UKCA & UMUI Tutorial 6

Adding new Chemical Reactions

UKCA currently uses two different methods of defining the chemical reactions solved in the model. The first is a backward Euler solver, and is used for the RAQ and StdTrop chemistry schemes where the solver itself is created by a code-writer. The second makes use of the ASAD chemical integration software package, and is used for the CheT/TropIsop, CheS/Strat, and CheST/StratTrop chemistry schemes. ASAD can use many different solvers, although currently it uses symbolic Newton-Raphson solver. In this tutorial we will only consider the ASAD framework, as this is easily extended by a user.

ASAD considers four different types of chemical reactions: bimolecular reactions, trimolecular reactions, heterogeneous reactions, and photolysis reactions.

Biomolecular Reactions

For most bimolecular reactions, it is sufficient to provide the $k_{0}$ , $\alpha$ , and $\beta$ coefficients that are used to compute the rate coefficient $k$ from the Arrhenius expression

$k=k_{0}\left({\frac {T}{300}}\right)^{\alpha }{\textrm {exp}}\left({\frac {-\beta }{T}}\right)$

The bimolecular reactions are defined in the ukca_chem_scheme.F90 routines using the ratb_t Fortran type specification, and are held in arrays. At the end of this routine the ratb_defs_scheme array is created from these, and if that scheme is selected in UKCA these reactions are copied across into the master ratb_defs array.

To format of this ratb_t type is

ratb_t('Reactant 1','Reactant 2','Product 1 ','Product 2 ','Product 3 ',&
'Product 4 ',   $k_{0}$ ,   $\alpha$ ,   $\beta$ , Fraction of Product 1 produced, Fraction of Product 2 produced, Fraction of Product 3 produced, Fraction of Product 4 produced), &

The specifications of the induvidual reactions are done as, e.g.

ratb_t('OH        ','C5H8      ','ISO2      ','          ','          ',& ! B144 
'          ',  2.70E-11,  0.00,   -390.00, 0.000, 0.000, 0.000, 0.000), & ! B144 IUPAC2009   
...
ratb_t('OH        ','HCl       ','H2O       ','Cl        ','          ',& ! B159 
'          ',  1.80E-12,  0.00,    250.00, 0.000, 0.000, 0.000, 0.000), & ! B159 JPL2011

The first reaction in these examples takes its kinetic data from IUPAC. Going to this website, this reaction is defined here.

Written by Luke Abraham 2013