Using cellular networks to measure air quality

To support efforts to curb health problems caused by air pollution, scientists and policymakers need to be able to reliably monitor air quality. But conventional ground-based sensors and aerosol monitors are too sparsely distributed to provide the coverage necessary for examining large areas.

Now Noam David and H. Oliver Gao at Cornell University have demonstrated that microwave signal data routinely stored by cellular networks offer an indirect yet dependable means of measuring air quality. The researchers knew that weather phenomena perturb the wireless links between cell-phone towers. In addition, atmospheric profiles turn out to be effective indicators of air quality: When temperature increases with altitude in the lower atmosphere, a condition known as temperature inversion, the air becomes stagnant and pollutants get trapped near the ground.

David and Gao’s novel system flags temperature inversions and the associated episodes of low air quality when it detects a phenomenon known as ducting. Layers of air have different refractive indices depending on their temperature, humidity, and pressure. Under inversion conditions, a transmitted microwave signal can get trapped under the warm overlayer, much like a light pulse experiencing total internal reflection in a fiber-optic cable (see the diagram below). The ensuing signal received at a given tower is therefore amplified.

The Cornell scientists were able to demonstrate the utility of their low-cost monitoring system at several locations in Israel by comparing their measurements of temperature inversions with measurements of common pollutants taken by traditional instruments. The researchers plan to conduct tests in other locations and extend the technique to monitor other atmospheric properties. (N. David, H. O. Gao, American Geophysical Union Fall Meeting, 12 December 2016 .)