Radon-220 calibration of near-surface turbulent gas transport

Activity concentration profiles of the short-lived radon isotope Rn-220 (half-life 56 seconds) in the lowest 50 cm above the soil are used to study near-surface gas transport processes. The experimental data are compared to profiles calculated by solving the one-dimensional diffusion equation for radioactive atoms with a linear increase of the eddy diffusion coefficient K with altitude according to K(z) = K-0 + K(z)z. The slope K-z in this model and the radon nux from the surface are continuously calculated from the activity measurements in time steps of one hour. Transport times for Rn atoms from an altitude z(1) = 5 cm to an altitude z(2) = 20 cm are typically between one and two minutes in stable meteorological conditions when the friction velocity u* is below 0.1 m/s.