It always bothers me that newspapers usually omit hyperlinks to relevant information. Here's a paper by the guy they interview in the article: https://www.repository.cam.ac.uk/handle/1810/245612

"[...] two-stroke scooters, vehicles that constitute a small fraction of the fleet, but can dominate urban vehicular pollution through organic aerosol and aromatic emission factors up to thousands of times higher than from other vehicle classes."

(PS: don't make that into an argument for cars and against scooters though. By reducing congestion, people opting for scooters can help all other road users pollute less. So electric scooters should be a priority - http://www.etatrust.org.uk/2014/05/scooters-vs-cars-vs-lorri... )