*3.4. Characterization of an Air Pollution Episode in Winter 2017*

During this research period, the particulate pollution incident that caused a wide range of impacts was identified in the winter of 2017. Figure 9 showed the hourly average PM10 concentration of four cities affected by air pollution incidents (3–6 January 2017), during which the hourly average PM10 concentration of all cities exceeded 120 μg/m3. The average daily concentrations from 3–6 January in Chengdu, Deyang, Ya'an and Meishan were 366 μg/m3, 245 μg/m3, 232 μg/m3 and 225 μg/m3, respectively. The average daily PM10 concentration in these cities was about 10–18 times higher than the WHO guidelines. During the period of heavy pollution, the average ratio of PM2.5/PM10 increased over Chengdu, Deyang, Ya'an and Meishan to 0.65, 0.68, 0.69 and 0.74, respectively. The ratio of the four cities is greater than 0.65, much higher than the annual average value of 2017 (0.59), indicating the dominance of fine particulate matter during the event.

**Figure 9.** Air transport clustering trajectory in Chengdu during heavy pollution period, and the color of red and orange represent two different trajectories. The right picture shows the hourly average PM10 concentration of the four cities during the event. The shaded part represents the arrival time of the peak concentration.

After clustering the downloaded backward trajectories, it was found that they mainly originated over the plateau area of the southwestern part in the basin. The air mass reached Ya'an first from the southwest. The concentration of particulate matter in Ya'an reached a peak with the value of 316 μg/m<sup>3</sup> at 20 o'clock on 4 January, and then the air mass continued to move northeastward. When the air mass moved to Chengdu, the concentration of particulate matter increased. Moreover, the highest concentration was observed in the Chengdu area, and the highest concentration may occurred over Chengdu and Deyang. Chengdu and Meishan peaked at around 15:00 on 5 January, with PM10 concentrations of 478 μg/m3 and 288 μg/m3, respectively. Deyang reached the maximum concentration of 324 μg/m3 during the pollution period at around 20 o'clock on 5 January.

It is worth noting that the backward trajectory changed the direction in DY and MY before arriving in CD. The further enrichment of particulate matter concentration from DY to MY and CD may be affected by climatic conditions. The adverse meteorological conditions in heavy pollution days, including high pressure, weak wind (0.7 m/s in average) and low temperature (10.5 ◦C in average), make the pollution track not easy to spread and can only flow inside the basin.

Southwest region is the industrial concentration area of Sichuan Basin, with developed secondary industry. In these cities of Southwest region, PZH is one of the four major iron ore areas in China. In 2018, the economic proportion of the secondary industry in PZH was 62.3%, the highest among all cities in Sichuan Basin. Previous studies have shown that there is a strong positive correlation between the secondary industry and PM2.5 concentration [58]. Dense industrial sources in southwest region and adverse meteorological conditions may be the main causes of heavy pollution events in the selected cities.
