A layer of haze filled China's Sichuan Basin on Jan. 23, 2014, when NASA's Terra satellite acquired a natural-color image of the phenomenon.
On the day this image was acquired, measurements from ground-based sensors at the U.S. Consulate in Chengdu reported PM2.5 measurements of 267 micrograms per cubic meter of air. Fine, airborne particulate matter (PM) that is smaller than 2.5 microns (about one-thirtieth the width of a human hair) is considered dangerous because it is small enough to enter the passages of the human lungs.
Most PM2.5 aerosol particles come from the burning of fossil fuels and biomass (wood fires and agricultural burning). The World Health Organization considers PM2.5 to be safe when it is below 25.
Haze in this region tends to worsen in the winter, when cold, heavy air traps pollutants near the surface. In this case, the haze likely was trapped in the Sichuan Basin by a temperature inversion. Normally, air is warmest near Earth's surface. Occasionally, a mass of warm air will move over cooler air so the atmosphere actually warms with altitude. Because the cool air doesn't have the energy to rise through the warm air, vertical circulation slows and air becomes trapped near the surface.
Image courtesy of NASA.