Code Manual for CONTAIN 2.0 - Federation of American Scientists

is used during evaporation, the solid materials are assumed to inhibit evaporation when the water
mass and the solid mass are comparable. The evaporation rate is under such conditions is assumed
to be
G evap = 1.037G tanh.(2mjmP) (7-13)
where G is the evaporation rate from a spherical aerosol given by Equation (7-12) and rq and ~ are
the liquid water mass and total mass of the aerosol particle, respectively. The amount of water
present on aerosols when the rate is strongly inhibited is typically not significant.
Nucleation centers can alter the condensation rate on aerosols considerably. Even if a large number
of aerosol particles are available to serve as nucleation centers, the condensation rate can decrease
once the particles have grown. In the CONTAIN fixed-grid method, an attempt is made to simulate
nucleation centers. A small amount of water aerosol is provided in the smallest diameter section
under condensing conditions when the mass concentration in that section is zero. This is intended
to promote condensation when it would not otherwise occur, or occur but at a significantly impeded
rate. The mass concentration added corresponds to
Q~UC=max(10-20, 0.001 “abstol” Q~) (7-14)
where Q~is the total suspended mass concentration, and “abstol” is the scale factor for the Runge-
Kutta absolute integration error(see Section 14.2.5). By default, “abstol” = 10-4. The added mass
concentration is typically a few orders of magnitude smaller than the integration error in the mass
concentration in a section.
7.2.2.2 MOvin~-Grid Model. This moving grid model is only used if the keyword SOLAER is
included in the global AEROSOL block input. Otherwise the fixed grid model is invoked by default.
The algorithm used is based on the method of characteristics. However, after the effects of
condensation are calculated over a system timestep, the aerosol is remapped onto the fixed grid in
order to incorporate the effects of the agglomeration and deposition calculations. The theoretical
development of the moving-grid model is discussed in detail in Reference Ge190.
In the moving-grid method, particle sections are followed as they grow or shrink from water
condensing on or vaporizing from the particles. Particles in a section are approximated as initially
having the same chemical composition, but different sections may have different chemical
compositions. There is essentially no constraint on the particle size or composition range covered
by a time-evolved section, and two or more sections of particles may overlap the same particle size
range.
For each timestep the growth or evaporization of a particle section is calculated based on an assumed
end-of-timestep steam concentration. The water mass balance is determined from the amount of
water condensed or vaporized and the assumed steam concentration. The code iterates on the end-oftimestep
steam concentration until the water mass balance is satisfied.
Rev. O 7-19 6/30/97