Abstract
Abstract. Detailed 222radon (222Rn) flux maps are an essential pre-requisite for the use of radon in atmospheric transport studies. Here we present a high-resolution 222Rn flux map for Europe, based on a parameterization of 222Rn production and transport in the soil. The 222Rn 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 222Rn soil profile measurements. Monthly 222Rn exhalation rates from European soils were calculated with a nominal spatial resolution of 0.083° × 0.083° and compared to long-term direct measurements of 222Rn exhalation rates in different areas of Europe. The two realizations of the 222Rn 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 222Rn 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 222Rn exhalation rates. Comparison with observations suggests that the flux estimates based on the GLDAS Noah soil moisture model on average better represent observed fluxes.
Highlights
One of the limiting factors for applying atmospheric 222Rn measurements for transport model validation is a reliable, high-resolution 222Rn flux map for the global continents, and on the regional scale for Europe
The Iberian Peninsula and areas in central Italy. For both soil moisture models, we find in many regions seasonal differences of the fluxes that are as large as a factor of 2
Using our observations at the Heidelberg Institut für Umweltphysik (IUP) site we find that the diffusivity differences during dry and wet conditions are as large as a factor of 20, leading to differences in the fluxes up to a factor of 7 (Table 1)
Summary
One of the limiting factors for applying atmospheric 222Rn measurements for transport model validation is a reliable, high-resolution 222Rn flux map for the global continents, and on the regional scale for Europe. The 238U content and the parameters influencing diffusive transport characteristics of the soil need to be known to properly estimate the variability of 222Rn exhalation rates (Schüßler, 1996) Taking these into account, Griffiths et al (2010) developed a high-resolution 222Rn flux map for Australian land surfaces. LópezCoto et al (2013) published a 222Rn flux map for Europe that uses numerical modelling of 222Rn transport in the upper soil layers Their input parameters were measured 238U activity concentrations from the Geochemical Atlas of Europe (Salminen, 2005) and other soil properties as well as modelled soil temperature and moisture data. Szegvary et al (2009) published a 222Rn flux map for Europe that solely uses γ -dose rate as a proxy for 222Rn exhalation rate
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