## 3 Results and Discussion
### Seasonal Variation of the Aerosol Layer over Saudi Arabia
Figure 8: Seasonal spatial distributions of N over Saudi Arabia from 2007 to 2019. Figure 9: Seasonal spatial distributions of AOD\({}_{1}\) over Saudi Arabia from 2007 to 2019. During the daytime, the seasonal mean spatial distribution of HTTH\({}_{1}\) was similar to that of HB\({}_{1}\). At nighttime, the values of HTTH\({}_{1}\) (spring: 1.95-2.17 km; summer: 2.12-2.37 km) are higher over the entire southern part of Saudi Arabia (Figure 15c,f). This variation can be explained by the substantial diurnal temperature differences during spring and summer and the enhancement of vertical convection caused by the Indian Ocean monsoon, resulting in the enhancement of vertical motion of the aerosol layer [34]. Compared with spring and summer, autumn (1.71-1.76 km) was slightly weakened, and winter (1.15-1.25 km) was the weakest. Interestingly, HTTH\({}_{1}\) in region III was the highest in the three regions during the summer daytime and was the lowest at nighttime, reflecting considerable diurnal temperature differences in the desert region and the greater impact on HTTH\({}_{1}\). Moreover, the seasonal average values of TAH during daytime and nighttime were the same (Figure 14c,f) and presented with the following trend: summer (during the daytime, 5.03 km to 5.34 km, nighttime: 5.54 km to 6.00 km) > spring (during the daytime, 4.17 km to 4.25 km, nighttime: 4.63 km to 4.92 km) > autumn (during the daytime, 3.52 km to 4.11 km, nighttime: 3.79 km to 4.28 km) > winter (during the daytime, 2.46 km to 2.96 km, nighttime: 2.90 km to 3.28 km); the value was slightly higher than the daytime value observed in the evening. The seasonal mean value of PAOD\({}_{1}\) (Figure 16a,d) in the three regions generally indicated the minimum value in summer (daytime: 0.59-0.66, nighttime: 0.56-0.60), followed by spring and autumn (daytime: 0.61-0.68, nighttime: 0.60-0.69), and the maximum value in winter (daytime: 0.72-0.76, nighttime: 0.69-0.77). This pattern can be seemingly explained by the characteristics of AOD\({}_{1}\) and N as discussed above, but by considering a  
Figure 11: Seasonal spatial distributions of PAOD\({}_{1}\) over Saudi Arabia from 2007 to 2019. Figure 10: Seasonal spatial distributions of HTTH\({}_{1}\) over Saudi Arabia from 2007 to 2019.  
roughly inverse proportional relationship [40; 46; 61]. The spatial distribution diagram also shows that PAOD\({}_{1}\) exhibits a certain correlation with N; the larger the value of N, the smaller the value of PAOD\({}_{1}\). Figure 14: Seasonal variation in the optical and physical properties of aerosol layer over Saudi Arabia (regions I, II, and III) from 2007 to 2019. Where (**a,d**) represent AOD\({}_{S}\), (**b,e**) represent HB\({}_{1}\), and (**c,f**) represent TAH. Figure 12: Seasonal spatial distributions of VDR\({}_{1}\) over Saudi Arabia from 2007 to 2019. Figure 13: Seasonal spatial distributions of CR\({}_{1}\) over Saudi Arabia from 2007 to 2019. As shown in Figure 16b,e, the seasonal average values of VDR\({}_{1}\) in spring (daytime: 0.21 \(\pm\) 0.08, nighttime: 0.16 \(\pm\) 0.07) and summer (daytime: 0.18 \(\pm\) 0.08, nighttime: 0.15 \(\pm\) 0.15) were higher than those in autumn (daytime: 0.14 \(\pm\) 0.07, nighttime: 0.12 \(\pm\) 0.07) and winter (daytime: 0.15 \(\pm\) 0.09, nighttime: 0.12 \(\pm\) 0.08), seemingly due to the severe sand-dust activity which led to the remarkable aspheric characteristics of aerosols [17; 38; 46; 62]. The seasonal mean value of VDR\({}_{1}\) in regions II and III was relatively close and higher than that in region I. The seasonal average value of CR\({}_{1}\) (Figure 16c,f) in the daytime did not exhibit significant seasonal variability (0.79 to 0.89), while the changes in the three regions were relatively small for each season. The seasonal variation of CR\({}_{1}\) at nighttime is significant (0.88 to 1.49), which shows the following pattern: \"summer > spring > autumn > winter\".