1.                      Model formulation and domain

1.1                   The continuity equation

 

The main prognostic equation in the LOTOS-EUROS model is the continuity equation that describes the change in time of the concentration of a component as a result of the following processes:

 

The equation is given by:

 

 

with C the concentration of a pollutant, U, V and W being the large scale wind components in respectively west-east direction, in south-north direction and in vertical direction. Kh and Kz are the horizontal and vertical turbulent diffusion coefficients. E represents the entrainment or detrainment due to variations in layer height. R gives the amount of material produced or destroyed as a result of chemistry. Q is the contribution by emissions, and D and W are loss terms due to processes of dry and wet deposition respectively.

 

In the model the equation is solved by means of operator splitting. The time step is split in two halves and concentration changes are calculated for the first half time step in the following order:

1.       chemistry

2.       diffusion and entrainment

3.       dry deposition

4.       wet deposition

5.       emission

6.       advection

Then for the second half time step the order is reversed. Note that if this cycle is repeated, two instances of the chemistry process are taken together with a whole time step. This can be computationally advantageous, because the time integration process does not have to be restarted for the second half time step.

 

In the following chapters these processes are described in more detail. Furthermore, the input data are described.

1.2                   Domain

The master domain of LOTOS-EUROS is shown in Figure 2.1. The boundaries of the domain are 35 and 70 North and 10 West and 60 East. The projection is normal longitude-latitude and the standard grid resolution is 0.50° longitude x 0.25° latitude, approximately 25x25 km. By means of a control file the actual domain for a simulation can be set as long as it falls within the master domain as specified above.

 

Figure 2.1 The domain of the LOTOS-EUROS modelling system. The example shows the average sulphur dioxide concentration (mg/m3) modelled for July, 1997.

 

In the vertical there are three dynamic layers and an optional surface layer. The model extends in vertical direction 3.5 km above sea level. The lowest dynamic layer is the mixing layer, followed by two reservoir layers. The height of the mixing layer is derived from meteorological observations and interpolated by the Free University of Berlin or obtained from ECMWF analyses. Mixing layer heights are input into the model every 3 hours. The model uses linear interpolation within the time interval of 3 hours. The height of the reservoir layers is determined by the difference between ceiling (3.5 km) and mixing layer height (See Fig 2.2). Both layers are equally thick with a minimum of 50m. In some cases when the mixing layer extends near or above 3500 m the top of the model exceeds the 3500 m according to the abovementioned description.

Optionally, a surface layer with a fixed depth of 25 m can be included in the model. Inclusion of this surface layer is especially useful when concentrations of primary constituents are to be simulated.

For output purposes, a diagnostic layer is used to calculate concentrations near the surface (reference height is usually 3.6 m, but it can be changed). It uses the concentrations of the lowest layer and calculates the vertical profile due to dry deposition.

 

 

Figure 2.2 An impression of the vertical grid system as function of the hour of the day. The surface layer of 25 m is optional.

 

1.3                   Run-options

 

LOTOS-EUROS currently describes the distribution of oxidants, aerosols and POP’s over Europe. Simulations for these components are often coupled but this is not always necessary. For example, one may be interested in ozone but not in aerosols. Therefore, LOTOS-EUROS has the ability to perform simulations in different set-ups as specified with a control file. The following options are available:

 

Oxidants

To calculate ozone and other oxidant levels over Europe a gas phase chemistry scheme must be chosen. LOTOS-EUROS includes the condensed CBM-IV mechanism from LOTOS and the CB99 mechanism from EUROS. These schemes describe photochemistry using 29 or 40 tracers, respectively. The only aerosol species calculated in these schemes is sulphate.

 

 

 

Secondary inorganic aerosol

The option to calculate SIA invokes a call to the aerosol equilibrium module, which describes the equilibrium between ammonium nitrate and its gaseous counterparts, ammonia and nitric acid. SIA calculations can only be performed in combination with the full oxidant scheme.

 

Secondary organic aerosol

This option invokes a call to the aerosol equilibrium module, which describes the formation of secondary organic aerosol (SOA). SOA calculations can only be performed in combination with the full oxidant scheme.

 

Primary aerosol

This option enables to switch on/off the calculations for primary aerosol components. At the moment, the primary components include primary PM2.5, PM10-2.5, Black Carbon (BC) and coarse and fine mode sea salt. The calculations for the primary components can be performed stand alone.

 

Sulphur-only

The sulphur-only option performs a simulation for SO2 and SO4 using predefined OH radical concentrations. Hence, the simulation comprises only 2 tracers and is very fast. The sulphur-only option can not be performed together with oxidant calculations as it does not make any sense.

 

POP’s

LOTOS-EUROS also contains a module to perform calculations for PAH’s and POP’s. The description of the model code for these compounds will be reported in a separate document. The code is based on the EUROS-POP module described by Jacobs en van Pul (1996).