European 222Rn flux map for atmospheric tracer applications

Abstract

Rn is commonly used as a natural tracer for validating climate models. Generally, a constant and homogenous 222 Rn source term ofatom cm−2 s−1 is assumed as a standard, sometimes reduced in northern latitudes. A tendency to overestimate measured 222 Rn concentrations by simulations with this standard assumption has often been found. To improve current models of atmospheric chemistry and transport a better source term for 222 Rn than currently used is necessary. This work aimed to establish a method for mapping the 222 Rn source term by using a commonly measured proxy, the terrestrial γdose rate. A relatively stable fraction (≈20%) of the total terrestrial GDR originates from the 238 U decay chain, of which 222 Rn is a member. In this study a regression model could be established by simultaneous measurements of 222 Rn flux and terrestrial GDR at locations in Switzerland and Germany. This model was validated on a regional scale by measurements in Finland and Hungary, at locations covering wide ranges of γ-dose rates. The predictions were within the error margin of measurements, and therefore considered to suffice to produce regional means of 222 Rn flux by using γ-dose rate as a proxy. To be able to develop a 222 Rn flux map for Europe, a base map for the γ-dose rate was necessary. For this instance, we used the large number of national γ-dose rate measurements, established after the nuclear reactor accident in Chernobyl in 1986. These data are composite values of terrestrial, cosmic and anthropogenic contributions and instrument background (self-effect). We extracted the terrestrial part of the total γ-dose rate provided by the EUropean Radiological Data Exchange Platform (EURDEP), which continuously udates and stores the data. Subsequently we produced annual, seasonal and weekly γ-dose rate maps for Europe (European Union, Norway, former Yugoslavia and Switzerland) with geostatistical methods. The regression model was then used to transform the terrestrial γ-dose rate maps into 222 Rn flux maps, using also additional information (organic/mineral soil, bare rock surface). Spatially and temporally resolved 222 Rn source maps for the European Continent resulted, with a spatial resolution of 0.5◦ x 0.5◦ . Previously made studies could be confirmed, and even more information was available now: modeled 222 Rn flux ranged from 0.03 to 1.76 atom cm−2 s−1 , with a coefficient of variation of 51% and half of the values were between 0.40 and 0.70 atom cm−2 s−1 . The weekly 222 Rn flux maps were applied in a simulation with the atmospheric transport model TM5, as well as the standard assumption ofatom cm−2 s−1 (with 0.5 atom cm−2 s−1 between 60◦ N and 70◦ N). The results from TM5 showed that our spatially resolved 222 Rn source term can improve predictions of atmospheric 222 Rn concentrations. In a case study in Gif-sur-Yvette (France) one week of 222 Rn concentrations were observed. The air mass trajectories turned (a) from areas with large (0.61 atom cm−2 s−1 ) to (b) areas with small (0.30 atom cm−2 s−1 ) 222 Rn fluxes. The standard assumption overpredicted atmospheric concentrations by (a) 70% and (b) 260%, while the simulation based on the new inventory followed the observation closely. On the basis of our approach we also produced 222Rn flux maps for the United States of America and the Russian Federation territory, which are still preliminary and await verification.