A modified band approach for the accurate calculation of online photolysis rates in stratospheric-tropospheric Chemical Transport Models J. E. Williams1, J. Landgraf2, A. Bregman1, and H. H. Walter2 1Royal Netherlands Meteorological Institute, De Bilt, The Netherlands 2SRON National Institute for Space Research, Utrecht, The Netherlands
Abstract. Here we present an efficient and accurate method for the online calculation
of photolysis rates relevant to both the stratosphere and troposphere for
use in global Chemistry Transport Models and General Circulation Models. The
method is a modified version of the band model introduced by Landgraf and
Crutzen (1998) which has been updated to improve the performance of the
approach for solar zenith angles >72° without the use of any
implicit parameterisations. For this purpose, additional sets of band
parameters have been defined for instances where the incident angle of the
light beam is between 72–93°, in conjunction with a scaling component
for the far UV region of the spectrum (λ=178.6–202.0 nm). For
incident angles between 85–93° we introduce a modification for
pseudo-sphericity that improves the accuracy of the 2-stream approximation.
We show that this modified version of the Practical Improved Flux Method
(PIFM) is accurate for angles <93° by comparing the resulting height
resolved actinic fluxes with a recently developed full spherical reference
model. We also show that the modified band method is more accurate than the
original, with errors generally being less than ±10% throughout the
atmospheric column for a diverse range of chemical species. Moreover, we
perform certain sensitivity studies that indicate it is robust and performs
well over a wide range of conditions relevant to the atmosphere.
Citation: Williams, J. E., Landgraf, J., Bregman, A., and Walter, H. H.: A modified band approach for the accurate calculation of online photolysis rates in stratospheric-tropospheric Chemical Transport Models, Atmos. Chem. Phys., 6, 4137-4161, doi:10.5194/acp-6-4137-2006, 2006.