## 4 Experimental Results and Analysis
### Time Series Trend Analysis  
Figure 7 presents the monthly average PM\({}_{2.5}\) concentration data of Nanning urban area from 2012 to 2023 in the form of a line chart. Through a detailed and in-depth analysis of these data, we can not only clearly sort out the long-term trend of PM\({}_{2.5}\) concentration during this period, but also combine known influencing factors such as holidays and epidemics to accurately analyze their specific effects on PM\({}_{2.5}\) concentration. From the overall change, during the period of 2012-2014, the PM\({}_{2.5}\) concentration was high at the beginning of each year, such as 75.4 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in January 2012, about 96 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in January 2013, and 120.4 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in January 2014. Although it declined in the following months, it still fluctuated at a high level. At that time, the industrialization process in Nanning urban area was advancing rapidly, industrial activities were frequent, energy consumption was high, and environmental protection measures were relatively weak. A large amount of industrial waste gas, dust and other pollutants were discharged into the atmosphere. At the same time, a large number of fireworks and firecrackers were set off during the Spring Festival, releasing a large amount of smoke, sulfur dioxide and other pollutants in a short period of time, causing the PM\({}_{2.5}\) concentration in the air to rise sharply. Since 2015, the PM\({}_{2.5}\) concentration in some months has shown a clear downward trend, especially in the summer months (June to August) from 2015 to 2017, when the concentration was generally low and stable. For example, the PM\({}_{2.5}\) concentration in July 2015 dropped to 25.5 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), and the concentration in the same period of 2016 and 2017 also remained at a low level. This is due to strict environmental governance measures, including strengthening the control of industrial pollution sources, requiring enterprises to install advanced waste gas treatment equipment, promoting clean energy, replacing high-pollution energy with solar energy, wind energy, etc., upgrading motor vehicle exhaust emission standards, and eliminating old vehicles, which effectively controlled pollutant emissions. However, in certain months of some years, such as National Day and other  
Figure 7: Monthly Average PM\({}_{2.5}\) concentration from 2012 to 2023.  
holidays, PM\({}_{2.5}\) concentrations will rise briefly due to factors such as increased mobility and traffic flow, and increased fume emissions from restaurants. The PM\({}_{2.5}\) concentration changes were special from 2020 to 2022 due to the impact of the COVID-19 pandemic. In the early stages of the pandemic, strict prevention and control measures such as factory shutdowns and vehicle restrictions significantly reduced industrial production and transportation activities, and pollutant emissions dropped sharply. In February 2020, PM\({}_{2.5}\) concentrations in many cities, including Nanning urban area, dropped to a relatively low level in the same period in history, about 32 \(\mu\)g/m\({}^{3}\). With the normalization of epidemic prevention and control, production and life have gradually resumed but are still restricted. Enterprises pay more attention to environmental protection, and public transportation operations are optimized, making PM\({}_{2.5}\) concentrations relatively stable during this period and the overall level is not high, and the seasonal fluctuation range is reduced. In addition, after 2019, the small peak concentration that might have occurred in October was significantly reduced or postponed. This may be because the epidemic affected holiday travel and gathering activities, reducing pollutant emissions, or it may be due to the public's improved environmental awareness and the positive impact of practicing green travel and energy conservation and emission reduction.