## 3 Results and Discussion
### Interannual Variation Characteristics of Aerosol Properties
In contrast to AOD\({}_{\text{S}}\), the value of AOD\({}_{1}\) at nighttime was higher than that during the daytime, which might be related to the characteristics of dust aerosols. Moreover, the temperature at nighttime is lower, the boundary layer is shallower than that in the daytime, and the ventilation coefficient is poor. Thus, the aerosols are more actively deposited during the night near the land surface [12] PAOD\({}_{1}\) represents the percentage of AOD\({}_{1}\) in AOD\({}_{\text{S}}\) (Figure 4a,c). The results show that region I (daytime: 0.70 \(\pm\) 0.32, nighttime: 0.66 \(\pm\) 0.35) > region II (daytime: 0.68 \(\pm\) 0.32, nighttime: 0.64 \(\pm\) 0.36) > region III (daytime: 0.64 \(\pm\) 0.32, nighttime: 0.63 \(\pm\) 0.36), with no significant difference (daytime: 0.06, nighttime: 0.03), and the value during the daytime is higher than that at nighttime. This indicates that the aerosols are mainly concentrated in the lower atmosphere, and the PAOD\({}_{1}\) value is associated with the average altitude of the region. The higher the altitude, the greater the PAOD\({}_{1}\) value. Simultaneously, it also shows that the layer of the bottom aerosol is mainly formed by vertical convection and the weak effect of the atmosphere. It should be noted that humans are more susceptible to the influence of aerosols during the daytime compared with nighttime [12]. HTH\({}_{1}\) represents the difference between the top height of the lowest aerosol layer (HT\({}_{1}\)) and the base height of the lowest aerosol layer (HB\({}_{1}\)). The average annual HTH\({}_{1}\) values are shown in Figure 3c,f of region I (daytime: 1.58 \(\pm\) 1.05 km, nighttime: 1.75 \(\pm\) 1.32 km), region II (daytime: 1.56 \(\pm\) 1.07 km, nighttime: 1.83 \(\pm\) 1.40 km), region III (daytime: 1.45 \(\pm\) 1.01 km, nighttime: 1.80 \(\pm\) 1.42 km), exhibiting only small changes during the daytime and nighttime. The value at nighttime is greater compared with the daytime, which is consistent with the TAH variations. VDR\({}_{1}\) is the ratio of the vertical polarization scattering coefficient at 532 nm to the whole aerosol scattering coefficient at 532 nm, which reflects the spherical and non-spherical degree of aerosol particles in the lowest aerosol layers. The lower the VDR\({}_{1}\) values, the  
Figure 4: Interannual variations of aerosol layers over Saudi Arabia (region I, region II, and region III) from 2007 to 2019. (**a,d**) the AOD proportion of the lowest aerosol layer (PAOD\({}_{1}\)), (**b,e**) the volume depolarization ratio of the lowest aerosol layer (VDR\({}_{1}\)), and (**c,f**) the color ratio of the lowest aerosol layer (CR\({}_{1}\)). more spherical the bottom aerosol particles, and vice versa [12]. The VDR could reflect the spherical characteristic of aerosols. Therefore, the VDR are mean values. In this study, the VDR\({}_{1}\) we used was the mean value of the lowest layers since almost 90% of aerosols exist in the lowest layers. Therefore, VDR\({}_{1}\) is almost the same as the VDR of the entire aerosol layer. Meanwhile, the VDR of the entire aerosol layer is not the product of CALIPSO directly. Therefore, VDR\({}_{1}\) could reflect the spherical characteristic of aerosols. The results are shown in Figure 4b,e for region I (daytime: 0.14-0.17, nighttime: 0.12-0.14), region II (daytime: 0.15-0.19, nighttime: 0.12-0.15), and region III (daytime: 0.16-0.18, nighttime: 0.13-0.16). These findings indicate the presence of more non-spherical particles in regions II and III, seemingly highlighting the large desert areas in these two regions affected by dust aerosols. These results also show that more non-spherical particles appear at nighttime than during the daytime, seemingly related to dry deposition at night [12]. The color ratio (CR) is defined as the ratio of the total attenuation backscattering coefficient (1064 nm) to the total attenuation backscattering coefficient (532 nm). CR reflects the size of the aerosol particles, implying that the larger the CR, the larger the aerosol particle size [58].