A NEW CALCULATION OF NITRIC-OXIDE PHOTOLYSIS IN THE STRATOSPHERE, MESOSPHERE, AND LOWER THERMOSPHERE

Photodissociation of nitric oxide in the middle and upper atmosphere i s examined using a line-by-line approach to describe absorption in the NO delta bands and O2 Schumann-Runge bands. The new analysis Of O2 ab sorption results in greater transmission of ultraviolet radiation in t he Schumann-Runge (5-0) band in comparison with previous studies, lead ing to increased rates for photolysis of nitric oxide in the delta(0-0 ) band. Reduced transmission in the O2 (9-0) and (10-0) Schumann-Runge bands produces smaller photolysis rates for the NO delta(1-0) band. A bsorption in strong lines of the NO delta bands is shown to make a non negligible contribution to atmospheric opacity at wavelengths which ar e important for NO photodissociation. Representative distributions of nitric oxide are used to quantify possible changes in the NO photolysi s rate over the course of a solar cycle. As a result of changes in the NO abundance in the thermosphere, modulation of the photolysis freque ncy at lower altitudes may be opposite iii phase to variations iii the solar irradiance. For solar zenith angles greater than 60-degrees, ph otolysis rates at altitudes below 100 km may be smaller during solar m aximum compared to solar minimum. A method is described which enables rapid calculation of NO photolysis frequencies, allowing also for effe cts of varying opacity by nitric oxide.