The menu is divided into three sections. In the first two the value of the
*INITD* namelist parameter is being set. In the upper portion of
the menu, the model is configured as either a full 3D particle or puff
model, or some hybrid combination of the two. The released particles or
puffs maintain their mode for the entire duration of the simulation.
Valid options are:

- 0 - 3D particle horizontal and vertical (
**DEFAULT**) - 1 - Gaussian-horizontal and Top-Hat vertical puff (Gh-THv)
- 2 - Top-Hat-horizontal and vertical puff (THh-THv)
- 3 - Gaussian-horizontal puff and vertical particle distribution (Gh-Pv)
- 4 - Top-Hat-horizontal puff and vertical particle distribution (THh-Pv)

Introduced with the September 2004 version are mixed mode model calculations, where the mode can change during transport depending upon the age (from release) of the particle. A mixed-mode may be selected to take advantage of the more accurate representation of the 3D particle or Gaussian approach near the source and the smoother horizontal distribution provided by one of the hybrid puff approaches at the longer transport distances. Valid options are:

- 103 - 3D particle (#0) converts to Gh-Pv (#3)
- 104 - 3D particle (#0) converts to THh-Pv (#4)
- 130 - Gh-Pv (#3) converts to 3D particle (#0)
- 140 - THh-Pv (#4) converts to 3D particle (#0)

Options 130 and 140 are the inverse of 103 and 104, respectively. Selecting
either one starts the simulation as puffs (#3 or #4) and after *CONAGE* hours
they convert to a 3D particle (#0). While the particle to puff conversion
is one-to-one, one particle is converted to one puff, the puff to particle
conversion results in one puff splitting into multiple particles. The exact
number will vary depending upon the available array space as defined by the value
of *MAXPAR*. Puff splitting can only fill up half of the available array
space. Therefore, if insufficient space has been pre-allocated, eventually splitting
will be turned off and the puff to particle conversion will become one-to-one. The
number of splits per puff is defined as 0.5 times the array size *MAXPAR* divided
by the number of puffs to be emitted over the duration of the simulation and where the
number of puffs is computed by *NUMPAR*NHRS/QCYCLE*. The split rate is computed at
each entry to the program (hourly) and may be turned back on if sufficient array space
again becomes available.

An example of the conversion (130) after 6 hours transport of a single Gaussian puff to multiple particles is shown below as a snapshot at 3 hours and 6 hours after release. On the left, the puff was converted to 50 particles (numpar=100) and on the right to 5000 particles (numpar=10000).

An option introduced with the January 2009 version (4.9), converts puffs or
particles to the Global Eulerian Model grid. The mass is
transferred to the global grid after the specified number of hours (*gemage*).
This approach should only be used for very long-range (hemispheric)
transport due to the artificial diffusion introduced when converting pollutant
plumes to a gridded advection-diffusion computational approach. The method is
ideal for estimating contributions to background concentrations. All mixed-
mode particles/puffs (not just 3D) will convert to the global grid if the
global option is selected from the special runs menu.

The third section defines the 3-D particle dispersion algorithm. The default is to use the HYSPLIT dispersion scheme. The STILT scheme was introduced in 2020. The STILT scheme is more complex module that includes a reflection/transmission scheme that preserves a well-mixed distribution of particles as they move vertically across model layer interfaces. The STILT scheme is computationally more expensive to run.