Methodology:
Time series analysis is an appropriate method to analyze air pollution, since the index of air quality is continuous as time. Rolling means and rolling deviations are made to help realize the trend of PM2.5 Value in Beijing. Event detection is also used with the sudden declining trend of time series plot of PM2.5 Value. After searching about it, it is confirmed to be the military parade in September in Beijing, when hundreds of factories were shut down and 2.5 million cars were banned. We can conclude that factory emissions and vehicle emissions are main factors of air pollution in Beijing. With the comparison of north and south cities, we can also conclude that coal burning is another main factor.