Photolysis mechanisms
This Guide describes photolysis mechansims used by GEOS-Chem.
Cloud-J
Cloud-J (Prather [2015]) is a multi-scattering eight-stream radiative transfer model for solar radiation based on the older Fast-J scheme. It was introduced into GEOS-Chem in 14.3.0.
From History of the Fast-J and FAST-JX photolysis codes:
Cloud-J version 7.3c now supersedes Fast-JX, and all new development for photolysis rates, whether in the Fast-JX core modules or the Cloud-J wrapper, will follow this notation….Cloud-J developed a new algorithm for cloud overlap based on the vertical decorrelation observed: Blocks of clouds in the altitude ranges 0-1.5 km, 1.5–3.5, 3.5–6, 6–9, 9–13, and >13 km are maximally overlapped while each block is de-correlated from the one below. Cloud-J also developed a fast code to completely define all independent column atmospheres (ICAs) for a range of cloud overlap rules. Other minor changes included the shift of the end of bin 18 from 850 nm to 778 nm to match the infrared bands of the solar heating code RRTMG-SW. Also, solar fluxes and cross-sections changed slightly with the updated solar spectrum reference data and improved averaging over wavelength, but changes in J’s and any of the Cloud-J stats are <1% and usually <<1%.
Cloud-J is the photolyis scheme used by all GEOS-Chem fullchem simulations.
Authors
Michael Prather (UC Irvine)
Lizzie Lundgren (Harvard)
Fast-JX
Sebastian Eastham implemented FAST-JX v7.0a into GEOS-Chem v10-01. As described in Eastham et al. [2014]:
GEOS-Chem uses a customized version of the FAST-JX v6.2 photolysis mechanism (Wild et al. [2000]), which efficiently estimates tropospheric photolysis. The customized version uses the wavelength bands from the older Fast-J tropospheric photolysis scheme and does not consider wavelengths shorter than 289 nm, assuming they are attenuated above the tropopause. However, these high-energy photons are responsible for the release of ozone-depleting agents in the stratosphere. The standard Fast-JX model (Prather, 2012) addresses this limitation by expanding the spectrum analyzed to 18 wavelength bins covering 177–850 nm, extending the upper altitude limit to approximately 60 km. We therefore incorporate Fast-JX v7.0a into GEOS-Chem UCX. Fast-JX includes cross-section data for many species relevant to the troposphere and stratosphere. However, accurately representing sulfur requires calculation of gaseous H2SO4 photolysis, a reaction which is not present in Fast-JX but which acts as a source of sulfur dioxide in the upper stratosphere. Based on a study by Mills (2005), the mean cross-section between 412.5 and 850 nm is estimated at 2.542 × 10−25 cm2. We also add photolysis of ClOO and ClNO2, given their importance in catalytic ozone destruction, using data from JPL 10-06 (Sander et al., 2011). Fast-JX v7.0a includes a correction to calculated acetone cross sections. Accordingly, where hydroxyacetone cross-sections were previously estimated based on one branch of the acetone decomposition, a distinct set of cross sections from JPL 10-06 are used.
The base version of GEOS-Chem uses satellite observations of total ozone columns when determining ozone-related scattering and extinction. The UCX allows either this approach, as was used for the production of the results shown, or can employ calculated ozone mixing ratios instead, allowing photolysis rates to respond to changes in the stratospheric ozone layer.
FAST-JX is still used by the GEOS-Chem Hg (mercury) simulation.
Authors and collaborators
Michael Prather (UC Irvine)
Oliver Wild (formerly UC Irvine)
Sebastian Eashtam (formerly MIT)