## 4 Conclusions  
In this paper, Saudi Arabia is divided into three regions according to topography, and a long-term study of CALIPSO satellite aerosol data is carried out. The temporal and spatial distribution characteristics of the optical properties of vertical aerosol layers in Saudi Arabia from 2007 to 2019 were studied. The annual and seasonal changes in the nine parameters during the daytime and nighttime were analyzed and discussed in this study. The annual average value of AOD\({}_{\text{S}}\) in region III is the highest. The highest value is likely related to the climate characteristics of the region that includes an expansive desert and is located in the border area with a scarce population and lack of dust control measures. From a seasonal perspective, spring and summer exhibited higher AOD estimates than  
Figure 19: The correlation between PAOD1 and N over Saudi Arabia from 2007 to 2019. (**a,d**) represent regions I, (**b,e**) represent regions II, and(**c,f**) represent regions III. Figure 18: The correlation between TAH and N over Saudi Arabiafrom 2007 to 2019. (**a,d**) represent regions I, (**b,e**) represent regions II, and (**c,f**) represent regions III. autumn and winter in the three regions, which was related to the frequent occurrence of dust weather in spring and summer. The HB\({}_{1}\) and TAH of the three regions exhibited an evident correlation with the topography of the region. Thus, it was inferred that the higher the altitude, the higher the annual mean values of HB\({}_{1}\) and TAH. The value of HB\({}_{1}\) was higher in the daytime than in the nighttime, while the value of TAH was higher in the nighttime than in the daytime. The annual mean for the spring and summer was slightly higher. The N values over region III were the highest and the lowest over region I, respectively. This pattern can be explained by the higher average altitude, lower temperature, relatively weak vertical convection, and lower aerosol load in region I, resulting in the formation of relatively weak vertical aerosol layers. The N values of the other three regions are generally higher in spring and summer, seemingly due to the higher air temperature and considering the enhanced vertical convection, causing aerosol stratification. The value of AOD\({}_{1}\) at nighttime was higher than that during the daytime, which might be related to the characteristics of dust aerosols. The VDR\({}_{1}\) values of regions II and III were relatively high, which may be attributable to the influence of large desert and dust aerosols in these two regions. The value at nighttime was higher than that in the daytime, which might be related to the dry deposition at nighttime. The high values in spring and summer can be attributed to the intensified dust activity. The CR\({}_{1}\) values over regions II and III are higher at nighttime and are likely driven by the prevalence of the dust aerosol type in the bottom layer. There was a weak positive correlation between HTH\({}_{1}\) and AOD\({}_{1}\) in the three regions, but the correlation was not significant (R\({}^{2}<0.5\)), indicating that the aerosol concentration was insignificant. There was a strong positive correlation between TAH and N in the three regions. It was suggested that the greater the value of N, the greater the value of TAH. PAOD\({}_{1}\) was negatively correlated with N.  
The results from this study can be used for improving the regional statistics regarding aerosol distribution over Saudi Arabia that lacks a nationwide aerosol monitoring network. In this way, our results can facilitate the prevention and control of air pollution in Saudi Arabia, thus improving the well-being of the Saudi Arabian population and promoting the economic development of the country. Conceptualization, Z.Z. ; software: B.S., M.P., S.I. and K.M.K. ; validation, Z.Z., M.B., J.L. and Y.C. ; investigation, Z.Z. ; writing--original draft, Z.Z. ; curation: B.S. and Z.Z. All authors read the manuscript, contributed to the discussion, and agreed to the published version of the manuscript.