**1. Introduction**

Nowadays, atmospheric particulate matter (PM) are receiving a lot of attention in atmospheric and environmental studies due to their complex and important impact on various problems, including human health [1,2], radiation balance and climate change [3–5], visibility degradation [6,7], and air pollution [8–10]. In recent decades, with increased economic and industrial development, PM have become the primary air pollutant in urban areas, and PM pollution, especially PM2.5 pollution (atmospheric particulate matter with an aerodynamic diameter less than 2.5 μm), has been found to be a severely impairing

**Citation:** Wu, H.; Chen, P.; Wang, T.; Xie, M.; Zhuang, B.; Li, S.; Li, M. Characteristics and Source Apportionment of Size-Fractionated Particulate Matter at Ground and above the Urban Canopy (380 m) in Nanjing, China. *Atmosphere* **2022**, *13*, 883. https://doi.org/10.3390/ atmos13060883

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 8 May 2022 Accepted: 27 May 2022 Published: 29 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

issue in China. So, considerable attention from governments and scientific communities has been drawn [11–15] and lots of studies have been focused on the mass concentrations and chemical properties of particles [16,17]. The chemical compositions of atmospheric PM consist of numerous numbers of species (metals, ions, and carbonaceous matter) and come from complex sources, including anthropogenic and natural sources [18]. Meanwhile, the formation processes involve complicated multiphase formation pathways that are not well understood yet [19,20]. So, it is significant to understand the formation and contribution to PM mass in urban areas.

Many previous efforts have been conducted to evaluate the size distributions and chemical compositions at ground level (0~20 m), which is significantly influenced by local emission sources such as fugitive dust, construction dust, and vehicle exhaust. In comparison, an observation of size-fractionated particulate matter (PM) on a height of 300–400 m would be meaningful to understand the influences from a large scale of the sources, transportation, formation, and removal mechanisms to particulate matter [21–23]. Nanjing is a megacity located in the most developed region of the Yangtze River Delta in China with a huge population and a large urban center. It is a perfect target to carry out the ground level and high-height level observations. Characteristics and source contributions of atmospheric particles were discussed by previous studies in Nanjing [24,25], but the chemical compositions and sources of PM in different sizes at ground level and high-height level have not yet been conducted. As the nocturnal planetary boundary height during the winter is often about 300 m, we chose Zifeng Tower (380 m) in Nanjing to be the observation site of atmospheric particulate matter in the lower boundary layer [26].

Compared to developed countries, the emission sources of atmospheric particles in developing countries are much more complex. An explicit knowledge of the source contributions and probable source locations is the first step for understanding and planning management strategies of particulate matter [27]. The receptor model is used for quantitative analysis of PM source contributions, as there is no limitation on pollution discharge, weather conditions, or terrain factors. Based on chemical analysis, the receptor models that could be chosen are either the chemical mass balance (CMB) method or a multivariate factor analysis model such as the positive matrix factorization (PMF) method. Both methods have been widely applied to evaluate the particles' source contributions [28–31]. Source profiles are necessary in the former method but useless in the latter method. However, there is no limitation in the number of samples in the former method, but a minimum number of samples should be prescribed in the latter method. The source profiles in Nanjing were obtained in a previous work [18], and due to the limitation of sample numbers, the CMB model was used in this study. Additionally, backward trajectory clustering analysis could be used to trace the source locations of emissions [32]. Moreover, backward trajectory clustering analysis combined with the results of receptor models have been proved to be a beneficial tool for identifying the major source locations [33–35].

In this study, a synchronous and parallel observation was carried out at the ground level and the 380 m platform on Zifeng Tower in Nanjing from November to December in 2016. The mass concentrations of size-fractionated PM were obtained, and the chemical compositions including elements, water-soluble ions, and carbonaceous matter (elemental carbon (EC) and organic carbon (OC)) were analyzed. The size distribution of chemical components and variations between the ground level and the 380 m platform were investigated. The CMB model was applied to apportion the emission source contributions of PM in different sizes. The characteristics of source contributions at ground level and above the urban canopy (380 m) were evaluated. Combined with backward trajectory clustering analysis, the potential transport pathways were traced at the two heights. Finally, based on the results above, we explored reasonable control measures that may be helpful for policymakers.
