Beryllium-7 content in rain: evidences for a semiarid environment

Abstract

Beryllium-7 (7Be) is a relatively short-lived radionuclide (half-life 53.3 days) which decays by electron capture either directly to the ground state of 7Li (89.56%) or to an excited state of 7Li (10.44%), which decays to the ground state of 7Li via gamma-ray emission at 477.6 keV. This allows us to easily quantify it by using gamma-ray spectrometers. Beryllium-7 has a cosmogenic origin and is produced in the upper atmosphere and lower stratosphere by high-energy spallation interactions of nitrogen and oxygen. It continuously enters to marine and terrestrial ecosystems via wet (over 90%) and dry (3 to 10%) deposition. Several factors can affect this input, such as production rate (which varies with latitude, altitude, and solar activity), stratosphere?troposphere mixing, circulation and advection processes within the troposphere and efficiency with which it is removed from the troposphere. After deposition, 7Be will tend to associate with particulate material (particle-reactive element). Its relatively short half-life, reactivity, and continuous and definable production rates make 7Be a potentially powerful tool for the study and description of several environmental processes such as soil redistribution, sediment sources assessment, concentration in air, air mass transport, study of metal scavenging and others. In order to use 7Be as an environmental tracer, the knowledge of its input from the atmosphere and its variability are needed. However, when its input is evaluated, divergent information may be obtained. For different regions, dissimilar environmental conditions and seasons of the year, 7Be rain water content shows a high variability, and the cause of these changes could be difficult to understand or explain. A high 7Be content has been reported for some environments for precipitations of a few millimeters and low 7Be contents for precipitations occurring after other precipitation event. These results have been explained by the atmospheric washing phenomenon and a reload rate can be estimated. Moreover, effects of rainfall rate on rain 7Be content have been reported with divergent results. Despite these, there is agreement that wet deposition on the ground can be estimated from the rainfall volume. This chapter summarizes the results obtained in evaluating the 7Be content in rainfalls for a semiarid environment characterized by a seasonal precipitation regime. For entire rain events, the effect of precipitation variables on 7Be content in rain water is evaluated and contrasted with other regions. For single rain events the changes of 7Be content and the effect of rainfall intensity is evaluated for each millimeter of rain fallen.