*3.1. Concentration Characteristics of PM2.5*

During the two sampling campaigns, the mass concentrations (from Telfon filters) of PM2.5 ranged from 29.11 μg/m<sup>3</sup> to 62.03 μg/m<sup>3</sup> in spring and from 24.46 μg/m<sup>3</sup> to 60.08 μg/m<sup>3</sup> in autumn (Figure 2). The overall concentration of PM2.5 is higher in spring than that in autumn. The concentration on individual days in autumn is higher than that in spring, which may be related to changes in meteorological conditions. The daily concentration levels of PM2.5 were all within Chinese National Ambient Air Quality Standards II (75 μg/m3). In addition, the total average concentration of PM2.5 in spring and autumn (44.85 ± 10.99 μg/m3) was higher than Standard II (35.00 μg/m3) (GB3095-2012), and was 3.0 times higher than the annual standard concentration in the USA (15 μg/m3). These values are lower than those of developed cities in the plains of China, such as Beijing, Tianjing and Shanghai [37,38]. Furthermore, the concentration of PM2.5 in Wenshan was lower than that in some plateau cities, such as Guiyang and Kunming [39]. In our previous research, we found that the dust emission volume of Wenshan was 1164 t, accounting for 52.9% of the total emissions. PM2.5 pollution may be associated with city construction, and the floor space of buildings under construction was 5.48 × 106 m<sup>2</sup> during 2016 in Wenshan. The mean concentrations of PM2.5 decreased from spring (48.00 ± 11.01 μg/m3) to autumn (41.64 ± 10.10 μg/m3). There were 10 and 5 days in spring and autumn, respectively, that exceeded the total average concentration, which means that were 62.5% of the days in spring exceeded the total average concentration, and it was twice that in autumn. This seasonal mean concentration variation is attributed to the primary influence of meteorological characteristics and source emissions.

**Figure 2.** The daily mass concentration of PM2.5 during sampling.
