This is Scientific American's 60-second Science, I'm Christopher Intagliata.
Air pollution causes millions of premature deaths around the world every year, according to an estimate by the World Health Organization. And one of the most significant drivers of air pollution in cities is exhaust that comes out of tailpipes from cars, trucks and buses. In an effort to tame its air pollution, the city of Hong Kong has deployed a system that can sense when a high polluting vehicle drives by. There are more than 150 sites for monitoring of vehicle emissions in Hong Kong, so that high-emitting vehicles can be identified quickly and enforced for repair. John Zhou is an environmental engineer at the University of Technology Sydney. And he's co-author on a new analysis of the program, in the journal Science Advances. He says the system works like this. Sensors at road level shoot beams of light across the road, to a reflector. The sensors then measure how much light is absorbed by pollutants, and roughly calculate emissions from passing vehicles.