*3.1. Temporal Variation Characteristics of PM2.5*

3.1.1. Temporal Variation Trend of PM2.5 Concentration

The variation trend of PM2.5 concentration in the study area was determined by calculating the Probability Density Functions (PDFs) and annual average concentrations of PM2.5 in the study area, from 2015 to 2019. As shown in Figure 2, the PM2.5 concentration in the study area expressed a downward trend from 2015 to 2019, which decreased by 27.17%, from 73.23 μg/m<sup>3</sup> in 2015 to 53.34 μg/m<sup>3</sup> in 2019. Although the annual PM2.5 concentration decreased, it still exceeded the Grade II standard of PM2.5 (35 μg/m3) in the Ambient Air Quality Standard (GB3095-2012) in 2019, which indicated that PM2.5 pollution in the study area was still severe. The frequency distribution of PM2.5 can be found in the PDF graph. From 2015 to 2019, the probability density curve moved to the left as a whole, indicating that PM2.5 concentration had decreased in all concentration intervals. The curves of 2015 and 2016 are similar, while those of 2017, 2018, and 2019 are similar. Compared with 2016, the occurrence probability of high concentration decreased significantly in 2017, resulting in a significant increase in probability in the low concentration intervals, and then remained stable. This sudden change may be related to the stricter air pollution control measures that were implemented in 2017.

**Figure 2.** Probability density function (PDFs) and annual concentration of PM2.5 from 2015 to 2019.

The mitigation trend was more significant in the context of concentration levels. In 2015, the average annual concentration of PM2.5 in all cities ranged from 34.6 to 106.42 μg/m3, but was 26.52–72.39 μg/m<sup>3</sup> in 2019. We can find that there was a large difference between different cities, with the maximum concentration being about three times that of the minimum. During the period of 2015–2019, the maximum concentration occurred in BD in 2015 and the minimum concentration occurred in WH in 2018. In addition, we also determined the statistics on the percentage of exceeding standard days in each city, from 2015 to 2019, as shown in Figure S1. In 2015, the average percentage of exceeding standard days in the study area was 37.45%, but it dropped to 15.66% in 2019. This apparent mitigation of PM2.5 pollution did not just start in 2015, it had been going on for a long time. Some studies on the long-term variation trends of PM2.5 concentrations have shown that it had been increasing since 2000 until reaching a peak in 2008, and then it fluctuated continuously and reached another peak in 2014 before decreasing since then [19]. It fluctuated after 2008 as the harm of PM2.5 pollution was widely known after the Beijing Olympic Games and China gradually entered the stage of economic restructuring [20]. China's government began to implement strict pollution control measures and regarded PM2.5 as a routine monitoring pollutant after issuing the Action Plan for Air Pollution Prevention and Control in 2013, which may be why PM2.5 concentration continued to decrease after 2014 [21]. As a large number of emission reduction measures have already been implemented, the reduction in PM2.5 will gradually reduce in the future. Therefore, the speed of pollution mitigation may be slowed down, and the spatial difference between cities would become narrower. From this aspect, Jiang et al. [22] reported that there was a spatial convergence trend for PM2.5 concentrations in the Beijing–Tianjin–Hebei region.
