北京12月中的空气污染红色警报再次牵动了大家脆弱的心肺，但是在NASA的空气污染色阶图中，北京天津似乎也就是个橙色级别，再看看石家庄才叫红得骄傲呢。NASA原文及滚动条互动图点这里。

Smog was particularly bad in cities like Beijing, which declared a five-day pollution “red alert” on December 16. The Chinese capital reached dangerous levels of more than 400 micrograms per cubic meter. In Shijiazhuang, capital of the Hebei province, airborne pollutants surpassed World Health Organization (WHO) guidelines by 100 times on December 19. (A safe level is 10-25 micrograms per cubic meter of PM2.5, according to the WHO.)

Fine, airborne particulate matter (PM) that is smaller than 2.5 microns (about one thirtieth the width of a human hair) can cause lung damage. Industrial practices like fossil fuel burning and agricultural fires produce most PM_2.5aerosol particles. Despite efforts to curb these emissions, China continues to struggle with its air quality.

From space, the smog appears gray. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured a natural-color image (top) of northeastern China on December 22, 2016. Heavy, gray smog shrouds parts of the country, while the brightest, whiter areas are likely clouds or fog. The second image shows the severity of haze as measured by satellite, with deepest reds indicating the most affected areas. The map is based on MODIS measurements of aerosol optical depth—how much sunlight the aerosol particles prevent from reaching the ground.

For full NASA post and interactive slider image tap here.