The Common Representative Intermediates Mechanism Version 2 in the United Kingdom Chemistry and Aerosols Model

We document the implementation of the Common Representative Intermediates Mechanism version 2, reduction 5 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, 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. 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. Plain Language Summary To understand the climate and predict how it will change in the future, we need to understand its chemical composition—the trace gases and small particles that exist in tiny quantities in the atmosphere. A key tool we use to do this are computer models which simulate the atmosphere and processes within it. Key processes include the formation of ozone, a harmful pollutant and greenhouse gas in the lower atmosphere. However, the chemistry involved in forming ozone is very complicated, so computer simulations of the atmosphere must greatly simplify the chemistry. These simple schemes may introduce errors in the model. We also have much more complex chemical mechanisms which simulate our best understanding of all chemical reactions, but these complex schemes require too much co