Can radon help to improve methane emission estimates? Results from a dual-tracer inversion

Abstract. A major source of uncertainty in inverse modelling of greenhouse gas emissions are deficits in atmospheric transport models, in particular in the description of vertical mixing within the planetary boundary layer (PBL). The properties of radon-222 (Rn) makes it a suitable natural tracer for vertical mixing in the PBL. When comparing the CH4 model-data mismatch (MDM), i.e. the differences between the observed and modelled CH4 concentrations, with the MDM of Rn, we found substantial correlations for several observation sites in central Europe in 2021 (the median CH4-Rn MDM correlation coefficient is 0.6), indicating that a large part of the CH4 and Rn MDM variability can be explained by common errors in the simulated (vertical) transport. We aim to exploit this information in a joint inversion for CH4 and Rn by taking into account prior uncertainties and making use of the fact that the transport model error is correlated between the two gases. We use simultaneous CH4 and Rn observations from 17 sites across central Europe in 2021. The dual-tracer CH4-Rn inversion yields lower CH4 fluxes in several countries covered by the observation sites compared to a single-tracer CH4-only inversion without Rn information. The differences in country-total CH4 fluxes between the dual-tracer and single-tracer inversions are on the order of a few percent and depend on the assumed uncertainty for the Rn prior fluxes. These findings underscore the importance of accurate Rn flux maps for fully leveraging the dual-tracer approach and enhancing the reliability of CH4 flux estimates.