Radiation and chemistry in the stratosphere: Sensitivity to O2 absorption cross sections in the Herzberg continuum

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Abstract
We propose that a significant overestimate of the molecular oxygen absorption cross sections in the important spectral window from 200‐220 nm is in large part responsible for the discrepancy between observed and modeled vertical profiles of some halocarbons (CFCl3 in particular), as well as for the long‐standing problem of simultaneously fitting N2O, CH4, CF2Cl2 and CFCl3 profiles with a single eddy diffusion model. Recent measurements of the direct solar flux in the stratosphere by J.R. Herman and co‐workers seem to support this idea. Replacing our current O2 cross sections in the 200‐220 nm range by values in better agreement with the results of the above group leads to a reduction in N2O, CF2Cl2 and CFCl3 concentrations (by factors of 0.70, 0.62 and 0.19, respectively, at 30 km), while CH4, H2 and CO profiles are essentially unchanged. Moreover, the predicted concentration of HNO3 above 30 km is reduced by ∼50%, yielding better agreement with observations. The reduction in O2 cross sections produces a 10‐20% decrease in ozone above about 35 km, but a fairly large increase (∼30%) near the peak around 20‐25 km. The changes in other stratospheric species are also briefly discussed.

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