Adjacent airspace with different initial concentration possible?

Hi,

I got my solid floor modeling to work, and am moving on to a slotted floor scenario:

There is a slotted floor that separates a pit (below) from a barn (above).  The airspace in the pit has an initial concentration of H2S = 156.9 ppm (the rest is air), and the airspace in the barn is 1,000,000 ppm air.  I tried setting up the model (attached) with a fluid subdomain for the pit, but Solidworks doesn't like the situation where there is no solid divider above.  Does anyone know how I can get this to work?  I'm trying to get the simulation to run with natural recirculation.  The velocity vectors seem to do this, but the gas evacuation/mixing is too fast if the initial concentrations are wrong.

Thanks,

Dan

SolidworksFlow Simulation