*3.3. Source Apportionment*

The source contributions of size-fractionated PM were calculated with the CMB model, which has been widely used in previous studies [64–66]. The source profiles including soil dust, the coal-fired power plant, vehicle exhaust, steel smelting, construction dust, secondary sulfate, secondary nitrate, and fugitive dust were used as model input for the CMB model. These profiles were obtained by previous work in the Yangtze River Delta [18]. Twenty-two species (Al, As, Ba, Ca, Cd, Cr, Cu, Fe, Mg, Mn, Pb, Zn, Ti, V, Cl−, NO3 −, SO4 <sup>2</sup>−, NH4 +, K+, Na+, OC, and EC), as well as the mass concentrations of size-fractionated PM, were used as ambient data for the CMB calculations. The source contribution estimates (SCEs) of different particulate sizes at Gulou and Zifeng are summarized in Table 5 and are also shown in Figure 7.


**Table 5.** Source contribution estimates (SCEs) of PM10, PM10-2.1, PM2.1, and PM1.1 mass concentrations with CMB model (μg m<sup>−</sup>3).

**Figure 7.** Source contributions of (**a**) size-fractionated PM at Gulou (μg/m3), (**b**) size-fractionated PM at Zifeng (μg/m3), and (**c**) PM10, PM10-2.1, PM2.1, and PM1.1 at both sites (100%).

The contributions of total emission sources to the size-fractionated PM accounted for varied from 97.3% to 89.9% at Gulou and from 99.2% to 92.2% at Zifeng. The three highest contributors of PM10 were found to keep consistent with nitrate, secondary organic aerosols, and sulfate arranging in order of contribution concentrations. To coarse particles, the three largest sources were secondary organic aerosols, the coal-fired power plant, and fugitive dust at Gulou, whereas the three largest sources were secondary organic aerosols, nitrate, and sulfate at Zifeng, indicating that the ground level would be more influenced by local sources such as fugitive dust, but at higher heights the formations of secondary inorganic and organic aerosols were more important. There was a similar contribution pattern in PM2.1 and PM1.1 at both heights, and the three largest emissions were nitrate, sulfate, and secondary organic aerosols. Interestingly, the fourth largest emission at Gulou was vehicle exhaust in PM10, PM2.1, and PM1.1, whereas it was the coal-fired power plant

at Zifeng in PM10, PM2.1, and PM1.1, also implying that the local source (vehicle exhaust) influenced more at the ground level.

As shown in Figure 7, the contributions to coarse and fine particulate matter were significantly different for specific sources. The construction dust (3.9–5.7% vs. 3.2–3.9%), fugitive dust (11.7–14.4% vs. 4.5–7.1%), and soil dust (3.0–4.1% vs. 1.4–1.6%) in coarse and fine particles showed larger contributions in the coarse size bin of PM, in which was found a similar tendency with the concentrations of crustal elements described in Section 3.2.1. The contributions of nitrate (12.7–16.8% vs. 22.2–22.5%) and sulfate (10.8– 14.0% vs. 17.3–19.6%) in PM10-2.1 and PM2.1 indicated an increasing trend as the particulate size decreased. However, the source contributions of secondary organic aerosols were 18.3–24.1% of PM102.1 and 15.5–17.1% of PM2.1.

As shown in Table 5, the contribution estimates of each source for PM10 and PM10-2.1 were larger at Gulou (20 m) than Zifeng (380 m), depending more on the higher concentrations of PM10 and PM10-2.1 at Gulou than Zifeng. By contrast, the source contribution estimates for fine particles such as PM2.1 and PM1.1 showed different features. Dusts, such as construction dust, fugitive dust, and soil dust, contributed a larger amount and proportion at Gulou than Zifeng since these dust sources were from ground and local areas. Vehicle exhaust also contributed larger concentrations at Gulou than Zifeng, which could reflect the influence of heavy traffic at the ground near the sampling site. However, the contributions of sulfate and secondary organic aerosols become larger with higher height, illustrating that the formation mechanisms of secondary aerosols played an important role at the height above the urban canopy. Similarly, the coal-fired power plant and steel smelting also contributed larger concentrations at Zifeng than Gulou, which might be caused not only by the industrial areas in the north of Nanjing but also by the transportation from regional areas. Furthermore, 36 h back trajectories were calculated every three hours for the sampling periods at 20 m and 380 m in this study. A total of 440 trajectories were used for clustering analysis in order to identify common atmospheric transport patterns. All trajectories were categorized into seven and six clusters at 20 m and 380 m, respectively. The average trajectories were shown in Figure 8. It could be found that there were more local sources (clusters two, three, and five in Figure 8a) at 20 m with lower wind speed, while there was more long-distance transportation (clusters one, four, five, and six in Figure 8b) from north of Nanjing with greater wind speed at 380 m.

**Figure 8.** Clustering analysis every three hours for the sampling periods at (**a**) 20 m and (**b**) 380 m.
