3.2.2. Water-Soluble Ions

Water-soluble ions are indispensable components in PM, taking a percentage of about 40–50%, especially in fine particles, where there was the largest part of all the compositions. The sulfate, nitrate, ammonium, and the rest of the water-soluble ions (RWSI) accounted for varied from 8.1–15.6%, 10.7–23.0%, 1.8–11.4%, and 5.9–17.0%, respectively, in different particulate sizes at Gulou, and varied from 6.9–15.7%, 8.7–20.0%, 2.8–10.3%, and 8.0–18.7%, respectively, in different particulate sizes at Zifeng. Most of the water-soluble ions showed the characteristics of larger mass concentrations in finer particles, except Na+, Ca2+, Mg2+, and F- , which showed the characteristics of higher mass concentrations in coarse particulate matter. As we know, sea salt was the major source of Na<sup>+</sup> and F- in coarse particles [49,50]. Therefore, the abundant Na<sup>+</sup> and F- in coarse particles were mainly from marine salt. However, the sum mass concentrations of Na+ and F- in coarse particles only accounted for about 2.56% of the PM10-2.1 mass at the ground level, indicating less contribution from sea salt aerosols to the particulate matter. In total, 88.8% of Ca2+ and 76.2% of Mg2+ were concentrated in PM10-2.1 at the ground, and the concentrations of Ca2+ and Mg2+ at Gulou (20 m) were about 100.5% and 52.4% larger than those at Zifeng (380 m), indicating that Ca2+ and Mg2+ were mainly from the sources of soil and construction dust [51,52] in coarse particles and local ground areas.

To evaluate the equivalence between cations and anions, the cations and anions were calculated based on the following equations, and the ratio of cations/anions (C/A) was applied to estimate the neutralizing level for size-fractionated PM at two heights. The average ratios of cations to anions at Gulou in nine particulate sizes (<0.43 μm, 0.43–0.65 μm, 0.65–1.1 μm, 1.1–2.1 μm, 2.1–3.3 μm, 3.3–4.7 μm, 4.7–5.8 μm, 5.8–9.0 μm, and 9.0–10 μm) were 0.99, 0.94, 0.93, 0.91, 0.98, 1.09, 1.35, 1.66, and 1.92, respectively. In addition, the ratios of C/A at Zifeng in nine particulate sizes from fine to coarse were 1.00, 0.87, 0.91, 0.92, 1.02, 1.05, 0.91, 1.84, and 1.47. Similar neutralizing relationships were shown at different height sites, which implied an acidic tendency of fine particles and an alkaline tendency of coarse particles. The loss of CO3 2- and HCO3 - , which were not analyzed, is a reason for the deficit of anions in the coarse particles.

$$\text{Cation} = \text{Na}^+ / 23 + \text{K}^+ / 39 + 2 \times \text{Mg}^{2+} / 24 + 2 \times \text{Ca}^{2+} / 40 + \text{NH}\_4^+ / 18 + (\text{CH3}) 2 \text{NH}\_2^+ / 46 \tag{3}$$

#### Anion = Cl−/35.5 + F−/19 + NO3 <sup>−</sup>/62 + 2 × SO4 <sup>2</sup>−/96 + HCOO−/45 + CH3COO−/59 + 2 × C2O42<sup>−</sup>/88 (4)

In further analysis, the relative contribution of stationary sources such as industrial and powered coal fire to the mobile source for size-fractionated PM was indicated by the ratio of NO3 <sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> [53,54]. As listed in Table 4, the average mass ratios of NO3 <sup>−</sup>/SO4 2− were compared at different height levels in different particulate sizes: 1.42, 1.33, 1.46, and 1.48 at Gulou (20 m) and 1.27, 1.27, 1.28, and 1.29 at Zifeng (380 m) in PM10, PM10-2.1, PM2.1, and PM1.1, respectively. The calculated ratios of NO3 <sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> at Gulou were higher than those at Zifeng, indicating that the mobile source played a relatively more important role to PM at the ground level than at 380 m. The average ratios of NO3 - /SO4 2- at Gulou showed a feature with a larger value of finer particles, suggesting that more contributions from the mobile source were to fine particles than coarse particles, whereas the mean ratios of NO3 <sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> at Zifeng were similar in PM10, PM10-2.1, PM2.1, and PM1.1, implying that the sources of nitrate and sulfate at higher heights were relatively fixed at different particulate sizes and would be dominantly driven by the secondary formation mechanisms. In addition, higher ratio values of NO3 <sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> were also found during the pollutant days: 1.44, 1.34, 1.48, and 1.51 at Gulou (20 m) and 1.43, 1.15, 1.48, and 1.54 at Zifeng (380 m) in PM10, PM10-2.1, PM2.1, and PM1.1, respectively, which indicated that the formation of nitrate played an important role under pollutant conditions. Comparing with other studies, the NO3 <sup>−</sup>/SO4 <sup>2</sup><sup>−</sup> ratios were similar to the work (1.50–0.54) in Beijing at 260 m height before and during the APEC [24] and were higher than those works (0.65–0.86) in Tianjin from

10–220 m height [10,41,54,55], but they were lower than works in Southern California (5.0) and in Denver (2.09) due to the lower usage of sulfur-containing coal in the USA than in China [56–58].

#### 3.2.3. Carbonaceous Components

Carbonaceous species are a very important part of particles. The mass concentrations of size-fractionated OC, EC, and the ratios of OC/EC are shown in Tables 3 and 4 and Figure 6. At ground level, the size distribution of OC was a trimodal mode with peaks at 0.43–0.65 μm, 1.1–2.1 μm, and 4.7–5.8 μm. EC had a bimodal distribution that peaked at <0.43 μm and 5.8–9.0 μm. At the 380 m level, the size distribution of OC was found to be a bimodal mode peaking at 0.43–0.65 μm and 3.3–4.7 μm, while EC only showed a single peak at 0.43–0.65 μm. It could be found that more carbonaceous components were concentrated in fine particulate matter at higher heights. To PM10 and PM10-2.1, the mass concentrations of OC and EC were larger at Gulou than Zifeng, indicating a decreasing trend with increased height. Interestingly, in contrast to PM2.1 and PM1.1, the mass concentrations of OC and EC were larger at Zifeng than Gulou, especially the organic acids such as CH3COO−and C2O4 <sup>2</sup>−, which could reflect much more influences of the formation mechanism of secondary organic aerosols at the higher level than at the ground. The size-fractionated ratios of OC/EC showed similar trends at Gulou and Zifeng with unimodal distribution. The peak of the OC/EC ratio at Gulou was 3.3–4.7 μm, while at Zifeng the OC/EC ratio peaked at 4.7–5.8 μm. At both sites, the ratios were larger than two, indicating the formation of secondary organic particles [59,60].

**Figure 6.** Carbonaceous species and ratios of OC/EC of size-fractionated PM at Gulou (20 m) and Zifeng (380 m).

In addition, the correlation between OC and EC was calculated, which could be a factor of common sources if the value was high. The correlation values (R2) showed an increasing trend with the particulate size decreased at both sites: 0.80 (PM10) < 0.81 (PM2.1) < 0.91 (PM1.1) at Gulou and 0.68 (PM10) < 0.76 (PM2.1) < 0.79 (PM1.1) at Zifeng. This result suggested that there were common sources of OC and EC in fine particles, while the sources were inconsistent in coarse particles. The OC of coarse PM might be contributed from the hygroscopic growth of water-soluble OC during the transport or the emissions of industrial sources in the regional areas [61]. The EC in the coarse particles could be formed by the resuspension of soil dust and the friction loss of tires [61–63].


**Table 3.** The average mass concentrations of main chemical compositions at each sampling height in PM10, PM10-2.1, PM2.1, and PM1.1 (μg m<sup>−</sup>3).

**Table 4.** The average ratios of cations/anions, NO3<sup>−</sup>/SO4 <sup>2</sup>−, and OC/EC for size-fractionated PM.


<sup>a</sup> GL represents Gulou site (20 m), ZF represents Zifeng site (380 m).
