need filter in ac duct

i am using a portable a/c unit and there is an exhaust duct. the duct is 6 inch.

looking for a carbon type filter. i was told when i bought my big carbon filter i needed to know the CFM of my fan.

is there a filter designed to use in a/c ducting or do i need to install an inline fan w/filter?

not sure what to do. always seem to find help here. pre-thanks for any help

The ac exhaust is on the radiator not the room air. Mine anyway.