When using STILT (ichem=8), cmass gets forced to 1 (mass) even when you set it to 0

[jawilli]

When using both HYSPLIT version 5.2.3 via the linux command line and the standard HYSPLIT GUI version 5.2.1 on a windows machine, the following was encountered when running backwards concentrations with STILT with the output desired in concentration not mass:

In command line by setting ichem=8 and cmass=0 in the SETUP.CFG file. Once HYSPLIT was run, resultant CONC.CFG file changed cmass=1 and output data in mass (but still labeled mass/m^3).
In GUI in advanced configuration setup set in-line chemical conversion module to STILT mode and concentration Grid packing Method set output Units to Concentration. Ran HYSPLIT. Resultant CONC.CFG file still changed cmass back to 1 for mass instead of 0 (concentration) and results were in mass also.

When setting ichem=6 with cmass=0 in SETUP.CFG in linux command line version and the equivalent "Divide output mass by air density to sum the mixing ratio" and output units in concentration in the GUI version, both versions did indeed output the desired concentration and the resultant CONC.CFG file did have the desired cmass=0.

It seems this could be a bug when chosing ichem=8 (STILT mode), that it does not allow you to output the results in concentration?

[christopher.loughner]

Correct, HYSPLIT forces CMASS=1 when run with ICHEM=8. However, running with ICHEM=8, particles are still divided by the air density to output a mixing ratio output field.