Mueller et al examine a four year ozonesonde record from Palau to determine the distribution of air parcels based on their deviation from background values of ozone and relative humidity.  They look at the seasonal variability and, using backtrajectories, the origin of these air parcels.  They find that background air (O3-RH+) dominates throughout much of the year, but that, particularly in FMA, O3+RH- air parcels make up a substantial fraction of the observed air parcels.   They find that the O3 likely originate from biomass burning or other anthropogenic emissions from southeast Asia, while the low RH likely results from large-scale descent in the tropics.  Potential vorticity analysis suggested no impact from in-mixing of mid-latitude stratospheric air. 

This paper is well-written and presents a convincing conclusions with a thorough analysis of their dataset, and it adds substantively to the debate on the origin of these air masses in the remote tropics.  While I recommend publishing the paper as is, I do have two very minor comments.

Line 68: In addition to Pan et al, since you mention CAST, you should also cite Harris et al, 2017.

Harris, N. R. P., and Coauthors, 2017: Coordinated Airborne Studies in the Tropics (CAST) Bull. Amer. Meteor. Soc., 98, 145–162, https://doi.org/10.1175/BAMS-D-14-00290.1. 

Section 3.2: Because convection would "reset" the ozone and water vapor abundance in air parcels to be more reflective of the remote Pacific background, what impacts will this have on your trajectory analysis since you are not explicitly including convection into your analysis?  Could this adversely impact your analysis?  (Based on your figures this does not seem to be the case but it might be worth a sentence or 2 of discussion explaining why this doesn't matter.)

Review of “Air Mass Transport to the Tropical West Pacific Troposphere inferred from Ozone and Relative Humidity Balloon Observations above Palau” by Katrin Müller and coauthors.

General comments

This study examined the transport processes in the Tropical Western Pacific (TWP) using a four-year record of ozonesonde data and trajectory analysis at Palau Atmospheric Observatory (PAO). The research methods are likely well-designed based on a comprehensive literature review, and research findings are clear and reasonably supported by a noble set of observations. Particularly, the tracer-tracer analysis between O3 and H2O (RH) and relevant back trajectory analysis provide meaningful insight on the air mass transport over TWP, a new finding in this field. The only concern is the readability of the manuscript. It is generally well-written, but some sentences are excessively technical and verbose. If the authors could put some effort into clarifying the sentences further, it would greatly benefit the readers of the ACP. I recommend a minor revision before the publication of the manuscript. Detailed comments and suggestions are provided below. 

Comments

1. Definition of background is confusing. 

In the air mass definition (section 3.1.3), the “background profiles” are defined as 20 and 83.3 percentile for O3 and RH, respectively. However, -5/+15 ppb (O3) and -20/+5 % (RH) ranges are also referred to as background again (in section 4.1.3). Although they are related, it is very confusing. Explicitly defining the latter as “background category” or “background group?” could be helpful, or there could be a better choice of word… Lines 298-303 are related.

2. Categorization

Fig. 3 shows four categories, but the actual analysis (Fig. 7) uses nine categories. I understand the authors’ intention to provide an easy example for the categorization, but it is actually very confusing…

Line 166: “we propose five qualitative categories…” Is it four? 

3. Physical interpretation

The “FMA” patterns in Fig. 9 and Fig. 10 are noble findings of this paper. The trajectory pattern seems well related to the seasonal “Gill-type pattern” (Gill 1980; Dima et al. 2005 for reality), and the source region is well matched with agricultural fire in South Asia (Yadav et al. 2017, etc). Some in-depth discussion may be beneficial.

Gill, A. E. (1980). Some simple solutions for heat-induced tropical circulation. Quarterly Journal of the Royal Meteorological Society, 106(449), 447–462. https://doi.org/10.1002/qj.49710644905

Dima, I. M., Wallace, J. M., & Kraucunas, I. (2005). Tropical Zonal Momentum Balance in the NCEP Reanalyses. J. Atmos. Sci., 62(7), 2499–2513. https://doi.org/10.1175/JAS3486.1

Yadav, I.C., Devi, N.L., Li, J., Syed, J.H., Zhang, G. and Watanabe, H. (2017). Biomass burning in Indo-China peninsula and its impacts on regional air quality and global climate change-a review. Environmental Pollution, 227, pp.414-427.

These are suggestions

Line 1-8: This background is somehow redundant with introduction, and authors may want to make it concise.

Line 29: Recommend “Air masses entering the stratosphere largely originate…”

Line 34: Recommend “tropospheric ozone (O3) concentration sheds light…”

Line 35: Recommend “both convective (…) and long-range transport processes in this region”

Line 37-40: The sentence is too long. “The hydroxyl radical…the local troposphere” part can be removed. 

Line 43: “humid, marine, and pollutant-free environment”

Line 49-50: “3.4% per day.” Is it an additional contribution? 3.4% loss rate may not make a lifetime “around 5 days”

Line 50-51: “Deep convective outflow and overturning processes lifting the clean boundary layer air to the Tropical Tropopause Layer (TTL)…”

Line 55: “enhanced O3 from the lower stratosphere against…”

Line 109-121: May need a little more detail. For example, is “diabatic heating rate” computed or provided? Are the spatial and temporal resolution enough for resolving convective transport?

Line 114: two tracers => O3 and RH?

Line 122, 125, 139: Figure numbers are cited randomly. I am not sure if it is ok... (normally, they are cited in increasing order). Fig. 2 and Fig. 3 can be switched, and “section 4.1.3” can be used instead of “Fig. 7”.

Line 181: “layers from balloon and aircraft profiles, different…”

Line 183: “for various atmospheric constituents”?

Line 184: “In approach…” => “Using a spike detection approach, they produced extensive statistics for the frequency of anomalous layers in the whole tropical Pacific region for two different seasons”

Line 187: “Besides…” => “Despite the statistical uncertainty, Stoller et al. (1999) recognized the importance of anomalous layers in tropical Pacific profiles and emphasized their role in atmospheric chemistry modeling due to their high occurrence”?

Line 193: “, the study found that approximately 50% of profiles are related to the layer with differing seasonal variations”?

Line 204-210: Please revise this part. It is important part, but barely understandable…

Line 210: exponentially weighted with height?

Line 211: “While Hayashi et al. (2008) defined the enhanced O3 layer using the 83.3th quantile, we used a less conservative approach in regard to O3 because the Palau background is characterized as a uniform… “

Line 228: “we did not assess the vertical structure of the anomalous layer…”

Linę 263: “strong TTL cycle”. The TTL ozone signal in May (column b/w M-J) is too deep to be simply explained by the annual cycle of the Brewer-Dobson circulation. The STE process cannot be excluded in the 12-15 km level. Although it doesn’t affect the main finding, the authors may want to add discussion or adjust the tone of the argument."

Line 268: reoccurring => recurring is a bit better choice.

Line 269: synoptical => synoptic 

Line 275: winter season => boreal winter (or FMA?)

Line 283-287: difficult to understand, please revise…

Line 301: “as a separate group despite the low population in Fig. 7a?”

Line 313: “As expected from the previous analysis, 𝛥O3+ 𝛥RH- are almost absent in ASO, emphasizing that the season represents a dominant background in the free troposphere.”?

Line 310 “A footprint of air mass transport to Palau is analyzed using 10-day backward trajectories for the study period (2016-2019) sorted by…”

Line 322: a day/profile => a profile?