Global
222Rn flux density distributions with a spatial resolution of 1° in latitude and longitude were estimated for each month in the period of 1979-2007 using an
222Rn exhalation model based on a porous media diffusion theory. This exhalation model calculated the
222Rn flux density as a function of the
226Ra content, soil water saturation, and soil temperature. The influence of the topographical ruggedness of the ground surface on the flux density was also considered in the model. The global average of the flux density during the 29-year period was estimated to be 21.3 mBq m
-2 s
-1. The annual variation in the globally averaged flux density showed a slight but steady increase since 1999. The seasonal variation in the globally averaged flux density was estimated to range from a minimum of 18.8 mBq m
-2 s
-1 in March to a maximum of 23.6 mBq m
-2 s
-1 in September. The regional averages for the annual flux density varied from 8.3 mBq m
-2 s
-1 for the northern part of North America to 33.5 mBq m
-2 s
-1 for Australia. The latitudinal distribution of zonally averaged flux density was found to be in good agreement with the latitudinal decline at high latitudes in the Northern Hemisphere. The spatial and temporal variations were caused mainly by variations in the soil water content and soil
226Ra content, but to a lesser extent by the soil temperature. The topographical ruggedness was found to increase the flux density by only 2% globally but by more than 10% in mountainous regions. Atmospheric transport simulations using the estimated flux density as input data showed that the simulated atmospheric
222Rn concentration was substantially lower than that measured at Hachijo Island, implying that the flux density of the present study with topographical ruggedness is still underestimated in the East Asian region.
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