Evaluation of GEOS-Chem model performance for simulating transport and deposition of cosmogenic 10Be and 7Be using different production models

Abstract

The cosmogenic radionuclides 7Be and 10Be are useful aerosol tracers for atmospheric transport studies. Combining 7Be and 10Be measurements with an atmospheric transport model, not only can improve our understanding of the radionuclide transport and deposition processes, but also can provide an evaluation of the transport process in the model. To simulate those aerosol tracers, it is critical to evaluate the influence of production uncertainties in simulations. Here we use the GEOS-Chem transport model to simulate 7Be and 10Be with different production scenarios: the default production scenario in GEOS-Chem based on an empirical approach, and two production scenarios from the CRAC: Be (Cosmic Ray Atmospheric Cascade: application to Beryllium) model. The model results are comprehensively evaluated with a large number of measurements including more than 490 sites for surface air concentrations and 300 sites for deposition flux. The model can reproduce the absolute values and temporal variability of 7Be and 10Be surface concentrations and deposition fluxes on annual and sub-annual scales. The simulations using the CRAC production scenarios yield a better agreement with the measured deposition flux (70% of data within a factor of 2) compared to the default production scenario in the GEOS-Chem model (59%).  This better agreement is also observed for the vertical profiles of air 7Be concentrations.  Independent of the production models, surface air concentrations and deposition fluxes from all simulations show similar seasonal variations, suggesting a dominant meteorological influence. Finally, we demonstrate the importance of including time-varying solar modulation in the production calculation, which can significantly improve the agreement between the model and measurements, especially from mid to high latitudes.