Haze hovered over eastern China on December 11, 2012. The haze, likely a combination of dust and air pollution, stretched from northern China across much of the coastal plain. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite acquired this natural-color image on December 11, 2012.
Throughout November and December, thick haze has often blanketed this part of China. One common cause of poor winter air quality is a temperature inversion, in which a layer of cold, dense air gets trapped beneath a more buoyant layer of warm air. As long as the temperature inversion persists, pollution builds in the trapped pocket of air near the ground.
Airborne particles are often measured in microns (also micrometers), or one-millionth of a meter. Particles with diameters of 2.5 microns or smaller—about one-thirtieth the width of a human hair—are believed to pose the greatest health risks because they can lodge deeply in the lungs, according to the U.S. Environmental Protection Agency.
In addition to the U.S. Embassy in Beijing, another site, China Air Daily, provides satellite images, ground-based photos, PM2.5 readings, and other air quality readings for multiple cities. Developed in response to a growing desire for more information on air quality, China Air Daily is a project of the Center on U.S.-China Relations at Asia Society.