## 3 Results and Discussion  
### Interannual Variation Characteristics of Aerosol Properties  
As shown in Figure 2 (Figure 2a: daytime; Figure 2d: nighttime), the annual average AOD\({}_{\text{S}}\) of the three regions from 2007 to 2019 were analyzed. The results show that the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Abbreviation** & **Parameter** & **Abbreviation** \\ \hline AOD of the lowest aerosol layer & AOD\({}_{1}\) & The base height of the lowest aerosol layer & HB\({}_{1}\) \\ The top height of the lowest aerosol layer & HT\({}_{1}\) & The color ratio of the lowest aerosol layer & CR\({}_{1}\) \\ AOD proportion of the lowest aerosol layer & PAOD\({}_{1}\) & Volume depolarization ratio of the lowest & VDR\({}_{1}\) \\ Number of aerosol feature layers & N & The top altitude of the highest aerosol layer & TAH \\ The sum of the AOD derived from all & AOD\({}_{\text{S}}\) & The thickness of the lowest aerosol layer & HTH\({}_{1}\) \\ aerosol layers & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The list of abbreviations of aerosol optical physical parameters in this paper. annual average AOD\({}_{\text{S}}\) exhibited an increasing trend from 2007 to 2009, which might be related to the economic development of Saudi Arabia and may also be influenced by the frequency of sandstorms [35; 44; 54]. The slight decrease reported in 2009-2019 corresponds to the relative slowdown in the economic development of Saudi Arabia, and few emission reduction measures have been undertaken to control events pertaining to aerosol accumulation comprising sand and dust in recent years. The annual average AOD in Saudi Arabia has seemingly varied over the years due to the sandstorm activities. This finding agrees with that stated by [PERSON] et al. [32], who reported the long-term MODIS estimate of the annual average variation trend of AOD for Saudi Arabia in 2002-2013. The annual mean value of AOD\({}_{\text{S}}\) was the highest in region III (daytime: 0.39 \(\pm\) 0.38, nighttime: 0.39 \(\pm\) 0.37), followed by region II (daytime: 0.35 \(\pm\) 0.36, nighttime: 0.35 \(\pm\) 0.35), and the value was the lowest in region I (daytime: 0.34 \(\pm\) 0.37, nighttime: 0.33 \(\pm\) 0.34). This finding can be explained by the fact that region III is an expansive desert located in the border area, with a limited population, exhibiting less accessibility to technology for the prevention and control of aerosol accumulation comprising sand and dust. Though a considerable portion of the area of region II is a desert with a relatively larger population and higher elevation than that of region III, it also includes Riyadh (the capital of Saudi Arabia). Considerable efforts have been aimed at the prevention and control of the dissemination of sand and dust in the region. On one hand, this seemingly explains the lower values of AOD\({}_{\text{S}}\) than that of region III. On the other hand, the average elevation of region I is higher, and the coast of the Red Sea is plain, which is less affected by the desert and the Mediterranean climate; hence, the AOD of region I is the lowest [17; 55]. In general, except that the average AOD value of region I was found to be slightly higher during the daytime than that observed during the nighttime (\(\sim\)0.1), and the difference between regions II and III was small. This may be attributed to the fact that region I is less affected by dust aerosols, and the anthropogenic activities in the region are more intense during the daytime than at nighttime, yielding enhanced anthropogenic aerosol emissions. In addition, lidar observations are affected by solar radiation, and daytime observations may be biased. The CALIOP aerosol layer data provided the top and bottom heights for each aerosol layer. This study was performed to examine the bottom height of the lowest aerosol layer (HB\({}_{1}\)) and the top height of the highest aerosol layer (TAH).