## 3 Results and Discussion
### Interannual Variation Characteristics of Aerosol Properties
Figure 2b,e show that  
Figure 2: Interannual variations of aerosol layers over Saudi Arabia (region I, region II, and region III) from 2007 to 2019. (**a,d**) The sum of the AOD from all the aerosol layers (AOD\({}_{\text{S}}\)), (**b,e**) the base height of the lowest aerosol layer (HB1), and (**c,f**) the top altitude of the highest Aerosol Layer (TAH). the interannual variation of HB1 in all three regions is similar, exhibiting an insignificant increase. However, the annual mean is considerably different (1.55 \(\pm\) 1.23 km, 1.42 \(\pm\) 1.13 km, 0.88 \(\pm\) 1.07 km during the daytime, 1.25 \(\pm\) 1.01 km, 1.05 \(\pm\) 0.89 km, 0.55 \(\pm\) 0.87 km at nighttime for regions I, II, and III, respectively). These results indicate that HB1 has a relatively higher correlation with the topography, suggesting that the HB1 values increase with increasing altitude [51]. Moreover, higher values of HB1 were observed in the three regions during the daytime compared with nighttime, which may be influenced by the temperature difference between daytime and nighttime. Of course, it cannot be ruled out deviations caused by solar radiation. Likewise, the TAH estimate also exhibits a stronger correlation with the topography. Figure 2c,f display that region I (daytime: 4.14 \(\pm\) 2.19 km, nighttime: 4.54 \(\pm\) 2.05 km), region II (daytime: 4.08 \(\pm\) 2.33 km, nighttime: 4.52 \(\pm\) 1.97 km), region III (daytime: 3.80 \(\pm\) 2.64 km, nighttime: 4.19 \(\pm\) 2.10 km) TAHs, and TAHs of all three regions exhibited lower daytime values compared with the nighttime values. These results are also similar to the previously reported results obtained for certain regions of China [11; 31]. It is important to point out that CALIOP S32 nm observations often miss the lower aerosol layer and may overestimate the bottom height, causing an error of the HB1 and HTH1 values [56; 57]. Figure 3a,d indicate that region III had higher annual average N values (daytime: 2.07 \(\pm\) 0.96, nighttime: 2.12 \(\pm\) 0.99), region I had the lowest (daytime: 1.82 \(\pm\) 0.85, nighttime: 1.95 \(\pm\) 0.91), along with region II (daytime: 1.89 \(\pm\) 0.87, nighttime: 2.01 \(\pm\) 0.91). It could be reasoned that the terrain characteristics of region I highlighted as mountainous and coastal plains with high average altitude, featured with low temperature, relatively weak vertical convection, and low aerosol loading [26]. In turn, this results in a relatively weak vertical stratification of atmospheric aerosols. In contrast, the average altitudes of regions II and III were lower, and these two regions mainly encompassed the desert areas marked by high temperatures, strong vertical atmospheric motion, and large atmospheric aerosol load. Therefore, the atmospheric vertical stratification was salient, yielding a high N value. These results show that the number of aerosol layers demonstrates a certain relationship with geographic characteristics and climate, which corroborates with the results obtained from Pakistan [12]. AOD1 represents the AOD estimate of the bottom aerosol layer, which is significantly affected by natural factors and human activities [31]. The AOD1 annual average change (Figure 3b,e) showed similar results to the AOD5. Namely, the values observed over  
Figure 3: Interannual variations of aerosol layers over Saudi Arabia (region I, region II, and region III) from 2007 to 2019. (**a,d**) The number of aerosol feature layers (N), (**b,e**) the AOD of the lowest aerosol layer (AOD1), and (**c,f**) the first layer is the thickness of the lowest aerosol layer (HTH1). region III were the highest (daytime: 0.21 \(\pm\) 0.21, nighttime: 0.26 \(\pm\) 0.34), followed by those observed over region II (daytime: 0.20 \(\pm\) 0.24, nighttime: 0.24 \(\pm\) 0.33), and region I presented the lowest value (daytime: 0.20 \(\pm\) 0.23, nighttime: 0.22 \(\pm\) 0.31). Interestingly, the difference in AOD\({}_{1}\) in the three regions was small, which might be related to the comprehensive influence of the topography, natural environment, and human activities.