Peering into China's thick haze of air pollution

As 2016 gave way to 2017, residents of Beijing, Tianjin, and many other northern Chinese cities suffered through the longest stretch of stifling air pollution ever recorded in the country. They choked through eight continuous days of thick, light-blocking haze, starting Dec. 30, 2016. This stretch of bad air began only a week after people in 70 northern Chinese cities were enveloped by similar days of haze composed of high concentrations of particles less than 2.5 μm in diameter (PM2.5).