*2.3. Air Quality Index (AQI)*

The air quality index (AQI) includes 24-h average measurement of PM2.5, PM10, NO2, SO2, CO, and 8-h average concertation of O3 and reflects the overall air quality [8,28,40]. Individual air quality index (IAQI) for six criteria pollutants is determined by using Equation (1), and the overall AQI is calculated based on the highest IAQI by using Equation (2) according to the instruction given in technical regulation on ambient air quality index (on trial) (HJ-633-2012) [19].

$$IAQI\_p = \frac{I\_{high} - I\_{low}}{\mathcal{C}\_{high} - \mathcal{C}\_{low}} \times \left(\mathcal{C}\_p - \mathcal{C}\_{low}\right) + I\_{low} \tag{1}$$

*IAQIp* = individual sub air quality index of the pollutant p

*Cp* = concentration of the pollutant p

*Chigh* = concentration breakpoint that is ≥ *Cp*

*Clow* = concentration breakpoint that is ≤ *Cp*

*Ihigh* = index breakpoint corresponding to *Chigh*

*Ilow* = index breakpoint corresponding to *Clow*

$$AQI = \max(I\_1, I\_2, \dots, \dots, \dots, \dots, \dots, \dots, I\_n) \tag{2}$$

In Equation (2), "*n*" indicates the number of criteria pollutants. When AQI is higher than 50, the highest IAQI is considered as a major pollutant for that given day [23,28,41–44]. Air Quality Index (AQI) has the following six categories:

Class I: 0–50 (Green), Good Class II: 51–100 (Yellow), Moderate Class III: 101–150 (Orange), Unhealthy for Sensitive Groups Class IV: 151–200 (Red), Unhealthy Class V: 201–300 (Purple), Very unhealthy Class VI: 300–500 (Maroon), Hazardous

## *2.4. Quality Assurance and Quality Control (QA&AR)*

Quality assurance and control procedures for ambient air quality data were strictly in accordance with Chinese Ambient Air Quality Standards (CAAQS) (GB-3095-2012) [17]. The daily average value was calculated when we have valid data for more than 16 h of that day (except for ozone, minimum 6-h value for 8-h ozone value); the monthly average was calculated only when we have 27 daily mean values; an annual value was calculated only when we have 324 daily mean values. Besides this, manual inspection was carried out to remove abnormal values, e.g., PM2.5 values higher than PM10 values.
