The presence in the atmosphere of radioactive debris, particularly of 90Sr and 137Cs from nuclear weapons tests, provided a unique opportunity to study the atmospheric transport. Currently, the depositions of these radionuclides have been significantly reduced to extremely low level (see Figure 5). On the other hand, the natural sources of atmospheric radioactivity used by scientists for improving the understanding of transport processes are 222Rn (radon-222), 210Pb (lead-210), and 7Be (beryllium-7). Because the global distributions of the source-sink terms of these natural radionuclides by latitude, longitude, and altitude are reasonably well known, their radioactivities can be easily measured to produce many data that are useful and available. The atmospheric 210Pb, which has a radioactive half-life of t1/2 = 22.26 years, is produced in the lower troposphere from the decay of 222Rn gas (t1/2 = 3.8 days) that is naturally emitted from the Earth's land surface as a result of uranium decay in soil. The atmospheric 7Be, which has a radioactive t1/2 = 53.44 days, is produced naturally by spallation reactions in the upper troposphere and the lower stratosphere. Atoms of 7Be and 210Pb attach themselves to nonreactive submicron-size aerosol particles and, therefore, act as aerosol-borne tracers. 222Rn acts as gas tracer. These tracers are used for assessing the characteristics of airflow and the transport of aerosols in the large- and global-scale atmospheric models. For instance, relatively high 7Be concentrations accompanied by low 210Pb concentrations could indicate subsidence of airflow from upper altitudes and vertical air movement. Atmospheric scientists have found these natural radionuclides to be useful tracers for validating transport models and studying atmospheric circulation, mixing processes, deposition or removal processes, air pollutant transport and ozone sources, and variability related to climate changes. For example, for validating global models scientists use 222Rn as a model input to simulate the global deposition of Pb, as shown in Figure 2. The results of simulation are then compared with the measurements. The model comparisons with the measurements help us improve our understanding of the atmospheric transport and the transport processes involved in atmospheric models.
The extensive database on 222Rn, 210Pb, and 7Be continues to provide the scientific community with tracer data used to verify global model simulations. The simulations of distributions of 222Rn, 210Pb, and 7Be radionuclides might establish the standards for how well a global model can represent the air concentration, and the aerosol concentration, and its deposition for air and an aerosol, respectively, passing through a monitoring site.
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