Wet deposition parameterization#

Overview#

Wet deposition in PALM is modelled using the parameterization of Berge and Jakobsen (1998). Scavenging rates for specific soluable gas and particulate species are calculated at the in- and below-cloud levels based on the field precipitation rate, through which concentrations of said species can be updated along each vertical column in the computational domain.

Theoretical foundations#

The convention employed in Simpson et al (2012) is used here to describe the parametrization of scavenging rates, expressed in ${\text{s}}^{-1}$, which consists of the following three calculations:

1. In-cloud scavenging rate ($S_{\text{in}}$),
2. Under-cloud scavenging rate for gas phase species ($S_{\text{sub}}$), and
3. Under-cloud scavenging rate for particulate species ($S_{\text{sub}}^{\text{aer}}$).

The in-cloud scavenging rate applies to all relevant gas and particulate species, and is calculated as follows:

${\displaystyle S_{\text{in}}=-\left(\frac{W_{\text{in}}P}{h_S{\rho }_w}\right)\chi }$(1),

where
$W_{\text{in}}$ [ ] is the in-cloud scavenging ratio,
$P$ [$\text{kg}$ ${\text{m}}^{-2}$ ${\text{s}}^{-1}$] is the precipitation rate,
$h_S$ [$\text{m}$] is the characteristic scavenging depth,
${\rho }_w$ [$\text{kg}$ ${\text{m}}^{-3}$] is density of liquid water, and
$\chi$ [ ] is the mixing ratio of a soluble component.

Similarly, the below-cloud scavenging rate for gas-phase species can be modelled using the relation below:

${\displaystyle S_{\text{sub}}=-\left(\frac{W_{\text{sub}}P}{h_S{\rho }_w}\right)\chi }$(2),

where $W_{\text{sub}}$ [ ] is the in-cloud scavenging ratio.

On the other hand, the scavenging of particle species through entrainment of rain droplets is determined using the following equation:

${\displaystyle S_{\text{sub}}^{\text{aer}}=-\left(\frac{\bar{E}AP}{V_{\text{dr}}}\right)\chi }$(3),

where
$\bar{E}$ [ ] is the raindrop particle collection efficiency,
$A(=5.2{\text{m}}^3$ ${\text{kg}}^{-1}$ ${\text{s}}^{-1})$ is an empirical constant based on a Marshall-Palmer size distribution (Laakso et al, 2003), and
$V_{\text{dr}}w$ [$\text{m}$ ${\text{s}}^{-1}$] is the rain drop terminal velocity.

Typically, the cloud layer is identified by using the mass fraction of liquid water, when it crosses a threshold of 10${}^{\text{5}}$ on a vertical column (van Zanten et al, 2011). Meanwhile, the characteristic scavenging depth ($h_S$) is set to a constant of 1000 m (Berge and Jakobsen, 1998), and a value of 5 $\text{m}$ ${\text{s}}^{-1}$ has been adapted for the rain drop terminal velocity (Simpson et al, 2012). In addition, the scavenging ratios ($W_{\text{in}}$ and $W_{\text{sub}}$), as well as, the particle collection efficiency ($\bar{E}$) are dependent on the particular soluble gas-phase and particle species. These are presented in the table below, adapted from Simpson et al (2012).

Species	$W_{\text{in}}\times {10}^{-6}$	$W_{\text{sub}}\times {10}^{-6}$	$\bar{E}$
${\text{SO}}_2$	0.3	0.15	N/A
${\text{HNO}}_3$	1.4	0.5	N/A
$\text{HONO}$	1.4	0.5	N/A
${\text{NH}}_3$	1.4	0.5	N/A
${\text{H}}_2{\text{O}}_2$	1.4	0.5	N/A
$\text{HCHO}$	0.1	0.03	N/A
$\text{ROOH}$	0.05	0.015	N/A
${\text{PM}}_{2.5}$	1.0	N/A	0.02
${\text{PM}}_{10}$	1.0	N/A	0.4

Namelist (_p3d) Options#

The PALM wet deposition model can be activated from the namelist (_p3d) file by setting the option chem_wet_deposition to .TRUE.. The model automatically matches the available gas phase and aerosol species listed in the table above with the species defined in the active chemical mechanism in the file chem_gasphase_mod.f90 by name. The user can further specify the intervals in which the scavenging rates are updated via the namelist option chem_wet_deposition_update_interval, which is defaulted to 300 seconds.

Normally, the wet deposition model extracts information cloud layer and precipitation from the build cloud model at each update interval (defined by the chem_wet_deposition_update_interval option) to re-calculate the scavenging rates. This dependence can be disabled by setting the namelist option chem_wet_deposition_override to .TRUE.. In this way, the vertical cloud layer and precipitation rate can be supplied directly by the user. The location of the cloud layer is specified through the options chem_wet_deposition_cloud_level_lower and chem_wet_deposition_cloud_level_uppers, and they are defaulted to 10 and 15 respectively, denoting the vertical grid level for the lower and upper bounds of the cloud layer. Meanwhile, the precipitation rate can be specified by the option chem_wet_deposition_rain_rate, which carries a default of 1.0 $\text{kg}$ ${\text{m}}^{-2}$ ${\text{s}}^{-1}$ per kilogram of air.

A summary of the namelist options are listed below:

_p3d_ Option	Default	Function
chem_wet_deposition	.FALSE.	.TRUE. activates the wet deposition model
chem_wet_deposition_update_interval	300	Seconds between scavenging rate updates
chem_wet_deposition_model_override	.FALSE.	.TRUE. to use user-supplied cloud and precipitation information
chem_wet_deposition_cloud_level_lower	10	User-supplied lower bound for the domain cloud layer
chem_wet_deposition_cloud_level_upper	15	User-supplied Upper bound for the domain cloud layer
chem_wet_deposition_rain_rate	1.0	User-supplied rate of rain precipitation for the whole domain

References#

• Berge, E., and Jakobsen, H.A.: A regional scale multi-layer model for the calculation of long-term transport and deposition of pollution in Europe, Tellus B, 50, 205-224, 1998.

• Laakso, L. et al: Ultrafine particle scavenging coefficients calculated from 6 years field measurements, Atmos. Environ., 37, 3605–3613, 2003.

• Simpson, D. et al: The EMEP MSC-W chemical transport model – technical description, Atmospheric Chemistry and Physics, 12, 7825–7865, 2012.

• van Zanten, M.C. et al: Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO, Journal of Advances in Modeling Earth System 3(2), 2011.