Fast-J2: Accurate simulation of stratospheric photolysis in global chemical models

Modeling photochemistry in the stratosphere requires solution of the equation of radiative transfer over an extreme range of wavelengths and atmospheric conditions, from transmission through the Schumann-Runge bands of O-2 in the mesosphere, to multiple scattering from tropospheric clouds and aerosols. The complexity and range of conditions makes photolysis calculations in 3-D chemical transport models computationally expensive. This study pesents a fast and accurate numerical method, Fast-J2, for calculating photolysis rates (J-values) and the deposition of solar flux in stratosphere. Fast-J2 develops an optimized, super-wide 11-bin quadrature for wavelengths from 177 to 291 nm that concatenates with the 7-bin quadrature (291-850 nm) already developed for the troposphere as Fast-J. Below 291 nm the effects of Rayleigh scattering are implemented as a pseudo-absorption, and above 291 nm the full multiple-scattering code of Fast-J is used. Fast-J2 calculates the mean ultraviolet-visible radiation field for these 18 wavelength bins throughout the stratosphere, and thus new species and new cross sections can be readily implemented. In comparison with a standard, high-resolution, multiple-scattering photolysis model, worst-case errors in Fast-J2 do not exceed 5% over a wide range of solar zenith angles, altitudes (0-60 km), latitudes, and seasons where the rates are important in photochemistry.