Thick smog blanketed China's capital city, Beijing, for many days, as seen by Metop-A.
08 September 2022
25 February 2014
The thick smog can be clearly seen as a large grey 'haze' in the Natural Colour RGB image from 25 February.
Although smog is common in parts of China, this was one of the most extreme and prolonged event for many years. Weather conditions (a temperature inversion) kept the pollution trapped nearer the ground than usual.
It was reported that the official smog reading for PM 2.5 — small airborne particles which easily penetrate the lungs causing major health issues and even death — stood at 501 micrograms per cubic metre. The World Health Organization's recommended safe limit is 25.
This lead to the China Meteorological Administration issuing an orange alert for air quality on 24 and 25 February, the second-highest alert possible. Schools were closed and residents were warned not to go outside.
This comparison shows the haze is more visible in the MODIS True Color RGB, from NASA’s Terra satellite, (right image) compared to the Metop AVHRR Natural Colour RGB (left image). This is because the MODIS RGB uses three visible bands, 0.4, 0.5 and 0.6 microns, which are all very sensitive to haze, whereas the AVHRR RGB only exploits the 0.6 micron band to detect the haze — smoke and haze are more transparent when you move to longer wavelength like VIS0.8 and NIR1.6.