Abstract
A global 2-D (latitude–altitude) model describing both the stratospheric circulation and the detailed formation and growth of stratospheric aerosols is used to simulate the stratospheric cycle of cosmogenic isotopes of beryllium, Be-7 and Be-10. These isotopes have been extensively used as tracers of stratospheric air, as well as of solar variability. The simulation of these isotopes is used to quantify the relative importance of transport, microphysic processes related to aerosols, and radioactive decay, on their concentrations. Calculations of model budget show that the vertical transfer of these isotopes due to the aerosol sedimentation contributes to about half of the stratospheric Be-10 flux into the troposphere, but is negligible for the Be-7 budget. The simulated residence time of these isotopes in the lower stratosphere is monotonically related to the age of air; however, this relationship is neither linear nor uniform, which biases the age of air inferred from these isotopes.
Highlights
Beryllium-10 (Be-10) and beryllium-7 (Be-7) are the radioactive isotopes of beryllium, with very contrasted half-lives: about 1.4 million years for Be-10 and only 53 d for Be-7
In the following we address two questions: (1) how much particle sedimentation impacts beryllium distribution, and (2) to what extent beryllium isotopes can be used for estimating the aerosol removal rate and the age of air? we want to test whether the classical views on the beryllium distribution in the stratosphere is supported by the model; that is, Be-10 distribution is mainly controlled by particle sedimentation, whereas Be-7 distribution is mainly controlled by its radioactive decay (e.g. Lal and Peters, 1967)
This study addresses the budget of these isotopes and the respective importance of transport, sedimentation, and radioactive decay, on determining their concentration and distribution in the stratosphere
Summary
Beryllium-10 (Be-10) and beryllium-7 (Be-7) are the radioactive isotopes of beryllium, with very contrasted half-lives: about 1.4 million years for Be-10 and only 53 d for Be-7. Isotopes concentrations ratios are much more sensitive to the difference between the radioactive decay rates of isotopes and to the time elapsed since the formation of the isotopes, than to transport processes such as advection of air masses or sedimentation of aerosols, which affect about both concentrations of cosmogenic isotopes. The top of the older versions of the ECHAM-HAM model was at 10 mb (about 31 km) (Heikkilaet al., 2008a, 2008b), preventing it from simulating correctly the timescale and pathways of the stratospheric general circulation (Bunzel and Schmidt, 2013) These global 3-D models have shown remarkable skills in simulating the tropospheric beryllium cycle We study the importance of the general stratospheric circulation, aerosol sedimentation and radioactive decay, in determining the beryllium isotopes concentrations and their distribution in the stratosphere (Sections 5 and 6). We study the impact of the aerosol sedimentation on the residence time of beryllium isotopes, and the bias on the inferred ‘age of air’ (Section 7)
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