grey lines are barely visible above land. Use another color for marine air masses.

Fig. 5. Please use the same ozone scale (0-120 ppb) for all the regions even if outliers are not always present. Axis thicknesses are too small. A grid would help.