How monitoring works

Beijing has 35 automatic air quality monitoring stations, including 23 in the city proper and the outer suburbs. The stations monitor major air pollutants such as sulfur dioxide, nitrogen oxide and PM2.5 - ultrafine particles measuring 2.5 microns in diameter or smaller.

The stations analyze the pollutants and transfer concentration data to a center that produces a daily report and air quality forecasts. In addition, the Beijing Municipal Environmental Monitoring Center publishes an air quality index and real-time concentration of major pollutants on its website and micro blog, and also on mobile phones and TV.

The air in Beijing is polluted on roughly half of the 365 days in the year and PM2.5 constitutes the major pollutant on 80 to 90 percent of those days.