From Istanbul to Beijing, this Winter is Very BadNASA has been monitoring nitrogen dioxide (NO2) levels across the Middle East and a portion of Asia with the Ozone Monitoring Instrument (OMI) on its Aura satellite, and it doesn't look good right now. As you can see in the image above, many of the bigger cities in the region have had pretty terrible air quality this winter. NASA explains why NO2 is a good proxy for other pollutants:
Nitrogen dioxide is a key emission from the burning of fossil fuels by cars, trucks, power plants, and factories; the combustion of fuel also produces sulfur dioxides and aerosol particles. When the weather is hot and sunlight strongest, NO2 emissions usually lead to the creation of ground-level ozone. In the winter, NO2 is less likely to breed ozone, but it does linger for a long time and contribute to fine particle pollution. Year-round, it is a good proxy for the presence of air pollution. (source)
Bad air quality during the winter is usually caused by a few factors combined. First, cold weather means that people and power plants (i.e., coal) are burning more fuel to heat buildings, so emissions are higher. But winter can also mean more frequent temperature "inversions," with the ground-level air being colder than the air higher in the atmosphere, trapping pollutants over cities.
Many parts of China are also struggling with bad air quality this winter, including Beijing, just like last year, and most of the time. Things are so bad that the government there is mandating rolling shutdowns of factories to try to reduce the smog.
The government had hoped that rain and snow would disperse the smog that has blanketed the city since last weekend, but an index monitoring of PM2.5 particulates revealed that it still stood at 400 in some parts of the city - down on last week's record score of 755, but still well above levels deemed hazardous to human health.(source)
These problems are pushing the city to formalize anti-smog regulations. Hopefully they will be adequate and will improve air quality there once and for all (and not with a reactionary band-aid approach). The U.S. went through something similar a few decades ago, and things have much improved since.