A recent study estimates that about 1.6 million people in China die each year—roughly 4,000 a day—from heart, lung, and stroke disorders due to poor air quality. Most of the nation’s lethal air pollution, including headline-grabbing toxins such as fine particulate matter (PM2.5) and ground-level ozone (O3), is produced in its coal-dominated energy and industrial sectors. But a substantial and growing contributor to the problem is road transportation; as private vehicle ownership and freight traffic increase, so, too, do ambient concentrations of pollutants from gasoline and diesel fuel exhaust.