Atmospheric chemistry & aerosols in UKESM1

The evolution of the composition of aerosol particles and reactive trace gases is simulated using the UKCA model. UKCA (UK Chemistry and Aerosols model) is a model developed jointly between NCAS and the Met Office to provide a coupled aerosol and chemistry model as a component of the UM atmosphere model. The Universities of Cambridge, Leeds and Oxford were the main academic partners.

Tracer transport and convective transport processes are handled within the atmosphere model. The main components of UKCA used by UKESM1 are:

  • Stratospheric and Tropospheric (CheST) chemistry and simplified aerosol
    precursor chemistry schemes;
  • Fast-JX photolysis model;
  • Newton-Raphson chemical solver;
  • GLOMAP-mode modal aerosol scheme;
  • RADAER scheme for direct radiative forcing of aerosols;
  • UKCA-ACTIVATE model for prediction of cloud droplet number from aerosol concentrations.

The CheST chemistry oxidation scheme combines the stratospheric chemical scheme described by Morgenstern et al. (2009) with the tropospheric chemistry of O’Connor et al. (2014) and a condensed isoprene chemistry (Pöschl et al., 2000). The Fast-JX photolysis scheme is described by Bian and Prather (2002). For some runs (e.g. CMIP terrestial carbon cycle simulations) the complex chemistry scheme will not be used, and the offline oxidants chemistry will be employed. This contains the aerosol precursor chemistry, but uses supplied oxidant fields (see Simplified aerosol chemistry in UKCA by C Johnson and A Sellar, UKESM Newsletter No 1, Aug 2015).

Figure 1. Ozone column for January from UKCA Strattrop model compared with CCMVAL/NIWA data.

The GLOMAP-mode aerosol scheme was described by Mann et al. (2010) with further developments made for UKESM described by Mulcahy et al. (2017), and is configured for UKESM1 to use four soluble size modes and one insoluble mode, with sulphate, sea-salt, black carbon and organic carbon as components. The RADAER scheme (Bellouin et al., 2013) calculates the aerosol optical depth properties from the number and mass concentrations in each mode. The indirect effects of aerosols on the radiation balance uses predictions of cloud droplet number concentrations ising the ACTIVATE model (West et al., 2014), and is based on a Köhler theory based parameterisation of Abdul-Razzak and Ghan (2000) with updraught velocities sampled from a distribution calculated from the turbulent kinetic energy.

A modal representation of mineral dust is currently under development, however UKESM1 will use a seperate scheme with six size bins described by Woodward et al. (2001). Other developments of UKCA for UKESM include the ability to import emissions of DMS and biogenic volatile organic carbons from the marine and terrestial carbon cycles, and an explicit water vapour feedback from chemical oxidation processes for the stratosphere.

For more information about improving the aerosol and radiative forcing for UKESM1 see: Improving aerosol processes and radiative forcing in preparation for UKESM1, by Jane Mulcahy et al, UKESM Newsletter No 4, Jan 2017.

Figure 2. Simulated 20 year annual mean aerosol optical depth (AOD) at 550nm from a UKESM prototype model. An annual mean climatology of level 2 AOD observations at 500nm from the ground-based sun photometer network ,AERONET, (Holben et al., 2001) at 67 worldwide locations are overlain.

Figure 3. Annual mean AOD at 550nm from the MODIS satellite. Satellite observations are derived from monthly mean level 3 products from the MODIS AQUA collection 6 (Sayer et al., 2014) dataset and cover the period 2003 to 2012.


  • Abdul-Razzak, H., and Ghan, S.J., 2000, A parametrization of aerosol activation 2. Multiple aerosol types, J. Geophys. Res., 105, 6837-6844.
  • Bellouin, N., Mann, G.W., Woodhouse, M.T., Johnson, C., Carslaw, K.S., and Dalvi, M., 2013, Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the Hadley Centre Global Environment Model, Atmos. Chem. Phys., 13, 3017-3044.
  • Bian, H.S., and Prather, M.J., 2002, Fast-J2: Accurate simulation of strattospheric photolysis in global chemical models, J. Atmos. Chem., 41, 281-296.
  • Holben, B.N., et al., 2001, An emerging ground-based aerosol climatology: Aerosol optical depth from AERONET, J. Geophys. Res., 106, 12067-12097, doi:10.1029/2001JD900014.
  • Mann, G.W., Carslaw, K.S., Spracklen, D.V., Ridley, D.A., Manktelow, P.T., Chipperfield, M.P., Pickering, S.J., and Johnson, C.E., 2010, Description and evaluation of GLOMAP-mode: a modal global aerosol microphysics model for the UKCA composition-climate model. Geosci Model Dev., 3, 519–551.
  • Morgenstern, O., Braesicke, P., O’Connor, F.M., Bushell, A.C., Johnson, C.E., Osprey, S.M., and Pyle, J.A., 2009, Evaluation ofthe new UKCA climate-composition model – Part 1: The stratosphere. Geosci. Model Dev., 2, 43-57.
  • Mulcahy, J., Johnson, C., Mann, G., Johnson, B., Sellar, A., Browse, J., Walters, D., Carslaw, K., Bellouin, N., and Jones, C., Implementation of the GLOMAP-mode aerosol scheme in the Met Office Unified Model Global Atmosphere 7.0 configuration, 2017, In preparation for Geosci. Model Dev.
  • Pöschl, U., von Kuhlmann, R., Poisson, N., and Crutzen, P.J., 2000, Development and intercomparison of condensed isoprene oxidation mechanisms for global atmospheric modeling, J. Atmos. Chem., 37, 29–52.
  • Sayer, A.M., Munchak, N.C., Levy, R.C., Bettenhausen, C., and Jeong, M.-J., 2014, MODIS collection 6 aerosol products: Comarison between Aqua’s e-Deep Blue, Dark Target, and “merged” data sets, and usage recommendations, J. Geophys. Res. Atmos., 119, 13965-13989, doi:10.1002/2014/2001JD022453.
  • West, R.E.L., Stier, P., Jones, A., Johnson, C.E., Mann, G.W., Bellouin, N., Partridge, D.G., and Kipling, Z., 2014, The importance of vertical velocity variability for estimates of the indirect aerosol effects, Atmos. Chem. Phys., 14, 6369–6393.
  • Woodward, S., 2001, Modeling the atmospheric life cycle and radiative impact of mineral dust in the Hadle Centre climate model, J. Geophys. Res., 106, 18115-18166.