Best respirator mask for polluted cities?

To actually do any good, you'd need a real respirator with the proper cartridges. This 3M web app should help you pick the right kind. http://www3.3m.com/SLSWeb/home.html?region=AMERICA%C2%AElId=20&langCode=EN&countryName=United%20States You're going to look rather silly wearing this around, though, and it would probably be a bit insulting to the natives. YMMV.

For years I used to use a Respro mask when cycling. It was a pleasant change not to get home and blow my nose to reveal a glob of black snot. I would post a link but the anti-spam measures on this board seem rather over zealous. Just search for Respro. (the only reason I don't use one any more is that I've moved somewhere far less polluted).

There are a variety of respirator ratings offered by NIOSH (us) here. Most people just use a mask with the lowest rating - N95 - unless they have a clear need to use something better. You can pick up such masks at the hardware store for very reasonable prices, and from what I've seen, there are enough Asians who wear masks in big cities that you won't stand out.