Often a smell for the first few minutes of air conditioning is caused by a clogged drain in the evaporator pan. I don't know about the i3 specifically, but sometimes they get clogged up with leaves or dirt. That sometimes means you have a little water in there that didn't drain out with some moldy crud in it. Again, I don't know about the i3, but in any other car I'd crawl underneath and see if there is a small rubber hose that's supposed to act as a drain and blow some air through it or something.