A new calculation of nitric oxide photolysis in the stratosphere, mesosphere, and lower thermosphere

Photodissociation of nitric oxide in the middle and upper atmosphere is examined using a line‐by‐line approach to describe absorption in the NO δ bands and O2 Schumann‐Runge bands. The new analysis of O2 absorption results in greater transmission of ultraviolet radiation in the Schumann‐Runge (5–0) band in comparison with previous studies, leading to increased rates for photolysis of nitric oxide in the δ(0–0) band. Reduced transmission in the O2 (9–0) and (10–0) Schumann‐Runge bands produces smaller photolysis rates for the NO δ(1–0) band. Absorption in strong lines of the NO δ bands is shown to make a nonnegligible contribution to atmospheric opacity at wavelengths which are important for NO photodissociation. Representative distributions of nitric oxide are used to quantify possible changes in the NO photolysis rate over the course of a solar cycle. As a result of changes in the NO abundance in the thermosphere, modulation of the photolysis frequency at lower altitudes may be opposite in phase to variations in the solar irradiance. For solar zenith angles greater than 60°, photolysis rates at altitudes below 100 km may be smaller during solar maximum compared to solar minimum. A method is described which enables rapid calculation of NO photolysis frequencies, allowing also for effects of varying opacity by nitric oxide.