Top-down estimates of greenhouse gas emissions by radon mass balance approach are obtained since the 1980s. One implementation of this approach relies on a stable nocturnal inversion layer in which concentrations of radon and greenhouse gases rise simultaneously over the course of several hours. This Radon Tracer Method (RTM) is a powerful tool to estimate emissions on a local-to-regional scale with monthly or seasonal resolution. Its potential is growing since greenhouse gas monitoring stations are increasingly equipped with high-precision radon detectors. In this context, the manuscript is a timely contribution to Atmospheric Measurement Techniques.

The manuscript presents the first in-depth evaluation of the RTM with all relevant issues illustrated by real-world examples from a range of locations. Particularly, the new idea to combine the RTM with stability classification extends possibilities, including to derive estimates for different fetch regions.

The manuscript's wide scope necessarily makes it lengthy (41 pages main text). Yet, the writing is easy to follow and there are no superfluous parts in text or illustrations. All is worth reading for those new to the RTM as well as to those, who have applied it before, but perhaps without thinking much about how the specific criteria they defined were affecting the informative value of the greenhouse gas emission estimate they derived. In this sense, the manuscript not only offers guidance to future studies, but also invites re-analyses of earlier ones.

Overall, the manuscript is well-balanced in its discussion of earlier work, scope for improvements, and caveats.

I found no major issue to flag. Nevertheless, I would like to encourage the authors to express in the Conclusions section their view on which insights might be transferable to the other three implementations of the radon mass balance approach they mentioned in the Introduction.

Technical issues

lines 141/142, assumption iii, "the target gas has no significant sinks over timescales associated with the measurement...":  To avoid the possible misunderstanding that a deposition flux might not be estimated by the RTM, expand to "no significant sinks in the atmosphere..."

line 373: Should the upper case 'delta' not be a lower case 'lambda'?

line 427: Not sure whether the acronym 'PTI' has been introduced in the text before?

lines 548-552: Three ways of estimating CH4 flux are introduced, labelled as i, ii, iii. In the Table, the labelling changes to 1, 2, 3, and in the text below the Table a 'method #2' is mentioned. Please stick with one kind of label throughout.

line 767: Delete 'from'.

line 807: Replace 'Those' with 'The'.

Figure 18: Most data points in (b) with values > 10 Bq m-3 are missing from (c). Please indicate the reason for that in the figure legend.

Figure 19, legend, second line: replace 'based on (a)...' with 'for (a) ...'?

Line 976: Add an 'of' between 'variability' and 'radon'.

This manuscript presents a comprehensive review of the nocturnal accumulation Radon Tracer Method (RTM) and the considerations involved in applying it under local conditions. The manuscript is well written and structured. It is particularly timely given the expansion of atmospheric observation networks aimed at quantifying greenhouse gas emissions and establishing robust emission baselines. In this context, the RTM provides an alternative top-down approach that can offer an independent constraint on emission estimates.

A particular strength of the manuscript is the detailed discussion of how choices regarding the accumulation window, atmospheric stability, measurement height, contributing fetch, radon flux and instrument performance can vary from site to site. The examples from different locations clearly illustrate how these factors affect the application and interpretation of the method. These examples and clarifications may encourage revisiting previous estimates and help improve the consistency and reproducibility of future applications.

Overall, I found no major scientific or technical issues with the manuscript. It provides a well-balanced discussion of previous work, methodological developments and limitations, with sufficient detail to support implementation of the method at new sites. I agree with the other reviewer that it would be useful to briefly consider how some of the insights presented here may apply to RTM implementations at different spatial and temporal scales. I recommend publication following minor revisions/clarifications.

Comments

Lines 70–80 and Conclusions

The Introduction briefly mentions the other three implementations of the radon mass balance approach. It would be useful for the Conclusions to briefly mention whether any of the considerations discussed here may also be transferable to those other approaches.

Lines 943–950

Given that these eight considerations form the main practical guidance of the manuscript, I wonder whether a short summary table could be useful here. The table could briefly list each factor discussed in Sections 3.1–3.8, the main consideration or recommended criterion, and which aspects should be adapted to local or site-specific conditions. I think this would make the manuscript particularly useful as a reference for researchers implementing the RTM at a new site, without implying a universal method.