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Open Access Publications from the University of California

Radon-222 as a test of convective transport in a general circulation model


The distribution of 222Rn over North America is simulated with a 3-d chemical tracer model (CTM) based on the meteorology of the GISS general circulation model (GCM). The GISS GCM has been used extensively for studies of climate change and global transport of chemical tracers. Simulation of 222Rn (e-folding lifetime 5.5 days) tests the ability of the model to describe the transport of pollutants in the boundary layer and the exchange of mass between the boundary layer and the free troposphere. Model results are compared to surface observations from 5 sites in the United States. It is found that the 222Rn concentrations are regulated primarily by dry convection. At night, the model underpredicts observations because it does not resolve the sharp222Rn concentration gradient which forms near the surface. In daytime, the predicted and observed concentrations are usually in good agreement, indicating that vertical mixing of surface air is reasonably simulated. Inspection of seasonal trends reveals, however, several significant discrepancies which are traced to anomalies in the GCM meteorology. In particular, the simulated222Rn concentrations over the northeastern United States are too high in the spring, because of excessive rainfall which suppresses dry convection, and too low in the fall, because of a severe drought which allows intense dry convection. Ventilation of 222Rn to the free troposphere is most efficient in the western half of the North American continent, due to intense dry convection, and is followed by rapid eastward advection of 222Rn in the upper westerlies. This transport mechanism produces a layer of high 222Rn concentrations in the upper troposphere over the eastern United States and over the western Atlantic Ocean in summer.

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