Inferring 222Radon soil fluxes from ambient 222Radon activity and eddy covariance measurements of CO2

We present a new methodology, which we call Single Pair of Observations Technique with Eddy Covariance (SPOTEC), to estimate regional scale surface fluxes of 222Radon (222Rn) from tower-based observations of 222Rn activity concentration, CO2 mole fractions and direct CO2 flux measurements from eddy covariance. For specific events, the regional (222Rn)surface flux is calculated from short term changes in ambient (222Rn) activity concentration scaled by the ratio of the mean CO2 surface flux for the specific event to the change in its observed mole fraction. The resulting 222Rn surface emissions are integrated in time (between the moment of observation and the last prior background levels) and space (i.e. over the footprint
of the observations). The measurement uncertainty obtained is about  15% for diurnal events and about  10% for longer term (e.g. seasonal or annual) means. The method does not provide continuous observations, but reliable daily averages can be obtained. We applied our method to in-situ observations from two sites in the Netherlands: Cabauw station (CBW) and Lutjewad station (LUT). For LUT, which is an intensive agricultural site, we estimated a mean 222Rn surface flux of (0.29+-0.02) atoms cm-2 s-1 with values > 0.5 atoms cm-2 s-1 to the south and southeast. For CBW we estimated a mean 222Rn surface
flux of (0.63+-0.04) atoms cm-2 s-1. Highest values were observed to the southwest, where the soil type is mainly river-clay. For both stations good agreement was found between our results and those from measurements with soil chambers and two
recently published 222Rn soil flux maps for Europe. At both sites, large spatial and temporal variability of 222Rn surface fluxes were observed which would be impractical to measure with an soil chamber. SPOT-EC therefore offers an important new tool for estimating region scale 222Rn surface fluxes. Practical applications furthermore include calibration of process-based 222Rn soil flux models, validation of atmospheric transport models and performing regional scale inversions of e.g. greenhouse gases via the SPOT 222Rn-tracer method.