A search for the seasonal variability on the depositional fluxes of 7Be and 210Pb

Investigations of the atmospheric fallout of 7Be and 210pb in many places around the globe indicate that there are seasonal variations on the alepositional 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 alepositional 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 alepositional fluxes. All the recently published ata on the alepositional fluxes of these radionuclides to date are synthesized. Also, the seasonal v riations of the alepositional fluxes of 7Be and 210pb due to the variations i the amounts of precipitation during these seasons are discussed. This study clearly shows that the alepositional 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 alepositional 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 he observed seasonal increase in the alepositional fluxes of 7Be durin•g spop• is not due to the troposphere-stratosphere exchange ofair masses during that season. The •Be/zxuPb activity ratio values re, main the highest in summer and this is attributed tothe increased rate of vertical transport of •Be (from the upper troposphere to the middle and lower troposphere) and 210pb (from the lower t oposphere to upper t oposphere) du to decreased stability of the troposphere during summer months. Copyright 1995 by the American Geophysical Union. Paper number 94JD02824. 0148-0227/95/94JD-02824505.00 Introduction a major influence on the depositional fluxes of 7Be and 210pb [Olsen et al., 1985; Dibb, 1989; Todd et al., 1989; Baskaran et al., It has been shown that here are seasonal v riations onthe 1993] even though production rate and transport a e important depositional fluxes of7Be and 210pb [Matsunami et l., 1979; even if they do not vary intime as precipitation does. Recently, it Olsen et al., 1985; Todd et al., 1989] (for other references, see was found that pulse rain events, with >_ 5 cm of rain in a single work by Baskaran et al. [1993]). The seasonal increases in the day, brought significant mounts of 7Be and 210pb. For example, depositional fluxes of these nuclides are commonly attributed to recent study on the depositional fluxes of 7Be and 210pb at the injection of 7Be-enriched stratospheric air to the troposphere Galveston, Texas, indicated that 4-6% of the total number of rainy [Olsen et al., 1985; Todd et al., 1989]. However, Feely_ et al. days, in the form of 4-6 heavy rain events, bring about 20-30% of [1981, 989] have addressed th seasonal variability of 7Be at the total nnual deposition of 7Be and 210pb [Baskaran et l., several locations around the globe from 7Be measurements of air 1993]. It has also been observed that the annual depositional samples and have observed that several additional meteorological fluxes of 7Be and 210pb increase with the increase inthe amount variables, uch as seasonal variability of the amounts of of precipitation [Nevissi, 1985; Baskaranet al.,1993]. precipitation and vertical mixing of lower and upper t oposphere, For a constant nual flux of 210pb or 7Be, any increase in the could explain the seasonal variability of 7Be concentration in depositional fluxes of 7Be and 210pb during one season in a year surface airat many locations. should theoretically lead to a minimum depositional fluxduring Even though a general linear relationship between the amount another season in the same year. However, the season(s) where fi s o 7 210 of precipitation and epositional uxe f Be and Pb has been the depositional fluxes of these nuclides are consistently minima at observed [Olsen etat., 1985; Dibb, 1989; Todd et al., 1989; different geographical lo tion is ot known. Inthe present paper, Schuler t al., 1991; Baskaran et al., 1993] in many places, a the seasonal variations f precipitation-normalized 7Be nd 210pb quantitative e aluation the precipitation-normalized seasonal depositional fluxes at Galveston and College Station, Texas, are variations of the depositional fluxes of 7Be and 210pb is lacking. discussed. A fairly large database from different regions of the ob s 7 210 The amount ofprecipitation in a particular ocation varies from gl e i available for the deposifional fluxes of Be or Pb, but season to season and from year to year. Since >90% of the simultaneous depositional flux data of both nuclides are limited. depositional fluxes of atmospherically de ivered nuclides are Since 7Be concentration in the air could be affected by derived through wet precipitation, the amount ofprecipitation has stratosphere-troposphere exc ange of air masses but not 210pb ' both nuclides together only can provide information on the specific processes that control the seasonal variations of the atmospheric depositional fluxes of these nuclides. Also, prior to 1980 many of the rain collectors were not acidified (as discussed in work by Turekian et al. [1983]). As a result, the reported depositional fluxes were lower than actual values and thus these