## 4 Discussion  
### Dynamic Changes in Air Quality Associated with Natural Factors and Anthropogenic Activities  
It is essential to investigate the effects of natural factors and anthropogenic activities on the dynamic variations in air quality. During the study period, the spatial and temporal dynamics of AQI and six pollutants indicated that air quality was related to seasonal changes. Concretely, air quality in the summer was substantially better than that in the winter, mainly because the climatic conditions in winter were harsh, particulate matter was not easily diffused, and energy consumption increased. In general, the climate in winter is relatively dry, the sunshine time short, and the atmospheric convection not active, which is not conducive to the diffusion of pollutants in the air [45]. In addition, winter jet stream can cause dust storms by picking up dry soil particles and carrying them over long distances and creating a temperature inversion [46]. A temperature inversion can lead to poor air quality as pollutants become trapped close to the surface, unable to disperse.  
Figure 9: Dynamic changes of the influence degree of industrial NO\({}_{X}\) emissions on PM\({}_{10}\) based on the functional linear model.  
In contrast, in the summer, with the more active atmospheric convective activity, and the arrival of the summer monsoon bringing widespread rainfall, the pollution is more obviously removed [47]. Heavy rain can carry particles from smog to the ground, or it can dissolve some harmful gases in the rain to reduce air pollution. However, O\({}_{3}\) is derived from the secondary conversion of NO\({}_{\chi}\) and volatile organic compounds at high temperatures and dry air [48]. Therefore, the hot weather in the summer might promote such a transformation. In addition, volatile organic compounds can undergo a series of reactions with NO\({}_{\chi}\) under the condition of ultraviolet light irradiation, improve the oxidation of the atmosphere, and cause an increase in the concentration of O\({}_{3}\).  
Industrial and vehicle emissions were found to be correlated with air pollution. Over the past decade, the economy of Fujian Province has been expanding, and its air pollution has also shown a fluctuating upward trend. With the development of the economy, the number of vehicles has increased, industrial zones have also sprung up in coastal areas, and a series of industrial clusters have formed. Industrial pollution sources and automobile exhaust pollution sources are the most serious sources of air pollution generated by human activities. The recent economic development of Fujian Province has been concentrated in the coastal areas. There are many large petrochemical parks, steel plants, nearly 20 large thermal power plants, over 30 waste incineration power plants, and other facilities distributed along the coastal areas. These emit a significant amount of pollutants. In addition, emissions from coastal ports, ships, and the logistics industry also contribute to pollution [49]. Against the backdrop of rapid urbanization and industrialization, the continuous expansion of cities has led to shorter distances between them, and urban expansion is positively correlated with air pollution [50]. As a result, pollutants are easily transported between coastal areas in Fujian Province, and their concentration declines slowly, leading to more severe pollution levels among cities in these coastal regions. However, from the end of 2019 to 2021, due to the impact of COVID-19 quarantine measures, factories shut down, transportation significantly reduced, and shipping halted, resulting in a noticeable improvement in overall air quality, which was consistent with the studies of [PERSON] et al. [51].