Fast-J2: Accurate Simulation of Stratospheric Photolysis in Global Chemical Models

Modeling photochemistry in the stratosphere requires solution of the equationof radiative transfer over an extreme range of wavelengths and atmosphericconditions, from transmission through the Schumann–Runge bands ofO₂ in the mesosphere, to multiple scattering from troposphericclouds and aerosols. The complexity and range of conditions makes photolysiscalculations in 3-D chemical transport models computationally expensive. Thisstudy pesents a fast and accurate numerical method, Fast-J2, for calculatingphotolysis rates (J-values) and the deposition of solar flux in stratosphere.Fast-J2 develops an optimized, super-wide 11-bin quadrature for wavelengthsfrom 177 to 291 nm that concatenates with the 7-bin quadrature (291–850nm) already developed for the troposphere as Fast-J. Below 291 nm the effectsof Rayleigh scattering are implemented as a pseudo-absorption, and above 291nm the full multiple-scattering code of Fast-J is used. Fast-J2 calculates themean ultraviolet-visible radiation field for these 18 wavelength binsthroughout the stratosphere, and thus new species and new cross sections canbe readily implemented. In comparison with a standard, high-resolution,multiple-scattering photolysis model, worst-case errors in Fast-J2 do notexceed 5% over a wide range of solar zenith angles, altitudes(0–60 km), latitudes, and seasons where the rates are important inphotochemistry.