Absorption Cross Sections of Stratospheric Molecules

Photoabsorption cross sections necessary for calculations of the equilibrium conditions in the stratosphere fall into two distinct classes: cross sections for molecular oxygen and ozone, which control the transmission of solar radiation; cross sections for minor atmospheric species which are optically thin to solar radiation, and which are needed to calculate their rates of dissociation.The principal absorption features of molecular oxygen are absorption bands of the Schumann–Runge system between 175 and 200 nm and a weak dissociation continuum which extends from 175 to 260 nm. The band structure consists of many sharp rotational lines, and it is necessary to calculate cross sections using measured band parameters. Two measurements of the line widths for these bands have obtained large line widths (∼1 cm−1) indicating predissociation. The agreement between the two sets of data is good for only a few lines. This has implications in the calculation of the transmission of solar radiation to the lower stratosphere. The continua have been measured by four groups. The results agree, within the respective experimental errors, near 220 nm, but disagree near 250 nm.Ozone has a continuous absorption spectrum between 175 and 300 nm with band structure above 300 nm. Four sets of data are available which agree within ±2%. The cross section above 300 nm is temperature dependent. The cross sections for the minor species are in general not as well known. In nitric oxide, carbon monoxide, ammonia, and sulfur dioxide, band structure dominates the absorption spectrum, and cross sections have been measured at insufficient spectral resolution. Other species, such as nitric acid, hydrogen peroxide, water vapor, carbon dioxide, nitrous oxide, and nitrogen dioxide, have continua over the entire spectra range from 175 to 300 nm. Cross sections for these species have been measured; however, cross sections for many molecules, e.g., N2O5, NO3, etc., have not been studied.