ATMOSPHERIC DEPOSITIONAL FLUXES OF BE-7 AND PB-210 AT GALVESTON AND COLLEGE-STATION, TEXAS

The bulk depositional fluxes of Pb-210 and Be-7 were measured at a coastal (Galveston) and an inland (College Station) station for about 3 years, between 1989 and 1991 The annual depositional fluxes of Be-7 and Pb-210 at Galveston during this period varied by a factor of about 2.5, between 8.9 and 23.2 disintegrations per minute (dpm) cm-2 yr-1, with a mean of 14.7 dpm cm-2 yr-1 for Be-7, and 0.67 and 1.71 dpm cm-2 yr-1 with a mean of 1.03 dpm cm-2 yr-1 for Pb-210, respectively. The precipitation-normalized Be-7 flux increases with increasing amount of precipitation. There is no systematic and consistent seasonal trend in the depositional fluxes for Be-7 or for Pb-210. The volume-weighted Pb-210 concentrations, when normalized to the amount of precipitation, seem to be constant over the time period of this, study. Four to six heavy rain events (> 5 cm) in a single day account for 20-30% of the annual deposition of Be-7 and Pb-210. Such events account, however, for only about 4-6% of the total number of rainy days in a year. The dry depositional fluxes of these nuclides appear to be a significant fraction of the bulk depositional flux only during the months when there is very little rain. The fraction of dry to total depositional flux of Pb-210 appears to be higher than that of Be-7. The strong positive correlation between Be-7 and Pb-210 depositional fluxes indicates that the flux of both nuclides is controlled by scavenging processes by local precipitation. This correlation also indicates that a major portion of the air masses that brings precipitation to Galveston and College Station is of continental origin. Our data therefore suggest that Be-7 and Pb-210 cannot be used as independent atmospheric tracers in our coastal station. This observation is consistent with those observed at many other continental and coastal stations.