## 5 Conclusions  
In this study, the temporal variation of global solar radiation in six important cities located in the solar-energy-available region of China was investigated. Furthermore, Harbin, as the most representative city, was selected in order to examine the correlation between global solar radiation, and other prominent meteorological and air quality factors, using wavelet coherence analysis. The results indicate that global solar radiation exhibits multi-scale characteristics, and there exists a correlation between global solar radiation and other meteorological factors, as well as atmospheric pollutants on various time scales. The research findings can be summarized as follows:  
1. From the macro perspective, the cities of Harbin, Shenyang, and Beijing, characterized by lower average annual temperatures, exhibit higher annual global solar radiation compared to Wuhan, Shanghai, and Guangzhou, which are geographically closer to the equator. Notably, among these six cities, Wuhan--renowned as one of China's four furnaces--possesses the lowest annual solar energy reserves. The coherence spectrum analysis of global solar radiation and other meteorological factors in Harbin reveals a strong correlation between global solar radiation and sunshine duration throughout the entire temporal domain. Although the atmospheric temperature is also positively correlated with the global solar radiation, the latter exerts dominant with more intricate influencing factors. This explains the phenomenon of lower global solar radiation in southern China, despite its higher annual average atmospheric temperature. 2. There exists a positive correlation trend in the annual cycle between global solar radiation and wind speed, with the latter leading by approximately 2 months. This suggests that Harbin's wind energy reserves are also at a low level during winter when global solar radiation is low. Consequently, combining wind power and solar power generation may not be a feasible energy supply strategy in Harbin. 3. The correlation spectra between global solar radiation and six common atmospheric pollutants indicate that, with the exception of O\({}_{3}\) and global solar radiation, the remaining five atmospheric pollutants exhibit a significant negative correlation with global solar radiation. This observation aligns with the actual conditions in Harbin, where decreased air transmission resulting from increased air pollution caused by heightened fossil fuel consumption during the cold winter has led to a reduction in levels of global solar radiation. By implementing comprehensive planning strategies that prioritize the utilization of natural gas and other clean energy sources instead of  
Figure 7: Wavelet coherent power spectrum between daily global solar radiation and air pollutants, including (**a**) RAD-CO, (**b**) RAD-NO\({}_{2}\), (**c**) RAD-O\({}_{3}\), (**d**) RAD-PM2.5, (**e**) RAD-PM10 and (**f**) RAD-SO\({}_{2}\). coal, oil, and other fossil fuels during periods when solar energy reserves are limited in Harbin's winters, not only can the overall availability of global solar radiation in the region be enhanced, but also some degree of air quality improvement can be achieved. * Global solar radiation exhibits a negative correlation with atmospheric pressure in the annual cycle, as well as with air humidity and precipitation over shorter time periods. Despite the unavoidable influences of the three meteorological factors on global solar radiation, these relatively stable correlation relationships can be utilized for the long-term and short-term prediction of global solar radiation. This present study provides insights into the dominant relationship between daily global solar radiation and six meteorological factors, as well as six air quality factors, on both long and short time scales. These findings are valuable for the selection process of relevant factors that influence changes in regional global solar radiation during prediction, and assigning appropriate weights based on their relative importance.