Smog shrouded Beijing and some other provinces again on Sunday, disrupting expressway traffic and flights.

Readings for PM2.5 exceeded 200 micrograms per cubic meter at 9 a.m. in some areas of the capital city, with visibility of less than 500 meters in the morning. Some flights at Beijing Capital International Airport were delayed because of low visibility.

Beijing stretches of expressways linking other cities including Kaifeng, Harbin, Shanghai and Tianjin were temporarily closed. Tianjin, Heibei and Henan were also hit with heavy smog.

The Chinese Academy of Sciences say they found nitrogen organic particles in a recent sample test of smog in Beijing. Experts say it is a dangerous signal for Beijing's air quality because nitrogen organic particles are one of main components of photochemical smog.

In the 1950s, photochemical smog was blamed for the deaths of hundreds of people in the US city of Log Angeles.