Impact of Cosmic Rays on Stratospheric Chlorine Chemistry and Ozone Depletion

Dissociation induced by cosmic rays of chlorofluorocarbons (CFC) and HCl on the surfaces of polar stratospheric clouds (PSC) has been suggested as playing a significant role in causing the ozone hole. However, observed stratospheric CFC distributions are inconsistent with a destruction of CFC on PSC surfaces and no significant correlation exists between ozone levels and cosmic-ray activity inside the polar regions. Model simulations indicate that this mechanism can have only a limited impact on chemical ozone loss and thus on the recovery of stratospheric ozone.

Figure 1

The variation over the time period 1979–2001 of cosmic-ray intensity (solid line, top panel), solar intensity (dashed line, top panel), and total ozone in spring averaged over the polar region (bottom panel). The pressure-corrected neutron count at Climax, Colorado, U.S.A., is shown as a measure of cosmic-ray intensity and the 10.7 cm (2800 MHz) solar flux adjusted to 1 a.u. in units of ${10}^{-21}\mathrm{W}{\mathrm{m}}^{-2}{\mathrm{H}\mathrm{z}}^{-1}$ as a measure of solar intensity. The polar total ozone values constitute averages over the latitude region $63°–90°$ for March (downward triangles) and October (upward triangles) in the northern and southern hemispheres, respectively. Total ozone measurements were taken from a series of TOMS satellite instruments [updated from Ref. 18; Newman, personal communication].

Figure 2

Scatter diagrams of the observed mixing ratios of ${\mathrm{N}}_2\mathrm{O}$ and CFC-11 (${\mathrm{C}\mathrm{F}\mathrm{C}\mathrm{l}}_3$) and ${\mathrm{N}}_2\mathrm{O}$ and CFC-12 (${\mathrm{C}\mathrm{F}}_2{\mathrm{C}\mathrm{l}}_2$), top and bottom panels, respectively, in winter 1991–1992. Shown are measurements inside (solid symbols) and outside (open symbols) the polar vortex. Measurements were made in December (circles), January (triangles), February (diamonds), and March (squares). See Refs. 19, 20 for detailed information on the measurements.

Figure 3

Results of model simulations of an airmass representative of the Arctic polar vortex in 1991–1992. Three cases are shown: dashed line, a simulation including DEA-induced heterogeneous destruction of CFC-11, CFC-12, and HCl (case a); dotted line, a simulation with DEA-induced destruction only of CFC-11 and CFC-12 (case b); and, solid line, a simulation without any DEA-induced heterogeneous chemistry (case c). The top panel shows the total simulated PSC surface area density (in $\mu {\mathrm{m}}^2/{\mathrm{c}\mathrm{m}}^3$), and the panels below the volume mixing ratios of key chemical species (where the active chlorine ${\mathrm{C}\mathrm{l}\mathrm{O}}_x$ is defined as $\mathrm{C}\mathrm{l}\mathrm{O}+2{\mathrm{C}\mathrm{l}}_2{\mathrm{O}}_2$ and total inorganic chlorine ${\mathrm{C}\mathrm{l}}_y$ as $\mathrm{H}\mathrm{C}\mathrm{l}+{\mathrm{C}\mathrm{l}\mathrm{O}\mathrm{N}\mathrm{O}}_2+\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}+{\mathrm{C}\mathrm{l}\mathrm{O}}_x$).