Is the intake for the air conditioning unit the same (i.e. under the hood near the windshield wipers) as the regular cabin air intake? What about in recirculation mode?
I'm not exactly positive about the BMW, but would assume it's just like any other car. The A/C uses the same fresh air locations as the rest of the system. When in recirculation mode, the air is forced to go back through and into the cabin interior with little air being drawn from the outside. In order for the air to be cool (when going through the A/C condenser) or warm (when going through the heater core), there is an object which is called a blend door which forces the air, in varying amounts, through one side or the other. If you want the air to be warmer, it forces more through the heater core. If you want it to be cooler, it forces more through the condenser core.
In recirculation mode, air from the cabin is sucked in by the blower motor that is passed through the HVAC unit back into cabin. The choice is based on user selection based on which the flap near the blower motor is adjusted to fresh air mode or recirculation mode.