I like scooters on an aesthetic level, but I don't know if it's true that they pollute less: my understanding is that most scooters use relatively dirty two-stroke engines, and that much of SE Asia's urban air pollution can be correlated to heavy scooter use.
>>woodru+(OP)
They used to be 2-strokes. Probably still are in many parts Asia and Africa.
But, in the US and EU, new scooters are (almost?) all 4-stroke today due to emissions regulations. Many are fuel injected for the same reason. I'm not sure if they're required to have catalysts - but that's a fairly simple fix (for new models).