**4. Conclusions**

The concentrations and chemical compositions of size-fractionated PM were sampled and measured from 16 November to 12 December in 2016 at Gulou (20 m) and Zifeng (380 m) in Nanjing, China. The characteristics of size-fractionated PM mass concentrations at different heights were described and the chemical components including elements, water-soluble ions, and carbonaceous species were analyzed and compared for different particulate sizes and sites. Furthermore, the source apportionment was carried out with the CMB model, and the characteristics of contribution estimates were discussed in this study.

The concentrations of PM10 and PM2.1 were 108.3 ± 23.4 μg m−<sup>3</sup> and 61.0 ± 18.8 μg m−<sup>3</sup> at Gulou and were 88.1 ± 21.1 μg m−<sup>3</sup> and 56.7 ± 18.6 μg m−<sup>3</sup> at Zifeng, which indicated a decreasing trend as the height increased. In contrast, the concentrations of PM1.1 were 40.9 ± 13.0 μg m−<sup>3</sup> and 44.8 ± 15.8 μg m−<sup>3</sup> at Gulou and Zifeng, indicating particles congregating in a finer particulate size bin at the higher level. The PM pollutant days were relative to the weather conditions, especially when wind speed was low.

All the species of size-fractionated PM detected in this study accounted for 82.7–87.9% of the total PM mass concentrations at Gulou and accounted for 83.1–83.4% at Zifeng. The crustal elements were concentrated more in coarse particles, accounting for 18.6% and 15.3% at Gulou and Zifeng. The water-soluble ions, especially nitrate (8.7–23.0%), sulfate (6.9–15.6%), and ammonium (1.8–11.4%), were dominant components of size-fractionated PM. The concentrations of nitrate, sulfate, and ammonium showed an increasing tendency with decreasing particulate size. Organic carbon was another dominant component of size-fractionated PM, accounting for 16.0–20.8% and 18.3–26.4%, respectively, at Gulou and Zifeng, which indicated larger contributions at the higher level.

The source contribution estimates (SCEs) calculated by the CMB model showed that the secondary inorganic and organic aerosols were the largest three contributors of PM10, PM2.1, and PM1.1 at both sites, but the fourth largest sources at Gulou and Zifeng were vehicle exhaust and the coal-fired power plant, respectively. To PM10-2.1, the largest three contributors at Gulou were secondary organic aerosols, the coal-fired power plant, and fugitive dust, which was different with Zifeng. Zifeng showed secondary organic aerosols, nitrate, and sulfate as the largest sources of PM10-2.1. These results illustrated that dusts including construction dust, fugitive dust, soil dust, and vehicle exhaust would contribute more at the ground level. In recent years, concentrations were decreased by 41.4%, 26.3%, and 24.8% of PM10, PM2.1, and PM1.1 in this study compared to the winter of 2010 at the same sampling site. These results showed an encouraging improvement in the control of particulate matter in Nanjing.

**Author Contributions:** Conceptualization, T.W. and H.W.; data curation, P.C.; funding acquisition, T.W.; investigation, M.X. and B.Z.; methodology, S.L. and M.L.; supervision, H.W.; visualization, P.C. and H.W.; writing—original draft, H.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (42077192, 41621005), the National Key Basic Research and Development Program of China (2020YFA0607802, 2019YFC0214603) and the Emory University-Nanjing University Collaborative Research Grant.

**Institutional Review Board Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
