Factors controlling temporal variability of near-ground atmospheric ²²²Rn concentration over central Europe

Abstract. Concentration of radon (²²²Rn) in the near-ground atmosphere has been measured quasi-continuously from January 2005 to December 2009 at two continental sites in Europe: Heidelberg (south-west Germany) and Krakow (southern Poland). The atmosphere was sampled at ca. 30 and 20 m above the local ground. Both stations were equipped with identical instruments. Regular observations of ²²²Rn were supplemented by measurements of surface fluxes of this gas in the Krakow urban area, using two different approaches. The measured concentrations of ²²²Rn varied at both sites in a wide range, from less than 2.0 Bq m⁻³ to approximately 40 Bq m⁻³ in Krakow and 35 Bq m⁻³ in Heidelberg. The mean ²²²Rn content in Krakow, when averaged over the entire observation period, was 30% higher than in Heidelberg (5.86 ± 0.09 and 4.50 ± 0.07 Bq m⁻³, respectively). Distinct seasonality of ²²²Rn signal is visible in the obtained time series of ²²²Rn concentration, with higher values recorded generally during late summer and autumn. The surface ²²²Rn fluxes measured in Krakow also revealed a distinct seasonality, with broad maximum observed during summer and early autumn and minimum during the winter. When averaged over a 5-year observation period, the night-time surface ²²²Rn flux was equal to 46.8 ± 2.4 Bq m⁻² h⁻¹. Although the atmospheric ²²²Rn levels at Heidelberg and Krakow appeared to be controlled primarily by local factors, it was possible to evaluate the "continental effect" in atmospheric ²²²Rn content between both sites, related to gradual build-up of ²²²Rn concentration in the air masses travelling between Heidelberg and Krakow. The mean value of this build-up was equal to 0.78 ± 0.12 Bq m⁻³. The measured minimum ²²²Rn concentrations at both sites and the difference between them was interpreted in the framework of a simple box model coupled with HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) analysis of air mass trajectories. The best fit of experimental data was obtained for the mean ²²²Rn flux over the European continent equal to 52 Bq m⁻² h⁻¹, the mean transport velocity of the air masses within the convective mixed layer of the planetary boundary layer (PBL) on their route from the Atlantic coast to Heidelberg and Krakow equal to 3.5 m s⁻¹, the mean rate constant of ²²²Rn removal across the top of the PBL equal to the ²²²Rn decay constant and the mean height of the convective mixed layer equal to 1600 m.