ATMOSPHERIC DEPOSITIONAL FLUXES OF BE-7 AND PB-210 AT GALVESTON AND COLLEGE-STATION, TEXAS
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abstract
The bulk depositional fluxes of 210Pb and 7Be 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 7Be and 210Pb at Galveston during this period varied by a factor of about 2.5, between 8.9 and 23.2 disintegrations per minute (dpm) cm2 yr1, with a mean of 14.7 dpm cm2 yr1 for 7Be, and 0.67 and 1.71 dpm cm2 yr1, with a mean of 1.03 dpm cm2 yr1 for 210Pb, respectively. The precipitationnormalized 7Be flux increases with increasing amount of precipitation. There is no systematic and consistent seasonal trend in the depositional fluxes for 7Be or for 210Pb. The volumeweighted 210Pb 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 2030% of the annual deposition of 7Be and 210Pb. Such events account, however, for only about 46% 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 210Pb appears to be higher than that of 7Be. The strong positive correlation between 7Be and 210Pb 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 7Be and 210Pb 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.
