NASA's Terra satellite captured this image of a long belt of smog stretching from Beijing (in the upper third of the image) to Shanghai (in the bottom right corner). That distance is roughly 1,200 km or about the distance from Boston, MA to Raleigh, NC. While smog and air pollution is fairly routine, it is less common to see smog stretch so far south.
In addition to ground-level ozone (smog), measurements taken at U.S. embassies in Beijing and Shanghai recorded elevated levels of particulate matter (soot) fewer than 2.5 microns in diameter (via Earth Observatory):
ground-based sensors at U.S. embassies in Beijing and Shanghai reported PM2.5 measurements as high as 480 and 355 micrograms per cubic meter of air respectively. The World Health Organization considers PM2.5 levels to be safe when they are below 25.
Fine, airborne particulate matter (PM) 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 of biomass (wood fires and agricultural burning).
At the time of the satellite image, the air quality index (AQI) reached 487 in Beijing and 404 in Shanghai. An AQI above 300 is considered hazardous to all humans, not just those with heart or lung ailments. AQI below 50 is considered good.
Smog and particulate matter tend to form due to problems during combustion. Uneven temperatures in a flame can lead to formation of pollutants like nitrogen oxides (NOx) that lead to smog, and particulate matter (or soot, unburned bits of fuel). These pollutants can be captured by cleaning the exhaust gas from a power plant (or industrial boiler), usually by recirculating exhaust gas back in to burn up any remaining fuel, or by passing it through a chemical process that strips out nitrogen oxides (using ammonia and a catalyst). These methods can be expensive and consume large amounts of energy, so operators tend to shy away from them. Instead, some power plants or factories will shut down completely to avoid producing air pollution.
Other space shots of Chinese pollution can be seen in "Smog shuts down Harbin, China", “Beijing’s air pollution as seen from space” and “China enveloped in smog, as seen from space. Again”.