A process-based 222radon flux map for Europe and its comparison to long-term observations

Detailed (222)radon (Rn-222) flux maps are an essential pre-requisite for the use of radon in atmospheric transport studies. Here we present a high-resolution Rn-222 flux map for Europe, based on a parameterization of Rn-222 production and transport in the soil. The Rn-222 exhalation rate is parameterized based on soil properties, uranium content, and modelled soil moisture from two different land-surface reanalysis data sets. Spatial variations in exhalation rates are primarily determined by the uranium content of the soil, but also influenced by soil texture and local water-table depth. Temporal variations are related to soil moisture variations as the molecular diffusion in the unsaturated soil zone depends on available air-filled pore space. The implemented diffusion parameterization was tested against campaign-based Rn-222 soil profile measurements. Monthly Rn-222 exhalation rates from European soils were calculated with a nominal spatial resolution of 0.083 degrees x 0.083 degrees and compared to long-term direct measurements of Rn-222 exhalation rates in different areas of Europe. The two realizations of the Rn-222 flux map, based on the different soil moisture data sets, both realistically reproduce the observed seasonality in the fluxes but yield considerable differences for absolute flux values. The mean Rn-222 flux from soils in Europe is estimated to be 10 mBq m(-2) s(-1) (ERA-Interim/Land soil moisture) or 15 mBq m(-2) s(-1) (GLDAS (Global Land Data Assimilation System) Noah soil moisture) for the period 2006-2010. The corresponding seasonal variations with low fluxes in winter and high fluxes in summer range in the two realizations from ca. 7 to ca. 14 mBq m(-2) s(-1) and from ca. 11 to ca. 20 mBq m(-2) s(-1), respectively. These systematic differences highlight the importance of realistic soil moisture data for a reliable estimation of Rn-222 exhalation rates. Comparison with observations suggests that the flux estimates based on the GLDAS Noah soil moisture model on average better represent observed fluxes