There are two choices, and I have no idea how they are any different.

 

The A signifies automatic. There is a post on here somewhere that describes the exact operation from the BMW TIS. Basically, there is a sensor that monitors the incoming air for hydrocarbons and other pollutants and activates the recirc when it detects those smelly things. Also, it will not activate under 40 degrees F to keep your windows from fogging up.

I keep it on all of the time in my car.

The recirc without the A is just pure recirc. Watch out your windows will fog up if is cold out.

 

The recirc with the "A" symbol allows fresh air in 99.9% of the time. It's a really poor icon, as it shows the recirculation arrow, even though it almost never switches to that mode. It should just be changed to "Auto".

As the previous post described, it only switches to recirculate when it detects high pollutants in the outside air. I have no idea how it does that.

 

It's gone one the 2010 for some reason :(

 

As strobyproby stated there are senors in the incoming air stream that sense the Polutants in the air and when it does it automatically switches to recirculate.

I know this works as where i live in central MA it very rarely switches but when i head into Boston, Hartford CT, Albany NY and other congested cities it switches many times.

I have posted to a thread here somewhere that explains all that it does and how it does it.