We document the implementation of the Common Representative
Intermediates Mechanism version 2, reduction 5 (CRIv2-R5)
into the United Kingdom Chemistry and Aerosol model
(UKCA) version 10.9. The mechanism is merged with the
stratospheric chemistry already used by the StratTrop mechanism,
as used in UKCA and the UK Earth System Model (UKESM1),
to create a new CRI-Strat mechanism. CRI-Strat simulates a
more comprehensive treatment of non-methane volatile organic
compounds (NMVOCs) and provides traceability with the Master
Chemical Mechanism (MCM). In total, CRI-Strat simulates the
chemistry of 233 species competing in 613 reactions (compared
to 87 species and 305 reactions in the existing StratTrop
mechanism). However, while more than twice as complex than
StratTrop, the new mechanism is only 75% more computationally
expensive. CRI-Strat is evaluated against an array of
in situ and remote sensing observations and simulations
using the StratTrop mechanism in the UKCA model. It is found
to increase production of ozone near the surface, leading to
higher ozone concentrations compared to surface observations.
However, ozone loss is also greater in CRI-Strat, leading to
less ozone away from emission sources and a similar tropospheric
ozone burden compared to StratTrop. CRI-Strat also produces more
carbon monoxide than StratTrop, particularly downwind of biogenic
VOC emission sources, but has lower burdens of nitrogen oxides
as more is converted into reservoir species. The changes to
tropospheric ozone and nitrogen budgets are sensitive to the
treatment of NMVOC emissions, highlighting the need to reduce
uncertainty in these emissions to improve representation
of tropospheric chemical composition.