No abstract

The bulk depositional fl uxe•s 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 21UPb at Galveston during this period varied by a factor of about 2.5, between 8.9 and 23.2 disintegrations perminute (dpm) cm -2 yr -1, with a mean of 14.7 dpmcm -2 yr -1 for 7Be, and 0.67 and 1.71 dpmcm -2 yr 1, with a mean of 1.03 dpm cm -2 yr '1 for 210pb, respectively. The precipitation-nomialized 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. v u w • 210 ß The ol me-e'ghted Pb concentrations, when normalized to the amount of precipitation, seem to be constant over the tim e peri od of this study. Four to six heavy rain events (> 5 cm) in a single day account for 20-30% of the annual deposition of 7Be and 211)I•. Such events account, however, for only about 4-6% of the total number of rainy days hi 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 them is very little rain. The fraction ofdry to total depositional flux of'21øpb appears to be higher than that of 7Be. The strong positive correlation between TBe and 2101• depositional fluxes indicates that the flux of both nuclides is controlled by scavenging processes by local precipitation. •Ihis correlation also indicates that a maj orportion of the air masses that brings p•ecipitation 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 staff ons. [Kuroda et al., 1962]), most of the 7Be nuclei that are produced in the stratosphere do not readily reach the troposphere except during spring when seasonal thinning of the tropopause takes place at midlatitudes, resultin• in air exchange between stratosphere and troposphere. Since Be is of cosmogenic origin, its flux to the Earth's surface has a latitudinal dependence [Lal and Peters, 1967]. Its concentration in the air increases with increasing altitude from the surface of the Earth, and its atmospheric flux to the Earth's surface should be independent of geography at any particular latitude [rurekian et al., 1983]. hi other words, the standing crop of 7Be in the atmosphere at a particular latitude should be the same irrespective of whether it is located over the ocean or a continent. h• contrast, 210pb is produced by radioactive decay froln its progenitor, 222pd•. This Pd• nuclide, w!fich is a noble gas isotope h• the 238U decay chah•, emanates primarily from land surfaces. The concentration of 2 !0pb in the air over the continents decreases with its elevation from the ground due to a decrease in the concentration of 222Rn [Moore et al., 1973]. As a consequence of its continental source...

[1] The depositional fluxes in the bulk and dry fallout as well as the concentrations of 7 Be and 210 Pb in aerosols were measured for a period of 17 months at Detroit, Michigan. The bulk depositional fluxes of 7 Be and 210 Pb varied between 3.11 and 63.0 dpm cm À2 yr À1 (mean of 11.4 dpm cm À2 yr À1 ) and 0.35 and 10.3 dpm cm À2 yr À1 (mean of 1.41 dpm cm À2 yr À1 ), respectively, and this variability in the depositional fluxes is attributed to the frequency and amount of precipitation and seasonal variations in the depositional fluxes. The dry depositional fluxes of 7 Be and 210 Pb contributed 2.1-19.8% and 3.6-48.6% of the bulk depositional fluxes, respectively. The higher precipitation-normalized enrichment factor for 7 Be during spring is attributed to the stratosphere-troposphere exchange of air masses during spring and large-scale atmospheric mixing during summer. We report that snow is more efficient than rain in removing 210 Pb from the atmosphere, and this is likely due to higher surface area of snow flurries compared to the rain droplets. The volume-weighted concentrations of 7 Be and 210 Pb are the highest reported in literature so far. The concentration of 7 Be in the air showed a seasonal increase during the spring and summer months, and no seasonal variation was observed for 210 Pb. The mean deposition velocities of aerosols calculated using concentrations of 7 Be and 210 Pb in air and precipitation are 1.6 and 1.1 cm s À1 , respectively, and the corresponding washout ratios are 948 and 637. There is no significant correlation between the aerosol mass and depositional velocities of 7 Be and 210 Pb in the air masses. We propose that a relatively minor portion of the aerosols actively participate in the removal of these nuclides from the air masses.

Investigations of the atmospheric fallout of 7Be and 210Pb 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 7Be and 210Pb 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 7Be and 210Pb due to the variations in the amounts of precipitation during these seasons are discussed. This study clearly shows that the depositional fluxes of 7Be and 210Pb 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 7Be during spring is not due to the troposphere‐stratosphere exchange of air masses during that season. The 7Be/ 210Pb activity ratio values remain the highest in summer and this is attributed to the increased rate of vertical transport of 7Be (from the upper troposphere to the middle and lower troposphere) and 210Pb (from the lower troposphere to upper troposphere) due to decreased stability of the troposphere during summer months.

No abstract