I don't know about the 10% figure, but regardless of whether you pick numbers from the high or low end of the range of estimates, the impact of lawn equipment is still staggering. 4 cycle engines make a huge difference, as do things like catalytic converters and scrubbers. 2-stroke engines, on the other hand, are both a horribly inefficient design to begin with, and also typically come fitted with absolutely no pollution controls.

Also, it's not just leaf blowers. It's also lawn mowers and other gas powered lawn equipment.