Code Manual for CONTAIN 2.0 - Federation of American Scientists

Table 7-1
Comparison Between Fixed- and Moving-Grid Models
Quantity Fixed-Grid Moving-Grid
Solute Effect Not Modeled Modeled
Kelvin Effect Not Modeled Modeled
Speed Faster Slower
Numerical Diffusion May Be Significant Reduced Relative to Fixed-Grid
Robustness High Low
grow faster than a particle with insoluble material. It may absorb water from the atmosphere even
if the atmosphere is superheated. Particle growth will continue until the water vapor pressure above
the particle surface is equal to that of the atmosphere.
The Kelvin effect considers the effect of the surface tension, which increases the water vapor
pressure in the particle over that for a flat surface. The smaller the particle, the greater the surface
area to volume ratio of the particle and hence the greater the effect of surface tension. For particles
of pure water or with water and insoluble material, the increase in water vapor pressure because of
the Kelvin effect will result in water evaporating from small particles in saturated environments. For
particles with soluble material, the Kelvin effect increases the water vapor pressure in solution over
that for a flat surface of solution. Thus, if a flat surface of a solution is in equilibrium with the _
atmosphere, the Kelvin effect would result in water vaporizing from a particle of the same
composition as that of the solution.
The moving-grid method has the computational advantage of reducing numerical diffusion compared
to the fixed-grid method. Numerical diffusion tends to smear discontinuous changes in the particle
size distribution function. For example, because of numerical diffusion, spurious particles may
remain in small particle size classes that have actually been swept clear of aerosols by condensational
growth. Similarly, numerical diffusion may cause spurious particles to remain in large particle size
classes that have been swept clear of aerosols by evaporation. However, reducing numerical
diffusion by using the moving-grid method may require an order-of-magnitude larger amount of
computer time than the fixed-grid method if the Kelvin effect is modeled.
With respect to selecting the appropriate method, if the solute effect or the Kelvin effect is to be
modeled, then the moving-grid method is the only choice. However, if these effects are not to be
modeled, the user may choose either method. Because the moving grid method is more accurate,
it should be used whenever numerical diffusion needs to be assessed. However, if the computer time
requirements are prohibitive, the fixed-grid method may be the only practical alternative.
To implement either method, the user must speci~ H20L as the last aerosol component in the
“mapaer” input under the global AEROSOL keyword. This is discussed in Section 14.2.5, Aerosol
Options. Within the fixed-grid method, one has either condensation or evaporation, but not a
Rev O 716 6/30/97