**1. Introduction**

In recent decades, with rapid economic development, industrialization and urbanization in China, the number of motor vehicles and the total energy consumption have increased, and atmospheric particulate matter (PM) has become one of the most significant air contaminants [1–3]. PM, particularly PM2.5 (aerodynamic diameter ≤2.5 mm), can exist in the atmosphere for a long time, which is conducive to its long-distance transport through the atmosphere and deposition towards remote areas. During long-range transport, PM2.5 carries abundant anthropogenic pollutants and has a serious impact on the global and regional climate, the visibility and composition of the atmosphere, the global biogeochemical cycle and the activation of cloud condensation nuclei (CCN) [4–6].

PM2.5 has been widely studied in recent years in China due to its potential impacts on air quality and human health. Water-soluble inorganic ions (WSIIs), organic carbon (OC) particles, inorganic carbon (EC) particles, and inorganic elements (IEs), as the main chemical components of PM2.5, have been extensively studied in China [7–9]. WSIIs are dominated by secondary inorganic aerosols (SIAs), including NH4 +, NO3 <sup>−</sup>, and SO4 <sup>2</sup>−.

**Citation:** Shi, J.; Feng, Y.; Ren, L.; Lu, X.; Zhong, Y.; Han, X.; Ning, P. Mass Concentration, Chemical Composition, and Source Characteristics of PM2.5 in a Plateau Slope City in Southwest China. *Atmosphere* **2021**, *12*, 611. https:// doi.org/10.3390/atmos12050611

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 22 April 2021 Accepted: 5 May 2021 Published: 8 May 2021

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

**Copyright:** © 2021 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/).

OC is composed of thousands of organic compounds and contains many toxic substances. Heavy metals are an important part of the inorganic elements comprising PM2.5, such as Fe, Zn, Cu and Pb [10–12].

Whether worldwide or only in China, it is essential to reduce PM2.5 concentrations to control their sources. The key point in formulating policies for the government to control PM2.5 pollution is the result of source apportionment and reliable source quantification [13,14]. In fact, PM2.5 is usually sourced by the emission of pollutants, and its classification is very complex, including its anthropogenic and natural sources and gas and particle phases [15,16]. In addition, PM2.5 forms secondary pollutants from primary emissions through photochemical reactions after being released from pollution sources, making it difficult to quantify its impacts [17–19]. The contribution levels of different sources in the air can be quantitatively estimated by using the receptor model. Generally, researchers have identified the possible sources of PM2.5 as traffic and industrial, coal combustion, biomass burning and secondary inorganic aerosol sources [20,21].

In recent years, most studies have generally focused on the Jing-Jin-Ji region and its coastal areas with severely degraded atmospheric environments in China [22–24]. Only a few researchers have investigated PM2.5 pollution in Yunnan Province, which is a remote southwestern region. More research has been conducted in areas such as Kunming and Yuxi [25–27]. Despite the economic backwardness of the remote southwestern mountains, there are still cases of PM2.5 exceeding the standard every year [28–30]. Therefore, we should pay more attention to these areas to improve their air quality.

Wenshan, a developing industrialized city in southwest China, has a high degree of air pollution, mainly resulting from the presence of PM2.5 in the atmosphere. Furthermore, Wenshan is located in the basin valley on the plateau, and the urban area is surrounded by high mountains, which aggravate particulate pollution. Wenshan is chosen as the study area to conduct PM2.5 sampling during the spring and autumn seasons at three monitoring sites. The concentrations of PM2.5, WSIIs, OC, EC and IEs are analyzed and discussed in the current study. Principal component analysis (PCA) is used for PM2.5 source apportionment to analyze the pollution sources. Potential major contributors were identified on the basis of PCA and local environmental background information. The details of the pollution characteristics and the results of PM2.5 source apportionment in this study can provide the local government with reasonable and effective measures to slow down atmospheric pollution with PM2.5.
