Zonal Mean Distribution of Cosmogenic Isotope (7Be, 10Be, 14C, and 36Cl) Production in Stratosphere and Troposphere

Abstract

Cosmogenic isotopes are produced by cosmic rays mostly in the middle-low atmosphere and then take part in the complicated processes of atmospheric transport and deposition that are different for the stratosphere and troposphere. Cosmogenic isotopes are continuously produced by galactic cosmic rays (GCRs) with a hard energy spectrum and sporadically by solar energetic particles (SEPs) with much softer energy spectra. The partition of the isotope production between the stratosphere and troposphere in different latitudinal zones depends on the spectrum of cosmic rays, geomagnetic shielding, and the tropopause shape. The exact zonal distribution of the isotope production is not accurately known and needs to be precisely modeled as well as the isotopes' transport and deposition in climatic reconstruction. Here, we present the results of the computations of 14C, 36Cl, 10Be, and 7Be cosmogenic isotope production in the Earth's atmosphere using the CRAC model. We provide zonal mean production rates separately for the stratosphere and troposphere in three latitudinal zones: tropical (0°–30°), midlatitude (30°–60°), and polar (60°–90°). The computations were performed for four scenarios: (a) production by GCR during a solar-cycle minimum; (b) production by GCR during a solar-cycle maximum; (c) SEP event with the hardest known spectrum (GLE#5); and (d) the strongest known soft-spectrum SEP event (GLE#24). The results confirm that, while all the latitudinal zones are relatively important for the isotopes produced by GCR, isotope production by SEPs in the tropical zone is small and can be neglected. The results are coupled to the SOCOL-AER2-BE chemistry-climate model and can be used for a simplified parametric modeling of the isotopes' atmospheric transport, for the conditions typical for the Holocene.