Setting the ICHEM variable to a non-zero value changes the model's internal configuration in terms of how it treats the pollutants. Some conversion options require additional modules and specific requirements in setting up the CONTROL file. See the linked discussion under each option for more information.
ICHEM is chemistry module selection index:
Additional Chemistry Options
MAXDIM is the maximum number of pollutants that can be attached to one particle. Otherwise, if multiple pollutants are defined they are released as individual particles. This feature is usually required with chemical conversion modules, which are not part of the standard distribution. However, the species 1->2 conversion option will automatically set MAXDIM=2. Both the standard point source and emissions file input routines are enabled to release multiple species on the same particle. In addition, for the daughter product calculation MAXDIM needs to be set equal to the number of daughter nuclides. Setting MAXDIM greater than one forces all the species on the same particle, with the limitation that the number of defined pollutants must equal MAXDIM. No other combinations are permitted. The advantage of this approach is that not as many particles are required for the calculation as in the situation where every pollutant is on its own particle. The limitation is that each species on the particle should have comparable dynamic characteristics, such that they are all gases or have similar settling velocities; otherwise the transport and dispersion simulation would not be very realistic. In the case of multiple species defined on a single particle, the gravitational settling routine will use the characteristics of the last defined pollutant to compute settling for that particle. Deposition, decay, and other removal processes are still handled for independently for each pollutant on that particle.