**1. Introduction**

With rapid urbanization and industrialization, air quality has been deteriorating in many cities in China, resulting in major environmental problems [1–3]. Particulate matter, such as PM2.5 (particles with diameter less than 2.5 μm) and PM10 (particles with diameter less than 10 μm), are regarded as the dominant pollutants influencing air quality [4]. PM2.5 components are complex, predominantly generated by human activities and natural release, and the former is more harmful. PM2.5 are mainly chemically formed, or condensed from hot vapor (i.e., diesel exhaust) and coagulated into fine particles. PM10, mainly derived from natural processes and imperfect combustion, and commonly affected by the suspension and transport of sand and soil particles [5,6]. Given the negative effects of particulate matter on human health, governments have implemented strict pollutant reduction measures, which were deemed as an effective way for air quality improvement [7]. The concentrations of particulate matter across timescales can provide insight into the myriad causes of observed variations in air pollution [8]. Data at the annual scale can reflect the effects of mitigation strategies implemented by the government [9]. Seasonal variations of air quality are conducive to parsing the contribution of meteorological conditions, and the

**Citation:** Tian, Y.; Zhang, L.; Wang, Y.; Song, J.; Sun, H. Temporal and Spatial Trends in Particulate Matter and the Responses to Meteorological Conditions and Environmental Management in Xi'an, China. *Atmosphere* **2021**, *12*, 1112. https:// doi.org/10.3390/atmos12091112

Academic Editors: Duanyang Liu, Honglei Wang and Kai Qin

Received: 19 August 2021 Accepted: 27 August 2021 Published: 30 August 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/).

intensity of emissions [10,11]. Studies have shown that particulate matter exerts a serious impact on the air quality in spring and winter [12]. In addition, the spatial heterogeneity of air quality can reveal the effectiveness of environmental regulation practices [13,14]. Though local emissions contribute to the air pollution, the meteorological conditions are also at play, affecting the accumulation, deposition, transportation, and diffusion of air pollutants [2,15]. The meteorological factors showed a non-linear relationship with the concentrations of particulate matter [16]. Therefore, the temporal and spatial characteristics captured by air quality and the relationship with meteorological conditions and environmental management also deserve further investigation.

Unregulated local emissions elevate the concentrations of particulate matter, especially in developing regions [17]. For example, in China, the large-scale emission sources of PM10 and PM2.5 include coal combustion, traffic engine exhausts, biomass burning, industrial activities, fugitive dust from construction activities and sandstorms [18,19]. Particulate matter as the primary pollutant (i.e., the concentration is higher than other atmospheric pollutants) with a higher proportion [12], its improvement plays a key role in improving air quality, especially in cities of the semi-arid and arid regions in western and northwestern China, where fugitive dust is the major component of aerosol particles.

Xi'an, the capital city of Shaanxi province, with an area of 10,108 km2 and a population of 13.0 million, is the most urbanized region in northwestern China. Due to the topography, meteorological conditions, and tremendous amount of atmospheric pollutants emissions derived from urbanization and industrialization, Xi'an has been plagued by severe air pollution over the last decade [6,20]. PM10 and PM2.5 contribute more than 90% of all air pollutants, which are regarded as the dominant controller of air quality in this region [12,21]. In recent years, atmosphere environment protection policies have been vigorously implemented in Xi'an, but few studies have assessed the effectiveness of environmental management in improving atmospheric pollution. The atmospheric pollution in Xi'an exhibits seasonal dependence. Bare surface and strong wind in spring, and biomass burning and coal combustion from intensive heating demand in winter accompanied with meteorological factors closely related to atmospheric pollution—all promote an abundance of particulate matter and visibility impairment [1,6,22]. Evaluation and improvement of air quality in the context of changes in energy consumption and rapid urbanization is conducive to the future development and sustainability of other megacities in northwest China.

In this study, PM10 and PM2.5 from 13 air monitoring stations in Xi'an from 2013 to 2017 were analyzed. We aimed to: (1) characterize the temporal and spatial variations of PM10 and PM2.5 in Xi'an; (2) analyze the relationships between particulate matter and meteorological conditions; and (3) explore the response of the concentration and emission intensity of atmospheric pollutants to the environmental management. This study aims to provide a scientific framework for atmospheric pollution management.
