It can track ozone, nitrogen dioxide, sulfur dioxide, bromine, and organic molecules such as formaldehyde, as well as tiny airborne particles called aerosols. TEMPO’s sensors detect tiny differences in the light reflected when sunlight strikes molecules in the atmosphere and gets absorbed at specific wavelengths. “And more observations mean we can have a better understanding of what is happening.” “It brings significantly more observations into the game than we had so far,” Pfister says. With TEMPO, scientists who have relied on once-a-day snapshots from orbit will soon have hourly images for most of North America, from smog-choked Mexico City to northern Canada’s oilsands. TEMPO is one of three geostationary instruments that together will create the first network dedicated to monitoring air pollution across much of the Northern Hemisphere. It will hover 36,000 kilometers above North America, orbiting in sync with Earth’s rotation. She is one of many pollution watchers eagerly awaiting the Tropospheric Emissions Monitoring of Pollution (TEMPO) instrument, which is mounted to a commercial communications satellite. Pfister should soon have a much clearer picture, thanks to a NASA sensor scheduled for launch on 7 April. Pinning down the sources of the pollution and understanding its hour-by-hour behavior is “very, very complicated,” says Gabriele Pfister, an atmospheric scientist at the National Center for Atmospheric Research.

Winds flow up and down the mountainsides like tides, sweeping away and returning the smog. Emissions from oil and gas wells to the north mix with car exhaust from the Denver area and sporadic wildfire smoke. The eastern flank of Colorado’s Rocky Mountains traps a confusing swirl of air pollution.

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