Scavenging and wet deposition

Pollutant scavenging is calculated by two methods,

depending upon whether the pollutant participates in the

cloud water chemistry. For those pollutants that are

absorbed into the cloud water and participate in the cloud

chemistry, the amount of scavenging depends on Henry's

law constants, dissociation constants, and cloud water pH.

For pollutants that do not participate in aqueous chemistry,

the model uses the effective Henry's Law equilibrium

equation to calculate ending concentrations and deposition

amounts. The rate of change for in-cloud concentrations


cld) for each pollutant following the cloud time scale (τcld)

is given by:

where α

i is the scavenging coefficient for the pollutant. For

sub-grid convective clouds, τcld is 1 hour and for grid


where ρ H2O is the density of water, W T is the mean total

water content (kg m-3), R is the Universal gas constant, and

T is the in-cloud air temperature (K). The washout time,

τ washout represents the amount of time required to remove

all of the water from the cloud volume at the specified

precipitation rate (Pr), and is given for the cloud with

thickness Δ zcld :

The accumulation mode and coarse mode aerosols are

assumed to be completely absorbed by the cloud and rain

water. Therefore, the scavenging coefficients for these two

aerosol modes are simply a function of the washout time:



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