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.