Fossil fuel powered vehicles emit significant primary PM, e.g. black carbon and primary organic aerosol (POA), and produce secondary organic aerosol (SOA), via atmospheric oxidation of precursor gases in the exhaust. We quantify, for the first time, secondary organic aerosol production from two-stroke (2S) scooters. Passenger cars and trucks, particularly diesel vehicles, are thought to be the main vehicular PM sources8. This needs re-thinking, as we show for the first time that elevated PM levels can be a consequence of ‘asymmetric pollution’ from two-stroke (2S) 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 1000s of times higher than from other vehicle classes. Further, we demonstrate that oxidation processes producing SOA from vehicle exhaust also produce potentially toxic ‘reactive oxygen species’.