The air in any urban area is not brilliantly clean, though it is significantly cleaner than it has been in the past (think smog). To filter out enough particulates to make a real difference, however, you are going to need a mask that restricts air-flow far more than the standard cycling ones. I have to wear particulate filter masks at work sometimes in the active areas, and I find it difficult to breathe after walking up and down a few flights of steps, never mind cycling.
