Carbon filter and air handler

I've been using a window unit for my100 sq Ft grow room and a mini-split system for my 300 sq ft office. The two rooms I built in my well insulated 1300 sq ft garage which leaves 900 sq ft remaining for the garage. Well my office mini split went out and I bought a Goodman 2 ton condensing unit located outside and air handler which I will locate in the attic. I'll rarely use the garage ducts but if a project comes up it would be nice to open the supply dampers for that area. The unit is cooling only.

My concern is could I use a carbon filter in the return duct of the grow room or should it be in my office supply duct and will it remove the shared return air smell? If the filter won't hide the smell then I'll have to go back to using the window ac unit for the grow room and the new air handler for my office air. I'll control the dampers with thermostats located in the 3 areas. I'm a retired electrician and hope to only need to use an AC company for the refrigerant lines as I am sure I can do everything else myself. Also I'm planning on running a fresh air duct for possibly part time operation. Any helpful comments will be greatly appreciated...

First off, for single pass scrubbing, exposure time is critical, therefore it is important that your airflow doesn't exceed the manufacturers single pass CFM rating. You may find that running a additional carbon filter as a scrubber in the grow area is required.

I would use the filtration on the return from the grow area, otherwise it would increase the size and cost of the filter.

This shows an example of the filter CFM ratings, the single pass would be for your return duct filter and the scrubbing CFM is for recirculating room air (the best way to kill odor IMO).