John P. Chesick
Haverford College Haverford, Pennsylvania 19041
Effects of Water and Nitrogen ~ioxides on the Stratospheric Ozone Shield
Ozone is produced by a sequence of reactions in the stratosphere a t altitudes between 15 and 50 km, and is responsible for repoval of almost all light of wavelength less than 3000 A from the light flux incident on the surface of the earth. Supersonic transport traffic in the stratosphere (above 15 km) will discharge quantities of water vapor, oxides of nitrogen, and assorted particulate matter. The most obvious concern is the possible and probable effect of the injections of species which might significantly reduce this ozone shield through catalytic chain reactions. Professor H. S. Johnston has presented in rather condensed form' the results of detailed kinetic analysis2 of this problem. He combined available data for atmospheric composition, reasonable estimates for levels of water vapor, hydroxy radicals, and oxides of nitrogen produced naturally and the increments in these expected from SST traffic, and current knowledge of rate constants and kinetics of the elementary reactions likely to be involved. It was thought possible to extract and expand the essentials of the analysis performed by Johnston for resenta at ion to students with a minimum background of chemical Emetics. This problem was seen to provide a useful classroom example of chemical kinetics and chemical reasoning applied to a much discussed problem which has been found to be of interest to students. It was thought that this case study might be of interest to other chemistry teachers and students. The figure and Table 1provide the least controversial, or rather the most definitely established, data for concentrations of 02,03, total gas pressure (M), temperature, and compilation of known rate constants for assorted elementary reactions. These constants are provided for the temperature a t 30 km altitude, which corresponds to the level of maximum (081. Most of the reactions in question have small activation energies and change little over the range of temperatures found in the ozone layer. The absolute temperature varies from 220°K at 15 km to 250°1< a t 40 km altitude. The product of light flux density times absorption probability/molecule for each of the photochemical reactions (a), (c) and (h), is defined as I., I, or Ih. These are calculable as a function of altitude using data from rocket studies of the solar flux in combination with the absorption spectra and density di~tribution~of the most important absorbers in the 2000-3000 A wavelength region, namely O2 and 03. The light flux and its spectral distribution a t a given
altitude are dependent on the total absorption occurring down to that altitude. Typical values for I,,I
