Two-stroke scooters are a dominant source of air pollution in many cities

Fossil fuel-powered vehicles emit significant particulate matter, for example, black carbon and primary organic aerosol, and produce secondary organic aerosol. Here we quantify secondary organic aerosol production from two-stroke scooters. Cars and trucks, particularly diesel vehicles, are thought to be the main vehicular pollution sources. This needs re-thinking, as we show that elevated particulate matter levels can be a consequence of ‘asymmetric pollution’ from 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. Further, we demonstrate that oxidation processes producing secondary organic aerosol from vehicle exhaust also form potentially toxic ‘reactive oxygen species’. Primary and secondary organic aerosols emitted by road vehicles are hazardous to health and climate, with diesel trucks and cars considered the main offenders. Platt et al. show that, despite constituting a small fraction of the fleet, two-stroke scooters can dominate vehicular pollution in some cities.