Atmospheric depositional characteristics of beryllium 7 and lead 210 along the southeastern Virginia coast

Abstract

Atmospheric depositional fluxes of 7Be and 210Pb were measured simultaneously in wet‐only and bulk deposition over a 29‐month period (October 1982 through February 1985) in Norfolk, Virginia (36°53′N, 76°18′W). Total precipitation accumulation during 1983 and 1984 was 134 and 129 cm, respectively, slightly above the normal yearly accumulation. The volume‐weighted average concentrations of 7Be in bulk deposition for 1983 and 1984 were 90 and 100 dpm/L, while those for 210Pb were 5.9 and 6.6 dpm/L, respectively. The annual depositional fluxes of 7Be and 210Pb in bulk deposition in 1983 and 1984 were 12.0 and 12.9 dpm/cm2 (for 7Be) and 0.79 and 0.85 dpm/cm2 (for 210Pb), respectively. There was a high degree of correlation between 7Be and 210Pb concentrations in wet‐only deposition. This was not anticipated, considering that the sources of these two radionuclides are different. These results suggest that 7Be and 210Pb cannot be used as two independent tracers for materials originating in the stratosphere versus those from the troposphere. Dry depositional fluxes for both radionuclides, estimated from differences in the wet‐only and bulk fluxes, were calculated to be a maximum of 12 and 11% of the bulk 7Be and 210Pb fluxes, respectively. On an annual average the washout ratio for 7Be in bulk deposition was 370, while that for 210Pb was 215. The annual average total deposition velocities for 7Be and 210Pb were estimated to be 1.3 and 0.7 cm/s, respectively. It is suggested that the apparent differences in the depositional characteristics (that is, total deposition velocity, washout ratio) for these radionuclides are mere artifacts of the manner in which they are calculated and do not necessarily reflect differential behavior in the atmospheric deposition of 7Be and 210Pb.