*3.1. Size-Fractionated PM Mass Characteristics*

During the sampling period, the average PM10 concentration was 108.3 ± 23.4 μg m−<sup>3</sup> and 88.1 ± 21.1 μg m−<sup>3</sup> at Gulou and Zifeng, respectively. The mean mass concentration of the PM10 concentration at ground level was 22.9% higher than that at high height (380 m), and the PM10 concentration at Gulou was higher at every sampling day than at Zifeng. The average PM2.1 concentration was 56.7 ± 18.6 μg m−<sup>3</sup> with the range from 34.6 μg m−<sup>3</sup> to 89.7 μg m−<sup>3</sup> at Zifeng. The average PM2.1 concentration was 61.0 ± 18.8 μg m<sup>−</sup>3, ranging from 32.4 μg m−<sup>3</sup> to 87.5 μg m−<sup>3</sup> at Gulou, which was 7.6% higher than that measured at Zifeng. At specific sampling days, the mass concentrations of PM2.1 were higher at high height (380 m). In contrast, the averaged mass concentration of PM1.1 at Zifeng was 44.8 ± 15.8 μg m<sup>−</sup>3, which was 9.5% higher than 40.9 ± 13.0 μg m−<sup>3</sup> at Gulou.

The more active secondary formation processes of aerosols at the higher level or the long-distance transportation from other regions may have caused these results, which will be explored and discussed in the following text. The coarse particles showed the same feature as PM10 at different heights. The summary statistics of measured PM10, PM10-2.1, PM2.1, and PM1.1 are presented in Table 1. The mass concentrations of PM10, PM2.1, and PM1.1 in this study were 41.4%, 26.3%, and 24.8% lower than those in previous study at Gulou, Nanjing during winter 2010 [24]. Meanwhile, compared with the similar research in a megacity of North China from December 2013 to January 2014 [41], the mass concentrations of PM10 and PM2.1 in Tianjin were 107.5% and 73.6% higher than those in this study. Overall, the air quality in Nanjing is better for these years, and a decrease trend of size-fractionated PM mass concentrations with increasing height was shown, except for PM1.1.


**Table 1.** Statistics of measured PM10, PM10-2.1, PM2.1, and PM1.1 at Gulou and Zifeng (μg m-3).

<sup>a</sup> Standard deviation.

Figure 2 shows the time series of observed size-fractionated PM mass concentrations at Gulou and Zifeng, which indicates that the time-scale variation trend was consistent at two heights. The correlation coefficient of PM10, PM10-2.1, and PM2.1 between Gulou and Zifeng was 0.79, 0.86, and 0.73, respectively. Further, in order to study the variations of particulate

concentrations and chemical compositions, the coefficient of divergence (CD) [42,43] could be used as follows:

**Figure 2.** Time series of size-fractionated mass concentrations at Gulou (20 m) and Zifeng (380 m).

$$CD\_{\mathcal{S}^z} = \sqrt{\frac{\sum\_{n=1}^{N} \left(\frac{\mathbb{C}\_{\mathcal{R}\xi} - \mathbb{C}\_{nz}}{\mathbb{C}\_{\mathcal{R}\xi} + \mathbb{C}\_{nz}}\right)^2}{N}} \tag{2}$$

where *g* and *z* represent the two sampling sites, and *N* is the total number of sampling days. *Cng* or *Cnz* means mass concentrations at different sites in the nth day. The value of *CD* shows the similarity of the mass concentrations of particulate matter at the two sites. The lower value represents the stronger similarity. When the value of *CD* is zero, it means the mass concentrations at the two sites are the same. [44]. In actual analysis, a *CD* value above 0.3 indicates a difference between each height, whereas a *CD* value less than 0.3 could be a sign of similarity between two heights [45]. To the mass concentrations, the values of *CD* for different particle sizes are shown in Table 2. All the *CD* values for PM10, PM10-2.1, PM2.1, and PM1.1 between Gulou and Zifeng are less than 0.3, which indicates that the mass concentrations of particles are similar at each height. The *CD* value of the PM10-2.1 mass concentration is relatively higher than those of PM2.1 and PM1.1, which suggests that the coarse particulate matter presents a larger variation between 20 m and 380 m. This might imply that local and primary sources contributed more to coarse particles at the ground than high height.



The sampling was conducted during November to December 2016, the most polluted time of the year. In this study, there is no 2.5 μm cut point, so 2.1 μm is defined as the threshold of fine particles, and the clear day standard for PM2.5, which is less than 75 μg m−<sup>3</sup> (Technical Regulation on Ambient Air Quality Index (HJ633-2012)), would be used for PM2.1 here. During the whole sampling period, there were four samples exceeding 75 μg m−<sup>3</sup> at the ground (26–27 November, 6–7 December, 10–11 December, and 12–13 December), and at Zifeng (380 m) there were two samples exceeding 75 μg m−<sup>3</sup> (26–27 November and 6–7 December). According to previous work, the beginning, process, and dispersion of particulate pollution could be influenced or driven by meteorological conditions [46–48]. The weather data observed at Gulou (20 m) were discussed with the concentration level. The hourly series of meteorological conditions including temperature (T), relative humidity (RH), wind speed (WS), and wind direction (WD) were shown in Figure 3. Under the weather condition of low wind speed, the PM concentrations would increase. The concentrations of PM2.1 at Gulou were higher than 75 μg m−<sup>3</sup> in 26–27 November, 6–7 December, 10–11 December, and 12–13 December when the daily maximum wind speeds were 2.9 m/s, 2.8 m/s, 3.8 m/s, and 3.9 m/s, respectively. However, the lowest concentration of PM2.1 was 32.5μg m−<sup>3</sup> in 4–5 December, with a much higher daily maximum wind speed (7 m/s). Additionally, the sampling day with low RH showed cleaner than the sampling day with high RH. The concentrations of PM2.1 at Gulou in 26–27 November and 12–13 December with higher daily average RH (69.6% and 79.1%) were much larger than that in 4–5 December with lower daily average RH (60.0%).

**Figure 3.** Time series of hourly (**a**) temperature and relative humidity and (**b**) wind speed and wind direction.

The average mass concentrations of size-fractionated PM manifested different features at different heights. The mass size distributions of PM with standard deviation at Gulou and Zifeng are shown in Figure 4. The distribution patterns of PM mass concentrations from 9.0–10 μm to 1.1–2.1 μm at two sites were similar, which showed an increasing trend with finer particulate size and the minimum concentration at 4.7–5.8 μm. However, the distribution patterns of particles smaller than the size bin of 1.1–2.1 μm were different. A bimodal distribution pattern could be found at Gulou with higher concentrations in the size bin of 1.1–2.1 μm and 0.43–0.65 μm. Otherwise, a unimodal distribution pattern could be found at Zifeng with a highest concentration in size bin of 0.43–0.65 μm. Particles in different sizes showed generally larger concentrations at Gulou than those at Zifeng, except in size bins of 0.65–1.1 μm and 0.43–0.65 μm, which caused the concentrations of PM1.1 to be larger at 380 m than at the ground. The average ratios of PM2.1/PM10 at Gulou and Zifeng were 0.56 and 0.64, indicating that more fine particulate matter were contained in PM10 at high height. The average ratios of PM1.1/PM2.1 at Gulou and Zifeng were 0.67 and 0.79, indicating in wintertime that the fine particulate matter at the top of the boundary

layer were finer than those at the ground level. The ratios of PM2.1/PM10 and PM1.1/PM2.1 for each sampling day at Gulou and Zifeng are shown in Figure 4.

**Figure 4.** (**a**) The average mass size distributions of PM with standard deviation at Gulou (20 m) and Zifeng (30 m). (**b**) The ratios of PM2.1/PM10 and PM1.1/PM2.1 at Gulou and Zifeng.
