Data:
The datasets used for this projects are hourly PM2.5 value of Beijing and some other main cities of China. They are provided by U.S. Department of State Air Quality Monitoring Program, which can be accessed at the official website: http://www.stateair.net/web/historical/1/1.html  But there are some errors in these datasets, some PM2.5 values are negative. Since PM2.5 is correctly defined as particulate matter with a mean aerodynamic diameter of 2.5 μm, so the negative PM2.5 values are wrong ones. Although this dataset contains only the information of PM2.5, analysis about air pollution would be persuasive if more information is contained for that PM2.5 is only one of the pollution particles. Negative values of PM2.5 Value are dropped and only the column of ‘Time’ and ‘PM2.5 Value’ are kept. Also, the values in ‘Time’ column are transferred into formal datetime data by using the function.