Review Report

Measurement Report: The Palau Atmospheric Observatory and its Ozonesonde Record - Continuous Monitoring of Tropospheric Composition and Dynamics in the Tropical West Pacific

By Müller et al.

This paper provides a first overview of the ozonesonde and radiosonde data taken at the Palau Atmospheric Observatory (PAO) in the tropical western Pacific for six years from 2016 to 2021. After describing various ground-based observations at the PAO, the authors discuss variations of ozone, relative humidity (RH), and other meteorological parameters at interannual and annual time scales. They also compare PAO ozonesonde data from those taken at SHADOZ stations and from an aircraft campaign data.

This manuscript, as a Measurement Report, is a very good introduction to the PAO ozonesonde measurements, and is written basically very well. I think that it will be acceptable for publication in Atmospheric Chemistry and Physics after considering my comments below.

My major comments are regarding Section 3.2.2 Comparison with CONTRAST.

Is the definition of RH the same between Pan et al. (2014) and this paper? I assume that this paper simply uses the radiosonde readings which are RH over liquid water, while it is probably not clearly written in the paper by Pan et al. (2014) whether they also used this, or RH over ice for temperatures colder than e.g. -20 deg.C. For the comparison in this Section 3.2.2, the same definition of RH should be used.

Related to this, the choice of definition of RH in other subsections of Section 3 might be an issue (e.g. Figure 7). If the authors use the RH over liquid water, RH values tend to be lower at higher altitudes (and never get near 100 %RH above ~5 km in the tropics; see e.g. Fujiwara et al., 2003, their Figure 1), which may give biased results when analyzing data for the whole troposphere simultaneously.

Fujiwara, M., M. Shiotani, F. Hasebe, H. Vömel, S. J. Oltmans, P. W. Ruppert, T. Horinouchi, and T. Tsuda (2003), Performance of the Meteolabor "Snow White" chilled-mirror hygrometer in the tropical troposphere: Comparisons with the Vaisala RS80 A/H-Humicap sensors, Journal of Atmospheric and Oceanic Technology, 20, Issue 11, 1534-1542.

Another comment for Section 3.2.2 is as follows. As the CONTRAST campaign was conducted during January-February, the result for the analysis of PAO ozonesonde data in this season only would also be interesting. Do you obtain similar results when limiting the season? In addition, is there any dependence on the ENSO phase?

Below are minor comments and suggestions.

Line 2 and line 42 : The latitude and longitude information of PAO should be shown here.

Line 128: “The pressure dependent background current correction” should be explained here, by showing the equation. This is because it looks this is the only point where the authors do not follow the published, recent recommendations for ozonesonde data processing.

Line 174: Vömel et al. (2016) discuss an update on the uncertainties of CFH measurements.

Vömel, H., Naebert, T., Dirksen, R., and Sommer, M.: An update on the uncertainties of water vapor measurements using cryogenic frost point hygrometers, Atmos. Meas. Tech., 9, 3755–3768, doi:10.5194/amt-9-3755-2016, 2016.

Lines 180-: Section 2.3: Reference papers should be shown, at least for Bruker 120 M and SFIT4 code.

Lines 186-187: “-1” should be superscript.

Line 197: / should be ,

Line 208: Change the locations of (

Line 241: MEI: Are positive (negative) MEIs indicating El Nino (La Nina) events. Please explain this here.

Line 274 vs. line 284: Use either El Nino/La Nina events or warm/cold ENSO events consistently to avoid confusion for non-experts on the ENSO terminology.

Line 368: Is there any contribution from the dependence on the radiosonde model used? (Kuala Lumpur is the only station where GRAW radiosonde has been used?)

Line 567-568: Müller et al. (2023) has been cited several times in the manuscript, but currently it is “to be submitted”. While I am very much looking forward to that paper, I point this out here to the editor who might have concerns from the editorial viewpoint.

Review of “Measurement Report: The Palau Atmospheric Observatory and its Ozonesonde Record - Continuous Monitoring of Tropospheric Composition and Dynamics in the Tropical West Pacific” by Müller et al.

Summary and General Comments:

This paper presents a detailed introduction to the Palau Atmospheric Observatory (PAO) with specific focus on the ~twice monthly ECC ozonesonde profiles that have been carried out since 2016. The paper is well written and provides excellent background information on PAO and its unique location, the ozonesonde data set, comparisons with nearby SHADOZ sites, the effects of large-scale climate oscillations, and examination of so-called “high ozone/low water vapor”, and vice-versa, features.

I was surprised and in principle do not agree with the pressure-dependent background for the ECC profiles here and as described in the Müller (2020) PhD thesis. As noted in the paper, this is certainly not the recommendation of the ASOPOS/ASOPOS 2.0 Panels. Nonetheless, the application of this correction avoids the near-zero ozone observations in the UT/LS in this region that previous work has shown to be false. For future studies of UT/LS low ozone amounts in the region, corrections for the slow reaction pathway as outlined in Vömel et al. (2020) and Smit et al. (2023; new reference, see below) and discarding of the pressure-dependent background should be examined. I would like to see more discussion on this data processing choice and the ECC slow time response as justification since it is a critical issue in the UT/LS above PAO. For example, what were typical background currents Ib0, 1, and 2 for this data set? Were background currents sufficiently low so that a pressure-dependent background current correction has little effect on the calculated profiles?

Smaller note: several times we are asked to consult a yet to be submitted Müller et al. (2023) paper. I suggest removing this reference for now, although by the time a final version of this paper is accepted, that may become available and it will be appropriate to add back in.

Recommendation:

I recommend this paper for publication after some additional discussion noted above, and very minor technical corrections as follows.

Specific and Line-by-Line Comments:

Line 61: Extra space after “/”

Line 164: I assume you have examined the pump temperatures for potential solution freezing/boiling effects as well, correct?

Line 193: Fix the quotations here

Line 212: analyses

Line 216: What type of radiosondes are launched there? If RS41 with its superior RH measurement, this would be an excellent data set for examining TTL RH.

Line 231: Thompson et al. (2021; https://doi.org/10.1029/2021JD034691) also used these distinguished characteristics to calculate regional ozone trends from SHADOZ data.

Figure 5 caption: “the a prominent layer” delete “the”

New paper to consider citing for discussion of the ECC “background” current and correction of the time response delay of the ozonesonde:

Smit, H. G. J., Poyraz, D., Van Malderen, R., Thompson, A. M., Tarasick, D. W., Stauffer, R. M., Johnson, B. J., and Kollonige, D. E.: New Insights From The Jülich Ozone-Sonde Intercomparison Experiments: Calibration Functions Traceable To One Ozone Reference Instrument, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1466, 2023.