Abstract
AbstractWe 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.
Highlights
Understanding chemical processes in the lower atmosphere is of vital importance for tackling the problems of air pollution and making accurate projections of how the Earth system will change due to human activity (Akimoto, 2003; Boucher et al, 2013; Monks et al, 2015; Sillman, 1999; Von Schneidemesser et al, 2015)
We focus on understanding how changes in chemistry affect gas-phase species which are important for describing the global tropospheric composition as drivers to climate: ozone, carbon monoxide, hydroxy radicals, methane lifetime and nitrogen oxides
We present a short evaluation against surface and remote sensing products, but the main focus is on comparing the performance of the Common Representative Intermediates (CRI)-Strat with the StratTrop mechanism, which is already well evaluated (Archibald et al, 2020), and on understanding how these changes are sensitive to the treatment of non-methane volatile organic compounds (NMVOCs) emissions
Summary
Understanding chemical processes in the lower atmosphere is of vital importance for tackling the problems of air pollution and making accurate projections of how the Earth system will change due to human activity (Akimoto, 2003; Boucher et al, 2013; Monks et al, 2015; Sillman, 1999; Von Schneidemesser et al, 2015). Many thousands of different NMVOC species have been identified in the atmosphere, and many more are yet to be discovered, making a complete representation of all NMVOC species and their chemistry in a model an impossible task (Goldstein & Galbally, 2007; Heald & Kroll, 2020)
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