UKCA-MODE aerosol scheme: Latest results and evaluation against observations
Status and Progress of Aerosol Scheme
Dr Graham Mann, University of Leeds
Introduction
The UKCA-mode aerosol scheme enhances aerosol-climate modeling capability in the UK Met Office Unified Model with four key respects:
- An aerosol microphysics scheme that describes both particle number and mass concentrations, enabling changes in the particle size distribution in time and space (the existing CLASSIC aerosol scheme in the Unified Model, like other 1st generation aerosol schemes, carries only mass per aerosol component).
- The ability to simulate particles of mixed composition (important for optical properties, deposition, particle reactivity, etc.). In the CLASSIC scheme all aerosol components are assumed to be in separate particles (externally mixed).
- Coupling of aerosol and gas phase chemistry (required to predict changes in particle composition and to use online oxidants).
- Include major components of the aerosol hitherto neglected: secondary organic material and the capability to simulate the complete inorganic composition (H+ /Na + /NH4+/SO42-/NO3-/Cl-).
Key new processes included are new particle formation (binary homogeneous H2SO4-H2O nucleation) and growth (by coagulation, condensation and cloud processing).
Configuration and status
The aerosol components included in UKCA-mode are sulfate, sea-salt, elemental carbon (EC), organic carbon (OC), dust, sea-salt and water. The standard version available at Unified Model vn6.6 or vn7.1 has 5 internally mixed size modes - 4 soluble modes (nucleation, Aitken, accumulation and coarse) spanning the dry diameter ranges <10nm, 10-100nm, 100nm-1micron, >1 micron respectively. Primary EC/OC is emitted into a 5th Aitken-insoluble mode and is aged by condensation & coagulation of soluble material into the soluble modes. At this initial UKCA release, dust is carried externally mixed from the other aerosol components in 6 size bins using the existing Woodward scheme. Several reduced versions of this configuration are also available via the UKCA UMUI panel. Example aerosol results from UKCA in the UM can be found 
here 
.
The aerosol chemistry extends the UKCA tropospheric chemistry scheme by also including 4 sulphur species: DMS (from oceanic phytoplankton), MSA, SO2 (from anthropogenic and volcanic sources) and H2SO4. Two organic aerosol tracers are also added to enable the production of biogenic secondary organic aerosol. The oxidation of an emitted monoterpene tracer to a condensable secondary organic gas phase species leads to additional particle growth upon condensation to existing aerosol.
The modal aerosol scheme was developed, tested and evaluated in the GLOMAP offline aerosol model. A range of results showing evaluation of simulated mass concentrations of sulphate and sea-salt , elemental & organic carbon , dust , Aerosol Optical Depth , mean particle size and size-segregated number , size distribution (column-integrated) , surface and profiles of CN concentrations and surface CCN concentrations . can be viewed via these links.
Benchmarking of UKCA-mode against the more bin-resolved GLOMAP aerosol scheme in the CTM gives added confidence to the simulated aerosol-climate effects. The following links shows benchmarking in GLOMAP of UKCA simulated CN concentrations , CCN concentrations , Aerosol Optical Depth and size distributions .
