Brief description of GLOMAP
Transport in the host CTM
The host model for GLOMAP is the TOMCAT global Eulerian grid-point chemical transport model that has been developed over the last 10 years. It is used extensively for tropospheric chemical studies. TOMCAT is forced by meteorological analyses (ECMWF or UKMO) updated every 6 hours. The model has a variable resolution of typically between 1ox 1o and 7.5ox 7.5o and 31 vertical levels between the surface and the tropopause. TOMCAT uses the Prather advection scheme (Prather, 1986) which uses second-order moments in the tracer field. The scheme has the great advantage of low numerical diffusion. As part of the APPRAISE project we are developing a new regional zoom version.Particle types
The model includes sea spray and sulfate aerosol, elemental carbon, organic carbon, dust, and a simple scheme for secondary organic material based on monoterpene oxidation products. As part of our QUEST project we are developing a new scheme for multicomponent inorgamic aerosol (ammonium, nitrate, etc).Chemical Processes
The host CTM has a full tropospheric chemical scheme involving over 40 species. Reactions can be selected from within a list and equations are solved with the ASAD chemistry solver (Carver et al., 1997). Currently in GLOMAP we are using a reduced sulfur cycle scheme and prescribed oxidant fields from a full model run. The sulfur scheme includes the main reactions controlling gas phase abundances of SO2, DMS, DMSO, MSA and H2SO4. As part of our SOLAS project, work is underway to couple the aerosol and chemistry schemes and to add new halogen chemistry.Gas and Aerosol Emissions
Emissions of SO2 are taken from the GEIA or AEROCOM inventories. DMS emissions are calculated using sea-surface DMS concentrations and a wind profile dependent surface-atmosphere flux. Size-resolved sea salt emissions are calculated either using the equations of Monahan and Gong or the more recent formulation of Mårtensson et al. [2003] based on 10m wind speeds and sea surface temperature.Aerosol Size Distribution
GLOMAP-bin uses a size bin scheme with a flexible number of bins geometrically spaced. The moving-centre scheme of Jacobson is used to describe particle movement on the grid. The primary aerosol quantities that are affected by microphysical processes are the molecules of each condensed species per particle and the number of particles per bin. Typically, we use 20 or more size classes between 3 nm and a few micrometres. GLOMAP-mode uses 4 movable size modes representing typical nucleation, Aitken, accumulation and coarse size ranges. These modes can be described as various internal and external mixtures.Microphysical Processes
The model includes binary H2SO4 aerosol nucleation, coagulation and hygroscopic growth. Coagulation is calculated using the non-iterative semi-implicit scheme of Jacobson. Processes are calculated using operator splitting manner, with timesteps chosen so as to minimise the error compared with a convergent baseline run. The model fully resolves the competition between nucleation and condensation as a sink for H2SO4. A range of nucleation rates from the literature have been implemented. Dry deposition is calculated for each size bin using deposition velocities that account for diffusion and sedimentation. We have recently included new nucleation schemes appropriate for the boundary layer.Cloud Processes
The host CTM has parameterised sub-grid scale convective transport and precipitation scheme (Tiedtke, 1989). Additionally, frontal (dynamic) rain is calculated using a scheme of Giannakopoulos (1998). The rainfall parameterisation and wet deposition schemes have been tested against observations and other models, with favourable results (Stockwell and Chipperfield, 1998; Giannakopoulos, 1998). In addition to these precipitating clouds, low level stratocumulus are included based on ISCCP fields. These low-level clouds are assumed not to remove aerosol by precipitation, but they serve as sites for aqueous oxidation of SO2. The number of aerosol particles activated is calculated using the scheme of Nenes and Seinfeld based on a prescribed stochastic updraught velocity.Aerosol removal in the model’s precipitating clouds involves processes of nucleation and impaction scavenging. These rates are calculated for each aerosol size class using a Marshall-Palmer raindrop spectrum that depends on the model’s precipitation rate.
Cloud chemistry based on cloud liquid water content and grid point concentrations of gaseous species is included. The addition of oxidised sulphur species to the activated aerosol is a key process for the development of a pronounced accumulation mode (see results).
