Knowledge Bank

Continuous absorption has been observed near $2400cm-1$ in solar absorption spectra obtained over long paths in the lower stratosphere. Synthetic continuum transmittances, calculated with absorption coefficients extrapolated to the low temperature of the stratosphere, are in fair agreement with the observed values and indicate that the continuous opacity is due to pressure-induced absorption by the fundamental band of $N2$ and far-wing absorption by distant lines of carbon dioxide. Because of the low concentration of water vapor in the stratosphere, the contribution of continuous absorption by $H2O$ is negligible at the altitudes considered. The strength of the continuum close to the R-branch band head of the $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C16C2\nu 3$ band is calculated to be sensitive to the $CO2$ far-wing line shape, and comparisons between observed and synthetic spectra are presented for different sub-Lorentzian line shape models. The need for further laboratory data at stratospheric temperatures will be emphasized.