Fast-j: Accurate simulation of in- and below-cloud photolysis in tropospheric chemical models

Photolysis rates in the troposphere are greatly affected by the presence of cloud and aerosol layers. Yet, the spatial variability of these layers alo ng with the difficulty of multiple-scattering calculations for large partic les makes their inclusion in 3-D chemical transport models computationally very expensive. This study presents a flexible and accurate photolysis sche me, Fast-J, which calculates photolysis rates in the presence of an arbitra ry mix of cloud and aerosol layers. The algorithm is sufficiently fast to a llow the scheme to be incorporated into 3-D global chemical transport model s and have photolysis rates updated hourly. It enables tropospheric chemist ry simulations to include directly the physical properties of the scatterin g and absorbing particles in the column, including the full, untruncated sc attering phase function and the total, uncorrected optical depth. The Fast- J scheme is compared with earlier methods that have been used in 3-D models to parameterize the effects of clouds on photolysis rates. The impact of F ast-J on tropospheric ozone chemistry is demonstrated with the UCI troposph eric CTM.