A SEARCH FOR THE SEASONAL VARIABILITY ON THE DEPOSITIONAL FLUXES OF BE-7 AND PB-210

Investigations of the atmospheric fallout of Be-7 and Pb-210 in many places around the globe indicate that there are seasonal variations on the depositional fluxes of these nuclides: high in winter in a few places, while in most places the high occurs in spring. However, these earlier studies did not address quantitatively the corresponding seasonal variations of the amount of precipitation during these periods. Data on the depositional fluxes of Be-7 and Pb-210 in Galveston, Texas, during the years 1989-1991 indicate that the seasonal variation is not uniform from year to year, and the amount of precipitation primarily controls the depositional fluxes. All the recently published data on the depositional fluxes of these radionuclides to date are synthesized, Also, the seasonal variations of the depositional fluxes of Be-7 and Pb-210 due to the variations in the amounts of precipitation during, these seasons are discussed This study clearly shows that the depositional fluxes of Be-7 and Pb-210 during fall were minimum, and most of the time the fallout maxima were confined to either spring and/or summer. Interseasonal variations of the depositional fluxes of these nuclides do not remain the same. The data presented in this paper in conjunction with earlier published data seem to indicate that the observed seasonal increase in the depositional fluxes of Be-7 during spring is not due to the troposphere-stratosphere exchange of air masses during that season. The Be-7/Pb-210 activity ratio values remain the highest in summer and this is attributed to the increased rate of vertical transport of Be-7 (from the upper troposphere to the middle and lower troposphere) and Pb-210 (from the lower troposphere to upper troposphere) due to decreased stability of the troposphere during summer months.