The Release Type is used to set the input parameters for the scenario being modeled. If you do not have enough information on the source term, you can run the "Unknown Material" scenario with a unit (1) mass of pollutant to get a footprint of the likely dispersion pattern until more information is known. For simulations longer than 24 hours, choose the "Unknown Material (Generic Mass - long duration)" option that will combine a fine resolution concentration grid near the source with a coarser resolution concentration grid well beyond the source location to give the benefits of each. For
both of these it is assumed the material is a gas or small enough particles that do
not settle due to gravity over the duration of the run.
For more information on the prescribed burn option, see https://www.arl.noaa.gov/hysplit/smoke-prescribed-burns.
The volcanic ash simulation uses a particle size distribution which may be
changed in a later entry page. The"Volcanic Ash (24h, 5level)" case has many inputs pre-set. Note eruption type is described in
Table 1 at the USGS Open-File Report 2009-1133 (Mastin et al.) at http://pubs.usgs.gov/of/2009/1133.