1. Wet Deposition
In LOTOS-EUROS wet deposition is treated in a
simplified way. As the meteorological input does not contain detailed information
on clouds the in-cloud scavenging of gases and aerosols is neglected. Hence,
below we describe the parameterisations for below cloud scavenging only.
1.1
Gases
The standard method to calculate wet deposition
for soluble gases is described below.
We define the following parameters:
M: mass (μg)
Cwater: concentration of component in water (rain), i.e. mass of component per volume of water (μg/m3)
Cgas: concentration of component in gas phase, i.e. mass of component per volume of air (μg/m3)
t: time (h)
Δt: time step (h)
V: volume (m3)
A: horizontal area (m2)
Δz: layer depth (m)
P: precipitation rate (m/h)
W: washout ratio, the ratio Cwater/Cgas
.
Exchange of mass takes place between gas in the air and the raindrops. Conservation of mass says:
. (1.)
Since
the volume of water is 
, we can write this equation for concentrations:
. (2.)
We
now assume that the process of falling rain from upper layers and mass getting
into the raindrops can be split (operator
splitting) in the following way: compute the water concentration at the end
of the time step in the uppermost layer, then assume that concentration to be
the input concentration for the next (lower) layer. Thus proceed to lower
layers. Defining
the water concentration of the layer above the current layer
(which has been computed in previous stages and is assumed constant in the current layer), then the operator splitting
leads to:
.
Eq. (2.) then reads:
. (3.)
Dividing
by 
and letting 
, we get the differential equation
, (4.)
with as solution:
Defining d, the concentration change within a layer due to wet deposition:
The
concentration in layer l+1 is
computed by accumulation of mass caught in rain in upper layers:
Note
that there is only exchange of mass to the raindrops in layer l, if the concentration in the falling
raindrops is still lower than 
(the restriction 
should hold).
The following algorithm is used to compute wet deposition:
Go from upper layer to below:
if (return_to_atmosphere[1] OR (not_return_to_atmosphere
AND 
))
The meteorological input for LOTOS-EUROS
supplies the amount of precipitation that reaches the ground. In reality,
precipitation is on average only 50% effective which means that half of the
rain drops evaporate before the drops reach the ground. This effect, which
redistributes tracer mass in an air column, is neglected in the current version
of LOTOS-EUROS
|
Component |
Λbc (*106) |
|
SO2 |
0.15 |
|
HNO3 |
0.5 |
|
NH3 |
0.5 |
|
H2O2 |
0.5 |
|
HCHO |
0.05 |
Table 6.1. overview of below cloud scavenging coefficients for gases
1.2 Aerosols
For particles the wet deposition is calculated following Scott (1979):
A = 5.2 m3 kg-1 s-1
P = precipitation rate [m/s]
Vrd = Fall speed of rain droplet [m/s]
E = Collection efficiency
|
SO4 |
0.1 |
|
NO3 |
0.1 |
|
NH4 |
0.1 |
|
PPM fine |
0.1 |
|
PPM coarse |
0.4 |
Table 6.2. Collection efficiency for aerosol particles in LOTOS-EUROS
1.3 Alternative scheme for below cloud scavenging of gases
LOTOS-EUROS also contains an alternative and simple parameterisation to
describe the below scavenging of gaseous species. The scavenging of a soluble
component C is given by:
Λbc = Below-cloud
scavenging coefficient
P = precipitation rate [m/s]
Δz =
scavenging scale depth [=1000 m]
The scavenging coefficients (Λbc) were
adopted from EMEP (2004; website) and are listed in Table 6.3.
|
Component |
Λbc (*106) |
|
SO2 |
0.15 |
|
HNO3 |
0.5 |
|
NH3 |
0.5 |
|
H2O2 |
0.5 |
|
HCHO |
0.05 |
Table 6.3. overview of below cloud scavenging coefficients for gases