Test of a northwards-decreasing 222Rn source term by comparison of modell and observed atmospheric 222Rn concentrations

Model-predicted atmospheric concentrations of 222Rn based on two different 222Rn source terms have been compared with observations in the lower troposphere. One simulation used a globally uniform 222Rn source term from ice-free land surfaces of 1 atom/cm2/s; the other assumed a northwards-decreasing source term (linear decrease from 1 atom/cm2/s at 30 deg N to 0.2 atom/cm2/s at 70 deg N). Zero emissions were assigned to oceans. The northwards-decreasing source term improved predictions at four out of six stations north of 50 deg N, reducing the mean prediction/observation ratio from 2.8 to 0.87. In the latitudinal band between 30 deg N and 50 deg N, the northwards-decreasing source term resulted in systematic under-prediction of atmospheric 222Rn, whereas the uniform source term provided predictions close to observations. Predictions based on the northwards-decreasing source term were significantly (p< 0.01) better than those based on the uniform source term for an averaged vertical 222Rn profile around 44 deg N, but were not for one around 38 deg N. The results indicate that a northwards-decreasing source term could be a more realistic representation of actual 222Rn emissions than a uniform 1 atom/cm2/s source term. However, the decrease in 222Rn source strength with increasing latitude might not begin at 30 deg N but somewhat further north. This hypothesis should be investigated through model-independent means