After China began its COVID-19 quarantine, a satellite map highlighted a dramatic reduction in air pollution over the country. When chemistry and earth and planetary sciences professor Ron Cohen, an air quality researcher, saw the image, he predicted a similar effect in the Bay Area. Now the EPA has confirmed that pollution levels around San Francisco have decreased by more than one third over last year. "It's much bigger than drops of anything else we've ever looked at," Professor Cohen says. He and his team have investigated the numbers for nitrogen dioxide, one of the key pollutants, finding that its levels were roughly cut in half from the week before the shelter-in-place order to the next week. He says they'll learn a lot more as new data becomes available. It will help the researchers not only confirm pollution levels in the air, but also what its sources are. "We think we know how much comes from cars, how much from trucks, how much from industry. And all of those things suddenly changed by different amounts. So we'll be able to check," he says. Link to video.