How much do scooters pollute?

Studies have shown that some motorcycles spew large amounts of air pollution ( a swiss study pegged the number at 16x hydrocarbons). While there are a number of questions about this study, I've been wondering about how well "modern" scooters fare.

Recently Paolo Timoni, President and CEO of Piaggo USA (Piaggo owns brands such as Vespa, Aprilia and others) published an open letter to US mayors. The letter was published as a full paged ad in the New York times.

Does anyone have any information about scooter pollution?