File:Haze over Northern China and the Korean Peninsula (MODIS 2021-11-26).jpg

English: A massive layer of thick haze blanketed the North China Plain, the Yellow Sea, and the Korean Peninsula in mid-November 2021. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired this true-color image on November 18. The gray haze began well inland, stretching for more than 1,100 miles (1,786 km) from west to east and more than 700 miles (1,127 km) from north to south. For much of this area, the ground underneath the haze was completely obscured from view from space, including Beijing, China and Seoul, South Korea.

Haze frequently occurs in this region, typically lasting somewhere between a day and a few weeks. While it can occur at any time of the year, it tends to worsen in the fall and winter when cold, heavy air traps pollutants near the land surface. This most often happens when a mass of warm air moves over cooler air (a temperature inversion), creating a barrier that the cooler air can’t rise through, so that the cooler air stays low and any pollutants in that cooler can’t disperse but accumulate over time.

Even without the added difficulty of a temperature inversion, the structure of the land is favorable for the accumulation of pollutants. The North China Plain is surrounded by mountains in the north and west, the Bohai Gulf (part of the Yellow Sea) to the east, while in the south the land is open and flat—and filled with industry and heavy population. Winds can easily carry pollutants northward until the flow is interrupted by the mountains and stagnation occurs.

Haze in this region comes from a complex mix of sources. These include industrial pollution, biomass burning, use of coal for heat, cooking fuels, and even dust from inland deserts and dry areas. The satellite record shows that, about a week before this extreme haze settled across the region, that dust was flowing eastward across China and mixing with a gray haze that had already begun to accumulate.