## 3 Results and Discussion
### Seasonal Variation of the Aerosol Layer over Saudi Arabia  
In this study, the optical and physical characteristics of the vertical distribution of aerosols during the daytime and nighttime in different seasons were studied (Figures 5-16). AOD\({}_{5}\) exhibited the highest values (see Figure 14a,d) in spring and summer, among which region I (daytime: 0.40 \(\pm\) 0.41, nighttime: 0.40 \(\pm\) 0.39), and region II (daytime: 0.47 \(\pm\) 0.43, nighttime: 0.47 \(\pm\) 0.44) presented with the maximum value during the spring and daytime. The three regions presented with their lowest values in winter (I/II/III, daytime: 0.27 \(\pm\) 0.31/0.25 \(\pm\) 0.29/0.25 \(\pm\) 0.27, nighttime: 0.26 \(\pm\) 0.32/0.24 \(\pm\) 0.29/0.27 \(\pm\) 0.31), which was consistent with the data reported in multiple previous studies. [PERSON] et al. [35; 44; 59] found that the dust peak in northwestern Saudi Arabia emerged in April and May, and the dust peak in the central and southern deserts emerged during May and June, respectively. The wind direction of dust and non-dust days in these areas has not significantly changed. This finding indicated that the dust aerosol was mainly generated over the local desert, and the resulting dust aerosol mainly formed in spring and summer. [PERSON] et al. [35] also found that dust aerosols over western Saudi Arabia were mainly associated with easterly winds from the Rual Khali Desert in spring and westerly winds from the Sahara Desert in summer. The high values of AOD in summer may be related to the rising temperatures, as high temperature and dryness play an important role in the rise in sand and dust [32; 38; 59]. Additionally, the water vapor content is directly related to the AOD atmospheric load. The higher the water vapor concentration in summer, the higher the atmospheric relative humidity and, hence, the higher the AOD values [18; 34]. The lower AOD values in winter may be attributed to precipitation in cold months which can help remove aerosols, or because the active intensity of wind-sand is lower than that during spring and summer [45]. the level of dust storms and is related to the level of economic and industrial development, the level of urbanization, and the level of automobile emissions, which is consistent with the research results reported by [PERSON] et al. [45]. Notably, the seasonal variation of AOD\({}_{1}\) values for all three regions is similar to that of AOD\({}_{\text{s}}\) (Figures 5 and 15c,f). The N values of the three regions (Figure 15a,d) are the highest during spring or summer, followed by autumn and winter, which may be due to higher temperature and enhanced vertical convection in spring and summer, resulting in aerosol stratification [60]. In each season, N values for region III (daytime: 1.78-2.23, nighttime: 1.97-2.20) than in area I (daytime: 1.62-1.91, nighttime: 1.73-2.02), and region II (daytime: 1.71-2.07, nighttime: 1.86-2.08) were recorded. Additionally, according to the spatial distribution map (Figure 8), the distribution of N values was relatively uniform, and the areas with relatively high values were primarily distributed in the plain terrain along the Red Sea coast and the eastern region, which might be related to particulate matter generated by seawater desalination and an increasingly intense dust activity in the eastern region [17; 18; 45]. From the spatial distribution map (Figure 6), it can be observed that the higher the geographical elevation, the greater the seasonal mean value of HB\({}_{1}\), and that the high value is mainly observed in the mountainous areas lying between regions I and II. The seasonal variations of HB\({}_{1}\) values (Figure 14b,e) during daytime and nighttime were markedly different. Compared with daytime (region I: 1.18-2.05 km; region II: 1.05-1.84 km), the seasonal variations of HB\({}_{1}\) values during nighttime (region I: 1.19-1.35 km; region II: 1.00-1.17 km) are not evident, which may be caused by low temperature and weak dust activity during the nighttime.