Denise Mauzerall arrived in Beijing this year at a time that was both horrifying and illuminating. The capital was facing some of its worst pollution in recent memory, and Mauzerall, a Princeton environmental engineering professor, was passing through on her way to a university forum on the future of cities.

“I took the fast train from Beijing to Shanghai, and looking out the window for large sections of that trip, you couldn’t see more than 20 feet,” Mauzerall recalled.