Cosmogenic isotope ~7Be: A case study of depositional processes in Rio de Janeiro in 2008-2009

摘要

We present here an investigation of the depositional processes of ~7Be-carrying aerosols in the troposphere through an analysis of the isotope concentration variability measured at Rio de Janeiro (Brazil) during 2008 and 2009, with weekly resolution. By adopting a simplified description of the equilibrium scenario in the troposphere and combining our isotopic data with regional meteorological parameters and numerical CRAC:Be7 model of cosmogenic production, it was possible to quantitatively reproduce the observed seasonal variability and estimate the typical ~7Be-aerosols residence time in the lower troposphere (t_r ~ 1 day) as well as the washout coefficient for each season (wet: A = 0.029; and dry: A = 0.023 h~(-1)). These results allowed us to discuss the origin of the seasonal patterns of ~7Be concentrations, indicating that the wet deposition modulation, caused by changes in the regional weather condition, is the most important forcing in both seasons. Even an anomaly in isotopic short-term measured variability can be explained when a detailed air-mass dynamic is considered. Our results indicate that anomalous events of tropospheric dynamic, such as occurrence of strong downward air flux, imprint information about air masses 3-D movement in the near-ground air ~7Be data, making ~7Be a useful tracer of its peculiar dynamics at local and synoptic scales in the troposphere. This study, limited to a single location and one year time interval, illustrates the potential of the method. A more detailed general study is planned for the future.