From GridLAB-D Wiki
air_density – Density of the air.
Remarks
This value is automatically set.
Default
air_density is set to be 0.0735 lb/ft<math>^3</math>
Bugs
The air_density is not adjusted according to the altitude given in the climate data (if any). See http://sourceforge.net/p/gridlab-d/tickets/554.