**1. Introduction**

Unprecedented economic activity, urbanization, industrialization, and motorization have deteriorated the ambient air quality in China [1–6]. China is the manufacturing hub of the world, with the majority of the industries in northwest China (NWC). Several studies have reported higher pollution levels in NWC due to increased industry, coal consumption, distinct topography, and adverse meteorology [2,5,7–11]. Increased pollution levels have attracted the attention of the general public, the scientific community, and relevant authorities because of their detrimental health effects [4,12–16]. To combat increasing pollution levels, China has made significant efforts, e.g., establishing Chinese ambient air quality standards (CAAQS) for six criteria pollutants [17], implementing the Atmospheric Pollution Prevention and Control Action Plan 2013 (APPCAP) [18], technical regulation on ambient air quality index (HJ 633–2012) [19], nationwide air quality monitoring, online data-sharing networks, etc. [20,21]. These measures have helped to reduce pollution to

**Citation:** Zaib, S.; Lu, J.; Bilal, M. Spatio-Temporal Characteristics of Air Quality Index (AQI) over Northwest China. *Atmosphere* **2022**, *13*, 375. https://doi.org/10.3390/ atmos13030375

Academic Editors: Célia Alves, Duanyang Liu, Kai Qin and Honglei Wang

Received: 23 December 2021 Accepted: 17 February 2022 Published: 23 February 2022

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some extent; e.g., from Ref. [22], a 12.3% reduction in fine particulate matter (PM2.5) was observed in China between 2013 and 2015, while other authors [23] observed a one-third reduction in PM2.5 from 2013 to 2017 due to APPCAP in China. In Ref. [24], it was observed that the annual average concentration (PM2.5), coarse particulate matter (PM10), sulfur dioxide (SO2), nitrogen dioxide (NO2), and carbon monoxide (CO) decreased by 27.9%, 23.8%, 51.2%, 10.6%, and 25.3%, respectively, in China from 2015 to 2019. Even after strict environmental regulations, the air pollution in some areas of NWC is beyond certain limits and causes serious health effects [25–28].

Most of the spatio-temporal studies in China focused on central China, north China, considered few parameters, fewer cities, mostly provincial capitals, and lacked detailed assessment, with few exceptions. Multiple studies, e.g., [3,29], observed higher PM2.5 pollution in northern China, western China, northwestern China, etc. due to increased industrial emissions, coal combustion, stagnant meteorology, etc. Similarly, other researchers [30,31] carried out a more detailed assessment, covered six criteria pollutants in provincial capitals, and observed higher pollution levels in the north and northwestern region. In Ref. [1], they analyzed the criteria pollutants (PM2.5, PM10, SO2, NO2, CO, and O3) in 336 cities of China, while other researchers [31] analyzed the criteria pollutants in 367 cities of China and observed higher pollution in industrialized areas of north China. Further, Ref. [2] thoroughly assessed the air pollution in NWC by analyzing the criteria pollutants in six cities of Gansu province in NWC and observed higher pollution in Lanzhou, the provincial capital, with increased energy consumption and industrial activity and facing serious health concerns [25].

PM2.5 is more dangerous than PM10 and is ranked as the first leading risk factor for disease in China, causing more than 1.1 million premature deaths, with the highest share being stroke, ischemic heart disease (IHD), lung cancer, lower respiratory infections, chronic obstructive pulmonary disease (COPD), etc. [32–34]. Other criteria pollutants, e.g., PM10, CO, O3, SO2, and NO2, cause multiple health disorders, e.g., headaches, dizziness, nausea, respiratory disorders, inflammatory reactions, reduced lung function, hampered neurological function, etc. [35–37]. Apart from jeopardizing human health, air pollution is also responsible for visibility reduction, economic losses, and climate change [13]. PM2.5 and O3 can cause gross domestic production (GDP) losses of 2.09% in health expenditure [38].

In view of such circumstances, we examined the spatial and temporal distribution of PM2.5, PM10, SO2, NO2, CO, and O3 in 53 cities located in five provinces of NWC for a period of four years (2015 to 2018). Besides the criteria pollutants, we also examined the PM2.5/PM10 ratio, Air Quality Index (AQI), AQI class distributions, major pollutant on a representative day, number of pollution days, and correlations among different pollutants to explicate the pollution status, spatial, and temporal distribution of air quality in NWC over time. This study provides useful information and a valuable reference for future research on air quality in NWC and is of considerable significance to environmental protection and human health.

#### **2. Materials and Methods**
