| 12 Nov 2025
Review of the Radon Tracer Method for GHG emission estimates: development, application guidelines, improvements, and caveats
Abstract. The Radon Tracer Method (RTM) is an established, independent top-down method that can be used to cross-check bottom-up greenhouse gas (GHG) emission estimates. Furthermore, as uncertainties of Atmospheric Transport Models are reduced, the RTM can provide a convenient means of quantifying continual improvement of inversion-based top-down GHG emission estimates. While the accessibility and perceived simplicity of the RTM drive its popularity, the technique is better suited to assessing long-term relative changes in GHG emissions than absolute changes, due to short-term soil moisture influences on simulated radon flux uncertainty. Considerations for applying the RTM, based on fundamental assumptions of the technique's development, are application and season specific, making the development of a "standard protocol" for its use challenging. After proposing a novel alternative means of applying the nocturnal accumulation RTM, which improves interpretation of findings, we use measurements from a range of contrasting sites to discuss the significance of the technique's eight key considerations: (i) nocturnal window definition, (ii) radon and target gas accumulation thresholds, (iii) radon-to-target gas regression linearity thresholds, (iv) measurement height, (v) the contributing fetch, (vi) spatial and temporal radon flux variability, (vii) RTM temporal resolution, and (viii) application specific selection of a suitable radon monitor. The insight provided by these examples to the flexibility (or otherwise) of the technique's considerations will clarify the implications if users choose to relax or ignore them, potentially making future RTM studies more directly comparable.
Competing interests: Some of the atmospheric radon monitors compared in Section 8 of this manuscript are separately available for commercial sale. SC and AG work for ANSTO, the organisation that develops and distributes two-filter dual flow-loop radon monitors, but every effort has been made to make the comparisons presented as objective and transparent as possible, focussing on suitability for purpose based on demonstrated performance. Aside from this, the authors declare no further conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
