*3.1. Temporal Variations in the Concentrations of PM10 and PM2.5*

The mean annual, seasonal, and monthly concentrations of PM10 were all higher than those of PM2.5 (Figure 2). Intra-annual variations in the concentrations of PM10 and PM2.5 exhibited similar characteristics during 2013–2017. From 2013 to 2017, the mean annual concentration of PM10 in Xi'an declined from 186.3 μg/m<sup>3</sup> to 131.7 μg/m3, with the lowest value in 2015 (128.0 μg/m3). The highest inter-annual decline rate was 19.2%, which occurred during 2013–2014. There was no significant difference (*p* > 0.05) between the mean annual concentrations of PM10 in 2015, 2016, and 2017 (Figure 2A). Similarly, the mean annual concentration of PM2.5 in Xi'an declined from 107.4 μg/m<sup>3</sup> in 2013 to 73.7 μg/m<sup>3</sup> in 2017, and the lowest value occurred in 2015 (58.1 μg/m3). The highest inter-annual decline rate was 27.9%, which occurred during 2013–2014. The differences between 2015, 2016, and 2017 were also nonsignificant (*p* > 0.05) (Figure 2B).

The maximum seasonal concentrations of PM10 (218.5 μg/m3) and PM2.5 (137.2 μg/m3) were recorded in winter, and the minimum (86.7 and 40.0 μg/m3) in summer. The seasonal concentrations of PM10 in spring were higher than those in autumn, while the concentrations of PM2.5 in spring were lower than those in autumn, and the differences were all nonsignificant (*p* > 0.05) (Figure 2C,D). The maximum monthly concentrations of both PM10 (239.2 μg/m3) and PM2.5 (151.9 μg/m3) occurred in January. The monthly concentrations of PM10 and PM2.5 decreased from January to June, with the lowest concentrations recorded in June (85.0 and 38.5 μg/m3). Concentrations then increased from June to December. The maximum inter-monthly decrease rate of PM10 and PM2.5 occurred during March–April (26.7% and 30.6%), while the maximum inter-monthly increase rates of PM10 and PM2.5 occurred during November–December (36.3% and 52.1%).

**Figure 2.** Annual, seasonal, and monthly variations in the concentrations of PM10 and PM2.5. (**A**): interannual variation of PM10; (**B**): interannual variation of PM2.5; (**C**): intraannual variation of PM10; (**D**): intraannual variation of PM2.5.
