*3.2. Chemical Composition Characteristic of PM2.5*

#### 3.2.1. WSIIs Levels

The concentrations of WSIIs were 11.75 ± 3.25 and 12.50 ± 3.40 μg/m<sup>3</sup> in spring and autumn, respectively. In PM2.5, the concentrations of K+, NH4 +, Ca2+, Mg2+, Cl<sup>−</sup>, F−, NO3 <sup>−</sup> and SO4 <sup>2</sup><sup>−</sup> were 0.67, 2.54, 1.28, 0.07, 0.24, 0.16, 1.25 and 5.53 μg/m3 in spring and 0.59, 3.17, 0.78, 0.07, 0.38, 0.22, 0.77 and 6.53 μg/m3 in autumn, respectively. The annual concentration of WSIIs was 12.15 μg/m<sup>3</sup> and occupied 26.91% of the total PM2.5. This result indicated that WSIIs were one of the main components of PM2.5. The mass concentrations of sulfate occupied 49.67% of the total WSIIs, followed by NH4 <sup>+</sup> (23.50%), Ca2+ (8.60%), NO3 <sup>−</sup> (8.37%), K<sup>+</sup> (5.21%), Cl<sup>−</sup> (2.52%), F<sup>−</sup> (1.56%), and Mg2+ (0.57%). The dominant compounds were secondary inorganic aerosols (SIAs, including NO3 <sup>−</sup>, SO4 2−

and NH4 +), with concentrations accounting for more than 80% of the total WSII mass of PM2.5. The concentrations of WSIIs in Wenshan are shown in Table 1 and compared to other typical cities, such as Kunming [30] and Guiyang [40] on the plateau and Beijing [41] and Nanjing [42] on the plain. Compared to other Chinese studies, most of the ionic species identified in research are found to be on the lower side. Compared with Kunming and Guiyang, the concentration of SO4 <sup>2</sup><sup>−</sup> was lowest, which is consistent with the lagged industrial development of Wenshan. These results show that the WSII concentrations at Wenshan were impacted more by local pollution sources (e.g., biomass burning, agricultural dust, construction dust, etc.) [43].

**Table 1.** Mean concentrations of WSIIs sampled in Wenshan in 2016 compared with data from other sites (μg/m3).


SIAs were the dominant ions in the PM2.5 component in both autumn and spring. The spring and autumn concentrations of SIAs follow the order SO4 <sup>2</sup><sup>−</sup> > NH4 <sup>+</sup> > NO3 − (Figure 3). One of the reasons is that industrial production leads to the incomplete combustion of fossil fuels, which increases the emission of the gaseous precursor SO2 [44,45]. Moreover, the geographical structure of urban areas is not favorable to pollutant diffusion in the atmosphere. Another reason is the high conversion rate of SO2 to PM2.5, which may be due to the relatively high humidity in autumn [46,47]. In addition, NH4 <sup>+</sup> was the most dominant cation in PM2.5 in the two seasons, and the emission of NH4 <sup>+</sup> originated from the nitrogen fertilizers used in agriculture [48,49]. The observed NO3 <sup>−</sup> levels were related to the synthetic action of various influencing factors, i.e., precursor NOX emissions, complex photochemical and heterogeneous reactions and gas-aerosol equilibrium [50,51].

**Figure 3.** Seasonal variations in SIAs and their ratios in PM2.5 in Wenshan.

To discuss the relative importance of mobile and stationary sources of SO2 and NOX, the mass concentration ratio of NO3 <sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> was used as an indicator [52]. The seasonal variation in NO3 <sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> in PM2.5 ranged from 0.16 to 0.32 and from 0.09 to 0.18 in spring and autumn, respectively, with an annual mean of 0.18 ± 0.07, which was lower than the values measured in Shanghai (0.43) [53], Qingdao (0.35) [54] and Taiwan (0.20) [55]. Therefore, with the increasing number of motor vehicles, the contribution of mobile sources is more important than before.

Ion balance calculations are frequently used to investigate the acid base balance of ions in PM2.5. The correlation of CE and AE and the variation in CE/AE in the two seasons were calculated. According to the electroneutrality of solutions, AE must be equal to CE [56]. The correlation coefficient between CE and AE for spring (R2 = 0.92) was higher than that for autumn (R<sup>2</sup> = 0.85), showing that cations and anions maintained better equilibrium during neutralization in spring. The average CE/AE ratios for autumn (1.72) were higher than those for spring (1.54), which indicates the basic nature of aerosols in which PM2.5 is alkaline in the two seasons in Wenshan [57,58].
