**3. Overview**

#### *3.1. Data Description*

The data used in this research are fetched from a weather website (http://www.tianqihoubao. com/aqi) that releases air quality monitoring data in China. The website contains daily data for different cities and records six kinds of major air pollutants, namely, *PM*2.5, *PM*10, *SO*2, *NO*2, *O*3, and *CO*. After data cleaning, we compute the Individual Air Quality Index (IAQI) for each pollutant using its concentration. Moreover, the Air Quality Index (AQI) of each piece of data is defined as the max IAQI.

The chosen data are dated from December 2013 to November 2016. We defined an annual period as the time from December of one year to November of the next year. Geographically, our data cover 88 cities in China (Figure 1), and we divide these cities according to their geographical locations into 8 regions: North China, Central China, South China, Southwest, Northwest, Northeast, East China, and East China coastal area.

**Figure 1.** Urban distribution of the research. Each circle represents a city. Color of the circle encodes the geographical division of the city.
