The light rail network in the Salt Lake City area already does a lot to improve air quality -- its three lines move more than 67,000 people a day along 45 miles of track, saving countless car trips and sparing the air tons upon tons of petroleum-powered pollutants.

But for the last four years, the trains, operated by the Utah Transit Authority have done even more: They've become air-sniffing sleuths, mapping out where and when different pollutants are present along the trains' route. University of Utah scientists recently published the findings of their ongoing study in Atmospheric Environment.

The TRAX project is the only known transit-based mobile air quality network in North America. Some results are unsurprising, such as spikes of carbon dioxide and nitrogen dioxide, byproducts of gasoline combustion, at street intersections. But the sensors also found methane emissions patterns that didn't correlate to daytime working hours, suggesting possible fugitive methane leaks.

"Our results suggest," the authors write, "air pollution and greenhouse gas emission monitoring and exposure assessment could be greatly enhanced by deploying instruments on public transit systems in urban centers worldwide."